• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Speaker snap or crack (no pop)

I have a K-12G that I'm using until I can build something better. When I turn off the amp, I sometimes get a snap or crack in the speakers. It is not a pop, nor is it a thump. It happens as the switch is thrown; there is no delay. It's not super loud; in fact, the amp can drive the speakers to a louder volume playing music. Nonetheless, I'd like to eliminate it.

So, after browsing the forums here, it seemed likely to me that the snap was caused by an arc at the power switch. I ordered this 0.01 uF ceramic 300VAC X/Y rated cap from digikey and installed it across the switch as shown below.

Well, I still get the snap, so either I did something wrong, or it's something else entirely. Any thoughts? I was worried about the 300VAC rating on the cap, but it was the highest rating I could find in an X/Y rated cap, and I thought I read somewhere that the surge rating was much higher (I could be wrong). I'm also assuming that this is how it should be soldered, and not across the primaries.
 

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M Gregg

Disabled Account
2010-06-28 11:04 pm
UK
sneakers,

you have only covered one contact, you have 2. Line / Neutral.

It could be back EMF from the primary winding so you could try a MOV across the primary of the Tx.

Remember that the cap across switch as shown means that you should not rely on it for isolation!
I would use the MOV first and not the cap unless this fails!

Regards
M. Gregg
 
X and Y caps:

http://www.illinoiscapacitor.com/pdf/Papers/EMI_RFI_interference_suppression.pdf

X and Y caps really do have to be rated for a.c. -- this isn't an area where an economic decision should trump safety. You can also get a "Power Entry Module" from a dead computer -- (servers usually have nice, hefty ones).

When the switch contacts get close, arcing occurs -- particularly for DC -- they're just like the plates of a capacitor. A cap across the contacts helps, but it is better if it has a resistor in series with it to dissipate the energy.

[IMGDEAD]http://www.tech-diy.com/quencharc.jpg[/IMGDEAD]

[IMGDEAD]http://www.tech-diy.com/quench_snub.jpg[/IMGDEAD]
 
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Thanks for all your help. I feel like I'm getting a good handle on how the amplifier itself works, but it's sometimes hard to find info on all the other stuff (like chassis layout, earthing, etc.) needed to make a working amplifier. I really appreciate the advice here.

Would this varistor be appropriate?
Littlefuse V220ZA05P
Varistor Voltage 220V
Current-Surge 400A
Max AC Volts 140VAC
Max DC Volts 180VDC

And just so I exactly understand: I want to put it across the two switch contacts attached to the transformer primary (the bottom 2 in my picture)?

If I went the resistor / cap route, is something like a 100R 1/2 watt carbon comp ok?

Finally, jackinnj, are you saying that my capacitor is insufficient? It is X1/Y2 rated.
 
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Simply putting the capacitor across the primary may be sufficient, provided that the route into the equipment is via the power supply. I generally do this as routine now, and get little trouble with switch-off clicks. The primary resistance will be sufficient as a snubber - no need for extra resistance here. The primary inductance is the most likely cause of switch arcing, as it will be much larger than the inductance of the mains leads.

If the noise is being radiated from the mains lead and then picked up on a signal lead then this won't work, although it won't do any harm.