| jazzpeter65 |
HI Guys!
I'm putting together a JFET guitar amp at the moment. Got the idea from Albert Kreuzer's page about the FET preamp he made for bass.
Have a look and let me hear your comments.
PS: The JFET used in the circuit should be BF245 or 2N3819
Look here for schematic:
http://www.bossaboa.dk/preamp.gif
or
http://www.bossaboa.dk/preamp.pdf
Best,
- Peter |
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| jazzpeter65 |
Updated schematic for FET guitar preamp:
I've made a few adjustments and eratas since the first version.
This preamp sounds great - forget about opamps for a moment a try this one. It's warm, smooth and tubey. Very simple circuit with few phase invertion stages.
Further ideas:
Effects loops
Spring reverb circuit
Overload/peak LED circuit
Parametric mid |
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| dimitri |
| I can suggest to you to increase R4, to decrease C1 (at least ten times). I also think it will be good to move "bright" switch to the input FET source. And as for me the Baxandall-type tone control suits better to "clean" channel, while in "distortion" channel the simple Fender tone control is OK. My suggestions cannot be applicable to you, as I don't know in what style do you perform. |
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| jazzpeter65 |
HI Dimitri,
Increasing R4 is a bad idea IMHO. Normally for guitar amps you see a 1M resistor here.
From what I knough (I'm not ingenier;-) this means that the input will be as noisy as a 1M resistor (called thermal noise??), Right?
Maybe better to decrease the input impedance to 500K if you can, to get the noise down?
C1 is too big - I agree. On most circuits you see a 100n here together with a 1M going to earth. The lowest note on guitar is 84hZ so the filter has to cut below this.
Moving the brightfilter to the input FET is a good idea. But how and where would you do that? Any ideas?
For me the Baxandall-type EQ is the best for guitar - no matter what style I play.
I think that the Fender passive tonecontrol is a mistake and a result of bad design. Leo might have stolen it from and RCA book without giving it too much thought. Today it's just how we percieve a guitar to sound.....
You see, when you place the controls of a fender EQ at 12'oclock it's not neutral at all, but cutting your midrange at ca. 750Hz (look at the plot).
Good for twangy countrymusic but not good for more midrangy playing.
To get a flat frequenze response you have to set bass and treble at 0, and the mid all the way up (see picture 2). Still it's not the same sound as a Baxendale. Sounds out of phase to me....
If I need the Fender curve I just turn down the mid on the Baxendal and get the same result.
Yes, I'm building the FET preamp as a "Clean" channel. For distortion I put a box( or two;-) infront. I don't ever you the "Drive" channel for that simple reason that all the effects that I put in front will be distorted Fx. reverb and delay:dead: .
Unless you use a effects loop afer the drive channel ofcourse:D
This FET pre is going to be used with both electric and acoustic (godin) guitars, so it should have the possibily to play 99% clean if I want it to. The FET's will hopefully add a nice and warm tubey coloring....
- Peter |
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| jazzpeter65 |
| ...and this is the Fender tonestack with bass and treble at 0. Mid is turned all the way up: |
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| jazzpeter65 |
Here I have simulated the Fender "neutral" curve with a standard Baxendale with only a bass and treble control. Both are turned about 75% up.
On the downside you can say that the Baxendale actually gains the signal, where the Fender only takes from "whats there allready".
Using quality components I don't think that noise is a problem today.
- Peter |
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| dimitri |
HI Peter,
Thank you for your comment, now I see you are a good company ;)
>Increasing R4 is a bad idea IMHO. Normally for guitar amps you see a 1M resistor here.
>From what I knough (I'm not ingenier;-) this means that the input will be as noisy as a 1M resistor (called thermal noise??), Right?
>Maybe better to decrease the input impedance to 500K if you can, to get the noise down?
No, the input impedance is shunted by pickup resistance, inductance and capacitance, so the input resistor will give rather small contribution in the overall noise. From the other side the pickup inductance and capacitance (with the cable and input capacitance) form a parallel resonant circuit with frequency 2-10kHz. If you use 0.5MOhm input resistor the resonance is almost damped, but if you use 5MOhm input resistor you can obtain up to +12dB resonant peak – so called voice (this is a small secret) – you can play with different caps in the range 500pF-2.2nF and put them in parallel with input resistor. By the way the 68kOm in series with input cap is too high/noisy. Use 2.2-4.7kOhm
>C1 is too big - I agree. On most circuits you see a 100n here together with a 1M going to earth. The lowest note on guitar is 84hZ so the filter has to cut below this.
If you design clean channel – what you actually did – don’t change it. Whether would you like to design distortion channel make the high pass filter 500-1000Hz or so, thus the low-frequency notes will be attenuated before distortion and will not give unpleasant roar.
>Moving the brightfilter to the input FET is a good idea. But how and where would you do that? Any ideas?
Use 10-47nF instead of C2 – it will be bright and shunt it by switch with your 220uF – it will be normal.
>For me the Baxandall-type EQ is the best for guitar - no matter what style I play.
Yes, I agree, but the e-guitar is not the source when we can speak about flat frequency response. I omit EQ at all and use octave equalizer in effect loop after gain stages.
>Yes, I'm building the FET preamp as a "Clean" channel. For distortion I put a box( or two;-) infront. I don't ever you the "Drive" channel for that simple reason that all the effects that I put in front will be distorted Fx. reverb and delay .
Yes this is bad idea to put reverb and delay before combo, because not only the direct signal will be distorted, but reflection signal also and it will be a complete mess, use them in the effect insert/return connection _only_.
>Unless you use a effects loop afer the drive channel ofcourse
>This FET pre is going to be used with both electric and acoustic (godin) guitars, so it should have the possibily to play 99% clean if I want it to. The FET's will hopefully add a nice and warm tubey coloring....
Yes, it will … |
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| dimitri |
You can find one of my active guitar electronics (active wire) on my page
www.angelfire.com/az3/dimitri/
and the figures of my other front-end electronics, which was published in ETI, 1995, June are below. May be somebody can help me to find this paper as my copy had gone with the hard drive to the trash can. |
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| dimitri |
frequency response:
1- S1 closed, S2- opened
2- S1 closed, S2 – closed (“bright”)
3,4,5 - S1 opened, S2 - opened, various position s of R2 slider
6,7,8 - S1 opened, S2 - closed (“bright”), various position s of R2 slider |
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| jazzpeter65 |
HI Dimitri!
Thanx for your nice reply. Godd company? Surtantly! I indeed need your help to develop my preamp, so please stay tuned:D
Before I comment your suggestions I would like you to elaborate and explain a little on the two GIF's your have attached.
Best,
Peter |
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| fernmeister |
interesting thread...
jazzpeter, could I ask, what sort of the programme are you using to generate those freq pictures?
thanks. |
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| jazzpeter65 |
The program I used for the EQ curves are called Tone stack calculator found at http://www.duncanamps.com/tsc/
Great tool for trying all kinds of tonestack for guitar out.
