Considering EVERY recording made goes through hundreds of electrolytic capacitors it seems pretty silly trying to use capacitors supposedly 'better for audio' 😀
Even sillier when we're talking instrument applications, and not even HiFi.
As for 'math' - this is the 21st century - haven't you thought of googling for 'capacitive reactance calculator'?
It just occurred to me 😉
as far as the circuit... I now want to add an internal volume control (a trim pot inside the guitar) as well as having an "external" volume control (accessible through a knob on the front of the guitar) for each piezo element. I want to do this so I can set an initial balance of the piezo elements, but still have control of the overall volume from the front panel. Anyways, this means two pots per piezo element. I've tried wiring two 10Ks in series right beside each other (at the location where there is a single 10K pot in each circuit right now) and they sound fine, don't seem to change the audio. Are there any other potential issues here?
Also, I want to wire in a HPF, to block the extreme low frequencies. Perhaps a first order low pass would work, with its gentle roll off. Generally, would it be best to place this filter before the buffer/gain circuit, or after? Maybe between the buffer and the gain?
Why not just turn the bass down on the amp you're feeding?.
Have you found a 'problem' with extreme low frequencies? - if so it's unlikely a first order filter would have much effect - unless the 'troublesome' frequencies are a LOT lower than any frequencies you want.
Have you found a 'problem' with extreme low frequencies? - if so it's unlikely a first order filter would have much effect - unless the 'troublesome' frequencies are a LOT lower than any frequencies you want.
I'm not feeding into an amp. I'm going direct into the soundcard and applying software EQ, compression, and FX. The problem is.... the software EQ I'm using doesn't yet have a HPF option.... it has parametric controls but is wanting in terms of how it filters low low bass frequencies (it allows bell curve cuts... but not that low). The software is not as developed as desktop software, it's created for the Apple's iOS... which is still "just getting there" in terms of professional software. But I'm sold on the portability factor, as well as on a sweet looper app called LoopyHD.
So far... the natural bass frequencies ARE too strong... the piezos are designed to pick up low vibrations as well.... and it might be the way the films are mounted on the guitar... but there's a lot going on down there... and I'd prefer to clean it up as close to "the source" as possible
I should also mention, I generally play "clean" guitar... so... there's no "covering up" with distortion... which is my concern for pristine highs and audiophile capacitors
Add a decent active HPF then, first order ones aren't going to be enough.
As for 'audiophile' capacitors if you want to waste the money, the choice is yours 😀
It's also VERY unlikely you would be able to either measure any difference, or hear any (in double blind tests) - it's mainly a question of "they were expensive, they must sound better".
But as you're feeding a sound card, the quality is probably not that high anyway?.
As for 'audiophile' capacitors if you want to waste the money, the choice is yours 😀
It's also VERY unlikely you would be able to either measure any difference, or hear any (in double blind tests) - it's mainly a question of "they were expensive, they must sound better".
But as you're feeding a sound card, the quality is probably not that high anyway?.
no, the soundcard is pretty good:
DUO-CAPTURE EX :: Products :: Roland
So I have created a new part to the circuit and revised the mixer to include an extra pot for each piezo. The new part is a "Multiple Feedback 2nd Order High Pass Filter" taken from the "Op Amps for Everyone" excerpt (Chapter 16). I haven't tried to work out the values yet, I first want to make sure the layout is sound.
[whoops, just noticed I made a small error with the power supply connections (close to the branch of the third piezo), please assume that is wired as in the previous schematics]
DUO-CAPTURE EX :: Products :: Roland
So I have created a new part to the circuit and revised the mixer to include an extra pot for each piezo. The new part is a "Multiple Feedback 2nd Order High Pass Filter" taken from the "Op Amps for Everyone" excerpt (Chapter 16). I haven't tried to work out the values yet, I first want to make sure the layout is sound.
[whoops, just noticed I made a small error with the power supply connections (close to the branch of the third piezo), please assume that is wired as in the previous schematics]
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Just my two cents on this, I would say keep the electrolytic, and if you do think a bit more ''definition'' , ''airiness'' or whatever is needed just put a smaller polyprop in parallel with the electrolytic. (maybe 1 or 2 nF)
The only way I can explain why that seems to work is that the electros don't seem to pass the high frequencies as easily. Is it impulse response different at higher frequency?, hysterisis? distortion? I am not decided on it and not even totally convinced...so it is just something to try.
And won't cost as much as a large value film cap.
Cheers
The only way I can explain why that seems to work is that the electros don't seem to pass the high frequencies as easily. Is it impulse response different at higher frequency?, hysterisis? distortion? I am not decided on it and not even totally convinced...so it is just something to try.
