New JFET guitar preamp project

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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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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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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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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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You're a great guy!

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
 
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
 
>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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Comment.......

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
 
FET biasing

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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>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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Parallel FX-loop?

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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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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Perfect explanation, Benny

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
 
benny said:
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.
attachment.php
 
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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