Pass A75 Troubleshooting - Page 4

Melon Head · 27th February 2011, 11:39 PM

Quote:

Originally Posted by Dmatuszak

this thing is so sensitive that even moving my hand over the circuit will drive the offset crazy.

This sounds like you have a ground loop problem.
Check that all your interconnected equipment is not creating a ground loop with your amp.
Also check the amp itself for correct grounding.

BobEllis · 27th February 2011, 11:51 PM

Mine acted the same way with the inputs shorted and nothing connected to the amp other than a power cord. The offset is set by balancing a constant current source against a constant current sink. The slightest air movement would throw the balance off. Moments after someone walked by my bench everything changed.

It boiled down to noticing how much the offset drifted when I opened the case and adjusting to that amount from zero.

Dmatuszak · 28th February 2011, 12:40 AM

I do have a scope. Haven't had it hooked up while setting the offset but it never displayed any evidence of oscillation before. I did check it a couple weeks ago so there was no reason to suspect a problem. At this point I'm just trying to tame this beast. It probably gave me fits when I built it 15 years ago too. But being new it was probably a little more tame. I just blocked out that memory. I just reworked the grounding,moving the star ground from the main cap plate to a star point next to the power supply where everything is tied together. I suspected maybe a grounding problem too but this change didn't make a difference. If .3V is the most I can set across R7,8 I guess I'll just live with it. Back to the bench.

BobEllis · 28th February 2011, 12:54 AM

.3V across R7/8 is rather low - only 6 mA in the VAS to drive the output stage. Are you saying you can't adjust it higher at all or aren't able to adjust it higher AND keep the offset under control?

Dmatuszak · 28th February 2011, 01:02 AM

I can adjust it higher but the offset is all over the place. The higher the R7,8V, the hotter it runs and the more unstable it gets. If I even move my hand or the screwdriver close to the circuit it goes nuts.

BobEllis · 28th February 2011, 01:23 AM

Sounds about right. As it gets hotter you need to give it more time to stabilize. Offset will change once the output stage heats things up.

Dmatuszak · 28th February 2011, 01:51 AM

ok. I hooked up the scope to watch the signal on the output. It's not perfectly flat but the funny thing is as soon as I connect the scope to the output the DC drops right down to about 5-4 mv and is stable.. Getting late but tomorrow I'm going to check the other side for comparison.

Melon Head · 28th February 2011, 05:19 PM

Quote:

Originally Posted by Dmatuszak

but the funny thing is as soon as I connect the scope to the output the DC drops right down to about 5-4 mv and is stable..

This does not surprise me

BobEllis · 28th February 2011, 09:42 PM

There are MANY gaps in my knowledge. Can you explain why strapping a few megohms across the output would make it stable? I can see how if the input differential had no DC path to ground it would cause all sorts of drift, but I can't wrap my head around how the output would do the same. Thanks.

Dmatuszak · 28th February 2011, 10:18 PM

I'm confused. when I look at the traces on the front end PCB there are 2 connections to circuit ground...1 for right and 1 for left right between the Output and -DRV connection. If you follow the traces they're not independent of each other, which suggests only 1 should be connected. connect em both and whalla...ground loop? I don't see the same issue with the ground connection terminals for the output zobel. Am I mad? Been at this a long time and I could be losing it. Connecting one side at a time I can dial everything in. Connect em both and it's a crap shoot.

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27th February 2011, 11:51 PM	#32
BobEllis diyAudio Member Join Date: Sep 2003 Location: Upstate NY	Mine acted the same way with the inputs shorted and nothing connected to the amp other than a power cord. The offset is set by balancing a constant current source against a constant current sink. The slightest air movement would throw the balance off. Moments after someone walked by my bench everything changed. It boiled down to noticing how much the offset drifted when I opened the case and adjusting to that amount from zero.

28th February 2011, 12:40 AM	#33
Dmatuszak diyAudio Member Join Date: Feb 2011 Location: Illinois	I do have a scope. Haven't had it hooked up while setting the offset but it never displayed any evidence of oscillation before. I did check it a couple weeks ago so there was no reason to suspect a problem. At this point I'm just trying to tame this beast. It probably gave me fits when I built it 15 years ago too. But being new it was probably a little more tame. I just blocked out that memory. I just reworked the grounding,moving the star ground from the main cap plate to a star point next to the power supply where everything is tied together. I suspected maybe a grounding problem too but this change didn't make a difference. If .3V is the most I can set across R7,8 I guess I'll just live with it. Back to the bench.

28th February 2011, 12:54 AM	#34
BobEllis diyAudio Member Join Date: Sep 2003 Location: Upstate NY	.3V across R7/8 is rather low - only 6 mA in the VAS to drive the output stage. Are you saying you can't adjust it higher at all or aren't able to adjust it higher AND keep the offset under control?

28th February 2011, 01:02 AM	#35
Dmatuszak diyAudio Member Join Date: Feb 2011 Location: Illinois	I can adjust it higher but the offset is all over the place. The higher the R7,8V, the hotter it runs and the more unstable it gets. If I even move my hand or the screwdriver close to the circuit it goes nuts.

28th February 2011, 01:23 AM	#36
BobEllis diyAudio Member Join Date: Sep 2003 Location: Upstate NY	Sounds about right. As it gets hotter you need to give it more time to stabilize. Offset will change once the output stage heats things up.

28th February 2011, 01:51 AM	#37
Dmatuszak diyAudio Member Join Date: Feb 2011 Location: Illinois	ok. I hooked up the scope to watch the signal on the output. It's not perfectly flat but the funny thing is as soon as I connect the scope to the output the DC drops right down to about 5-4 mv and is stable.. Getting late but tomorrow I'm going to check the other side for comparison.

28th February 2011, 09:42 PM	#39
BobEllis diyAudio Member Join Date: Sep 2003 Location: Upstate NY	There are MANY gaps in my knowledge. Can you explain why strapping a few megohms across the output would make it stable? I can see how if the input differential had no DC path to ground it would cause all sorts of drift, but I can't wrap my head around how the output would do the same. Thanks.

28th February 2011, 10:18 PM	#40
Dmatuszak diyAudio Member Join Date: Feb 2011 Location: Illinois	I'm confused. when I look at the traces on the front end PCB there are 2 connections to circuit ground...1 for right and 1 for left right between the Output and -DRV connection. If you follow the traces they're not independent of each other, which suggests only 1 should be connected. connect em both and whalla...ground loop? I don't see the same issue with the ground connection terminals for the output zobel. Am I mad? Been at this a long time and I could be losing it. Connecting one side at a time I can dial everything in. Connect em both and it's a crap shoot. Comments?