Quirk when measuring damping factor
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 22nd August 2014, 09:07 PM #1 Conrad Hoffman   diyAudio Member   Join Date: Mar 2007 Location: Canandaigua, NY USA 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. __________________ I may be barking up the wrong tree, but at least I'm barking!
rayma
diyAudio Member

Join Date: Apr 2011
Quote:
 Originally Posted by Conrad Hoffman 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.

 22nd August 2014, 09:29 PM #3 llwhtt   diyAudio Member   Join Date: Jun 2008 Location: SoCal Don't most of the various SWTPC amps have voltage gain in the output stage? Could be the reason. Craig
forr
diyAudio Member

Join Date: Dec 2004
Location: Next door
Quote:
 Originally Posted by rayma 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.

 22nd August 2014, 10:27 PM #5 Conrad Hoffman   diyAudio Member   Join Date: Mar 2007 Location: Canandaigua, NY USA 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! __________________ I may be barking up the wrong tree, but at least I'm barking!
rayma
diyAudio Member

Join Date: Apr 2011
Quote:
 Originally Posted by Conrad Hoffman 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.

 23rd August 2014, 12:24 AM #7 Conrad Hoffman   diyAudio Member   Join Date: Mar 2007 Location: Canandaigua, NY USA 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. __________________ I may be barking up the wrong tree, but at least I'm barking!
rayma
diyAudio Member

Join Date: Apr 2011
Quote:
 Originally Posted by Conrad Hoffman 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.

Elvee
diyAudio Member

Join Date: Sep 2006
Quote:
 Originally Posted by Conrad Hoffman 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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 23rd August 2014, 03:06 PM #10 Conrad Hoffman   diyAudio Member   Join Date: Mar 2007 Location: Canandaigua, NY USA 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? __________________ I may be barking up the wrong tree, but at least I'm barking!

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