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I'm building MIDI controlled effects pedal that autofades the blend amount of a send-return effect loop. Still pretty much a noob on the analog side, so bear with me.

Firstly I need to split the signal into two - one for the effects loop and one for the dry signal.

This is done by a simple buffer using the TL071 opamp and simply drawing two audio paths from its output.

After the split I control each signal path volume with a TI PGA2311 chip.

The problem: The output from the input buffer seems to be unsuited for the volume-chip. The buffer's output is perfect and clean sounding by itself, and so is the PGA2311 when bypassing the input buffer. When used together, the signal is completely crushed an low sounding. I'm still a beginner, and especially when it comes to impedance talk.

The datasheet of the PGA2311 says input buffering is not necessary, but I'm pretty convinced I need an input buffer before splitting an audio signal, otherwise it might put a load on the guitar pickups.

Any thoughts?

Both chips run on +-5 V. Guitars output is around 200-300 mV.

The bad output sound is a lower and very choked sound. Not just loud and overdriven as you'd expect if the input volt into the PGA volume-chip were too high. I'd like that.

First image illustrates how buffer end volume-chip each sound good seperated enter image description here

Second image: bad sound when connected like this enter image description here

Jesper Kragh
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    Can you post schematics? – bobflux May 23 '22 at 06:57
  • We can't guess what you connected and how and with what kind of signal levels. Please provide more info, and ask a specific question that is answerable, as currently you seem to be asking for thoughts. The only thought is that can you ever get TL071 working with only a +/- 5V supply. – Justme May 23 '22 at 06:58
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    If you are a beginner you may have some error that we cannot guess, maybe even cannot believe it before we see someone really has done it. I think you help yourself best by inserting a well readable (not a blurry smartphone photo taken in bad light) schematic of your circuit. A photo of something messy built on a breadboard is useless without a schematic. –  May 23 '22 at 07:13
  • Got it! Images added in question :) – Jesper Kragh May 23 '22 at 09:07
  • So what are the input voltage levels and to what gain setting the PGA is set to? – Justme May 23 '22 at 09:23
  • From the guitar comes 300 mV into the buffer. The PGA sounds choked at all gain settings when the buffer is before it. Not loud and overdriven -that would have be great. But choked and really bad.. – Jesper Kragh May 23 '22 at 09:31
  • What does "choked" mean? How do you know that the guitar signal is 300mV? What is the measured guitar signal level when not using the buffer? What is the measured signal level at the output of the buffer? – JRE May 23 '22 at 09:56
  • The schematics are also more block diagrams. It is unclear if the diagrams are really like that or not. For example, grounds are not connected together, and if the DRY signal is OK even though the PGA output is not. It is also unclear where the supplies come from, the 5VA and 5VD must be within 0.3V or chip damages. – Justme May 23 '22 at 10:05
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    Is the sound from the DRY wire still ok when the connection between the opamp output and PGA2311 input exists? –  May 23 '22 at 13:46
  • Add datasheet links. Do you need a coupling capacitor on the PGA2311 input or is DC coupling allowed? –  May 23 '22 at 15:28
  • I suspect the circuit is built on a breadboard and is oscillating like most breadboard circuits do. – Audioguru May 23 '22 at 16:06
  • Have you considered the impedances involved? I believe that IC has an input resistance of 10K, and they recommend that the source impedance is 600Ω or less. Connecting a guitar to that will probably load the guitar signal quite a bit. When you add the buffer the guitar signal is no longer loaded down. Do you have a scope to look at the signal level going into the IC with and without the buffer? – GodJihyo May 23 '22 at 16:26
  • Thanks everyone. I'm a fool. It's working now. The DRY sgnal was clean and good souding even when opamp and PGA was connectet, but I had the DRY connected to the same output as the FX SEND. This is obviously not the way to mix the two signals. Why that is I still have to learn..? – Jesper Kragh May 24 '22 at 07:42
  • Found my answer here https://electronics.stackexchange.com/questions/510184/why-are-resistors-necessary-in-a-summing-audio-circuit – Jesper Kragh May 24 '22 at 10:27

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