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Old 20th January 2012, 08:18 PM   #1
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Default Strange.................

Just rewired my Aikido linestage e88cc/6sn7.....Sounds nice and really quiet..Problem is when i switch it off and touch either the 6sn7 bases or the output caps i get a loud buzzing from power amp.Any ideas????????
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Old 20th January 2012, 08:47 PM   #2
DF96 is offline DF96  England
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It's called mains hum. Your body is acting as one plate of a capacitor. The other plate is your mains live wire. Quite small capacitance, but the input impedance of most power amps is quite high. When the linestage is turned on it has a low output impedance so that greatly reduces the effect.
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Old 21st January 2012, 06:59 PM   #3
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Could also be parasitic oscillation; the additional capacitance from your finger could be tipping the system just out of stability. Now, fingers usually dampen rather than excite systems, since there's a lot of resistance in the body, but if it's oscillating at a relatively low frequency (~MHz rather than 100MHz+), the impedance is probably quite high, in which case your finger will look like a good capacitor.

In either case, however it's picking up the noise, if through some radio-frequency voodoo, or just plain old finger-buzz, it might be worth double checking your layout, make sure inputs and outputs are far apart, that sort of thing.

Tim
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Old 21st January 2012, 10:57 PM   #4
kevinkr is offline kevinkr  United States
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Nothing strange, once off the pre-amp has a high source impedance and as DF96 explained you are capacitively injecting signal into your amps.

Now the biggest question arises as to why you aren't really following good power sequencing practice which is to turn off the power amps first, then the line stage and finally everything else. Power up should be reverse sequence.
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