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Author Topic: HELP-I'm really confused now :(  (Read 9135 times)

John Roberts {JR}

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Re: HELP-I'm really confused now :(
« Reply #10 on: May 02, 2015, 10:39:27 AM »

Don't ignore the obvious, maybe one amp has 10V of DC in it's output.

Generally the scope is the best way to measure DC in the presence of Large AC. VOM can be confused and read wrong.

The waveform would probably need to be visibly distorted to contain 10V of DC content in the actual signal.

It's always something.

JR
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Jeff Carter

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Re: HELP-I'm really confused now :(
« Reply #11 on: May 02, 2015, 02:38:09 PM »

Probably a stupid question, but the signal ground of the scope is tied to chassis ground, right? Does that do anything screwy to one amp implementation that it wouldn't to the other?
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Darren Scaresbrook

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Re: HELP-I'm really confused now :(
« Reply #12 on: May 02, 2015, 11:56:04 PM »


The class D amp has the usual "switching supply" noise and that reads 5V on the HP meter with nothing plugged into the input of the amp.

Ivan, why do you think this is usual? 5v is WAY more than I have ever measured of the switching artifacts remaining after filtering on the output of an amp.
Darren

John Rutirasiri

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Re: HELP-I'm really confused now :(
« Reply #13 on: May 03, 2015, 01:36:28 AM »

The scope is an old 20Mhz analog model-just a basic scope.

I think what you're seeing is aliasing...regardless of the freq of your test signal, you have 200KHz artifacts (switching freq)  in the class D amp waveform.  And there are probably harmonics at higher freq.  Depending on your horizontal scale, on a 20Mhz scope you may not have enough bandwidth/data points.

Did you try increasing the horz scale to show fewer cycles but faster sample rate?

J. Rutirasiri
« Last Edit: May 03, 2015, 01:42:11 AM by John Rutirasiri »
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Steve M Smith

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Re: HELP-I'm really confused now :(
« Reply #14 on: May 03, 2015, 03:15:39 AM »

Did you have a dummy load on the output when you did the test?

That would be my question too.


Steve.
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Ivan Beaver

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Re: HELP-I'm really confused now :(
« Reply #15 on: May 03, 2015, 09:58:43 AM »

That would be my question too.


Steve.
I answered that earlier-yes it was an 8 ohm load.
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Art Welter

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Re: HELP-I'm really confused now :(
« Reply #16 on: May 03, 2015, 01:15:25 PM »

I answered that earlier-yes it was an 8 ohm load.
Ivan,

The answer to Jeff Carter's question in #11 is needed.

If the class D amp does not reference chassis ground (as in one that is already "bridged" for each side's operation) and you were using chassis and "+", it should read lower output (-6, not -3.5 dB) compared to "-" and "+". 
If you subtract the 5v offset from the 20V you read, the output would be 15V, 6 dB down from 30V, as would be expected if only seeing one leg of a bridged output.

Art
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Ivan Beaver

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Re: HELP-I'm really confused now :(
« Reply #17 on: May 03, 2015, 02:16:11 PM »

Ivan,

The answer to Jeff Carter's question in #11 is needed.

If the class D amp does not reference chassis ground (as in one that is already "bridged" for each side's operation) and you were using chassis and "+", it should read lower output (-6, not -3.5 dB) compared to "-" and "+". 
If you subtract the 5v offset from the 20V you read, the output would be 15V, 6 dB down from 30V, as would be expected if only seeing one leg of a bridged output.

Art
I could have sworn that I answered the load question earlier-but it doesn't appear that I did-sorry-my bad.

I should have included that in the original post.

Yes the class D design is a "bridged" design-so not referenced to ground.

I do need to look into this a bit further next week-to be sure.

When I was saying 20V, that was the "peak" of the waveform (original signal PLUS the parasitic osciallation), so the "signal" level would be lower.

It was a Friday-and other things going on as well------------
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John Roberts {JR}

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Re: HELP-I'm really confused now :(
« Reply #18 on: May 03, 2015, 02:53:23 PM »

I could have sworn that I answered the load question earlier-but it doesn't appear that I did-sorry-my bad.
You did ... 8 ohm dummy load.
Quote
I should have included that in the original post.

Yes the class D design is a "bridged" design-so not referenced to ground.
Bridged does not automatically mean floating... generally both outputs are directly or indirectly referenced to ground. If a bridged output you need to put two probes on it.

If the output is completely floating there could be some odd rectification going on in the scope electronics.
Quote
I do need to look into this a bit further next week-to be sure.

When I was saying 20V, that was the "peak" of the waveform (original signal PLUS the parasitic osciallation), so the "signal" level would be lower.

It was a Friday-and other things going on as well------------

It's always something...

JR
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David Sturzenbecher

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Re: HELP-I'm really confused now :(
« Reply #19 on: May 03, 2015, 04:27:56 PM »

I could have sworn that I answered the load question earlier-but it doesn't appear that I did-sorry-my bad.

I should have included that in the original post.

Yes the class D design is a "bridged" design-so not referenced to ground.

I do need to look into this a bit further next week-to be sure.

When I was saying 20V, that was the "peak" of the waveform (original signal PLUS the parasitic osciallation), so the "signal" level would be lower.

It was a Friday-and other things going on as well------------

Sounds like a speakerpower amp to me...may be easier to just say that.  Have you talked to Brian at all about it?
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Re: HELP-I'm really confused now :(
« Reply #19 on: May 03, 2015, 04:27:56 PM »


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