Baxendale is there too - it's called "james".
- Peter |
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| jazzpeter65 |
Dimitri, you wrote:
>Use 10-47nF instead of C2 – it will be bright and shunt it by >switch with your 220uF – it will be normal.
Wouldnt it be easier just to move the bright cap and switch to the gain pot?
- Peter |
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| dimitri |
>I would like you to elaborate and explain a little on the two GIF's your have attached.
Could you please ask what is incomprehensible?
>Wouldnt it be easier just to move the bright cap and switch to the gain pot?
Yes you can do it, you will get different sonic signature, so you can try what would you like best. In my preamps frequently I use both “bright” switches: in the source(cathode) of the first stage and by bypassing of the “gain” pot.
I try to explain. The second stage, being JFET or tube based, will have a substantial input current when being overdriven by positive half of waveform. The capacitor C3 will be charged by this current through part of R7+R1 and move the working point of the second stage to the negative gate-source voltages. This effect can be described like a small compression or sustain. If you use “bright” by bypassing of the “gain” pot this compression/sustain will not be pronounceable at high frequencies. |
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| dimitri |
| And Mr.Lund, on your site http://www.bossaboa.dk on the photo on the starting page – where are you – on the left? ;) |
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| jazzpeter65 |
Yes, I must admit. I'm the guy with the guitar. Sorry, this site is only in Danish so far...;-)
- Peter |
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| jazzpeter65 |
Dimitri,
I get the point about the different placement of the bright switch. I never thought about it this way. Very interesting. I'll try both exsamples to see what I like the best.
It seems that you have designed quite a few guitar preamps yourself. Would you like to share some of them?
- Peter
PS: What I really like about desiging for instruments is that it's all about sound - not measurements....
If it sounds right...it is right!;)
- Peter |
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| jazzpeter65 |
Dimitri;
Concerning active guitar electronics:
I was just wondering why you brougt the two circuits of yours into this thread.
What has active guitar electronics to do with the preamp? What is the connection?
Best,
- Peter |
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| dimitri |
| The guitar amp must work in heavy conditions, say from +10C (club with bad heating) up to +60C (summer stage or when some stupid idiot put your amp on the top of tube combo ;)). When you open datasheet for 2N5457, you can find that drain current changes ~ 20% in that temperature range. The sound of your amp will change with temperature. I show my design which is actually the same common source JFET stage, then i/v converter with servo, no need for temperature compensation and no caps. |
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| jazzpeter65 |
Dimitri;
It's not my intension to use the 2N5457 FET in my preamp.
First I tryed BF245, but they sounded a little dull and lifeless to me. I had a few 2N3819 lying around and they sounded much better. Much more brilliance and no grain.
Try to look at their data and tell me what you think.
http://www.ligo.caltech.edu/~vsanni.../FET/2N3819.pdf
- Peter |
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| jazzpeter65 |
Dimitri,
The guitar pre I'm working on is basically two (inverting) gain stages and one source follower/impedance converter.
I'm wondering if the gain comming from it will be enough to drive a poweramp? Will it have the nessary voltage swing and current output to drive it?
Or would adding a FET buffer be better?
I just found an interesting FET guitar pre at: http://www.sugardas.lt/~igoramps/grass_preamp.htm
I looks like it has a SRPP circuit at the output.
Jack Ormann utilizes this circuit in his mini-booster (took the idea from an old National appnote. See: http://www.generalguitargadgets.com/fet_an_32.pdf). Should give well over 20 Db of clean gain. Look at: http://www.muzique.com/amz/mini.htm or
http://kickme.to/fatboost for schematic
The National Semiconductor circuit is a jfet derivative of an expired patent of a vacuum tube circuit for a "Series balanced Amplifier", US Patent 2,310,342 issued Feb. 1943.
What is you opinion?
Best,
- Peter |
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| jazzpeter65 |
| ...Here's the appnote: |
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| dimitri |
| Dear Peter, as it seems that nobody else are interested, I offer to continue by :mail: |
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| leadbelly |
| quote: | Originally posted by dimitri
Dear Peter, as it seems that nobody else are interested, I offer to continue by :mail: |
I'm very interested, just do not have much to add. So, if you do move off forum, please at least post results! |
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| Devil_H@ck |
I second that, I've been following this with great interest. I'm just not good enough with electronics to be of real help here.
Please, do continue :). |
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| dimitri |
Hi, leadbelly & anthony
May be you have some specific question? |
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| AGGEMAM |
Now now there, we could just be lurking here, but still interested in seeing how this evolves ..
Please continue in the open and not via mail. |
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| leadbelly |
| quote: | Originally posted by dimitri
Hi, leadbelly & anthony
May be you have some specific question? |
No, I just like reading the discussions. I do have a design in mind that I would like to build someday, but that day is a long way off, and that is a super-simple hybrid guitar amp. I am thinking along the lines of a 2 stage preamp made with 6922's (so it can be powered with lower voltages) feeding a chip power amp, so you know where my interests lie. |
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| Barre |
Hi,I've recently discovered this page here and am very glad it exists!!! I've been looking for a FET-preamp design for my guitar for a long time now. I don't know FET's that well yet to desiogn one myself.
Is there a final schematic of the amp now? Or shoukd I continue to follow the first schematic you've posted here?
For me there is not really a tone section needed,since I've got an amp who has great options in this way.
Anyway,for the tonesection in you're pre-amp I'd like to experiment with a Cab Sim I found on the net.
Any comments?
Cheers
Barre |
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| jazzpeter65 |
HI Barre!
I'm too much involved in playing music at the moment to try out the FET preamp presented here. Go ahead a try it out. It should work fine. Otherwise you will have Dimitri at this thread - he's a specialist concerning FET's and Tubes...
You could also try out the Skiff Grass preamp as a tube/FET hybrid. It has been built with succes.
Please post your exsperinces here. I'll get back to building mine later this year.
Best,
Peter |
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| dimitri |
Hi Barre,
What style do you play and what pickup/guitar/combo do you have? What would you like to obtain in the long run? |
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| Barre |
The style I play is metal,sort of metallica-style I'd say.
I own a BC-Rich guitar with standard humbuckers and use a Marshall Valvestate 100R combo (100W)
I set the amp for a "as neutral sound possible" because the music I play contains a lot of clean played parts.
This means I'm not looking for any distortion in a pre-amp what-so-ever. If I need distortion,I'll use my Zoom multieffects unit.
My idea of using the Cab Sim in a guitarpreamp is becuase I thought it sounded good whan I heard it online.
So I thought,why not building it in a guitarpreamp?
So my first idea is:
*guitar to zoom pedal
*zoom pedal to ownmade preamp (with the cab sim in it).
*preamp to the Valvestate where you usually plug in your guitar.
But I also have further ideas that may be better.
My valvestate has an FX loop (send and return) and experimenting with that has learned me that using a external preamp and putting that into the return of my amp, gives much better sounds.
