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I'm working on a digital guitar effect pedal, but wanted to implement a "dry through" circuit so that I could mix the digitally effected/wet signal with the original unaffected/dry signal at the output. This might be a simply be input buffer and summing amp at output, but I might be totally wrong.

I'm struggling with a search term for that which is giving me any useful results, so thought I'd ask here. I think there might be a lot of input/output impedance issues in replies and that's a dark art to me. I think generally guitar effects pedals try to have a high input impedance and a low output impedance, so that maximum signal voltage is transferred to the following stage.

So I thought as I'm splitting the signal, at the input, that I should use an op amp, (maybe you can use an BJT transistor), to 'buffer' the input signal and give the input impedance which the guitar is expecting. After that buffering Amp I can split the signal into two paths. One (dry) would go directly to a summing amp at the output, whilst the other (wet) signal would go through the digital signal path to the summing amp at the output.

I think I'd need a dual gang potentiometer at the output to effect the mix ratio, but that's about it. I don't want to get into phase issues but I guess that might be inevitable.

So in short buffing amp at the input and a summing amp at the output, with a dual gang pot for mix ratio. All the time naively avoiding figures for impedance and phase issues.

My question is: does that sound about right for a dry through mix circuit or totally wrong?

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  • \$\begingroup\$ even a rough block diagram would go a long way towards explaining your thoughts \$\endgroup\$
    – jsotola
    Nov 14, 2020 at 20:59
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    \$\begingroup\$ You're on the right track except for "I think I'd need a dual gang Pot at the output to effect the mix ratio, ..." Most dual-gang pots are designed for stereo volume control. You want a single balance or cross-fade pot to allow you to go from dry through blended to wet only. Usually these are wired with the wiper to ground. Look for stereo mixer cross-fade circuits. Maybe electricdruid.net/single-vca-crossfader? \$\endgroup\$
    – Transistor
    Nov 14, 2020 at 21:21
  • \$\begingroup\$ ... or the very simple diystompboxes.com/smfforum/…. \$\endgroup\$
    – Transistor
    Nov 14, 2020 at 21:27

2 Answers 2

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If you have the real estate on the stomp box, I suggest a DPDT phase switch so you can switch between the mix being additive (sum) or subtractive (difference).

You might prefer volume control pots on both sides, that's pretty easy to implement with the op-amps. Alternately, yeah, one for the mix, but I think maybe a volume pot as well. For a low-impedence out, a small signal transformer will work wonders even if you hate what it does for the ultrasonics.

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  • \$\begingroup\$ and if this is a combination distorted/flanged/chorus into a noise gate, I wanna hear it! \$\endgroup\$ Nov 14, 2020 at 21:37
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The most common opamp summing configuration is inverting (with or without some gain).

IF the input buffer also is inverting (with or without some gain),

AND IF the phase shift through the digital section is zero,

THEN the net phase shift through the box is zero for both paths.

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