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Class AB Amp Possibilities??
Class AB Amp Possibilities??
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Old 20th May 2020, 04:32 PM   #11
Freecrowder is offline Freecrowder  United States
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Join Date: Jun 2007
Class AB Amp Possibilities??
So want tom show some interesting results from my balanced attempt with the Pass AB. Ran into some issues with the Doug Self preamp as source. Feeding SE input and using balanced output. You can see from attached pics that the gain between the two phases is not identical. One is 1.25vac and the other is 1.09vac at 1000hz. So looking at the output across 5.5 power resistor on scope I see two signals. I have not completely thought thru that buts seems like I should have seen one signal that is sum of two outputs. HMMM. someone set me straight. You can actually see that as it clips the higher gain signal clips first. I'm assuming this is all related to the fact that the two phases aren't matched in gain. I'll need to troubleshoot gain stages. But interesting that the AB puts out ~430W into 5.5ohm at clipping. Plenty for the Maggies.
Attached Images
File Type: jpg IMG_0028.jpg (29.1 KB, 102 views)
File Type: jpg IMG_0027.jpg (26.8 KB, 102 views)
File Type: jpg IMG_0030.jpg (31.7 KB, 104 views)
File Type: jpg IMG_0031.jpg (27.5 KB, 105 views)
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Old 21st May 2020, 12:05 AM   #12
kasey197 is offline kasey197
The Dastardly Dad of Three
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Join Date: Sep 2009
Class AB Amp Possibilities??
Not sure about the preamp differeing levels

but how did you connect the scope to the bridged amplifier outputs ? Earths ?
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Old 21st May 2020, 03:13 PM   #13
Freecrowder is offline Freecrowder  United States
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Join Date: Jun 2007
Class AB Amp Possibilities??
So Got the preamp issue taken care of. It was ok but scope leads had to adjust capacitance. Everything good now with that. But output still looks the same.
Scope is connected to one side of power resistor and ground is on other side of resistor.
Scope is floated.
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Old 21st May 2020, 03:20 PM   #14
kasey197 is offline kasey197
The Dastardly Dad of Three
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Join Date: Sep 2009
Class AB Amp Possibilities??
Two channel scope if you can get one would be super handy... I’ve played with bridge and floating outputs on amps and they’re sometimes seem just idiotic how they behave when being measured.

The most reliable solutions for me were to keep the grounds intact, and use the scope A-B function to display the composite waveform. If your waveform doesn’t clean up, it looks like you have some oscillation going on...

Last edited by kasey197; 21st May 2020 at 03:22 PM. Reason: Punctuation...
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Old 21st May 2020, 05:25 PM   #15
Freecrowder is offline Freecrowder  United States
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Join Date: Jun 2007
Class AB Amp Possibilities??
Good point. It is a 4CH scope so can do. But started looking at SE input and output from single channel and seeing some things that aren't right. I will post some more pics when I get a chance. looks like there is some strange behavior that will have to be addressed before going further.
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Old 21st May 2020, 05:56 PM   #16
Freecrowder is offline Freecrowder  United States
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Join Date: Jun 2007
Class AB Amp Possibilities??
So what is attached is short video of scope screen. This phenomena only shows up at lower frequencies~100hz and lower. This shot is 20hz. It looks like crossover type distortion. Do not understand why it travels around the sine. Tried cranking up the bias just to try and it makes no difference. Just plan on injecting signal and tracing circuit to find issue. Anyone seen anything like this.
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Old 21st May 2020, 05:57 PM   #17
Freecrowder is offline Freecrowder  United States
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Join Date: Jun 2007
Class AB Amp Possibilities??
Is there a way to attach short video?? Lasts 5 seconds
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