This way, I don't use the Valvestate's preamp, just it's poweramp.
I don't know why,but I like this sound beter.
So another idea would be:
*guitar to zoom
*zoom to ownmade FET preamp (with the cab sim in it).
*ownmade FET-preamp to return of the Marshall Valvestate.
Since I don't know much about FET's yet (we still have to see that in school), I wonder if the posted cabsim can be placed in the FET-preamp posted here...and so replacing the toneregulationcircuit that was in it at first.
So I'd try to build the circuit posted here (if it works,don't know if it's the final schematic here) with the cabsim I posted.
But don't know if it will work.
And if this is possible, is the pre-amp strong enough to feed the return of my Valvestate???
That's about it I guess.
I hope I've explaind it well
;)
Regards and thanks for your reply
Barre |
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| Barre |
Hi,
Actually I don't expect you to check what I described up there.
A closer look at the Cab Sim above has shown me that it's just a combination ah filters (HP and LP).
So I think I if I substitute that in the pre-amp above instead of the givven tonecircuit,that will do. :)
So actually I there's just one question left.
Wil that pre-amp be able to drive the return of my Valvestate (since I don't use the pre-amp of the Valvestate then)?
And my biggest question:
Can you give me some online resources that teach FET's?
I already know transistors a bit, so I'd like to teach myself some FET basics.
Thanks again
Barre |
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| dimitri |
| Thank you Barre, for your explanation, I will think and answer, have a hard day today ... I have a schematic of Marshall Valvestate so I get your point ... |
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| Barre |
Thx Dimitri.
Please do take your time.
I will build this project this summer (when there is no school) and I also prefer to build something after figuering out how things work ;)
I've never found a schematic of my valvestate on the web,but maby I just have to look harder
Thanks again and once more,there 's no rush.
Barre |
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| Barre |
Thanks once more Dimitri.
Wel,the cab sim was just an idea.
But you're probably rigt.
I don't use a cabinet, but just the speaker in my combo.
So I guess my question reduces itself to: Can I find a FET-preamp that can drive the poweramp of my valvestate? ;)
The cabsim maby was a silly idea...but until someone gives you some advice,the only thing you can do is experimant.
I'm still looking for "that" sound with which I wanna play.
I'd be glad to try the FEt-preamp posted here,but want to be sure that the schematic is good and not noisy and strong enough to drive the return of the valvestate.
Thanks for the links. |
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| dimitri |
The block schematic of the guitar amp is drawn in enclosed figure, here
1 is pre-equalization, 2 and 3 – amplifying stages, 4 – post equalization
I prefer several gain controls as in Roland combos, for versatility |
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| dimitri |
| The block 1 is the pre-equalization; the main idea is to amplify only hf components and to emphasize the pickup resonance. The simple JFET stages mentioned above is not good as they inevitable will be clipped with high outputs pickups. Please note that JFET stage use power supply of dozen volts, not hundreds, as tube stage. Use of guitar volume control is not recommended as it often changed tone simultaneously with volume. For the block 1 I can recommend op-amp solution from Peavey, with the gain varied from several dozens to less than unity. With rather small cap in feedback loop, the lf gain remains unchanged and only hf gain will be varied. You can play with input cap to shift the resonance of pickup, the input resistance should be more than 1MOhm. |
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| dimitri |
| For the given JFET Idss - drain current with source short-circuited to gate should be measured, then Rd – drain resistor can be calculated as (Ec-3V)/2Idss. After with gate shorted to ground the variable resistor might be connected in source network as Rs and adjusted in that way that the drain voltage becomes several volts higher than Ec/2. Then Rs should be measured and replaced by fixed resistor. |
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| dimitri |
| For the amplifying stages the obtained Rs and Rd values for given JFET should be used, I don’t advise to shunt source resistor Rs with a cap, otherwise only even harmonics will be generated. You can play with resistor between gate and input, capacitor between gate and ground and diode between gate and ground. You can use Si, different LED diodes, their series connection, or different diodes in parallel and back-to-back. For the post equalization block you can use three-band Baxandall tone control, mentioned earlier by Peter Lund. Please note that if you need gross overdrive no tone control will be able to remove unpleasant hf distortion (“sand”) from the sound. The second order low-pass filter, tuned on 2.8kHz can be inserted after amplifying stage 3 to remove unwanted fuss. |
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| Barre |
Thanks once more...
You're help is really not measurable!
The block diagram of the preamp you suggest seems very interesting and I'll do some research on it and definately try to build my disign after your block diagram!
One little question. You say you can add diodes in betweeen gate and ground (source). Why is that?
I've seen that in some schematics to get distorion/cliping. (like back to back diodes,LED's, even PN-junctions of transistors,...)
Or is it to obtain stability of the FET-circuit (I know in a transistor common emittor design ,sometimes diode(s) are used between base and emittor/ground to get a stable circuit or to make a constant current source...)?
Thanks for the really valuable information!
I have two weeks vakation now and with you're post, I have some great stuff to investigate and try out now!
Barre |
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| dimitri |
Barre,
This is solely for purposes of getting extra disto, you may start without any and add them later |
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| Barre |
I've been doing a lot of thinking,research and simulation today.
When I determine the dimensions of the FET-stages like you explaind so well, I just have a little problem.
I first measure Idss and then place the calculated Rd in the circuit.
I take the gate away from the source (ground) and leave it open and add a potmeres as Rs.
Then you should adjust Rs so that the drain voltage is a bit more than Ucc/2,right?
From where do you measure that? Is it the voltage from D to S or from (I used a +15/-15V powersupply because of the opamp in the first stage)) d to ground or from D to -15V?
I think it's from D to the ground....
I enjoy testing you're ideas.
I hope designing and simulating will be done by the end of the week so building can be started :D
As for block 4,is this a FET bufefrstage?Any hints on that?
That's the stage going to the return of my Marshall.
Once more,I can only thank you for your valuable help!
Kind regards,
Barre |
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| dimitri |
>I take the gate away from the source (ground) and leave it open and add a potentiometer as Rs. Then you should adjust Rs so that the drain voltage is a bit more than Ucc/2, right?
No, no, adjust Rs with gate _shorted_ to ground.
>I think it's from D to the ground....
yes, you are right, use bipolar +15/-15V supply for op amps and +15 for jfet stages.
Block 4 is post equalization, you can use Peter tone control |
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| jazzpeter65 |
Barre,
Before you build this circuit consider to add a parallel effects loop just before the last gain stage - not a serial, as you have proposed.
From my exsperiens, it's better to "mix in" the effect, than breaking the signal path. It simply gives a much better guitarsound because you leave your original signal "intact". I have built two guitaramp with parallel loops and they work great.
Also consider to try the SRPP-output as I suggested before. It may sound good for guitar - lot's of clean gain.
- Peter |
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| Barre |
So is this a proper way to bias the FET stages?
