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Old 22nd August 2014, 09:07 PM   #1
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Default Quirk when measuring damping factor

Never spent much time measuring or worrying about damping factor, but decided to give it a try. I'm driving the DUT with a second power amp and isolation resistor, and then calculating the impedance and damping factor. No problem there.

Here's the quirk. With most amps, the residual signal seen at the DUT output is a clean sine wave. With my SWTP Tiger amp clone that signal is a bit asymmetric and distorted, enough to be obvious on the scope.

What do you think causes that? The amp works normally and has low distortion, though the DC offset is higher than people today like to see, maybe 100 mV or so.
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Old 22nd August 2014, 09:12 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by Conrad Hoffman View Post
Never spent much time measuring or worrying about damping factor, but decided to give it a try. I'm driving the DUT with a second power amp and isolation resistor, and then calculating the impedance and damping factor. No problem there.

Here's the quirk. With most amps, the residual signal seen at the DUT output is a clean sine wave. With my SWTP Tiger amp clone that signal is a bit asymmetric and distorted, enough to be obvious on the scope.

What do you think causes that? The amp works normally and has low distortion, though the DC offset is higher than people today like to see, maybe 100 mV or so.
Some amps by nature can't sink current, maybe that is one.

Last edited by rayma; 22nd August 2014 at 09:33 PM.
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Old 22nd August 2014, 09:29 PM   #3
llwhtt is online now llwhtt  United States
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Don't most of the various SWTPC amps have voltage gain in the output stage? Could be the reason.

Craig
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Old 22nd August 2014, 09:56 PM   #4
forr is offline forr  France
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Quote:
Originally Posted by rayma View Post
Some amps by nature can't sink current, maybe that is one.
Can you detail how they look like ?

For calculating damping factors, I found the method using the voltage change when loading a low value resistor connected to ground much more reliable.
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Old 22nd August 2014, 10:27 PM   #5
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I think the output stage has a gain of 2. Maybe that's related to the issue. I also realized that the amp has output fuses- I need to recheck with 500 amp solid copper buss bars in place and see if that changes the situation!
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Old 22nd August 2014, 10:43 PM   #6
rayma is offline rayma  United States
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Originally Posted by Conrad Hoffman View Post
I think the output stage has a gain of 2. Maybe that's related to the issue. I also realized that the amp has output fuses- I need to recheck with 500 amp solid copper buss bars in place and see if that changes the situation!
Shorting the fuses might be kind of risky - you could increase their current rating. Are the output fuses included in the nfb loop? If not, that could help.
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Old 23rd August 2014, 12:24 AM   #7
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Hey, risk, what's the worst that could happen. The house is fused. I figure I can fix anything I built, or at least that was what I always told them at the McIntosh clinics when they tested my stuff back in the '70s.

Anyway, found it. It was fuse related, but not the fuse itself. I actually had to clean the fuse holder "spinner" and fuse ends with DeOxit to get the connection stable. After that, no more odd waveform.
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Old 23rd August 2014, 01:01 AM   #8
rayma is offline rayma  United States
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Quote:
Originally Posted by Conrad Hoffman View Post

Anyway, found it. It was fuse related, but not the fuse itself. I actually had to clean the fuse holder "spinner" and fuse ends with DeOxit to get the connection stable. After that, no more odd waveform.
Yes, in older equipment that's a persistent problem. I have an old Pioneer receiver that I use as a radio, and the switches keep acting up.
Even Cramolin hasn't been able to fix them, but it's not worth the time and effort to do much more.
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Old 23rd August 2014, 07:42 AM   #9
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Conrad Hoffman View Post
Never spent much time measuring or worrying about damping factor, but decided to give it a try. I'm driving the DUT with a second power amp and isolation resistor, and then calculating the impedance and damping factor. No problem there.
Yes, that is the proper and most accurate method

Quote:
Here's the quirk. With most amps, the residual signal seen at the DUT output is a clean sine wave. With my SWTP Tiger amp clone that signal is a bit asymmetric and distorted, enough to be obvious on the scope.

What do you think causes that? The amp works normally and has low distortion, though the DC offset is higher than people today like to see, maybe 100 mV or so.
In this case, it was fuse-related, but sometimes it is revealing: that is the case when the OP stage has a variable non-linear impedance dominating the distortion factors. Always good to know.
Regarding fuses, I always try to include them in the FB loop: even in perfect condition those little bastards manage to be non-linear.
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Old 23rd August 2014, 03:06 PM   #10
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I've always wondered why people don't put fuses in the loop, as the fuse is a significant source of resistance and low frequency non-linearity. What do you have to consider if the fuse is moved inside? Until it blows, probably not much. After it blows, no feedback. Should it by bypassed with a medium value resistor?
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