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Core Audio 6BC4 buffer problem
Core Audio 6BC4 buffer problem
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Old 5th November 2011, 06:53 PM   #1
paulrerrie is offline paulrerrie  United Kingdom
diyAudio Member
Join Date: Jan 2008
Default Core Audio 6BC4 buffer problem

Hi. I have a Core Audio (or VALAB?) buffer that uses 6BC4's. I replaced the fixed input attenuator with a 50K pot. I have two odd things happening:

1) With the pot at minimum (0 ohms), there is audible noise from the right channel speaker, almost like an oscillation.

2) As I increase the pot to max, a DC offset appears at the right channel output (1.2V at max - enough to trigger the DC detect in my DC coupled ARCAM amp). This offset is only present with an input signal playing (1KHz from a PC).

Since adding the pot, I also replaced the valves and replaced the output coupling caps - these were 10uF film caps connected across large bipolar electrolytics. They are now just 22uF polyprops (Mcaps - nothing too exotic).

The only items after the caps are a resistor with back to back zener diodes across it and a muting relay.

Its annoying as I would like to use the DC coupled inputs of the amp.

Any ideas appreciated, Paul
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