If I teke Rs 1k or more than Ud is way over 15V/2 and then the signal always is attenuated instead of amplified.
If I use Rs 82 Ohm, then I get amplification and Ud is 8V apprix.
So,the first stage you suggested is with an opamp.
I simulated that and got HUUUGE gain at it's output.
That means all following FET stages will be clipped,no?
So the amplifier,I thought about an amplification of 1 to 5 or 1 to 10, is that good?
Maby I'm wrong with my simulation ;)
I use a sinewave with 1V amplitude.
What signal sould I use when I design a circuit for my guitar?
Peter, I've build the "minibooster" on the Skiffgeras preamp' output,and indeed...sound very clean...even with a powersupply of just 15V.
Thanks
Barre |
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| dimitri |
>If I use Rs 82 Ohm, then I get amplification and Ud is 8V approx.
Good, 82 Ohm is the right value.
>I simulated that and got HUUUGE gain at it's output
The amplification of the first stage is varied from zero(slider in right position) to 50(slider in left position). I enclose the figure with values to start with. You can change input cap in the range 1…10 nF and feedback cap(s) in the range 0.47..22uF. When the switch is closed it will be normal, while when it is opened it will be bright – the low frequencies will be attenuated due to small 0.47u cap.
>What signal sould I use when I design a circuit for my guitar?
For simulation you can use something like 50-100mV, but you should be aware that it can be up to several volts during short time when you hit the strings. |
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| Barre |
Thanks for your help sofar!
I will post my schematic once it's finished AND working as my thanks to your help!
I'm not sure about the parallel FX loop though.
That's the kind on my Marshal VS now...and it's cool...but...
YOu have to knwo I make my distorton with a multieffects unit..and puting that in the FX loop...effects only (without disto) sound ok,but using -say -a rithm distortion, it really doesnt' sound good at all.
So I'm thingking of having one small input-gain stage,then the FX-loop ('cause I don't wanne ruin my ZOOM-effectsunit ) and then one mor gan stage and then the post EQ.
If everything goes well,I'l build it this summer.
Now I've got some exams to worry about:eek:
See yah later
Barre
btw,I wanna share the FX-loop I found on the net;..maby it's usefull for others. |
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| FredM |
Combo side:
Ok - I am baffled! How the hell does this circuit work, and what does it do? - I am an ingeneur so I should be able to work it out.. but this one defies me! :confused: |
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| dimitri |
FredM wrote> How the hell does this circuit work
Which one? |
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| benny |
hi,
i'm no engineer... actually i'm just a teenage high school kid, but i think this circuit is actually pretty simple.
for a start, you ask what it is... it's a buffered parallel effects loop...
i think it works like this:
from preamp in, signal is split and follows what i drew in on diagram below as red and green paths. red path goes to effects, and back, green is dry signal... the mix pot i highlight in blue has the center tab conected to earth... when rotating, you are bleeding a different amount to earth of each effected and dry signals as they are attached to the outer tabs...
all the switching is just to change levels. the TL071's would be there as buffers.
the only other part i think is a bit confusing is the 10ohm resistor on the earth conection for the send... but i think this is there to break any ground loops that might ocour.
does this make sense??? if i make a mistake, someone point it out, but i think that's how this circuit works. pretty simple.
cheers. |
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| dimitri |
Yes, switching is for adjusting the input level, which your FX can cope. In shown position of loop level switch the input signal is attenuated for guitar effect with 100mV sensitivity and amplified later, after FX
Yes, 10 Ohm is there to break any ground loops |
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| FredM |
| quote: | Originally posted by benny
hi,
i'm no engineer... actually i'm just a teenage high school kid, but i think this circuit is actually pretty simple.
cheers. |
Hi Benny - Good explanation of that circuit (better than many engineers I have worked with could have given!) - Alas, not the circuit I was having problems with!
Can you tell me what this is?
Best regards,
Fred.http://www.diyaudio.com/forums/attachment.php?s=&postid=320292 |
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| alchemy |
hello, i'm interesting by this messages because i search to build a preamp for play guitar and recorded with PC, i have some questions, the thunderchief is a preamp and simulator of amplifier but the orginal message of peter is just a preamp, no simulation
it's possibly to used only the cabsim for preamp and plug in the sound card in line enter? |
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| Barre |
The thunderchief will do as pre-amp if you want to record to pc.
If you want to keep your circuit simple and only use the runoffgroove cab sim, you will have to build an extra gain stage.
(herefor you might try the opamp design Dimitri presened earlier!!!Realy good one!)
That's my opinion after building the CAB sim an checking it out.
Barre |
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| alchemy |
thats i want is a good preamp for plug my guitar in pc, with good clear sound and if it's possible an option for a simulation of amplifier/hp and a loop fx, but a want to switch this.
i don't want tube for the moment. |
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| alchemy |
nobody are interest by this subject :bawling: :xeye:
in preamp for guitar what the difference between j201, 2N5457 or others jfet |
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| Giora |
Hello, all.
I am recording my guitar direct to PC. A have a high quality instrument( Les Paul), and cheap op-amp preamps ruin the sound.
I should probably go straight to the tube preamp, but before
that I think to try J-FET.
I know it is a stupid question, but how close J_FET comes to simulationg the tube behavior ?
I don't use lot of distortion, need instead deep bluesy sound with a lot of sustain.
Last, but not least: Even if you put J-FET or tube preamp,the sound still goes thourgh op-amps in the sound card. Wouldn't it ruin the sound ?
Sorry for unprofessional questions :) |
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| James D. |
| quote: | | how close J_FET comes to simulationg the tube behavior ? |
Ahh How long have you got?
I guess the quick answer is - for 'clean' operation it can be very close.
For overdrive or distorted operation... then better than a bipolar but not really that close.
And when compared to a tube pre-amp engineered for the traditional 'toob' sound - then not a lot...
So it depends what you want from the pre-amp as to which is best.
The caveat here is that if you really understand what generates a particular sound from a pre-amp then you can use any active device to generate any sound... However this is very very rare and I for one can't do it.
ciao
James |
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| MrGuitardeath |
Hey I'm certain I'm too late but there is a much much simpler JFEt pre out there that performs better.
http://www.diyguitarist.com/Guitars...uitarPreamp.htm
Go to that link. It links you to the originator of the circuit, but Paul has modified it. So far this site has been very helpful and so has the owner.
I've read a lot about this preamp online and it is said to be like sticking a 12AX7 in your guitar.
I've also spoke to a few ppl now that have built and use it regularly. They seem to swear by it.
I'm not that experienced or educated in electronics yet but I have learned that a lot of these simple circuits based on old world transistors and IC's give the richness and vibrance we're always looking to increase in our tone.
......after trying numerous dual op amps in my Crate head a 40 year old chip ended up giving me some of the best overall performance. The NE5532. I researched it's specs though and out of all the chips I was using that had specs close to a 12AX7 tube, the 5532 was by far the closest.
