Need some help with stereo to mono scheme....

[dschwartz]

Hello all electronic wizards!
I'm designing a stereo-able pedal, with a TRS input.
Tip and ring signals are each buffered by a bjt buffer, into inverting opamp gain stages. And the mono signal can be switched between gain A and gain B via jfets.
If a mono cable is used, the ring buffer is grounded, so i get a mono signal with a/b switching....
If a stereo cable is used, the tip signal can be switched A/B, but the ring is always B.

My issue is that in mono configuration, channel B has lot less gain, and i suppose that is because the ring buffer is acting like a low impedance dc source and making C1 act like a LPF and eats part of the signal..i tried adding a resistor in line with C1, but it affects the gain of the system if used in stereo...
I tried using an opamp, but it just kills the signal if grounded..

I'm scratching my head heavily with this.. I can't find a simple solution...

[anotherjim]

Maybe the JFET's are not perfectly isolating when off. Have you got reverse diodes to the JEFT gates?
https://www.premierguitar.com/articles/jfet-switching-1

[dschwartz]

Maybe the JFET's are not perfectly isolating when off. Have you got reverse diodes to the JEFT gates?
https://www.premierguitar.com/articles/jfet-switching-1

Yes, checked that..reversed diodes in place, switch voltage buffered by 4049 inverter..
Jfets are 2sk208 and seem to work very well.
If i remove C1 the balance is perfect...
Maybe if i change the gain stage into a non inverting high impedance input stage, the effect of the buffer sinking current could be less noticeable? What do you think?

[anotherjim]

I'm thinking it would be difficult to stop it loading via C1. How about routing C1 to the opamp inverting input via a mixing resistor? There won't be any signal in C1 when the ring is grounded by a mono jack and it won't interfere with the tip channel from the JFET.

[PRR]

Seems you want another switch?

[dschwartz]

Yeah i tried that, but still have -6dB or so on the B channel..
According to LTspice it should work, but it doesn't..maybe the jfet on resistance is interfering..
I'm thinking on adding another buffer before channel B. And connecting the ring buffer through a cap and resistor to it, together with the jfet output..in theory, if the ring is grounded, i get no signal from the ring buffer and will act as a "second bias" resistor.

In LTspice it works..and i have a spare opamp on the board, so maybe it will work without adding much stuff...

[dschwartz]

Seems you want another switch?

Yes, thought about that, but the user would have to remember to activate that switch if stereo is used.. i want it to be "smart" so it won't need any user action if stereo is needed.

[dschwartz]

Hmm. even better maybe
The only drawback i see is extra noise from the 47k resistor and the buffer..but could be too low to bother.

[11-90-an]

have both of them have the same control signal? (each with its own reverse diodes, of course)

[dschwartz]

have both of them have the same control signal? (each with its own reverse diodes, of course)

When used in stereo, channel A can be switched from A to B, and channel B is always B.  I ommited the output switching in the drawing, where B output is connected to the ring ( through a resistor) and A/B (with jfet routing) to the tip

[anotherjim]

Yeah i tried that, but still have -6dB or so on the B channel..
According to LTspice it should work, but it doesn't..maybe the jfet on resistance is interfering..

If the amps have gain and if their input resistors are too close to the JFET on resistance, then the path gain may well be wrong. Obviously, the amp with the higher gain will be the most wrong. The amp R-ins should be at least x10 the path impedance via the JEFT's?
Do the opamps have to be inverting?

[dschwartz]

Yeah i tried that, but still have -6dB or so on the B channel..
According to LTspice it should work, but it doesn't..maybe the jfet on resistance is interfering..

If the amps have gain and if their input resistors are too close to the JFET on resistance, then the path gain may well be wrong. Obviously, the amp with the higher gain will be the most wrong. The amp R-ins should be at least x10 the path impedance via the JEFT's?
Do the opamps have to be inverting?

Yeah, the input resistor is 33k so, it shouldn't be a problem
And yes..they have to invert, there is another inverting  gain stage after that, and since it will have "thru" options, i rather have the gain stages in phase with the input...also, i like inverting stages more
If my opamp buffer strategy fails, I'll try with NI...

[dschwartz]

Ok.. I'm embarrassed..
It works fine now..i had an error

EDIT...it wasn't an error....still investigating