Tuesday 11 August 2020

Ampeg vh140c Channel A Pre Amp

 This one was quite a mess to get right since some of the schematics online was missing parts and/or had the wrong values. After some head scratching I worked from the factory schematic of the actual amp. 

However, did some mods without changing the charcter of the sound to much. First 9v/-9v operation is used simply because there wasn't a big difference in sound compared to stock 15v/-15v, maybe a bit less headroom (I have added a 15v version of it at bottom of this post but it is not verified).

Added some power filtering and changed a non polarized cap to standard electrolyctic (3,3uF) and changed gain pot to audio taper for better sweep.

Stock units had three 4558 op amps but to my ears TL072's perform much better. Also a buffer up front is mandotory because this thing will squeal with controls dimed.

First up schematic:

Verified 9v/-9v version

15v/-15v version which is using a the more robust LT1054 charge pump.


  1. Hi fuzzhead. Which buffer do you recommend?

    1. Hi fuzzhead. Which diode can i use instead of 15v zener?

    2. For those high gain pre amps and amp emulators I've been using buffered overdrives up front lately like tubescreamers or a Boss OD.
      With the overdrive bypassed it is acts like a buffer that cures the the noise. But with the TS/OD switched on(set at low gain) it really helps to tighten anf thicken up the pre amp. It still acts as a buffer as well.

      As for standalone buffers I've been recommended Klon buffer but haven't tried it yet.

    3. As for the Zener diode it is there protect the charge pump from higher voltage than 15v. LT1054 can handle maximum input voltage at 15v so I think you need a 15v zener. You could try 12v zener but you wont have 15v/-15v by then.

    4. Hi fuzzhead. Thanks for the tips. I will try the 12v zener and the Klone buffer.

    5. S msall update. I just boxed up my board and with shorter input and output wires the noise and oscillation is almost gone. Some microphonic feedback is present when all the knobs is dimed but overall it is a big improvement. Tried it with a buffer upfront and the sound is almost identical.

  2. Any sound demos of this thing? How close does it sound to the amp?

  3. Anyone know how much current this preamp draw approx?