Help eliminating pop in variable nfb switching circuit

[hasserl]

I've been adding a 100k pot to the nfb circuit of Carvin VT50 amps for quite awhile. Owners of those amps really like the effect, but have been asking me to make it footswitchable for a long time. I've blown them off all along, because I really didn't see the benefit to it, when I use this effect I set the control to where I want it and that's it. But after a lot of nagging I finally relented and on this last mod job I've added a relay to switch between the stock circuit and the 100k pot. I tapped into the filament supply circuit for power and ground the relay trigger circuit thru the footswitch. It works just fine except for a loud pop when switching. Below is a schematic of the mod:



I have added a 1M resistor from the input to R71 to the output of the pot (basically bypassing the whole thing) hoping it might help with the pop, but it did not reduce it at all. The pop happens both when engaging and disengaging the relay. It makes no difference where the volume control is set, the pop does not get quieter or louder, it remains the same regardless where it is set. The same with the 100k nfb pot, it doesn't matter where it is set, the volume of the pop remains the same.

Any ideas of how I could get it to quiet down?

TIA
Hasse

[Satamax]

Hasse.

Make the NFB resistor bigger, up to the double of the value of the pot in series with the original NFB resistor (so 220 k in this case) and put that straight in the circuit, and parallel switch a 220K pot to that nfb resistor. That should cure the pop imho.

[hasserl]

Hey Satamax, I corrected the schematic above to show the 1M resistor I installed to keep the circuit loaded while the switching is being done. Is this similar to what you were describing? Unfortunately, it does not cure the popping noise.

[Satamax]

Sorry i replied quickly and didn't make any sense, the simpler solution imho!

[hasserl]

You know, I was thinking of this overnight and came up with the same idea. I think I wil redo it to this configuration and see it that cures the pop. I suspect that any switching in the nfb circuit just might cause some kind of pop.

[Satamax]

You know, I was thinking of this overnight and came up with the same idea. I think I wil redo it to this configuration and see it that cures the pop. I suspect that any switching in the nfb circuit just might cause some kind of pop.

May be adding a cap to ground would lower the pop, or, in series with a res to ground or! I can't remember how to avoid pops!

[hasserl]

May be adding a cap to ground would lower the pop, or, in series with a res to ground or! I can't remember how to avoid pops!

Yes, I've been thinking of how to add some capacitance to relive the pop, but since this is the nfb loop a cap to ground would function like a Presence, sending hf's out of the loop. I don't think that I want that, I already have a Presence control.

I'll have to think about it some more.

[hasserl]

Well, I redid the whole thing like this:



I've still got the popping going on every time I switch the relay, either On or Off.

Any other ideas on this? Anyone?

I thought about bypassing the switch with a cap, as the cap may suppress any sparks across the contacts of the relay. But that would just make a high pass filter around the pot, not something I want to do.

[Satamax]

May be try a 1MΩ between 6 and 8?

[bob p]

i think that a pop should be expected.

have you tried bypassing the entire array of resistors with a coupling cap?

essentially, you'd put the two leads of the cap across the points noted on your schematic with the arrows.

[hasserl]

i think that a pop should be expected.

have you tried bypassing the entire array of resistors with a coupling cap?

essentially, you'd put the two leads of the cap across the points noted on your schematic with the arrows.

Not yet, but I'll try it today. Any ideas on the value of teh cap? I'm thinking of going small, ~150pf, to give same capacitance to absorb the energy during the switching, but not large enough to bypass everything around the resistance and leave it ineffective. But I'm open to suggestions.

[hasserl]

Well that didn't work! A cap across those points sends the amp into a horrible low frequency oscillation.

[oldtone]

hasserl, in TV broadcast situations we used a reverse biased diode across the relay coil to kill the inductive kick. Bob