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Great Diy Buffer/preamp For Piezo Pup Guitars


Paul Marossy

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I built this little gizmo to try out with my acoustic-electrics and my Parker Mojo Nitefly. It really works pretty well and is very easy to build. It should work well with any instrument that has a piezo - from bass to violin.

http://www.diyguitarist.com/Guitars/PiezoBuffer.htm

It won't substitute for an amplifier that is designed specifically for acoustic guitars, but it does make one sound a whole lot better when plugged into a regular guitar amp.

I used it this morning for about an hour and I must say that it makes a huge difference with my Parker Mojo Nitefly's piezo pickups. It actually sounds like an acoustic guitar when plugged into something designed for conventional magnetic pickups. Piezos need to see a very high input impedance, so the FET satisifies this requirement and the output impedance is more along the lines of a stompbox, so it drives things made for electric guitar pretty well.

Now I have an idea to combine this buffer circuit with my stereo splitter box for an all in one unit. Could be pretty handy for Parker Fly owners...

I thought this might help someone here at the forum, so I thought I'd share it with y'all. :D

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That's an interesting thing...I would have thought that the Parker already had adequate pre-amping/buffering to run it's peizos on board....what's it's circuitry doing if not that?

But another preamp design is always welcome. Is this a better circuit than say a Tillman on unity gain? Oh...and could this be used in a similar way as the Tillman, and could extra gain be arranged easily for use as an active preamp for conventional electric guitar pickups?

Nice layout as always...psw

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But another preamp design is always welcome. Is this a better circuit than say a Tillman on unity gain? Oh...and could this be used in a similar way as the Tillman, and could extra gain be arranged easily for use as an active preamp for conventional electric guitar pickups?

Nice layout as always...psw

One thing that interests me about that circuit is the high value of the 220k source resistor. That should make it super low current, and hence a battery should last for ages. But does it cause any issues driving past cable capacitance or into amp and eeffects inputs downstream?

The Tillman uses very much lower resistors (and is a different configuration). I have had success with the Tillman design, multiplying all resistors by x3 to extend battery life, and it is still a very effective buffer.

John

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The main difference between this circuit and the Tillman is that this circuit configuration is a source follower. Notice how the output signal is taken at the source instead of at the drain like in the Tillman. As a result, there is no voltage gain. It's purely a unity gain impedance buffer... it has high input impedance and low output impedance and Vout=Vin. It's probably a lot more stable and less reliant on the individual FET characteristics than the Tillman circuit. So if all you need is a buffer to prevent the loss of highs or the loss of signal level, this is a better solution than the Tillman.

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That's an interesting thing...I would have thought that the Parker already had adequate pre-amping/buffering to run it's peizos on board....what's it's circuitry doing if not that?

I think it's a buffered preamp, but I am pretty sure that it is not designed to be run into a regular guitar amp - it's meant more for going to an amplifier designed specifically for acoustic guitars or a PA system. This buffer makes a huge difference when plugged into a digital effects unit or similar (or electric guitar amp). Yeah, the inputs are high impedance, but they probably aren't designed for piezo pickups. They can handle the bandwidth, though, so this buffer really is an improvement over just running the piezo straight into the input.

if all you need is a buffer to prevent the loss of highs or the loss of signal level, this is a better solution than the Tillman.

That's really all that it's designed to be - just a buffer to prevent loading the piezos down and killing the dynamics. This buffer really makes things sound more clear, if you will. Maybe crisp is a good word to describe it.

One thing that interests me about that circuit is the high value of the 220k source resistor. That should make it super low current, and hence a battery should last for ages. But does it cause any issues driving past cable capacitance or into amp and eeffects inputs downstream?

Yeah, a battery should last a very long time. Another nice little feature. I haven't verified what possible issues there might be with effects inputs downstream. I imagine it shouldn't really be an issue - as I see it, the buffer should only serve to make things sound better... :D

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One thing that interests me about that circuit is the high value of the 220k source resistor. That should make it super low current, and hence a battery should last for ages. But does it cause any issues driving past cable capacitance or into amp and eeffects inputs downstream?

