Signal merge without a mixer??

[John Egerton]

Hi guys, thanks for all your help with my small clone project...

I need help with something else now...

I have made a theremin in a plastic enclosure with a radio antenna sticking out of it and can be turned on with a dpdt switch...

My idea is to have this thing in my pedalboard which will inevitably have its signal affected by things like flangers and delays in my setup. However there is one problem... There is only an output on a theremin and no input and so I need a method of adding this thing to my setup.

Basically I don't want however I achieve this to have any effect on the tone of my setup when it is either on or off and I do not want volume levels altering for my setup.. Only a level knob for the theremin to match the level of the guitar...

Anyone know how I can do this?

Thanks

John

[R.G.]

Sorry - you want a mixer. Fortunately, not much of one.

You want a mixer with unity gain for the pedalboard and variable gain for the theremin input. The simple mixer I posted on Leper's archive back in the mid 90's would do it. It wasn't new with me then, and wasn't new with any of the versions posted on the web since.

You take an opamp with a resistor for feedback, say 100K. Into the inverting input go resistors from the various sources, as many as you like. The input resistors determine your gain by the G=Rf/Rin formula, so for unity gain, you use a 100K input resistor. If you want gain of ten (unlikely) you use a 10K, if you want a gain of 1/10th, you use an input resistor of 1M.

You can use a volume pot in front of the input resistor to do an infinitely variable gain.

All of these have to be decoupled from the opamp bias, so you'd use a capacitor in front of any input resistors, and connect the (+) input to the bias voltage. A capacitor at the output keeps DC off the output.

If you worry about absolute phase, you can put an gain-of-one inverter after the mixer, with Rf=Rin= 100K. Same comments about bias, capacitors and DC levels.

That ought to do it.