I ended up using it in the input buffer stage. Then gain one and gain two. I believe I have either an OPA2604 in my clean channel or an OPA2228. Both very tubey sounding. The 5532 just had great clarity and overall improvement everywhere rather than only in specific areas like the other chips gave.
I then used an OPA2228 in my reverb. No other chip that exists could outperform this chip for reverb circuits. It's imaculate.
Anyway my point is even op amps can give a tube sound in the right circuit and better yet in tube emulation circuits. |
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| gtech |
from Dimitri post:
>>>>You can find one of my active guitar electronics (active wire) on my page
www.angelfire.com/az3/dimitri/
and the figures of my other front-end electronics, which was published in ETI, 1995, June are below. May be somebody can help me to find this paper as my copy had gone with the hard drive to the trash can.
Attachment: eti069501.gif
This has been downloaded 926 time(s).<<<<
I've been following this thread on and off since the day it started, and I find your preamp design very interesting Dimitri.
I am already using Don Tillman's FET Preamp Cable, and Jack Orman's Mosfet Booster in a box.
But I always wanted to combine both a FET preamp in the cable and an opamp preamp in the box. At first, I just wanted to add a simple preamp in the FET Preamp Cable box, but I find your design better. Except the +/- 15V which I could bring down to +/- 9V (2 x 9V batteries).
Do you see any problem with that option to use +/- 9V with your circuit?
I want to make a pcb layout for that purpose.
Thanks
Gilles |
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| dimitri |
It is not a problem +-9V, you should only check current through Zener diode and omit current sink (resistor from the output of the op amp to negative rail) for battery life.
I have some reprints left from the latest Berlin AES convention named "Triode Emulator," which I can send to interested persons by ordinary mail free of charge, just wrote to ddanyuk at usa dot net. Please no request for preprint in electronic form, I don't have it. The design for a low-noise amplifier is presented. The amplifier has a tube-like transfer characteristic and produces harmonic distortion components that are similar to triode preamplifier. FET common source stage without source resistor has quadratic transfer characteristic. FET common source stage with rather large source resistor has about linear transfer characteristic. For some intermediate value of source resistor the transfer function becomes close to three halves power law, which is typical for vacuum triode. |
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| gtech |
Ok, Thanks. I began working on a layout for the article in the ETI of 1994 this evening.
I'm interested about the article for the tube emulation... I sent you an email
Another source of info.
http://members.tripod.com/~gabevee/sstubepre.html
Gilles |
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| Giora |
Hi, Peter !
You wrote that you've based your design on Albert Kreuzer bass
preamp.
>I'm putting together a JFET guitar amp at the moment. Got the >idea from Albert Kreuzer's page about the FET preamp he made >for bass.
I looked into his schematic on the net, I wonder:
What are the things that makes a difference between guitar preamp and bass preamp ? :)
Input impedance seems to be the same, Albert has some tone swithes that you ommited ( ultra low and bass cut), but that would not contribute a major difference in the tone of the circuit, woudl it ?
Your circuit looks identical to the Albert's, except for the swithes that I mentioned.
I admit, I did not see ANY schematic of bass preamp before, this is why I am asking about this.
Regards, George. |
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| Barre |
quote from Dimitri:"I have some reprints left from the latest Berlin AES convention named "Triode Emulator," which I can send to interested persons by ordinary mail free of charge, just wrote to ddanyuk at usa dot net. "
Bring it on I'd say!!!Stuff like that is interesting enough for me to study;)
(versaevelbart@hotmail.com)
Cheers
Barre |
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| Barre |
I've started a new topic on simulating tubedistortion.
Maby some of you wanna check that out!
I didn't post it here, 'cause it doesn't use FET's... It's a totaly different idea.
Why did I post it?
I wanted to cher it + maby some improvements may come out of it!
Check it!
http://www.diyaudio.com/forums/show...&threadid=38141
Barre |
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| Marito |
Hi Peter(or whatever)
Please send me an e-mail at makoko1900@yahoo.com.ar including the Jfet guitar preamp schematic of the page www.bossaboa.dk/preamp.gif because I don´t open this page from my country.
Thank you very much and congratulation for your excellent forum.
Greeting from Argentina and sorry for my English.
Marito |
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| xavmdq |
Hi, I have a something offtopic question. Why not to use an OPAMP with unity gain for buffering guitar pickups? For example using TL081/2 or some instrumentation amplifier.
From http://www.muzique.com/lab/buffers.htm I see this circuit http://www.muzique.com/images/buff8.gif
Someone build it?
I know the clipping is not like tubes but if typical pickups have a peak to peak voltage between 4V and 6V, clipping occurs?
Sorry for my english.
Javier
Argentina |
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| Barre |
Hello,
Why not this and why not that.
It's just a matter of taste, if not you'll open up a can of worms and end up in a endless discussion...not getting any whiser out of it.
I've made several own opamp buffer designs, even some with bootstrapping... They all worked fine, nothing special.
My own personal experience is that the high input impedance picks up some noise and their extreme open loop gainproduct isn't neccesary to make a simple guitarbuffer.
Just a point to pay attention to.....dump you're TL072/082...they are damn old and noisy compared what the marked offers nowadays!!!
As for anything else....they work.
FET's are my personal taste though....and I still cannot give a scientificl explanation. |
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| JP Hugsley |
Javier,
If you plan to use the TLO81 in the buffer circuit you describe, you may
run into some issues. First, I would recommend that you emulate the
inherent capacitance of a guitar chord by placing a 330pF capacitor from
the input cap to ground. This will optimize the tone of your pickups by
"resonating" with their inherent inductance. The input resistance is
the parallel combination of the two 1.0M Ohm resistors (500K Ohm) which
is about as low in resistance as you would want to go for this
application; any lower and you begin to "damp-out" the pickup, thus,
affecting the tone.
Secondly, for reason having to do with feedback control theory, you
would want to add an additional resistor at the output of the buffer
(between 22 and 100 Ohms should suffice). This will prevent the buffer
from oscillating when the "phase margin" is diminished by the guitar
chord capacitance.
This "instability" issue is one reason why I would NOT choose to use a
non-inverting op amp buffer in an on-board guitar preamp. I would
rather use a combination amp configuration using discrete amplifiers;
either a JFET or a high-beta transistor, as a common-source or common
emitter input stage followed by an emitter follower in the output stage.
If, however, you like the op amp solution (it is simpler), I would
choose another one that has less input noise voltage and less power
supply current drain. The TLO81 has 25nV/rootHz noise voltage and about
3.5mA current draw. There are other amplifiers available with better
noise specifications and the current draw would give you about 160 hours
of life on a healthy 9.0Volt battery (not great).
BTW: A discrete solution as previously described, can be designed with
over 3,000 hours of battery life and better noise specifications
(depending on the input transistor and circuit design). Food for
thought.
In additon: Please let me know if you want me to post a schematic of the two-stage discrete preamp. I would be happy to do so.