Yeah, a battery should last a very long time. Another nice little feature. I haven't verified what possible issues there might be with effects inputs downstream. I imagine it shouldn't really be an issue - as I see it, the buffer should only serve to make things sound better... :D

What I was concerned about was roll off due to cable capacitance. I find the usual types of buffers such as Tillman, and the versions Ive built to be very good at fixing this by lowering the output impedance of the guitar. The concern I have is that this would raise the output impedance.

With your circuit with the high 220k resistor, I am not sure exactly what the output impedance would be, but it would be at least closely realted to that 220k value. Guitar cables of say 20' can have a capacitance of 600-800pF (based on 30-40pF per foot), so if 220k was the output impedance, then treble roll-off could start at a frequency as low as 900Hz. My understanding is that a circuit like that would work just as well with say 22k there, in which case the roll off would start at 9000Hz, which is above guitar frequency range. Current drain would still be less than 0.25mA, so still very good battery life.

As you can tell, Im interested in this theory but I am not an electonics engineer, so I am grateful and interested in any corrections to the above.

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Hi JohnH

Perhaps you've hit on a clue as to why Paul is getting a better sound from his Parker (which has an onboard preamp) with this "box". Perhaps the amp likes the higher impedance that's coming out...just a thought?

If you have other preamp designs I for one would be interested if you've a mind to post or link 'em. as i said...

Of course I'm always looking for stuff for the sustainer preamp.

alway's on the lookout...psw

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With your circuit with the high 220k resistor, I am not sure exactly what the output impedance would be, but it would be at least closely realted to that 220k value. Guitar cables of say 20' can have a capacitance of 600-800pF (based on 30-40pF per foot), so if 220k was the output impedance, then treble roll-off could start at a frequency as low as 900Hz. My understanding is that a circuit like that would work just as well with say 22k there, in which case the roll off would start at 9000Hz, which is above guitar frequency range. Current drain would still be less than 0.25mA, so still very good battery life.

I'm not versed enough in frequency response and roll-offs to say one way or the other, but I would think that the output cap size of the circuit would have more to do with the frequency roll-off than the cable capacitance would. I haven't tested this thing extensively, but it sounds real good to my ears with a 10 foot long cable. Above all, I am having trouble understanding how the source resistor affects the frequency response... :D

Whatever the case, it sounds a lot better with the signal going thru the buffer and on to an electric guitar amp than otherwise.

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OK, I think I get it now. Since it's a source follower, the source resistor more or less sets the output impedance. That part I get. But I am still confused about one thing: wouldn't a 22K resistor make it much worse?! :D

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uh just a question. Have you ever thought of just tweaking the onboard preamp to your liking. I had a fishman that everyone was like thats great its the best. and i gutted it and put my own circuit inand its a lot more responsive. but i like to destroy things. however in all seriousness. i mean has anyone put a piezo in there guitar and plugged straight in with no preamp. cool sounds but its not a reliable tone. adding a preamp obviously amps it up and eq's it which means the pickup isn't working as hard. but unless your guitar was pickup less the piezo was working the same before you built the new preamp now i am sure it helps it out without a doubt cause hearing is believing. but plugging my passive strat into a booster set at unity makes it sparkle so much more. and pluggin my active jackson into the same booster set for again unity still helps it sound so much better. again paul this isnt' a stab man my main point is that so many times diy sounds better cause we take the time instead of the almighty dollar so we let our ears be the judge instead of our pocketbooks. and the fact is most active preamps in acoustic guitars suck or have too wide of sweeps to be effective unles you spend hours and hours mastering your particular system.. personly i like a volume knob on mine or nothing at all. i plug into my pig and control all my sounds from there and to me and my people we see m to get better results that way. but again to each his own. also i owned a parker man back when they first came out. I liked the neck but i didnt' have a manual with that thing and i could never get the piezo's to sound good at all. and for that matter the whole guitar didnt' sound good to me but again thats me. and i only paid 100usd for it so i sold it to l and m music for 1100usd or so. but thats years ago and neither here nor there.

ciao.