And won't cost as much as a large value film cap.
Cheers
Thanks shanx, I'll test it out... and see if i can hear a difference. I'll try adding a small parallel PP cap to all the electrolytics in the circuit.
on reflection I can also record the two versions, post them online... to let others listen as well
on reflection I can also record the two versions, post them online... to let others listen as well
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It's a common thing to do, but it's pretty debatable if it makes any difference or not - and even assuming you think it does, it would probably be a different story in double blind tests.
You might consider that most of the best sounding amplifiers of all time don't do such things.
If you've got the test equipment?, try measuring it - first without the extra capacitors, then with them.
I haven't any notion if it makes a measurable difference, and even if it did it may not be audible difference..that's why ''definition'' and ''airiness'' in quotes, as I do not have test equipment to measure those parameters(!) Just use an electrolytic with a small low cost film if you want to try ;-)
I am not comfortable with much in that 2N5457 buffer/source follower...I'm going to be lazy and skip pages 5-13...I apologize if I say something someone else already did.
Here is a simple JFET guitar buffer: Discrete FET Guitar Preamp
Check out the Runoff Groove Peppermill effects box...a lot of gain, but can be pretty transparent too. Peppermill.
Also go back to MSI and look at their CM-01B contact mic data sheet...it has the schematic posted. It uses a 2N7002 as source follower (no drain resistor, and 10k source resistor). Despite being a buffer, it has alot of output - I drove a guitar amp and (a separate experiment) a Boston Acoustics powered subwoofer with no satellite speakers...plenty loud...I was looking for 'sweet spots' for mounting piezos.
One more thing to learn from the MSI CM-01B schematic...it's low corner freq is 5 Hz. They use so-called 'noiseless biasing' for the MOSFET with relatively large resistors 300k, and a 100 Megohm gate resistor to the midpoint of the 300k-300k bias divider...
To get a 5 Hz lower corner freq based on the internal sensor capacitance and 100M, that means the internal sensor is very small (low capacitance), yet it has a very strong signal. It uses a small polymer piezo under a silicone rubber coupler. The spring effect of the rubber coupler apparently causes a resonance around 5 kHz. My point about this contact mic as an example is that there is a lot of signal being produced by a tiny polymer piezo, so don't dismiss yours yet...
Here is a simple JFET guitar buffer: Discrete FET Guitar Preamp
Check out the Runoff Groove Peppermill effects box...a lot of gain, but can be pretty transparent too. Peppermill.
Also go back to MSI and look at their CM-01B contact mic data sheet...it has the schematic posted. It uses a 2N7002 as source follower (no drain resistor, and 10k source resistor). Despite being a buffer, it has alot of output - I drove a guitar amp and (a separate experiment) a Boston Acoustics powered subwoofer with no satellite speakers...plenty loud...I was looking for 'sweet spots' for mounting piezos.
One more thing to learn from the MSI CM-01B schematic...it's low corner freq is 5 Hz. They use so-called 'noiseless biasing' for the MOSFET with relatively large resistors 300k, and a 100 Megohm gate resistor to the midpoint of the 300k-300k bias divider...
To get a 5 Hz lower corner freq based on the internal sensor capacitance and 100M, that means the internal sensor is very small (low capacitance), yet it has a very strong signal. It uses a small polymer piezo under a silicone rubber coupler. The spring effect of the rubber coupler apparently causes a resonance around 5 kHz. My point about this contact mic as an example is that there is a lot of signal being produced by a tiny polymer piezo, so don't dismiss yours yet...
http://www.meas-spec.com/downloads/Contact_Microphone.pdf
If it asks you to register to see the datasheet, look closely as they also allow you to decline to register...
If it asks you to register to see the datasheet, look closely as they also allow you to decline to register...
Did you mean "not comfortable" as in "unfamiliar with" or as in "I don't like it"?
Thanks for all the info Murray, I'm looking into it.
I am not saying it won't work...I'm sure the person who posted it originally did so after successful use, but I meant I didn't like it...some of the same things others commented on...and I felt more helpful giving other ideas/links than trying to half-analyze something I didn't build from the comfort of my armchair...a figure of speech I overuse, as there are no arms on my usual PC chair ;O).
It just seemed unnecessarily complex in its simplest form...and is becoming moreso...this is the pot calling the kettle black...I am guilty of over-complicating things myself...
Your call...I think there are other way to accomplish what I think you want,,,
It just seemed unnecessarily complex in its simplest form...and is becoming moreso...this is the pot calling the kettle black...I am guilty of over-complicating things myself...