JP Hugsley |
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| xavmdq |
Hi Hugsley,
Thanks for the info. Well, I'll make the buffer using JFET. Please, post the schematic if you can.
I built one using a JFET in common drain (I took it from the last stage in Albert Krauser design. After the 10th transistor it worked well. My idea is, based in Vcc, Vp and Idss, Zin, Gain, to calculate bias resistors. Right now I have 2 exams and I don't have much time, but once I have free time I'll investigate the JFET biasing.
Thanks!
Javier |
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| JP Hugsley |
Hello Javier and Marito,
Sorry it has taking me so long to post a buffer amp design but I had to think this one through. I decided to post a high-quality, discrete, two-stage buffer amplifier designed specifically as a 9.0V, low-current, on-board guitar preamp. I also decided to do this in two installments: the first on choosing and evaluating a JFET and the second as a description of the complete preamplifier.
I noticed that there are several postings about what JFET to use for the best "tone" and I'm sure everyone has their favorites but I decided to use a J201, N-channel JFET for the first stage of the preamp. This device was chosen mainly because it fit the criteria for the preamp design but also because I happened to have a couple laying about the shop.
I have attached three (3) JPEGs: 1) The first shows an evaluation technique used to determine the Idss and Vgs-off of the J201 (JFET_Evaluation01), 2) the second shows the transfer characteristics of the specific J201 used in the design of the first stage of the preamp (JFET_Transfer01) and, 3) the last JPEG is a schematic of the first stage design of the preamp (JFET_Amp01).
It's a fairly straight forward process to determine the Idss and Vgs-off of a JFET and, if the topology of choice is a self-biased amplifier, it is highly recommended that such a process is applied due to the large variation in JFET characteristics; even within the same part type!
Manually determining the Idss and Vgs-off of every JFET would, of course, be a difficult thing to do in a manufacturing environment but, in the case of a "onesy-twosey" home project, it's not a problem.
I used a 9.0 volt battery as a power source and two 1.5 Volt batteries for the -3.0 V control-voltage. When the Gate and the Source of the JFET are "shorted" together and 9.0 volts is placed across the Drain and the Source you have, in essence, a constant current source; the current measured in this configuration is the Idss. In the case of the J201 I used, the Idss was 510uA.
By placing a negative voltage to the gate of the J201 with 9.0 Volts across the Drain and Source, I varied the negative control potentiometer until I had zero current through the JFET. This negative voltage applied across the Gate and Source was the Vgs-off; in the case of the J201 I used, the Vgs-off was -0.77 Volts.
These are good values for the buffer-preamplifier project because I wanted to design the first stage with a bias current of no more than 100uA and still be high-up enough the transfer cure to be linear with "small" signals.
In order to obtain an Id current of 100uA in the J201 using the self-bias JFET amplifier topology (very similar to the standard triode bias technique), I required a Vgs voltage of -0.38 Volts. To achieve this I placed a 3.9kOhm resistor from the Source terminal of the J201 to ground.
In order to have the Drain of the J201 biased at around mid power supply voltage (4.5V), I placed a 43.0 kOhm resistor from the 9.0V supply to the JFET Drain terminal. In addition, I placed a 1.0M Ohm resistor from the Gate terminal to ground to maintain a "high-impedance" at the input of the amplifier.
With the configuration shown in the Jpeg, I achieved a voltage gain of about 6.8 without the 100uFd by-pass capacitor across the Source resistance. In addition, I was able to achieve a voltage gain of about 23 with the 100uFd bypass capacitor across the Source resistor.
The question you are probably asking yourself is, "why do I need gain in a buffer amp'. The answer will become obvious in the next installment.
In the mean-time, please let me know if you have any questions about what I have posted so-far. I'll answer anything I can before we go on to the final design.
Thank you for your patience, JP Hugsley. |
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| JP Hugsley |
JFET Evaluation JPEG
JP Hugsley |
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| JP Hugsley |
JFET Transfer Characteristic JPEG
JP Hugsley |
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| unclejed613 |
as far as driving capacitive loads goes, the best way to drive into a high capacitance with unity gain buffers made with op amps is to use several (4 is convenient with a quad op amp such as a TL084) in parallel...
http://www.linear.com/pc/downloadDocument.do?id=25632 |
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| xavmdq |
Hi JP Hugsley!
Because I still have to do an exam, I only post to thank you for explanation. After exam I'll read your post carefully and make you questions.
Thanks!
Marito: I'll also look that design. Thanks. |
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| xavmdq |
Hi JP Hugley,
Just to add something to your explanation:
If you have different Vp and Idss values, you will need to use the following equations (if you didn't use this equations please correct me):
Vgsq = Vgs for operation point
Vgsoff = Vp = Vgs for Id=0
Idss = Id for Vgs=0
Igss = inverse saturation current (not considered in calculations)
Vp <= Vgsq <= 0V
Rs = -Vgsq / Idq
Vo = Vdd - Id*Rd => if we want Vo=Vdd/2 then
Rd = (Vdd/2) / Id
I have a few questions:
1-
Why do you choose Id = 100uA?
2-
Using FET equation (Vgs related to Id and viceversa):
Id = Idss * (1 - Vgs/Vp)^2 or
Vgs = Vp * (1 - sqrt(Id/Idss))
and with Vp=-0.77V, Id=100uA and Idss=510uA, I get:
Vgs=-0.43V, but you get -0.38V
3-
I calculate the minimum and maximum Vin using:
Vin min = Vp = -0.77V
Vin max = Idss * Rs = 1.989V
DVin = 2.76V
It's enough for piezoelectric pickups? I don't remember but I saw a signal from a piezoelectric pickup and it had a peak voltage of 3V (DVin=6V).
4-
With a gain of 6.8V and taking into account DVin=2.76V we have a variation in the output of DVo=18.77V (DVo must be below Vdd-(Vgsq-Vp)=9V-[-.38V-(-.77V)]=8.61V or 7.61V to have room for battery discharge). |
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| JP Hugsley |
Hi Javier,
Yes, you are correct; what you have written is the equation quantifying the "square law" relationship between id and vgs. But you must realize that the equation is only an approximation that will agree usually to within 10% of the actual value of id based on Vgs-off, Idss, VDS and vgs. A more accurate equation for the transfer relationship of a JFET exits but it is based on the geometries, doping densities, doping gradients and voltage gradients of the JFET itself. If is much easier to use the "square law" equation but, better still, it is more accurate to determine the transfer relationship of your particular JFET experimentally for use as a "self-biasing" amplifier.
The exponent for the transfer relationship between id and vgs you used was "2" (hence, "square law") however, since the relationship is only an approximation, the exponent can actually vary between 1.5 and 2.5 for a more accurate fit. The "square-law" equation is only an approximation.
