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Old 9th April 2013, 02:17 PM   #1
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Default amp buzz in one channel on inrush / power on

Hello,

One channel of my amplifier has a significant buzz/hum when initially turned on, then subsides after 1-2 seconds.

What can be the cause of this?

Please help me find the problem... if it is even a problem

both channels do it, however one is significantly louder than the other.

What should I look for?
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Old 9th April 2013, 03:27 PM   #2
Kosmin is offline Kosmin  Europe
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Do you have an oscilloscope? A way to look after the problem is watching the voltage across an artificial load to see what happens there and after that digging more in the amp.
Do you have a schematic of your amp? How is the power supply built? Is there any softstart?
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Old 9th April 2013, 05:00 PM   #3
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There was a soft-start on a relay, but I disconnected it as it became unreliable (occasionally needing a tap with a screwdriver to engage). I intend to rebuild that with a manual switch instead. I understand it's due to inrush current and started the same time I disconnected the soft-start, but my only concern is that the one channel is significantly louder, and would like to diagnose this if it is a problem before proceeding to re-implementation of the soft-start switch, I'd rather not ignore it by masking it. Also no oscilloscope on hand.

Can tube filaments cause this, maybe I'll start by swapping L to R tubes see if there's a change to the other channel.

BTW all diodes and capacitors seem to balance well enough without dismantling for accurate measurements...for now.
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Old 9th April 2013, 05:02 PM   #4
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I'll post schematics later,

It's an 805 driven by a 300b design.
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Old 9th April 2013, 05:55 PM   #5
DF96 is offline DF96  England
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Could be poor grounding, injecting charging pulses into signal ground. Charging pulses may be bigger during the warm-up phase.
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