Phase Linear Turnoff Thump

d3imlay · 4th February 2005, 07:26 AM

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.

Jan Dupont · 4th February 2005, 11:57 AM

I know these problems from different amps, and might be able to give you some hints

Which series II are we talking about ????

d3imlay · 4th February 2005, 01:53 PM

Phase Linear 400II is what I'm working on now. the 700II is identical except the power supply goes from + 80V to + 100V.

Jan Dupont · 4th February 2005, 02:05 PM

As I recall, the schematic I have on my web is the Series I.
Could you please email me the Series II schematic ?

Netlist · 4th February 2005, 06:54 PM

Drive PCB schematic can be found here: http://hometown.aol.com/phasetek/index.html

/Hugo

d3imlay · 5th February 2005, 05:07 AM

The phasetek site lists only the 400, not the 400II.

ACD I'll send you the schematic.

Jan Dupont · 5th February 2005, 08:47 AM

d3imlay: Thanks, I'll wait for it

Netlist · 5th February 2005, 08:51 AM

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

djk · 5th February 2005, 08:03 PM

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.

4th February 2005, 07:26 AM	#1
d3imlay diyAudio Member Join Date: Sep 2004 Location: Ohio	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.

4th February 2005, 11:57 AM	#2
Jan Dupont diyAudio Member Join Date: Apr 2003 Location: Copenhagen, Denmark	I know these problems from different amps, and might be able to give you some hints Which series II are we talking about ???? __________________ Jan Dupont (formerly known as ACD) B.Sc.EE Personal audio website with free schematic and service manual downloads www.audio-circuit.dk

4th February 2005, 02:05 PM	#4
Jan Dupont diyAudio Member Join Date: Apr 2003 Location: Copenhagen, Denmark	As I recall, the schematic I have on my web is the Series I. Could you please email me the Series II schematic ? __________________ Jan Dupont (formerly known as ACD) B.Sc.EE Personal audio website with free schematic and service manual downloads www.audio-circuit.dk

4th February 2005, 06:54 PM	#5
Netlist diyAudio Moderator Join Date: Jan 2003 Location: Belgium	Drive PCB schematic can be found here: http://hometown.aol.com/phasetek/index.html /Hugo __________________ Toothpaste is bad for the teeth; it’s too sweet.

5th February 2005, 08:47 AM	#7
Jan Dupont diyAudio Member Join Date: Apr 2003 Location: Copenhagen, Denmark	d3imlay: Thanks, I'll wait for it __________________ Jan Dupont (formerly known as ACD) B.Sc.EE Personal audio website with free schematic and service manual downloads www.audio-circuit.dk


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4th February 2005, 01:53 PM	#3
d3imlay diyAudio Member Join Date: Sep 2004 Location: Ohio	Phase Linear 400II is what I'm working on now. the 700II is identical except the power supply goes from + 80V to + 100V.

5th February 2005, 05:07 AM	#6
d3imlay diyAudio Member Join Date: Sep 2004 Location: Ohio	The phasetek site lists only the 400, not the 400II. ACD I'll send you the schematic.

5th February 2005, 08:51 AM	#8
Netlist diyAudio Moderator Join Date: Jan 2003 Location: Belgium	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 __________________ Toothpaste is bad for the teeth; it’s too sweet.

5th February 2005, 08:03 PM	#9
djk diyAudio Member Join Date: Feb 2001 Location: USA	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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