There are two reasons why I chose a bias current of 100uA. First of all, it is in a fairly linear region of the transfer curve derived experimentally for the J201 and, secondly, it allows a very low current drain from a nine volt battery (essential for an "on-board" guitar preamp). The gain of the J201 is a little less with a low ID value (proportional to the square-root of ID) but this is not a problem in our application.
Due to the fact that the "on-board" preamp in our discussion is being designed more for an electric guitar with an electo-magnetic pickup (not a piezoelectric pickup), gain is needed in the first stage. The "on-board" preamp is intended, specifically, for a Stratocaster, single-coil, electromagnetic pickup but can be modified for other applications. Of course, if the gain is too much for your application, you can always add a "pad" or attenuator to the first stage. But we can customize this later with real listening tests.
If you are worried about the gain being excessive for transients from the pickup you can "buy" yourself 6.0 dB of "headroom" by using two 9.0V batteries for an 18.0 Volt system but this may not be necessary. You may want to first conduct an A/B comparison test between a 9.0Volt and 18.0 Volt system. "Real World" evaluation and testing is essential in determining what best suits you're application.
Please let me know if this helps. Thanks, JP Hugsley |
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| xavmdq |
Hi JP Hugley:
| quote: | | There are two reasons why I chose a bias current of 100uA. First of all, it is in a fairly linear region of the transfer curve derived experimentally for the J201 and, secondly, it allows a very low current drain from a nine volt battery (essential for an "on-board" guitar preamp). The gain of the J201 is a little less with a low ID value (proportional to the square-root of ID) but this is not a problem in our application. |
Ok. Then the first step is to use a FET with Idss=200uA or more and follow these steps:
- Fix Id (Idq).
- With transfer characteristic obtain Vgsq
- Calculate Rs = -Vgsq / Idq
- DVin = function of Vgsq, Rs, Vp
- Calculate Rd = (Vdd/2) / Id for Vo = Vdd/2
- Voltage gain is: Av ~ 2 * (Idss / Vp) * (Vgsq/Vp –1) * Rd
Is this correct?
| quote: | Due to the fact that the "on-board" preamp in our discussion is being designed more for an electric guitar with an electo-magnetic pickup (not a piezoelectric pickup), gain is needed in the first stage.
|
But, I have a piezoelectric pickup (PP) (the guitar actually have electromagnetic pickup, EP, but I added PP and thats why I need a buffer to mix the two signals). I don't touch the original circuit (EP, 9V battery, vol + tone controls): my idea is to mix both signals using this buffer. In both, I have the same signal level (2-3V peak).
Please, continue with the following part. Thanks. |
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| JP Hugsley |
Hi Javier,
I've been off-line for a week and I am just now getting back to DIY.
The calculations that you have written and your line-of-reasoning are correct. However, please understand that there are many JFET's available all with different characteristics. You must choose the one that you feel is best for your application. In my case, my primary concern is for "battery life"; for that reason, I decided on the J201 which can be biased between 50uA and 250uA depending on it's particular transfer characteristics. There are other JFETs that are optimized for noise and/or linearity but they may have much higher Idss and Vgs-off (pinch-off) values and, depending on the bias point, may drain a 9 Volt batter in less than 100 hours. With an overall current draw specification of 250uA for the entire on-board circuit, a nine volt battery can last over 2,000 hours.
For example, if you have a JFET with an Idss value of 3mA and a Vgs-off of -6.0 Volts, you may want to self-bias a common source amplifier with a Iq of 1mA. If you have two such amplifiers on your on-board circuit, that would be a total quiescent current draw of 2mA which would translate to a 9.0 Volt battery life of about 250 hours.
In any case, you would most likely bias your amplifier at about 1/3 the Idss value for best linerity (only a rule-of-thumb). For best results, you could, with a simple voltage source and current meter, determine the transfer characteristics of your particular JFET and, using Excel, graphically illustrate the transfer curve of any JFET you like (the accuracy would depend on the number of data points and the sophistication of your curve-fitting software).
The most striking realization you would have from such an excercise is discovering the large variation in JFET characteristics even among JFETs of the same part type.
Would you be interested in doing such an excercise? Would you like for me to send you some graphic samples of such an excercise?
Please let me know.
BTW: I have never designed a preamp for a piezoelectric pickup. It would help to know the source impeadance of such a pickup. Do you have this information? If not, could you give me the name and model of your guitar, maybe I could reseach this information myself.
Thanks, JP Hugsley. |
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| sepulchre |
Hi guys,
First, I'm nowhere near you in the theory department but I have been building electronics for years and know a bit about it - enough to get me by.
But I'm having a problem biasing the Drain on this FET preamp circuit - not the one shown above, but Donald Tillman's:
http://www.till.com/articles/GuitarPreamp/
He recommends trying a few JFETs to get one with which the Drain bias will be 5 - 7 volts. I tried 10 of them and said bias is always 8.4 to 8.66 volts.
This bias is measured Drain to Ground, right?
I found this article:
http://www.diyguitarist.com/Guitars...uitarPreamp.htm
in which a trim pot is use for the 2.2k source resistor to make biasing easier. When I tried it the drain bias could indeed be set to around 6 volts and the circuit does work . . . Way TOO well. The amplification is far more than the 3.5 db described in the original article. I've seen the circuit used by lots of people successfully and I can't figure out why mine doesn't work. I've checked everything, even building two of them just to make sure I got it right.
I'm curious about how all the FETs I tried come out so similar and yet out of range.
Any ideas on why or how I can get this little beast in line?
I suppose I should just start over using the circuit shown above, but I don't like getting beat by such a simple device that everyone else seems to have no problem with.
Any info or pointers would be much appreciated.
Thanks,
Ken |
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| JP Hugsley |
Hi Ken,
The Fet circuit shown in Don Tillman's article is very similar to the circuit that I had written about in my previous postings. If you have not read my previous postings, this may be a good time to do so.
In any case, the amplifier is a self-biased, common-source design using the J201 JFET. The key term here is "self-bias", which simply means that the amp will bias up to a steady-state (or, quiescent) condition that is totally dependent on the devices "transfer characteristics" (again, this is explained in my previous postings).
In any class-A, linear amplifer (e.g. the Tillman design) you should bias the device so that the DC value at the output (in this case, the Drain) is equal to between 1/2 and 2/3rds that of the power supply (in this case, a nine-volt battery). This will ensure a relatively "equal" swing of an AC signal above and below the DC value at the Drain before "clipping" occurs.
When Tillman said that the DC value should be between 5 and 7 volts, he was referring to the voltage you would measure relative to the negative terminal (or "zero" volt reference) of the nine-Volt battery. This would mean that you have between 4 and 2 volts across the 6.8KOhm Drain resistor. According to Ohms law, this would also mean a bias current between 0.588mA and 0.294mA through the J201 (in the interest of conserving battery-life, I biased my amp to 0.1mA).
