Passive a/b switch "pop"

[Howard]

Hi  :-) I have "designed" a passive a/b switch box using a 4PDT. This allows me so separate each output and eliminate ground-loop hum. Also I short the output not in use in order to keep that amp silent. Works as intended, but adds a new problem. As most tube-amps have a small (0,X) DC voltage on the input, when releasing the short/switching to that channel, the DC raises and makes a sharp peak that gets amplified, thus causing a loud "thump" in the amp/speaker at switchpoint. What component can I add at each output in order to (at least) soften the phenomena? Howard

[Howard]

Would a pull-down resistor help? Or a cap i seriell that blocks the DC? Or perhaps a cap into the short-line so that the DC doesnt get affected? Again - what values? I do not have these parts so they have to be ordered. Thanks :-)

[ghostsauce]

http://www.fulltone.com/sites/default/files/manuals/fultone-a-b-switcher.pdf

^this is the one I used... No ground loop, muted input for inactive amp, and only need a 3pdt. Took me forever to find one worth building, so I understand your frustration, but this guy is completely noiseless.

[Howard]

Thank you! Nice design. However I notice that the outputs have common ground. This may, as it does for me, cause ground loop hum. It then does not matter if the respective input gets muted, as this is a marginal ground-potential issue between the amps that causes a tiny ac-mains current that again causes hum. I have discussed this issue with som small-scale makers of such switch-boxes, and though some are aware of the issue, they claim they wery seldom gets reports about this phenomena from customers. In my design there is no common ground, and I maintain use of robust metall-jacks by insulating them from the chassis by use of shoulder nylon washers.

[R.G.]

See comments in the other thread.

[ghostsauce]

Hahaha, oh man... I need to get more sleep.  In my defence it's been a while since I built it.

[PRR]

[Howard]

Thanks! I found a 400V 22nf cap that I replaced in my F Junior II (Bills mod), and connected it in paralell to the input of my various tube-amps (Champ 600, Excelsior & Jr ). At mid range volum-settings the hum from an open cable was redused to a hiss that is of no real annoyance. Also the pop was evened to an acceptable level. Anyway - Caps & Resistors are incoming :-)

[Howard]

Here is the prototype in a 1590B-ish enclosure. Has the unthrustworthy Tayda 4PDT, and still lacks the 0,1uF caps - so it pops. Will report back when finished. Need a better switch too, but they are hard to come by it seems. Would appreciate links to quality switches :-)

[GibsonGM]

Depending on where you're located, Howard:  www.smallbearelec.com

Great place to get parts, he's involved with the DIY community on here, so we have the chance to "keep our shopping local" rather than go to a huge, huge distributor.   

[Howard]

Thanks! I got those isolating shoulder-washers from Smallbear El. I will check his inventory for switches. Im located in Norway. Seems I did not manage to update my profile in Tapatalk

[Howard]

Here is a different approach to an a/b box. Got hold of some russian military surplus 3p3t rotary chickenhead switces. The popping is significantly reduced and so there is no need to add components.