Question about mixer schematic for ABY pedal

[Elegost]

Hello folks,

I have built a dual channel ABY pedal.  The idea is, it takes 2 inputs and has 2 outputs, and lets you choose either or both to be active.  It also has switches to lift the ground on one output, and to sum both inputs to the "A" output.  Each channel has a buffer, for which I used the GGG dual buffer circuit.

Having built this already, I get a little bit of crosstalk, and some weird interactions when I have it in "Y" mode, and having done some reading on the AMZ mixer, I'm wondering if I need input resistors to prevent interaction between the two channels.  And, if' i add some input resistors, do i then need to change my buffer circuits to provide makeup gain?

Here is the schematic, any feedback would be much appreciated

[Elegost]

Any advice?  Do I need input resistors to prevent crosstalk in Y mode?  Will they reduce gain that I need to make up somehow?

[PRR]

Yes, input series resistors would reduce interaction. See the classic Fender 2-jack input: 68k resistors. Much higher will raise hiss, much lower then two guitars will interact funny.

The "best" answer will be to buffer before and after any mixing.

I am not understanding the and/or/Y switching. It seems messy to my eye.

> crosstalk in Y mode?

Not unnerstanding this neither. Isn't "Y" supposed to crosstalk?

[Elegost]

Sorry if the schematic is hard to read.

I'm splitting a bass signal and shifting one side up an octave, then running it thru a distortion pedal. Let's call that the "guitar" signal. Then I run the unmodifed bass signal and the "guitar" signal into the two sides of this pedal. The and/or switches let you pick whether to let through the bass, the guitar, or both. The output jacks go to two different amps. The Y switch is for if I only have one amp, it sums both to the A output.

Crosstalk is probably the wrong word, interaction is maybe a better word

If I put 68k resistors on the inputs, will that reduce volume?  do I need to mod the buffer circuits for makeup gain?  Can I do that by putting a resistor in the feedback loops of the TL072?

Thanks for the help!

[antonis]

I am not understanding the and/or/Y switching.

Me also..

Can't figure out the purpose for deleting lower buffer when Y switch set on upper position..
(that makes a simple mixer with unbalanced inputs..)

>If I put 68k resistors on the inputs, will that reduce volume?<
Less than 10%..(for each individual input)
68/(733+68) = 0.915, where 733k is the equivalent resistance of 3 2M2 parallel non-inverting input resistors..

>do I need to mod the buffer circuits for makeup gain?<
See above..

>Can I do that by putting a resistor in the feedback loops of the TL072?<
No, 'cause by simply putting a resistor in each feedback loop you'll still have buffers of unity gain..
If you insist to makeup gain, you have to modify existing configuration to non-inverting op-amp with "some" gain..
Place a resistor, Rf say, between TL072 output & inverting input and another resistor, Rg say, between inverting input & Ground.. The gain should be 1 + Rf/Rg..
(more practically, you should also place a capacitor between Rg & GND to form a HPF and to roll-off DC to unity and another capacitor in parallel with Rf for forming a LPF..)

[Elegost]

Hmm ok yeah I figured it wouldn't be simple to get gain out of this. Maybe I will try the 68k resistors ands see if the 10% loss is a problem before I tear the pedal apart.

I see what you guys mean abt the Y switch now I drew the input connection wrong I will redraw it tonight.

[antonis]

Maybe I will try the 68k resistors ands see if the 10% loss is a problem before I tear the pedal apart.

0.7dB loss shouldn't be considered "problem", should it..?? 

[Elegost]

Ok i updated the schematic so the "Y" switch is noted properly.  Does this make more sense?

Yeah 0.7db is probably fine!

[Elegost]

Update, adding the 68k input resistors fixed the problem!

Here's a picture of the final product