Your call...I think there are other way to accomplish what I think you want,,,
Well I have actually revised the circuit quite a lot since the beginning of this thread. I've attached a schematic of the buffer circuit I am currently using (see example "A" of my attachment).
In recent research though, on buffer circuits in particular... I came across some information at:
Basic Buffers
This suggests that a revision may be in order, where the input resistor attached to a biased voltage will increase the headroom (see example "B" of my attachment). Any thoughts on this revision are greatly appreciated, if it will have audible benefits, etc
Additionally, it was suggested on the page that the particular JFET I use is not particularly important. Any comments on this?
In recent research though, on buffer circuits in particular... I came across some information at:
Basic Buffers
This suggests that a revision may be in order, where the input resistor attached to a biased voltage will increase the headroom (see example "B" of my attachment). Any thoughts on this revision are greatly appreciated, if it will have audible benefits, etc
Additionally, it was suggested on the page that the particular JFET I use is not particularly important. Any comments on this?
Some Results:
Been awhile, but thought I'd post some results.
So I decided on a circuit, built it, installed it in my guitar, but was not happy with it so I revised it again and built it again from scratch, then reinstalled it. Now I'm fairly happy with the results. "ComboCircuit-rvsn12" is the circuit I settled on. One surprising thing I found was that the resistor R3, before the first JFET, was necessary to block RF. I was paranoid, so I added C1 for additional protection against RF as well. I added these because without them (specifically R3) I would clearly pick up some radio station.
I couldn't hear a difference between electrolytics VS film capacitors though so I didn't worry too much about them.
In general, I found that R2 added headroom (and thus also some clarity) and Q2 added clarity to the bass frequencies (though Q2 DOES need a lower IDSS or you get a sort of mild "phase effect" sound happening).
I still get a tiny bit of hum if I'm not touching the strings of my guitar... I'm not sure if this can be helped or not... but it's not too loud, so I can learn to live with it.
There's also a bit of hiss/noise that I'll learn to live with for now, but for the next version (which won't be for awhile), I read from a couple sources that it's best to use a non-inverting amplifier stage as there is less noise at output, please correct me if I'm wrong.
Loop1
- is the sound of all three piezos, wired in parallel, straight to my soundcard (switched to accept a low impedance signal)
Loop2
- is the sound of all three piezos, wired in parallel, straight to my soundcard (switched to accept a high impedance signal)
Loop3
- the three piezos through the finished circuit, each with their own buffer circuit but wired before a single summing opamp
Loop4
- three piezos, each with their own buffer circuit, but the high and low piezos wired through one side of the OPA2134, and the middle piezo wired through the other side of the OPA2134 (so, separate amps), and these two signals wired to different outputs on the guitar, then recorded separately and mixed with some stereo spread
Loop5
- what my old magnetic pickups sound like (bridge humbucker)
now I can work on some software processing and then I'll be ready to record some music.
Been awhile, but thought I'd post some results.
So I decided on a circuit, built it, installed it in my guitar, but was not happy with it so I revised it again and built it again from scratch, then reinstalled it. Now I'm fairly happy with the results. "ComboCircuit-rvsn12" is the circuit I settled on. One surprising thing I found was that the resistor R3, before the first JFET, was necessary to block RF. I was paranoid, so I added C1 for additional protection against RF as well. I added these because without them (specifically R3) I would clearly pick up some radio station.
I couldn't hear a difference between electrolytics VS film capacitors though so I didn't worry too much about them.
In general, I found that R2 added headroom (and thus also some clarity) and Q2 added clarity to the bass frequencies (though Q2 DOES need a lower IDSS or you get a sort of mild "phase effect" sound happening).
I still get a tiny bit of hum if I'm not touching the strings of my guitar... I'm not sure if this can be helped or not... but it's not too loud, so I can learn to live with it.
There's also a bit of hiss/noise that I'll learn to live with for now, but for the next version (which won't be for awhile), I read from a couple sources that it's best to use a non-inverting amplifier stage as there is less noise at output, please correct me if I'm wrong.
Loop1
- is the sound of all three piezos, wired in parallel, straight to my soundcard (switched to accept a low impedance signal)
Loop2
- is the sound of all three piezos, wired in parallel, straight to my soundcard (switched to accept a high impedance signal)
Loop3
- the three piezos through the finished circuit, each with their own buffer circuit but wired before a single summing opamp
Loop4
- three piezos, each with their own buffer circuit, but the high and low piezos wired through one side of the OPA2134, and the middle piezo wired through the other side of the OPA2134 (so, separate amps), and these two signals wired to different outputs on the guitar, then recorded separately and mixed with some stereo spread
Loop5
- what my old magnetic pickups sound like (bridge humbucker)
now I can work on some software processing and then I'll be ready to record some music.
