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Old 14th April 2009, 12:42 AM   #771
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by Sheldon
Same transistors as AndrewT. Everything else exactly as spec'd. I have no problem powering up the amp. It's only probing the front end, or connecting something to the input.
To make sure I've given the complete picture: I omitted C1, as per post 573.

Sheldon
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Old 14th April 2009, 12:55 AM   #772
syn08 is offline syn08  Canada
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Quote:
Originally posted by Sheldon
Added 100pf in parallel with the 100pf in place. Square wave shows no overshoot at all. Clean, but as expected more rounded leading and trailing edge, with the transition now around 36kHz.

However, if I plug and unplug the output from the signal generator, fuses go poof.
Power up and slowly drive the amp into overload/clipping. Is it stable or does it start screaming blue murder, eventually blowing the fuse?

I've see such a behaviour in amps that have the nasty property to decrease the phase margin, eventually becoming unstable, with the input signal increase. Low beta transistors may create such problems.
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Old 14th April 2009, 08:59 AM   #773
roender is offline roender  Romania
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Sheldon,

Please try to put the scope over 10ohm input resistor and tap with a screwdriver into input RCA ground. If nothing happened then tap into active input wire.
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Old 14th April 2009, 04:28 PM   #774
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by roender
Sheldon,

Please try to put the scope over 10ohm input resistor and tap with a screwdriver into input RCA ground. If nothing happened then tap into active input wire.
OK, this is pretty interesting. With the scope probe attached across the 10R input resistor, the amp seems stable. I can plug and unplug the generator, on or off, no problems. Can drive the amp to clipping with and 8R load and 1 kHz sine or square wave, no problem (can't ramp it too slowly or hold it there too long, as I've only a 20W resistor). It clips at about 25V, so that seems normal. Works the same, whether I have the probe at the input end or ground end of the resistor.

Then, to do the control, I disconnected the probe and tried to insult the input with a screwdriver and with the generator input. The amp no longer cares?!!!

I doubt this has any significance, but I had a bunch of extra UF4007 diodes, so I used those for the antiparallel diodes across the 10R. Could the capacitance of the safety diodes cause a problem?

Sheldon
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Old 14th April 2009, 04:39 PM   #775
roender is offline roender  Romania
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Sheldon,

Just to be clear, now the amplifier is stable with two antiparallel diodes over 10ohm input resistor?
Also, why it clips at only 25V? What are your supply voltages? How big is your output transformer?

Cheers,
M
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Old 14th April 2009, 05:11 PM   #776
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by roender
Just to be clear, now the amplifier is stable with two antiparallel diodes over 10ohm input resistor?
The diodes were always there. Nothing was changed from the last time I blew fuses by fiddling with the input RCA. But now I don't have a problem. Or, I should say, that the only problem is that I've no idea was was causing the original issue. Static, noise from some other source?

Quote:
Originally posted by roender
Also, why it clips at only 25V? What are your supply voltages? How big is your output transformer?
It clips at 25vrms output. Should have made that clear. Supply is 34Vdc at the rail. Pooge caught me being sloppy on this issue a few posts ago. Sorry for not learning my lesson well.

Sheldon
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Old 14th April 2009, 05:20 PM   #777
roender is offline roender  Romania
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Quote:
Originally posted by Sheldon


The diodes were always there. Nothing was changed from the last time I blew fuses by fiddling with the input RCA. But now I don't have a problem. Or, I should say, that the only problem is that I've no idea was was causing the original issue. Static, noise from some other source?

Sheldon
Strange
You were smelling some sulphur smoke in the air when the amp was unstable?
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Old 14th April 2009, 05:28 PM   #778
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by roender


Strange
Did you smell some sulphur smoke in the air when the amp was unstable?
No, but that doesn't mean much. My sense of smell (which used to be very acute) is pretty much shot. Beginning to wonder about my sanity now too.

Also, the instability issue seemed instantaneous. The rail fuses would go in less than a second of contacting the wrong thing in the front end or tapping the RCA connector together. Not much time to heat anything up. No obvious signs of burnt components. Amp works fine, output looks fine.

Sheldon
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Old 14th April 2009, 05:53 PM   #779
roender is offline roender  Romania
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Quote:
Originally posted by Sheldon

... Not much time to heat anything up. No obvious signs of burnt components. Amp works fine, output looks fine.

Sheldon
That's because the OPS SOA is overrated
Let's wait and see how Andrew will solve this issue. Maybe he will discover something ...
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Old 14th April 2009, 06:30 PM   #780
tvicol is offline tvicol  Romania
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Sheldon,

Add a 2,2nF - 4,7nF cap across R25=10ohm.

Regards,
Tibi
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