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This is a relatively well known ABY circuit available on the internet.

CLICK HERE, REMOVED IMAGE FOR COPYRIGHT REASONS

I have two questions about this circuit : 1) What does the RC network (in the secondaries) do in the output of the transformer? My guess is that it's some sort of filter. 2) what is the point of the two resistors (on the opamps driving the primaries) on the opamp. They don't seem to be providing any gain?

Below is my attempt at an isolated buffer. The aim is to provide a potentiometer giving attenuation (not to silence, just to allow adjustment based on the output differences of various guitar pickups) in one direction and some gain in the other (about 2x gain).

enter image description here

As you can see I've just added some gain to the opamp, and a pot on the output. LTSpice shows this working perfectly. I've left the original RC network in place. What effect on that will the POT I've added have? Is there a better way to do this?

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  • \$\begingroup\$ You already have a simulation, run it and observe the difference \$\endgroup\$ – PlasmaHH Aug 13 '18 at 12:51
  • \$\begingroup\$ Without a real idea of the concepts.. I could be swapping components in and out and not really knowing why the results are what they are. \$\endgroup\$ – Richard Aug 13 '18 at 12:57
  • \$\begingroup\$ The RC part could be a snubber network. Like a reversed diode next to a relay but for AC. \$\endgroup\$ – Michael Koeppen Aug 13 '18 at 13:06
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    \$\begingroup\$ @Richard: That schematic says clearly on the bottom "No permission for local copies or serving from Web sites other than http://www.geofex.com". Did you get permission? If not ... \$\endgroup\$ – Transistor Aug 13 '18 at 13:22
  • \$\begingroup\$ @Transistor I did not obtain permission. My reasoning is that this circuit is so ubiquitous on the internet already that I am simply one of thousands who have already republished this diagram. I deliberately left the creators text on there so that everyone can see where it came from. RG Kean absolutely deserves credit for his work. I'm not in any way attempting to undermine him, I just wanted to make it easier for people to answer my questions. If removing this picture and replacing with a link is advisable then I will do that. \$\endgroup\$ – Richard Aug 13 '18 at 14:18
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I'm not sure about the purpose of the RC network. It provides a 10k load at frequencies above 16 kHz, and a rising impedance at frequencies below that, becoming an open circuit at DC. It seems rather pointless in this application.

The two resistors at the input of the opamp are used to balance the input bias currents, eliminating one potential source of DC offset in that stage. It isn't clear why the same precaution wasn't taken with the first stage.

It's possible that these are example of "cargo cult" design, in which components get copied from one circuit to another without any real understanding of their purpose.

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  • \$\begingroup\$ The series RC looks like a snubber, and is a prime cargo cult suspect! \$\endgroup\$ – awjlogan Aug 13 '18 at 13:15
  • \$\begingroup\$ @awjlogan: Such a circuit (with very different component values) is often used directly at the output of an amplifier in order to improve its phase margin with reactive loads (see the example circuits for the LM386, for example), but I don't see how that applies here. \$\endgroup\$ – Dave Tweed Aug 13 '18 at 13:19
  • \$\begingroup\$ Possibly they've have seen it on the secondary side of trafos and so thought they should have one too... \$\endgroup\$ – awjlogan Aug 13 '18 at 13:26
  • \$\begingroup\$ David, This is the creator (Kean's) second attempt at this circuit, he says that the second version improves the frequency response of the cheap transformer he sourced. What effect will my adding the POT have on the RC network? Will it adversely affect the frequency response? How can I have the POT without affecting the RC network operation? I can't really use another opamp without loosing isolation. \$\endgroup\$ – Richard Aug 13 '18 at 14:21
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To me, the RC part looks like an "anti-pop" circuit. I think it might help to reduce the "pop" sounds when the circuit will be switched on and/or when a cable will be plugged in.

It can also act as a Boucherot cell or a Zobel network, avoiding high frequency oscillation when no load is connected.

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  • \$\begingroup\$ This seems possible. OP's circuit is mean to act as an A/B/Y box in front of a guitar amplifier, not a speaker, and thus shouldn't see the same issues with impedance and stability -- unless it's to account for the inductance of the transformer? Interesting circuit, I'm putting it in my toolbox for sure. \$\endgroup\$ – esilk Aug 13 '18 at 14:48
  • \$\begingroup\$ Even if this network has been made to be placed in front of a loudspeaker, it may be possible that these oscillations can occur with the inductance of the transformer indeed. I'm not very familiar with all this so I'm not sure but I do know that a transformer and a loudspeaker are somehow similar electrically speaking \$\endgroup\$ – J. K. Aug 13 '18 at 15:08

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