Take a look at this article on so called 'noiseless biasing.
http://www.geofex.com/Article_Folders/modmuamp/modmuamp.htm
What you appear to have is a very high impedance voltage divider biasing the jfets along with the 1k 'RF choke' resistor, and the 1 pF 'RF bypass'. Those high resistances are contributing thermal or Johnson noise which increases with resistance. This is a grossly oversimplified summary of a huge topic I am not knowledgeable enough to explain. I'm disclaiming more to avoid making an incorrect statement than to avoid misleading you...if that makes any sense....just don't take this summary as a fact in all scenarios involving noise reduction, as 'it depends...'.
The idea behind noiseless biasing is to set the bias or operating point with a much lower resistance voltage divider, then bypass (capacitor) the lower resistor. In general it's a good practice to have a bypass capacitor from the 'undivided' DC supply to ground as well, so on a schematic it can look like you are bypassing each resistor of the divider. It looks like, although that's not the primary intent.
If you look at the noiseless biasing examples, there is a higher resistance that sets the input impedance you want, while biasing the jfet(s) from a low resistance divider that is inherently quieter. I've seen anywhere from 10k-300k resistors in the divider, and 0.1-10 uF as bypass, and an 'input impedance resistor' from 470k to 20M ohm or more depending what you need (ranging from magnetic guitar at the low end to small piezo at the high end). If you read about 'rail splitter' techniques of producing dual polarity DC supplies from a single supply, (HeadWize site?) there are some discussions of what effect such dividers have on other performance factors (I never gave them a moment's thought until reading about it). You don't need dual polarity but the discussion of the divier and bypass capacitor selection may be useful...just to be aware of what difference it makes (after all, why would you want 10k or 300k resistors.
You should look closely at how the signal and high-value resistor are coupled to the JFET input as it can be easy to misinterpret what purpose each connection serves. It's a very simple technique, but it solves a number of issues simultaneously, and the jfet connected to the signal the wrong way can give you some but not all the benefits of this technique. I think the noise reduction is the easiest feature to inadvertently defeat.
I think a small value bypass capacitor works better closer to the jfet, as the piezo itself is already a relatively large ceramic capacitor. 1 pF across what is probably several hundred to several thousand pF of piezo is not the right place.
I can't think of an article that explains the placement of a RF stopper resistor and small RF bypass capacitor...maybe Francis Deck's bass preamp (FDeck on the TalkBass forum) article. Hopefully someone else can refer you to an explanation.
This is another area where arbitrary choice of component values and placement can give you unintended defects. Additional capacitance following a piezo (like long cable runs), or too much extra capacitance can cause an impedance divider analogous to resistor dividers, reducing signal level. Sometimes this can be useful for too hot a signal coming in, but I think it only causes significant division when it's a significant fraction of the piezo capacitance. Combining noiseless bias and RF blocking and bypass are all good techniques, but it might be best to carefully interpret a published circuit that uses all these techniques. If you try to mimic a partial method from a lower-Z magnetic pickup preamp, or a jfet preamp that just sends a resistor to ground for a piezo where the input impedance doesn't need to be extremely high, you may have component values scaled wrong. What's right or wrong is all relative to details of the particular design.
I can't tell you exactly what will solve all this because I've read more about it than I've actually followed through on building...I keep reading and reading and not finishing much...
Hopefully this is helpful...I'm a bit vague because I don't have specific values for your situation or remember enough at the moment...hopefully you won't fall into the trap I set myself of adding more ideas which impede progress...but high-impedance piezo preamps can be frustrating.
I just read something today (Analog Devices online op amp design book, chapter 6 by Walt Jung about audio applications) how much easier it is to keep noise and hum under control with a differential input. With a JFET input, this easily doubles the complexity, but with an op amp circuit, you already have differential capability and need a few more resistors.
http://www.geofex.com/Article_Folders/modmuamp/modmuamp.htm
What you appear to have is a very high impedance voltage divider biasing the jfets along with the 1k 'RF choke' resistor, and the 1 pF 'RF bypass'. Those high resistances are contributing thermal or Johnson noise which increases with resistance. This is a grossly oversimplified summary of a huge topic I am not knowledgeable enough to explain. I'm disclaiming more to avoid making an incorrect statement than to avoid misleading you...if that makes any sense....just don't take this summary as a fact in all scenarios involving noise reduction, as 'it depends...'.
