[sdiy] Patch pal style audio amp

[brianw]

A single transistor circuit is really only going to work well for a signal like Gate, where it's just On or Off, and you use the transistor to change the voltage or even invert the polarity.

For an audio signal, a single transistor is either going to give you very low quality, or require a good number of support components that will have to be carefully designed to avoid distorting the audio. This is not the easy route.

BBob's recommendation to use op-amps is, by far, the easiest. This is exactly why the op-amp was invented. You can even buy a dual op-amp in a single package. These will run off batteries, but you might want to select low-power op-amps and adjust the circuit to deal with any tradeoffs or headroom differences.


As for your applications, you might find that you need two different solutions:

E) The desktop modular is going to take ±10V peak-to-peak audio, or maybe half that amplitude. This is the one where BBob's gain recommendations should be spot-on.

D) The desktop synths might not expect modular voltage levels, so you might distort the audio input when feeding the synth with boosted levels intended for modular. In my admittedly limited experience with synths that have audio *input* (samplers), the expectation is usually for line level audio. It's very likely that each synth will be different. There's not necessarily any standard audio input level for desktop synths. If you tell us more about your desktop synths, it might be possible to find out what audio input levels are best.

Brian


On Jul 17, 2022, at 8:14 PM, frey at radioles.com wrote:
> Tx for the reply. I am going into desktop modular device that’s eurorack spec. I am coming out of a Crudman voltage controlled cassette player into a EOWave Quantrantid Swarm. I have a few desktop synths that I want to run audio into in this way, not an actual eurorack spec powered case.
>  I have a bunch of passive patchpal thingies in the style Quincas has built. I want to make something self contained so batteries because not passive. I have seen amp circuits using one transistor. Could this work? JoeF.
> On Jul 17, 2022, 10:37 PM -0400, bbob <fluxmonk at gmail.com>, wrote:
>> I use 2 inverting op-amps in series, with a gain on the first one at about 5 (since you're in eurorack, you already have a dual power supply)
>> 
>> On Sun, Jul 17, 2022 at 10:25 PM JoeF <frey at radioles.com> wrote:
>>> Hey there all, I bet someone has already made a small battery powered amp to go from line level to a hot input signal for Eurorack modular standard?
>>> If not, then can a I use a single transistor amp to do this? Will the input drag it down or load it up? TY for any advice or help. JoeF.
>>>