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yeah court date and carwreck insurance meeting took a toll on me as well as the breakin at the shop. i am off thursday. will hollar back then.

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OK, I think I get it now. Since it's a source follower, the source resistor more or less sets the output impedance. That part I get. But I am still confused about one thing: wouldn't a 22K resistor make it much worse?!  :D

Paul,

I thought Id write this out for everyone to see. Applogies to any Grandmothers who all ready know how to suck an egg!

Ideal amp stages have high input impedance to take very little signal, but low output impedance, so they can drive whatever is downstream. In th eFET circuit above, 22k source resistor will result in lower output impedance than the 220k, though of course, if it sounds great, thats fine!

In general however, the issue of treble rolloff due to cable capacitance is also affected by this. The rolloff starts to become significant at a frequency where the cap impedance matches the output impedance. Cap impedance decreases with increase in frequency, so with a low output impedance, the roll-off frequency is higher. To work out this frequency, with say a 22k output impedance:

cap impedance = output impedance

1/(2Pi x F x C) = R

F = 1 / (2Pi x R x C)

C = 800pF = 800 x 10^-12

R = 22,000

F = 9042 hZ - ie quite high, which is usually what you want

The equivalent with R =220k is 904hZ, which is significantly into the guitar frequencies. It might of course, sound great if a bit of treble roll-off is wanted.

John

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Ah, I see what you're getting at now. You learn something new everyday! I'll have to do some testing between the two resistor values and see what happens. BTW, this circuit was designed by a bass player, so that 200K would probably be perfect for bass, but maybe less than optimum for guitar, as you have pointed out.

Thanks for taking the time to explain this to me! :D

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Simple way to find out, Paul - just patch in a parallel 22k across that 220K, and see if your high end improves! FWIW, I usually use a 10K "load" for bipolar emitter followers or JFET source followers, and it's always worked pretty well, but it's more out of habit than any mathematical understanding. I do know that the output impedance is a bit more complex than simply the source resistor, and output impedance is what interacts with cable capacitance to roll off high end.

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I don't think you're going to find there's a huge amount of treble boost, just probably added presence and air, but depending on what you're driving with it (like an inductor wah, or a vintage fuzz), there might be quite a bit of difference (not that most people use a Crybaby with their piezos, but, hey, it could happen!).

Wasn't there a thread over at Aron's about variable output impedance and the general results a couple of weeks ago? Might be interesting to reread that when/if somebody gets around to it.

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Actually, I am one weirdo that likes to use a wah pedal with piezo pickups. But that's just to cop a filtered kind of Michael Hedges tone. I just keep it in one spot after I find it. :D

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In evaluating the performance of this source follower, there are two concepts to keep in mind:

output impedance

and the ability to drive current into a load.

Output impedance just tells you how much the output voltage changes when you put a load on the circuit, assuming you are driving the circuit with a small signal. With the source follower, the output impedance is determined by the FET characteristics (the inverse of the transconductance). The 200 K source resistor is in parallel, but it is so large that it is the FET that counts. The output impecance of the FET source follower is low enough to prevent high frequency loss.

The 200K source resistor limits how much current flows through the FET, and thus how much signal voltage it can deliver into a load. Driving the 1 Mohm input impedance of your amp is not a problem. Driving the cable capacitance might be.

If your cable is 800 pf, this is about 40 Kohm at 5 KHz. You can probably only get about a volt peak to peak before clipping with this circuit at 5KHz, but several volts at 50 Hz. Since guitars do not have much output at 5 KHz, this is not a problem.

The unmodified Tillman circuit uses an unbypassed source resistor that makes the circuit pretty stable against variations in the FET.

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Well, I changed that source resistor to 22K. I can hear a very subtle change in the mids/high mids as a result. It probably would be much more noticeable with a 20' or longer cable. Better safe then sorry, I guess. :D

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