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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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In Reply to: Re: Reducing Noise by Choosing Impedances posted by Charles Peterson on December 31, 2005 at 00:55:10:
Charles, I wish to say that your 'second guessing' amp design only confuses the situation. Do you see a THX logo on your amp? This means that the amp follows THX guidelines. One guideline is the voltage gain of the amp must be calibrated to: .1V=1W or 1V=100W. We have no choice in this.
Of course, a 50K input impedance is a compromise between 5K (somewhat possible with added distortion) and 500K, which was the standard tube input resistance. It was selected as a compromise, since we sometimes have tube preamps driving these power amps.
As I said before, the actual resistance value of the amp is set at 1 meg ohm by a 1 meg resistor at the input. The noise of the amp is composed of the input stage (perhaps 200 ohms equivalent noise) and the series input and feedback resistor (2.8K total). This makes about 3K equivalent input noise. Of course adding the equivalent of 12.5K of added noise will increase the noise, and 50K added noise (as the way you measured, will increase it even more. Remember folks, noise adds as the square root of the added resistance, so you have to add the resistances together and then take the square root to get what should be the actual noise increase.
I hope this helps to clarify the situation.
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Follow Ups
- Re: Reducing Noise by Choosing Impedances - john curl 12:14:52 12/31/05 (2)
- Re: Reducing Noise by Choosing Impedances - Charles Peterson 23:18:42 12/31/05 (1)
- Re: Reducing Noise by Choosing Impedances - john curl 14:33:45 01/01/06 (0)
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