Vox - AC30 Amplug  [schematic]

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
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amonte
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Post by amonte »

Two of them actually, but they don't seem complete. I haven't seen them over here though so I thought I would share.

One thing I don't get at all is what is happening off of U1A with all of the transistors. Not sure what that's accomplishing.

Still, I had been wondering about this circuit for a long time. I was going to go through it with my meter and get the cap readings. Not sure if I would (or could!) attempt a straight up clone, but maybe something with a similar setup (Buffer-> Soft Clipping Stage -> Tone stack -> Hard Clipping Stage -> Cut Control -> Recovery stage -> Volume) might be interesting to try.
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Amplug_1.png
Amplug2.png

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Post by B Tremblay »

amonte wrote:One thing I don't get at all is what is happening off of U1A with all of the transistors. Not sure what that's accomplishing.
http://www.muzique.com/lab/gyrator.htm
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Post by amonte »

Ah! Thanks! I appreciate the heads up.

I'm going to take a wild guess that these were added to give the output more of a "speaker like" response.

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Post by darthoverdrive »

Nice find, thanks for posting. :applause:

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Post by Emanuele »

to me it's harder to figure out what's happening on U5B..there's a part of signal that skips this opamp, and it doesn't seem to be a gain stage..

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Post by mozwell »

do you have the capacitor values ?

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Post by amonte »

Sorry, I don't. I will go back and see if I can find them in the other forum.

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Post by rgm »

Measured values after desoldering (take with care):

C61: 10nF
C62: few pF (?)
C63: about 2.2nF
C64: about 470pF (?)
C65: 22nF
C66: 1nF
C67: few pF (?)
C68: about 1nF
C69: 220nF
C70: 220nF
C71: 22nF
C72: few pF (?)
C74: 1uF
C73: 10nF
C75: 22nF
C1: 100nF
C3: 1nF
C4: 10uF / TANT. POL.
C5: 74nF (?)
C8: 74nF (?)
C9: 10nF
C10: 100nF
C11: 10nF
C12: 4.7nF
C14: 1uF
C15: 1uF
C16: 1nF

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Post by rgm »

amonte wrote:Two of them actually, but they don't seem complete. I haven't seen them over here though so I thought I would share.

One thing I don't get at all is what is happening off of U1A with all of the transistors. Not sure what that's accomplishing.

Still, I had been wondering about this circuit for a long time. I was going to go through it with my meter and get the cap readings. Not sure if I would (or could!) attempt a straight up clone, but maybe something with a similar setup (Buffer-> Soft Clipping Stage -> Tone stack -> Hard Clipping Stage -> Cut Control -> Recovery stage -> Volume) might be interesting to try.
They are gyrators made with transistors. They wanted to shape a complex eq. curve to emulate the cabinet response and they make it pretty well. http://en.wikipedia.org/wiki/Gyrator

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Post by sonicwarrior »

amonte wrote:Two of them actually, but they don't seem complete.
It's only one: Rev 0B is a correction of Rev 0A. The source is (German!):
http://forum.musikding.de/vb/showthread ... 30-zerlegt
But you can view them only as a registered user.

The schematic goes only until the Aux In point so the headphone amp and Aux In are missing.

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Post by blackcorvo »

sonicwarrior wrote: It's only one: Rev 0B is a correction of Rev 0A. The source is (German!):
http://forum.musikding.de/vb/showthread ... 30-zerlegt
But you can view them only as a registered user.

The schematic goes only until the Aux In point so the headphone amp and Aux In are missing.
[UPDATE] The original forum from that link has moved, here's that post in their new forum: http://musikding.rocks/wbb/index.php/Th ... #post18986

Also, has anyone checked the voltages on the op-amps? Because I'm skeptical they would even be able to drive the clipping LEDs if they were only being powered directly by the batteries...
From the circuit board photos on that forum, it looks like it has a switching power supply, since there's an inductor next to the power switch, the only 16v-rated electrolytic in the board next to it, and an IC on the other side of the board in the same area. Maybe they are stepping up the voltage to power the preamp (possibly to 9v)? Hard to really tell otherwise, since you can't see the IC's name on those pics.
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