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Old 27th August 2012, 10:31 PM   #1
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Default Signal path on Classd Audio amp

I have the ClassD audio Spiral Groove amp. It has fixed resistors instead of pots to adjust the gain. Are these resistors in the signal path? They are metal film which I will replace with nude Vishays if they ARE in the signal path. No schematic is available. Thanks
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Old 27th August 2012, 11:13 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

For an non-inverting amplifier both feedback resistors are not in the signal path.
However they are clearly the most critical resistors of the amplifier stage.

For inverting they clearly are in the path, but the reality is they are the same.

rgds, sreten.
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Old 27th August 2012, 11:17 PM   #3
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The Spiral Groove amp is a custom made amp by classDaudio. As far as I know classDaudio uses Vishay Beyschlag resistors, and since it is custom made I can only assume that resistor values are matched to perfection. And that doesn't necessarily mean to eachother but for total performance.

I'm also curious why you'd invest in Vishay Z-foils over Vishay Beyschlags for a class D amplifier as it's principle advantages are resistance to ambient temperature changes which doesn't occur much in a class D amplifier and resistance to changes in loaded power which is miniscule in the input section anyways. So there really don't seem to be much reason to change already optimized components.
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Old 27th August 2012, 11:19 PM   #4
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Thanks sreten. I am new to class d amps but understand that the gain setting resistors are between the input buffer and the pwm section. I would assume that in a tube circuit this would be between the input tube and the driver. Regards
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Old 27th August 2012, 11:22 PM   #5
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I have been using Vishays for twenty years. I have yet to find a circuit whose SQ cannot be improved by their use. I also think they are less microphonic than metal film resistors. Since these are gain setting resistors tracking is important and the TC is a major contributor to this. Regards
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Old 27th August 2012, 11:27 PM   #6
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Quote:
Originally Posted by john dozier View Post
I have been using Vishays for twenty years. I have yet to find a circuit whose SQ cannot be improved by their use. I also think they are less microphonic than metal film resistors. Since these are gain setting resistors tracking is important and the TC is a major contributor to this. Regards
Z-foils are metal film resistors. Those is your amp are Vishay metal film resistors. And I assure you the linearity of any off the shelf metal film resistor (even if it was that) would be an order of magnitude better than the active components in the amp.

I'm sure that z-foils have their applications. This isn't one as far as I can see.
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Old 27th August 2012, 11:27 PM   #7
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BTW many years ago, I wrote a white paper for John Cunningham of Texas Components (who make Vishay bulk foil resistors) outlining their advantages in both analogue and digital circuitry. The points I made showed up in Vishay ads in several major audiophone publications. Vishay never gave me attribution or money but John did send my a nice pair of Stentorian drivers for my efforts. Regards
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Old 27th August 2012, 11:28 PM   #8
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In "Linear Audio" there is an excellent test of resistors, and their distortion. The bulk metal resistors came out on top by an order of magnitude. Regards
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