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Yeah, I was a skeptic going in. But lo and behold, opamp saturation never sounded so good, except for the LM308 of course. Definitely try other opamps. Now that I think about it, I should breadboard one with a couple of LM308s. Billy must be double jointed. I have to slide my hand to play...

Good question! R14 biases the linearizing diodes on U2C. Since U2A's and U2C's inputs are wired in parallel (pin 4 to 13, pin 3 to 14), the two OTAs can share one set of linearizing diodes. If we look inside the OTA, we can see that the linearizing diodes (D2 & D3) are connected to the input pins.

The Moog Lab Series L5 amp has 2 OTAs in the preamp. One is a compressor and the other is an OD. The Aphex Aural Exciter II also used an OTA to generate mild distortion. The both do it by feeding signal to the amplifier input and to the gain control (Iabc) input. I breadboarded the Aural...

I had to make a couple of edits to sheet 1. I had forgotten C3 and I had a duplicate ref des. This is the correct schematic. Here's the next rev. I put the two OTAs in parallel and put the linearizing diodes to work. I had to fiddle some resistor values to get the gain range back where I...

Maybe Marshall got the idea for parallel channels from the same place as Colombo: an actual Marshall amplifier. At first glance, I am not too impressed with the opamp-based design. I'll probably breadboard it at some point so I can compare it with the Plexi Breed.

The charge pump noise is at 40KHz, so I would not expect that to be audible, buuuuut, if audio laced with 40KHz noise is then sampled by a BBD or PT2399, then there is a good chance that the 40KHz gets aliased down into the audio band. That, I have heard. I just read that post and it says a...

Yeah, if only switching power supply design was that simple. :LOL: Bypassing is good, but unless the high-freq currents and EM fields are constrained, the switching noise will propagate. That being said, my breadboard is remarkably noise-free.

I does, but there might be a work-around. Another thing on my to-do list is to see if I can make a viable "no charge pump" version. The trivial solution is to simply run this circuit on +18V and create a Vref.

I suppose one of the buffers could be used to buffer the output. Not really necessary. but doable. I was planning to try the parallel OTA trick once I had everything dialed-in. It will reduce the noise from the LM13700 by 3dB. Whether that will be audible is debatable. The LM13700's noise...

Most of the compressors we run into are feedback-type compressors. The output level is sensed and that is used in a feedback loop to adjust the gain. The Engineer's Finger compressor is a feed-forward type. The input signal level is sensed and that is used to adjust the gain. There is no...

Good questions. There is no hysteresis. but there is a slow-ish release. At first, I was concerned about the long time constant between R15 & C6/C7 (my schematic). It's a trade-off between release time and ripple. If we shorten the release time too much, by reducing R15 or C6/C7, then there...

Yes. No difference when using just the OTA. The Darlingtons come in handy in filter circuits but are not needed here. Close. The OTA (U2A) acts like a current-controlled resistor. The peak detector (Q2-Q5) outputs a current proportional to the peak amplitude at the output of U1B. That...

Back when I was a young lad, my friend & roommate Richard bought a Black Finger. As per usual, I would cajole him into letting me take it apart & trace the circuit with the the promise that he'd get it back intact. A very interesting circuit indeed. Years later he sold it to me and I still...

I'll keep that in mind. I bought a matched pair of JJ's.

Don't pay too much for them. Shop around. I bought some from a guy in the UK thru eBay.

Every time you change a transistor, you should check and adjust the biasing. This is especially true with Ge. Otherwise you're changing more than one thing when you swap out a Ge transistor. Q3's bias has a major effect on tone. Q3-C should be around 4.6V.

The two leakage currents I measure are Iceo and Ices. Ices is basically the same as Icb since E & B are shorted together when measuring Ices. The one I pay the most attention to is Iceo (collector current with B not connected to anything). How much is not enough? How much is too much? How...

Yes.

Getting the right amount of light on the LDR is the critical. LDRs are EXTREMELY sensitive to light. It is easy to put too much light on the LDR and if that happens, then it recovers (resistance goes up) very slowly. A diffuse LED is better because it spreads the light more evenly over the...

Correct. The DC bias can come from anywhere, as long as it's the right voltage. Check out the biasing in the Big Muff II. Stage 1 biases stage 2. Stage 2 biases stage 3.