The idea behind noiseless biasing is to set the bias or operating point with a much lower resistance voltage divider, then bypass (capacitor) the lower resistor. In general it's a good practice to have a bypass capacitor from the 'undivided' DC supply to ground as well, so on a schematic it can look like you are bypassing each resistor of the divider. It looks like, although that's not the primary intent.
If you look at the noiseless biasing examples, there is a higher resistance that sets the input impedance you want, while biasing the jfet(s) from a low resistance divider that is inherently quieter. I've seen anywhere from 10k-300k resistors in the divider, and 0.1-10 uF as bypass, and an 'input impedance resistor' from 470k to 20M ohm or more depending what you need (ranging from magnetic guitar at the low end to small piezo at the high end). If you read about 'rail splitter' techniques of producing dual polarity DC supplies from a single supply, (HeadWize site?) there are some discussions of what effect such dividers have on other performance factors (I never gave them a moment's thought until reading about it). You don't need dual polarity but the discussion of the divier and bypass capacitor selection may be useful...just to be aware of what difference it makes (after all, why would you want 10k or 300k resistors.
You should look closely at how the signal and high-value resistor are coupled to the JFET input as it can be easy to misinterpret what purpose each connection serves. It's a very simple technique, but it solves a number of issues simultaneously, and the jfet connected to the signal the wrong way can give you some but not all the benefits of this technique. I think the noise reduction is the easiest feature to inadvertently defeat.
I think a small value bypass capacitor works better closer to the jfet, as the piezo itself is already a relatively large ceramic capacitor. 1 pF across what is probably several hundred to several thousand pF of piezo is not the right place.
I can't think of an article that explains the placement of a RF stopper resistor and small RF bypass capacitor...maybe Francis Deck's bass preamp (FDeck on the TalkBass forum) article. Hopefully someone else can refer you to an explanation.
This is another area where arbitrary choice of component values and placement can give you unintended defects. Additional capacitance following a piezo (like long cable runs), or too much extra capacitance can cause an impedance divider analogous to resistor dividers, reducing signal level. Sometimes this can be useful for too hot a signal coming in, but I think it only causes significant division when it's a significant fraction of the piezo capacitance. Combining noiseless bias and RF blocking and bypass are all good techniques, but it might be best to carefully interpret a published circuit that uses all these techniques. If you try to mimic a partial method from a lower-Z magnetic pickup preamp, or a jfet preamp that just sends a resistor to ground for a piezo where the input impedance doesn't need to be extremely high, you may have component values scaled wrong. What's right or wrong is all relative to details of the particular design.
I can't tell you exactly what will solve all this because I've read more about it than I've actually followed through on building...I keep reading and reading and not finishing much...
Hopefully this is helpful...I'm a bit vague because I don't have specific values for your situation or remember enough at the moment...hopefully you won't fall into the trap I set myself of adding more ideas which impede progress...but high-impedance piezo preamps can be frustrating.
I just read something today (Analog Devices online op amp design book, chapter 6 by Walt Jung about audio applications) how much easier it is to keep noise and hum under control with a differential input. With a JFET input, this easily doubles the complexity, but with an op amp circuit, you already have differential capability and need a few more resistors.
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Thanks for the response and info Murray. I must say, it'll take me awhile to get through all of it. I like to do things in depth, and there's a lot of info there, so... yeah, could take a while. I've kinda switched into audio software mode for the time being. Anyways, thanks again.
-Brett
-Brett
So... I've spoken too soon. *I've been testing out my guitar in different locations, with different audiocards. I find that with a particular audiocard, I am getting strong mains hum (60Hz)... BUT ONLY WHEN THE AUDIOCARD IS POWERED VIA THE WALLWART (ie. if I power the audiocard via batteries, there is NO hum).
This happens with only my guitar plugged directly into the audiocard, which is plugged into the mains power. Here, I mean to clarify that no other devices are involved, that there is no ground loop between different devices.
From what I've researched thus far, I'm thinking of trying to add a resistor to ground at the input stage. Any other ideas for this circuit (same circuit as posted above, revision12)... and how to reduce mains hum?
This happens with only my guitar plugged directly into the audiocard, which is plugged into the mains power. Here, I mean to clarify that no other devices are involved, that there is no ground loop between different devices.
From what I've researched thus far, I'm thinking of trying to add a resistor to ground at the input stage. Any other ideas for this circuit (same circuit as posted above, revision12)... and how to reduce mains hum?
How about this?
Huminator: Power Supply Filter
I'd prefer to fix it in the circuit... but this might be an alternative?
Huminator: Power Supply Filter
I'd prefer to fix it in the circuit... but this might be an alternative?
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