Good impedance match for Precision Fidelity C-7AR


When using my PF preamp with either my tube or SS amps, I have only two volume settings I can use, low or too loud, at between 8 o'clock to 9 o'clock on the volume pot. At the higher of the two volume settings, the preamp seems as if it is outputting full voltage. The input impedance on the tube amp is 100Kohm, and the main output impedance on the PF is 600Ohm - 7Kohm. Is this a classic case of impedance mismatching? Would it be worthwhile to see if there is a problem in the PF unit, or, should I check into locating a better matching amplifier?
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Ghosthouse,
Thanks for replying. I will try using them on the inputs in varying places. I appreciate the suggestion about Harrison Labs attenuators, and I may try those, also. It may be that the gain needs to be lowered in the tube stage before the passive stage, and output.
Pix4work, looking at some photos I found of the interior of the PF it appears that the volume control pots may be the unsealed kinds (unless the Alps you mentioned are upgraded or different than the ones I see in those photos). So it may be possible for you to remove the top cover and squirt into them one of the Caig/DeoxIT products, or some other tuner or contact cleaner, that may help with the noisiness issue, and possibly even with the abruptness of the volume change at low settings.

Applying the Rothwells at the input jacks of the power amp would not result in any impedance compatibility issues with respect to the amp itself, given the amp's 100K input impedance. And at low settings of the volume control I would expect the output impedance of the PF to be toward the low end of the 600 ohm to 7K range you mentioned, which also shouldn't cause any issues. However insertion of the Rothwells at the amplifier inputs (which appears to be the only reasonable place in your system where they could be used in conjunction with a phono source, unless the PF provides a tape loop into which they could be inserted, including a tape output having relatively low impedance) would, by causing you to operate the volume controls at higher settings, raise the PF's output impedance, perhaps to a couple of K or so. Or maybe even more at deep bass frequencies, given that the volume controls are probably preceded in the PF's circuitry by coupling capacitors at the outputs of the tube phono section, which probably raise the output impedance significantly in the deep bass region.

The 10 db version of the Rothwells, in combination with the 100K loading of your amp, would have an input impedance in the vicinity of 30K based on measurements I've performed. Which given the foregoing would seem to suggest that adverse effects at the frequency extremes might occur to a slight degree, but I wouldn't know how to explain effects that were more than slight (assuming the Rothwells were inserted directly into the output jacks of the power amp, with no intervening cables).

The Harrison Labs attenuators Ghosthouse mentioned, btw, have much lower impedances than the Rothwells, according to measurements he provided in a past thread, and would not be suitable for this application.

As far as gains are concerned, I couldn't find definitive specs, but it appears likely that your power amp (rated at 75 watts) has a fairly high sensitivity rating (corresponding to a low sensitivity number) of 0.75 volts, and it appears likely that the PF has an overall gain when used with a phono source of 50 db (which is on the high side for use with a 4.5 mv cartridge). 50 db is a voltage multiplication of 316 times, which will raise a 4.5 mv cartridge output to about 1.4 volts, nearly twice what is required to drive the amp to full power (although depending on other variables, including speaker efficiency, listening distance, preferred volume levels, etc., that it itself is not necessarily unreasonable). But the musical peaks of recordings having particularly wide dynamic range may result in cartridge outputs of several times as much as that 4.5 mv. So it seems understandable that your volume controls have end up being operated toward the low end of their range, where (as I mentioned earlier) volume changes tend to occur in relatively large increments.

Hope that helps. Regards,
-- Al
Almarg,
Thank you very much for your excellent reply, and especially for taking the time to do the measurements. I will definitely try cleaning the pots before I consider replacing them. The Grado Sig 8MX can also be replaced with a lower output cartridge, and fortunately there good choices available at a reasonable price. I really do appreciate people like you taking the time to share your knowledge with thoughtful answers, as that is what keeps this community of audio enthusiasts going. Thanks again, and I will let you all know what I find out in the coming days.
Follow-Up:

After checking, there was one large high voltage capacitor which was leaking a small amount, and, after replacing and breaking in the replacement for a few hours, the PF is back in fine operation once again. Thank you to everyone who contributed to the discussion, I appreciate your very helpful comments and efforts.

Pix4work