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I am looking to modify the input of a small stereo amplifier with a TRS input jack to have two TRS input jacks instead.

I want it to handle the following two scenarios.

Scenario 1. Two TS plugs are inserted in each TRS socket. Audio signal from one plug should go to left speaker and audio signal from the other plug should go to the right speaker. I am not looking to mix the signals together, I want them to play separately in each speaker.

Scenario 2. One TRS stereo plug with stereo audio signal is plugged to one of the TRS sockets (let's say left). The other TRS socket is left unplugged.

Scenarios that I don't care about to simplify the circuit: Two TRS plugs inserted in each TRS sockets. One TRS plug inserted in one sockets, one TS plug inserted into the other. Nothing inserted into left input socket.

The TRS plugs are normally closed and have both NC and NO contacts for Tip, Ring and Sleeve. If possible I don't want to use any additional switch.

This is what I tried.

T is for Tip
R is for Ring
S is for Sleeve
IL is for Left Input Jack
IR is for Right Input Jack
O is for Output Jack

T(IL) → 10kΩ resistor → T(O)
R(IL) → 10kΩ resistor → R(O)
S(IL) → S(O)

T(IR) -> 10kΩ resistor → R(O)
R(IR) unplugged
S(IR) → S(O)

It kind of works, but the audio output level is dramatically reduced. I supposed it's because the resistor value I chose was too high. I will be able to try again with a lower value after the weekend, but I was hoping someone to tell me I'm doing something stupid and should not waste more time on that.

The output impedance of the device I want to connect is 44 Ω. I don't know the input impedance of the small stereo amplifier.

There are probably some areas that I am not considering, like impedance matching. I was hoping someone would suggest some further reading to understand the problem and possible solutions.

EDIT: I was able to play around with it again today on the breadboard and connected it the following way:

  • T(IL) → 330Ω resistor → T(O)
  • R(IL) → 330Ω resistor → R(O)
  • S(IL) → S(O)
  • T(IR) → 330Ω resistor → R(O)
  • R(IR) → 330Ω resistor → T(O)
  • S(IR) → S(O)

It appears to work as I wanted and opened up a scenario where two stereo signals are connected, but one has channels switched. There's minor volume decrease (much less than with 10k) and I don't hear any audible sound degradation for my use case. However, I can't tell if there is a signal bleed on the speakers and don't know how to test.

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  • \$\begingroup\$ Please show the diagram of the sockets/receptacles you have or intend to buy. Preferably, link to a datasheet which is a better source of information than your writing about it. But what you describe sounds same but in reverse as in vintage synths that have either dual mono output or single stereo output for connecting to a mixer input. \$\endgroup\$
    – Justme
    Commented Apr 13 at 10:06
  • \$\begingroup\$ The sockets I have are similar to Rean NYS215. \$\endgroup\$
    – michoch
    Commented Apr 13 at 10:52
  • \$\begingroup\$ Are you open to buffering the signals with opamps? \$\endgroup\$
    – Dave Tweed
    Commented Apr 13 at 12:36
  • \$\begingroup\$ I would like this to be passive. \$\endgroup\$
    – michoch
    Commented Apr 13 at 12:59
  • \$\begingroup\$ Yes, there will be some bleeding between channels because of the non-zero output impedance of the source. But if you can't tell, does it really matter? \$\endgroup\$
    – Dave Tweed
    Commented Apr 13 at 13:28

1 Answer 1

Reset to default
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Here is a circuit that will do what you want.

enter image description here

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    \$\begingroup\$ Simple enhancement: connect the "R" contact of the "Right" jack to the switched "T" contact of the "Stereo or Left" jack. This gives the "channel swap" feature if you plug a TRS into the "Right" jack. \$\endgroup\$
    – Dave Tweed
    Commented Apr 13 at 15:18

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