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Old 15th April 2013, 02:44 PM   #1
raul_77 is offline raul_77  Europe
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Default AC coupled feedback

The valve (tube) amps have the feedback AC coupled, as taken from the secundary of the output transformer.

The SS amps have the feedback DC coupled, not only the AC signal (music) are taken (how it should be), a DC from the output can be superimposed.

This are indesirable, as write years ago in the french magazine L'Audiophile:

Document sans titre

In the datasheet of the TDA2004 the feedback are as the '(e) variante' drawing:

http://www.datasheetcatalog.org/data.../378181_DS.pdf

Click the image to open in full size.

Someone knows more AC coupled feedback SS amps?
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Old 15th April 2013, 02:53 PM   #2
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If you read the Application Note, this range of amplifiers is designed for Car Audio use and have an open loop gain of 90dB. There is internal DC feedback and uses a bootstrap to help with the non linear output stage. If there was no DC feedback it couldn't work!
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Old 15th April 2013, 03:13 PM   #3
raul_77 is offline raul_77  Europe
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Hi Harleyjon,

Thanks for your explanation, I was wrong to see the scheme.

But the question remains: someone know AC coupled feedback SS amps?

Last edited by raul_77; 15th April 2013 at 03:16 PM.
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Old 15th April 2013, 03:25 PM   #4
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As I understand it, SS amps have to be balanced midway, that provides for equal + and - signals with respect to either 0volts or ground with a capacitor. A valve amp is biased at each stage and does not require DC feedback. The greater the negative feedback, SS and valve, the less the gain and overall noise. A happy medium is therefore achieved, if possible.
Hope that helps.
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Old 15th April 2013, 04:44 PM   #5
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by raul_77 View Post
But the question remains: someone know AC coupled feedback SS amps?
Here's one:
It's the Power amp for the Peavey Pacer100 guitar amp and the PA100 4 channel PA mixer, both a very conservative 45W RMS@8 ohms.
Simple and robust, many still in use.
Attached Images
File Type: gif PeaveyPacer100Pwr.gif (13.8 KB, 56 views)

Last edited by JMFahey; 15th April 2013 at 04:48 PM.
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Old 15th April 2013, 04:50 PM   #6
raul_77 is offline raul_77  Europe
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Hi Harleyjon,

Yes, but have some SS amps with AC coupled feedback, how it appears in the L'Audiophile article, also I think that YBA have some, whit the feedback across a small transformer.

I have curious if there are other designs with this feature.
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Old 15th April 2013, 04:52 PM   #7
raul_77 is offline raul_77  Europe
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Hola JMFahey. Gracias!

Hi JMFahey. Thanks!
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