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Switching scheme with muting and LEDs for guitar tube amplifier

Thread starter #1
Hi guys, I have a problem with designing a channel switching system for a tube amplifier. I usually go for relays with flywheel diodes, with the relay coil ground connected to the switch. However, I implemented this solution in a 3 channel amp and I can not tolerate the popping noise. I looked around the web and it seems that the best way would be to use a muting circuit (some also suggest to use an optocoupler). My specs/requirements are:
1) the amps has three channels with separate gains, masters and tonestacks.
2) The are 4 relays. R1, R3, R4 are tied together, and R2 is separate.
3) when all relays are off, the mode is "Crunch". When R1, R3, and R4 are on, "Lead" mode is on. When R2 is on, "Clean" mode is on.

Right now, this is triggered with 3 position switch from the front panel of the amp. It works, but it pops. My idea is to keep the front panel switch, add muting, add LEDs (currently not present), and add a footswitch, possibly using a 5 pin DIN port. My current footswitch works with a stereo jack but has only two footswitches, and to change from CRUNCH to LEAD I have to press one of the buttons twice. I'd ideally like to have three separate buttons for each channel.

Do you guys have any idea on the circuit that is most suitable for this application? I've attached a sample circuit that I drew using NPN switching (which is a completely topic for me) instead of the current system... I also attached the signal path for the amp. What do you think?
I've always built single channel tube amps so far, so this is new territory for me. Forgive me for my noobiness in the area, but this is quite exciting to learn

Thanks for the help.

Current system


Proposed system with LEDs and with cure for POP


Signal path in the amp
 

cowboybob

Well-Known Member
Most Helpful Member
#2
Could be that I'm just dense, but I'm not at all sure what the "pop" is that you reference (I assume an audio effect during switching) and how it is that the second circuit you've provided cures it.
 

rjenkinsgb

Active Member
#3
but I'm not at all sure what the "pop" is that you reference
Same here..

If it's due to noise pickup from the relay coil/switch wiring, I'd first try low value series resistors and small parallel capacitors (across the coils) in the control circuit, to remove any "spike" or flywheel pulse from the control wiring.

Your transistor circuit will not work as drawn, as the transistors have a base threshold of about 0.6 - 0.7V and the various parts involved in pulling the bases down will not get that low - eg. the LEDs will be conducting at about 1.8V or more.

You could possibly make it work with a three resistor combination on each base:
Power to input or control (as shown for the first two) then from there to base and from base to 0V, forming a potential divider between the control signal and base.

If that was eg. 5:1 then the transistor threshold would become more like 3V, allowing a LED or diode to pull low and turn it off.
 

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