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Old 4th February 2005, 06:26 AM   #1
d3imlay is offline d3imlay  United States
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Default Phase Linear Turnoff Thump

Over the years I've repaired more amplifers than I could count. I seemed to hook up with the Phase Linears and have specialized in them. Hey, it provides steady work!

I often thought that I could cure the output turnoff thump of the Series II amps by modifying the power supply for the opamp to keep it powered up until the main power supply caps discharged.

I recently set out to accomplish this task. After some experimentation, I put an outboard power supply on the opamp supply so that the opamp would never lose power. One channel was dead quiet while the other still had a thump. I compared the voltage stages in both channels and found the last voltage stage in one channel shutdown and that appeared to cause the thump. I swapped semiconductors between channels and that made no difference.

Anybody care to offer a guess? I can provide a pdf of the schematc if necessary. Yeah, I could put a relay in, but I'd rather address the real problem.
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Old 4th February 2005, 10:57 AM   #2
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I know these problems from different amps, and might be able to give you some hints

Which series II are we talking about ????
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Old 4th February 2005, 12:53 PM   #3
d3imlay is offline d3imlay  United States
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Phase Linear 400II is what I'm working on now. the 700II is identical except the power supply goes from + 80V to + 100V.
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Old 4th February 2005, 01:05 PM   #4
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As I recall, the schematic I have on my web is the Series I.
Could you please email me the Series II schematic ?
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Old 4th February 2005, 05:54 PM   #5
Netlist is offline Netlist  Belgium
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Drive PCB schematic can be found here: http://hometown.aol.com/phasetek/index.html

/Hugo
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Old 5th February 2005, 04:07 AM   #6
d3imlay is offline d3imlay  United States
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The phasetek site lists only the 400, not the 400II.

ACD I'll send you the schematic.
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Old 5th February 2005, 07:47 AM   #7
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d3imlay: Thanks, I'll wait for it
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Old 5th February 2005, 07:51 AM   #8
Netlist is offline Netlist  Belgium
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I thought the 400II had the same drive PCB except for the minor changes listed in the lower left corner of the 700II schematic. (PL36)
And of course less output devices.
I'd be happy to receive your schematic too.

/Hugo
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Old 5th February 2005, 07:03 PM   #9
djk is offline djk
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The clue to the thump lies with the bootstrap current source for the Vas, the cap may be dried out.

If the bias is not set properly this cap will not charge properly either.
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