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Old 25th September 2011, 10:09 PM   #1
Arroyo is offline Arroyo  United States
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Default Cause of dc thump at turn on

Have an amp which exhibits a hard turn on DC voltage at turn on.
Can see voltage up to 10 or more volts on the unloaded output at turn on.
This settles down after the amp is on for 10 or more seconds. This is also
only on one channel. The amp has seperate power supplies with bridges
for each channel. Any directions to look at ? Some of my ideas have
ranged from a bad bridge rectifier ( leaky) to damaged transistor in one
of the drive sections. Voltages appear fine at the outputs ( reference
to other channel). Any ideas as to locating this would be helpful..
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Old 25th September 2011, 10:27 PM   #2
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Are you saying this amp has recently developed a thump at turn on but is otherwise OK or has it always had a turn on thump and you want to eliminate it?

John
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Old 25th September 2011, 10:32 PM   #3
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What is the brand and model of the amp?
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Old 25th September 2011, 10:35 PM   #4
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How much 'thump' voltage when the output is loaded?
A 'thump' usually means that one side of the bipolar power supply is coming up before the other side of the supply. Could be that the electrolytic capacitors in the power supply have lost some capacitance.
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Old 25th September 2011, 11:37 PM   #5
ODougbo is offline ODougbo  United States
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A friend repaired a similar problem with new caps and had to wash the board down with alcohol – a common problem with Adcom 565. He did a nice job, the amp(s) are repaired and work perfectly.
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Old 26th September 2011, 12:48 AM   #6
Arroyo is offline Arroyo  United States
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Default DC offset thump

Hi I appreciate the responses and it does appear that the caps are on the
way out. On observing discharge after power off the caps on the defective
channel have substantially less voltage remaining after a short period of time
IE bad channel 13 volts remained charged versus the good channel having
a charge remaining above 30 volts. I experienced this condition starting
after trying to drive some Apogee Calipers for a short while at lower volume
and a loud hum occured. I have not tried to drive the Apogees again but
am also looking to replace the bridges as well. ( could be the cause....).
In response as the amplifier it is an early 90's PS Audio Delta 100..
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Old 26th September 2011, 01:21 AM   #7
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It may not be the caps. Another Delta 100 owner brought out the same fault over at the PS Audio forum. One of the former PS Audio designers attributed it to the input jfet breaking down over time. Click for PS Audio forum on Delta 100.

It would help if you have the schematic.
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