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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 think that you are making more of the situation than what really is important.
First, the noise floor will usually be set by the volume control which should be located in a preamp that has some gain. If this is the situation, then the preamp pot is much more important as a noise contributor than the power amp pot. It is true that the power amp pot can be replaced by fixed resistors, a lower (or higher ) value pot, or the pot simply removed. Actually, just removing the pot would give you the best performance as the input impedance would jump to 1 Meg ohm and the path would be uncompromised by any pot position.
For the record, the pot is a convenience and should not be used, if you don't need it. It will always add some extra noise to the circuit, unless it is on maximum gain. We don't put pots in the JC-1 power amp, for example.
Virtually all preamp and amp circuits should be tested with a SHORTED INPUT. You can easily make a shorting RCA plug with Radio Shack parts and a shorting wire between hot and ground. IF you want, you can insert a 100 ohm resistor between hot and ground of the RCA plug, but any higher value is probably going to just add extra noise to any measurement that you might make.
In your case, the HCA-1000, you have a low noise j-fet input that is NOT affected by input impedance as far as noise increase of the amp itself is concerned, but you still have to short the input for measurement and simulation of actual operation as the Xover will supply a low impedance to the amp, when connected to it. A few hundred ohms, one way or another, is not very important in this case.
If you are willing to tolerate added distortion, then SE's recommendation of a transformer volume control, would give you excellent RFI protection and low noise. I doubt that you will actually get a better signal to noise ratio in practice, but in principle the transformer is a pretty good idea.
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Follow Ups
- Re: Reducing Noise by Choosing Impedances - john curl 10:24:13 12/31/05 (0)
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