Posted by Wayne Parham [ 65.69.120.160 ] on March 15, 2005 at 17:54:03:
This evening, I took a few minutes and installed some power supply filter components. I took measurements before and after, to compare the differences. I also ordered a matched set of Светлана 6BM8 tubes, so I'm anxious to compare those with the others I already have.
I added 0.022uF 200v mylar capacitors across each diode and an LC filter (1mH/47uF) just before R1 in the power supply. The diode switching shunt caps worked wonders to stop the slight hint of 8kHz buzzing - It's completely gone now. But the LC filter did almost nothing to reduce 120Hz noise and I'm not sure it's worth the trouble. Here's a chart of measured values:
8kHz buzz without 0.022uF caps with caps
==================================================================
L channel output 0.15mV 0 - unmeasurable
R channel output 0.15mV 0 - unmeasurable
power supply output 0.6mV 0 - unmeasurable
120Hz hum without 1mH/47uF LC filter with LC filter
==================================================================
L channel output 0.18mV 0.12mV
R channel output 0.15mV 0.10mV
power supply output 2.0V 1.4V
I still have to replace a coupling cap, cathode resistor and bypass capacitor on one side, because I have used substitution values and they're haphazardly placed. So there is that to consider. Or the difference might be due to circuit layout. We'll see.
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