1
\$\begingroup\$

I am experimenting a bit and have built the circuit shown below to mix two input signals. It works great across the full audio spectrum.

enter image description here

Now I’m trying to figure out a way to switch the signals so that one or the other can be switched off/on individually without any strange artifacts/pop from DC/audible distortion, etc. at the output, while still allowing for removing any large DC bias at the inputs.

I have tried numerous attempts at placing switch contacts, and this is made a bit complex by the single supply. The requirement to bias the summing amp adds DC to the right of the 220uF caps, so while adding large resistors to ground to the right of the caps doesn’t affect the bias or response much, if the switch is placed after the large resistors and before the 1k resistor, every operation requires the cap to recharge, and distorts the signal until it is charged. Placing the switch before the caps means if a long time goes between switching, the signal is distorted until the cap fully charges.

Can someone give me some suggestions on how I might accomplish this?

UPDATE: Thanks to the replies, I have modified the circuit as follows, and tested successfully with pushbutton switches.

enter image description here

Regarding the type of contacts for S1 and S2, assuming use of an analog switch or other FET type: Which would give better muting performance without unwanted side-effects, using an SPST switch and simply breaking the connection, or would it be better to use an SPDT switch and both break and shunt the input signal through a resistor to ground? As recommended in various locations on the web, I have tried simply shorting the input directly to ground, but I've never gotten satisfactory performance this way, probably due to the lower input impedance of the next stage.

\$\endgroup\$
  • \$\begingroup\$ You might need to buffer each one of the inputs individually, and place the switches between the buffers and the summing amp. \$\endgroup\$ – Don Joe Oct 6 '17 at 2:48
  • 2
    \$\begingroup\$ Use 100K to Vcc/2 after each cap before CMOS switch. \$\endgroup\$ – Sunnyskyguy EE75 Oct 6 '17 at 3:48
  • \$\begingroup\$ You could take a look at this schematic of the Boss DS-1 (specifically Q6, C2, C3) where FETs are used to switch between bypass and active. As Tony mentioned, caps are used which are always biased at the correct level to minimise pops. \$\endgroup\$ – loudnoises Oct 6 '17 at 5:43
  • \$\begingroup\$ @TonyStewart.EEsince'75: Simple and elegant solution! Thanks!! And thanks everyone else for the responses. I've tried muting this circuit by simply shunting to ground through a parallel switch, and that doesn't attenuate enough. If i go CMOS, should I use an SPST in series and just break it and leave both sides open, or is it better to go 2:1, break the connection and connect the input to ground through some resistance? \$\endgroup\$ – User7251 Oct 6 '17 at 8:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.