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Old 27th August 2013, 09:55 PM   #21
Lid55 is offline Lid55  Canada
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Yes Nigel is right, the level of the piezo film is very low... it is possible to get piezo film that outputs a higher level, but right now it costs 20x the money... because it's non standard. I've already tried the buffer circuit alone extensively, it sounds great (balanced frequency spectrum), but in order to hear it I have to max my computer's external audio card, and the amount of noise is unacceptable.

Nigel, do you know... if I ran the signal through both sides of the OPA2134... would any of the circuit component values change the second time around (i.e. after the first opamp in the 2134)?
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Old 28th August 2013, 08:19 AM   #22
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Quote:
Originally Posted by Lid55 View Post
Yes Nigel is right, the level of the piezo film is very low... it is possible to get piezo film that outputs a higher level, but right now it costs 20x the money... because it's non standard. I've already tried the buffer circuit alone extensively, it sounds great (balanced frequency spectrum), but in order to hear it I have to max my computer's external audio card, and the amount of noise is unacceptable.

Nigel, do you know... if I ran the signal through both sides of the OPA2134... would any of the circuit component values change the second time around (i.e. after the first opamp in the 2134)?
You don't need a high impedance input on the second amp, as it's fed from a low impedance source (the first opamp).
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Old 31st October 2013, 09:48 PM   #23
Lid55 is offline Lid55  Canada
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Hey there, I've finally found some time to dig into this circuit, my updated circuit is in the PDF attachment. I've added annotations (small green text) regarding what I've found in my research, if you want to read them you have to zoom in.

So far, I've found that using the buffer circuit really helps in balancing the lows, so I've ruled out using the opAmp circuit alone.

Any notes regarding potential optimizations of this circuit are appreciated.

Specifically, I am looking for information regarding the main tone shaping components in the circuit. Would it be fair to call C1, C3, C5, and C8 all "audio coupling capacitors? Also, would I be right to call this an AC circuit, or is it also necessary to think about it in terms of DC?

Thanks for the responses thus far,
- Brett
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File Type: pdf CombinedPiezoBufferAndOpAmpCircuit.pdf (54.2 KB, 38 views)
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Old 1st November 2013, 03:01 PM   #24
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Originally Posted by Lid55 View Post

Specifically, I am looking for information regarding the main tone shaping components in the circuit. Would it be fair to call C1, C3, C5, and C8 all "audio coupling capacitors?
Yes, all are coupling capacitors - although there seems no point in having both C3 and C5 (or R5 at all).

You don't use coupling capacitors for tone shaping, their action is far too gentle.
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Old 1st November 2013, 03:32 PM   #25
Lid55 is offline Lid55  Canada
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Thanks for the info Nigel. I found online the equation C(new) = (C1*C2)/(C1+C2)... and calculated the result to be .018uF.... so... then... I would just leave the .022uF capacitor and get rid of C3 and R5?

I haven't figured out measuring impedance yet... but I'm thinking that might be the next step.
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Old 1st November 2013, 03:57 PM   #26
mjf is offline mjf  Austria
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hello.
R5 can be seen as a damping res against hf.......

you should swap the position of c8 /r10 (or you will have some dc current through the pot).......

if you do not really need the pot (r10) you can leave it out.......
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File Type: jpg piezopre1.JPG (53.9 KB, 67 views)
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Old 1st November 2013, 03:59 PM   #27
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Originally Posted by Lid55 View Post
Thanks for the info Nigel. I found online the equation C(new) = (C1*C2)/(C1+C2)... and calculated the result to be .018uF.... so... then... I would just leave the .022uF capacitor and get rid of C3 and R5?
Assuming you're wanting a 3dB/Oct bass roll off then 0.022 should be fine, but it's not really very much of a roll off.

Quote:

I haven't figured out measuring impedance yet... but I'm thinking that might be the next step.
Measuring the impedance of what?.
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Old 1st November 2013, 04:07 PM   #28
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Originally Posted by mjf View Post
hello.
R5 can be seen as a damping res against hf.......
It's not a valve amplifier
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Old 1st November 2013, 07:54 PM   #29
Lid55 is offline Lid55  Canada
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Originally Posted by Nigel Goodwin View Post
Assuming you're wanting a 3dB/Oct bass roll off then 0.022 should be fine, but it's not really very much of a roll off.
So would a higher value cap create a steeper roll off? For example, a 1uF cap might provide a 6dB/Oct bass roll off? I thought a cap here would more affect the cutoff frequency... or does it control BOTH cutoff frequency AND roll off "Q"/slope? Ideally, I'd like a fairly steep roll off at about 60Hz.



Quote:
Originally Posted by Nigel Goodwin View Post
Measuring the impedance of what?.
I was thinking it would be enlightening to measure impedance at the output of the first circuit and the input of the second circuit. I was thinking this would help with knowing the right value capacitor to place there... for the most transparent audio signal.

Last edited by Lid55; 1st November 2013 at 08:05 PM.
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Old 1st November 2013, 08:03 PM   #30
Lid55 is offline Lid55  Canada
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Originally Posted by mjf View Post
you should swap the position of c8 /r10 (or you will have some dc current through the pot).......

if you do not really need the pot (r10) you can leave it out.......
well I need the pot (at least SOMEWHERE in the circuit)... as this is how I am adjusting the volume after the circuit is installed in my guitar....

so simply swap C8 with R10? mjf, I noticed you swapped the polarity of C8 as well... was the original drawing of the circuit (that I got online) wrong? I've tested the original circuit out already and the cap hasn't exploded... doesn't this prove then that some directional caps be used unidirectionally?
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