It is interesting to note that the J201 JFET I evaluated for the amplifier in my previous postings would not have worked well with Mr. Tillman's design. However, Tillman does say this when he mentioned that not just any J201 would work with the values he chose for his circuit. He suggests that you try a handful of them before you can find one that works well.
On the other hand, by evaluating the JFET and determining it's transfer characteristics (not a difficult thing to do as explained in my pervious postings) you can customize Mr Tillman's common-source, self-bias design for optimal performance (or, in other words, excellent symmetry about the Drain DC bias value).
Please let me know if this helps; if you have any questions, please do not hesitate to ask.
Thanks, JP Hugsley |
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| sepulchre |
First, thank you for your response!
Yes, I have read your previous postings on the subject though I didn't understand everything. I'm getting back into electronics after retiring from a career of programming so I need to brush up on terminology and theory. Also, much of this is new to me as most of my experience has been with opamps where my main considerations were gain and cutoff frequencies.
I think that perhaps I should consider using your schematic rather than Mr. Tillman's for a few reasons:
-It would conserve battery life a bit more.
-Am I correct in my impression that yours is not quite as picky about the particular JFET used?
-I like your gain figures better (6.8). And..
-I've exhausted all 10 of my transistors searching for one with the proper characteristics. There's no electronics store near me so I must wait for ordered parts.
I will try evaluating the FETs I have and study more carefully how to customize the circuit for proper performance
.
One question: If I use your circuit would the 10mfd decoupling cap across the battery still be a good idea?
You've been a great help and I really appreciate your time and effort.
Thanks again,
Ken |
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| sepulchre |
Mr. Hugsley,
After rereading your posts I realize now that the schematic you posted is the first stage of a guitar preamp. I thought it was the whole preamp due to its similarity to Donald Tillman's. Would it work without a second stage? What would be the advantages of a second stage?
Thanks,
Ken |
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| JP Hugsley |
Hi Ken,
There is really no difference between the design that I posted and that of Mr. Tillman. They are both common-source JFET amplifiers; the difference is in the device itself. Even though they are both J201 devices, the variation in IDSS and VGSoff among J201 type JFETs is rather large. If you refer to a data sheet, you may be surprised to see that the IDSS for the J201 can be between 0.2mA and 1.0mA. In addition, the VGSoff can be between -0.3V and -1.5V.
From what I can tell, the device used in the Tillman design is on the high-side of the specification scale. In other words, the device he used most likely has an IDSS of 1.0mA and a VGSoff of -1.5V. The reason that I believe this is because he mentioned that the bias current in his circuit is about 0.5A. This means that the VGS is about 1.1V (0.5mA*2.2KOhm = 1.1 Volt) and the voltage drop across the Drain resistor (R3) is 3.4 Volts (0.5mA*6.8KOhm = 3.4Volts). This, of course, means that the Voltage at the drain of the J201 is 5.6 Volt (9Volt - 3.4 Volt = 5.6Volt).
(If any of this is not clear, please let me know and I'll explain it with a sheet of equations and diagrams).
The J201 device that I used in my circuit had a measured IDSS of 510uA and a VGSoff of -0.77Volts. If I had used my J201 in Mr. Tillman's design, it would not have worked very well (if at all). The bias conditions for my design were chosen to fit the particular J201 I had in my shop; this is what must be done when using the self-bias technique to ensure proper operation. From what you have told me, I believe the J201s that you have in your possession are probably more middle-of-the-road devices rather than the high-end-of-the-spectrum device Mr. Tillman used. In other words, there is nothing wrong with your JFETs, you just have to change the circuit values to best suit them.
Try changing the values of Mr. Tillman's design with one of your JFETs by making R3 = 20KOhm, R2 = 2.0KOhm and R4 = 10KOhm. R3 and R2 will give you a gain of nearly 7.0 (like my design) but R4 will load-it-down to a gain of about 2.0 (this is how you "tame the thing" as you called it).
In any case, don't throw away your JFETs, just evaluate them for IDSS and VGSoff as I described in my previous postings and then select component values to ensure optimal operation (biasing).
Again, if any of this in not clear, please let me know and I will make an effort to produce a paper that is more professional and concise.
Also, a 10uFd decoupling cap is a good idea to have on the circuit board as near to the active device(s) as possible. This will provide a zero-Ohm Voltage source for your circuit and help prevent issues with stray impedances.
Also again, the second-stage device on the design I was talking about in my postings will be an emitter-follower using a BJT. The advantage to an emitter follower stage is to isolate your active device and tone circuitry from the un-predictable loading effects of your guitar cable and guitar amp input impedance. This will become clearer in a future posting but, first, you should evaluate your JFETs and then optimize your common-source circuit design.
I hope this helps.
JP Hugsley. |
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| xavmdq |
Hi JP Hugsley:
Sorry for delay to answer (In this week I have 2 exams and I don't have much time).
The guitar is a Steinberger Synapse EMGSYN-GP-1. I don't have information about piezo. Check this page to see a picture.
In these days I'll make transfer curves using 5 or 6 2N3819 I have (In my city there are no J201 FET). I check a 2N3819 and obtain an Idss=1.3mA and Vp=-2.5V.
In these post and following I'll show my actual circuit. My idea is to replace equalizer with jfets (if possible) to reduce current consumption and the possibility to adjust circuit elements to achieve the best performance and low consumption. |
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| Andronico |
| Hi, my name is Julián and live in Buenos Aires, how many J201 do you need ? |
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| xavmdq |
Hi Julian,
I need between 10 and 20 (I have to select the best for this circuit). My mail is xaviermdq arroba yahoo.com.ar.
Javier
| quote: | Originally posted by Andronico
Hi, my name is Julián and live in Buenos Aires, how many J201 do you need ? |
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| sepulchre |
@ J. P. Hugsley
Hi,
I have evaluated my JFETs, writing their Idss and Vgsoff values on masking tape for future reference. I'm still not clear on how exactly to calculate the surrounding components for optimal results. I'd like to bias the amps at 0.1ma as you have to conserve batteries. I've read and reread your posts and though I have a much greater understanding of how the amp works, I guess I'm missing something. Can you point me to equations I can apply to calculate values for target currents and gain factors?
I'm happy to say that I built an amp following your recommended values as closely as I could: R3 = 21.5 k, R2 = 2.15k and R4 = 10k. This gave a voltage at the Drain of slightly over 6 volts. I know that's a little high but the amp does work so I tried it in my guitar. It's still slightly strong, overdriving the digital effects device I use unless the volume on the guitar is turned down to half or less, but usable. I noticed that when the guitar volume is turn down to 1 or 2 (out of 10) the high frequencies are being damped. Should I put the preamp between the pickup and the volume control? Also, should there be a DC blocking capacitor at the output?
Anyway it's working for now and I await your post of the second stage so I can build the complete preamp.
I really appreciate all your help and patience with one in unfamiliar territory. Instead of blindly following a schematic and a parts list with no explanation I am coming to understand how these work and learning to design my own.
Thank you very much,
Ken |
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