DIY Audio Source Switch Box: Avoiding a loud pop through speakers

[chrisng]

Valve amps are different because of the transformer coupling to the load (so the same would/could apply to solid state if it was an unusual transformer coupled design). With no load such a transformer could generate very high voltage transients if signal (and also power on and off transients) was present and that can cause breakdown of the insulation on the transformer windings.

Hi Jonathan,

When I built an amp switcher for tube amps, I used a make-before-break switch to ensure that both tube amps were always connected to the load when switching.

If I understand correctly your design uses a form C contact relay for switching and when the relay contact moves between the NO and NC contacts the tube amplifier will disconnect from the load, how do you solve this problem?

 

[indianajo]

when the relay contact moves between the NO and NC contacts the tube amplifier will disconnect from the load, how do you solve this problem?

I did not switch my ST70 but I did sometimes walk across the speaker wires and yank the wire out of the spade lug. I solved this problem by putting 1000 ohm 3 watt resistors across the 16 ohm and ground terminals, inside the chassis. Both channels Sonicly invisible. If both tube amps are so equipped, no problem.
These days a MOV with ~300 v rating in the came place could accomplish the same result. If a tube amp is too pristine to be modified, the resistors or MOV's could be installed at the amplifier inputs of the switch box.

 

[ecksrhae]

hey all, in the interests of not starting a new thread for something almost the same, do any of the suggestion in here change if I'm building a box to take 6 TRS inputs (3 stereo field microphones) with no phantom power to two (one stereo) XLR ourputs? Trying to get to a spot where I can put my MixPre in a waterbag and only risk this switch to the weather (the field mics are contact mics and hydrophones up to the weather themselves). so far I've set to design something using the RY-12W-K relays and protection diodes. Any further suggestions?

 

[indianajo]

so far I've set to design something using the RY-12W-K relays and protection diodes.

Relays cause pops on low voltage signals, as for example dynamic mikes (SM87). The hydrophone dpdt Teledyne relay I used in 1976 in oil exploration was certainly a failure due to the noise. Grounding the input with another relay before during and after the switch over did not help. Back diode on the relay coil did not help. Pop is less likely a problem on condensor mikes (1-2 vac). Pro way to switch line level signals is a CDS optocoupler as the nsl-32sr3. One for each channel, driven by a 20-30 ma dc current on the LED. CDS cell turns on & off slowly enough to not pop. See post #7, 9, 11.
External switches, there are IC's that sense the capacitance of a finger on a touch pad. Total non contact way to control things in bad weather. Cheaper, pushbotton switches with rubber seals.