Amp preamp impedance matching...can anyone explain?
Ideally the input impedance of the amp should be 10 or more times greater than the output impedance of the preamp, at the frequency for which preamp output impedance is highest. Otherwise audible frequency response irregularities MIGHT result. If, as is often the case, the highest output impedance of the preamp across the audible frequency range is not known, and only a nominal output impedance is specified (perhaps based on a frequency of 1 kHz), I suggest using a ratio of 50 or more, and preferably 75. Many tube preamps, and some solid state preamps, use a coupling capacitor at their outputs, which can cause their output impedance to be much higher at deep bass frequencies than at higher frequencies. Impedance incompatibilities are most likely to be encountered when using a tube preamp with a solid state power amp. If the power amp has an input impedance of around 47K or more, it is unlikely that there will be an issue, even with a tube preamp. Regards, -- Al |
Onhwy61, that would apply to situations where the component providing the signal is specifically designed to be able to drive 600 ohms, without a significant increase in distortion or a significant impact on bandwidth or frequency response. Most consumer gear will not meet those requirements. I believe that Atmasphere is a notable exception. According to posts Ralph (Atmasphere) has made in the past, a benefit that would result from loading the balanced outputs of his preamps with a low impedance like 600 ohms is minimization or elimination of cable effects and cable differences. Best regards, -- Al |
Thanks for chiming in, Bombaywalla, and saving me some time :-) I found some information specific to the CJ PV-5, though. The schematic and parts list can be found here. It can be seen that the output coupling capacitor is two capacitors in parallel, totalling 2.35 uF. At 20 Hz, that corresponds to an impedance of 3388 ohms, based on 1/(2piFC). (That figure will be vastly lower at mid-range and treble frequencies, consistent with the 200 ohm nominal output impedance). It is most likely safe to assume that the overall output impedance at 20 Hz will be just a little higher than the impedance presented by the capacitors at that frequency, so let's call it 3500 ohms. 22K/3.5K and 24K/3.5K are ratios that are between 6 and 7, lower than the ideal of at least 10. So there may be a slight impact on the deepest bass frequencies, which may or may not be perceptible depending on the deep bass extension of your speakers, and on room acoustics. If your speakers have good deep bass extension, my instinct would be to play it safe and avoid the B&K and the Aragon, although it's a close enough call to be debatable. Regards, -- Al |
07-24-12: DistortionsNo, gain and impedance compatibility are two separate issues, although if the amplifier's input impedance is too low in relation to the preamp's output impedance (which it is not in this case) there will be a very slight reduction in overall gain. I see that in stock form, prior to the recent modifications, your preamp had a line stage gain of 26 db. That in combination with the amp's 30 db gain, as you realize, is way too high, especially if you are using a digital source having the typical full scale output of 2 volts or so. I see that you have another thread in progress on amplifier replacement and other possible approaches, and you've already received an intriguing input from Steve McCormack himself! Regards, -- Al |
Lpw, as I indicated earlier what is important is the preamp's output impedance at the (audible) frequency for which that output impedance is highest. That figure will often occur at 20 Hz, especially in the case of a tube preamp (due to the coupling capacitor that most but not all tube preamps have at their outputs), and in the case of a preamp having an output coupling capacitor will very often be much higher than the specified value. The specified value can be assumed to be based on midrange frequencies, unless explicitly indicated otherwise. According to Stereophile's measurements: Ref 3: The output impedance was also to spec., at 635 ohms balanced and 326 ohms unbalanced in the treble and midrange, but rose to 1437 ohms and 625 ohms, respectively, at 20Hz.SLP-05: The SLP 05's output impedance is specified as a usefully low 400 ohms. However, I got significantly higher values for the unbalanced output of 15001600 ohms in the midrange and treble, rising to 3400 ohms at 20Hz, with similar if slightly lower figures for the balanced output.The specs for your X250.5 indicate an input impedance of 30K balanced and 20K unbalanced, which are the same as the input impedance specs for many of the other Pass amps. At some point in the past, though, their unbalanced input impedance may have been spec'd at 15K, lower than it really was and is. See the specs and measurements in Stereophile's review of the XA30.5. That discrepancy perhaps contributed to some misconceptions. The XP-10 and XP-20 are spec'd as having output impedances of 1K balanced, 150 ohms unbalanced. Presumably balanced connections would be used between all of these particular components. Based on the foregoing numbers for balanced connection, the Cary's output impedance rise at deep bass frequencies results in it not meeting the ideal minimum ratio of 10 that is applicable if the preamp's maximum output impedance across the audible frequency range is known (30K/approx. 3.4K = 8.8). Although it comes close enough to probably be marginally acceptable in some systems (depending on the deep bass extension of the speakers, for one thing). The Ref 3 should be no problem, in terms of impedance matching. 30K/1.437K = 21. Although I couldn't find measurements, it can be presumed that the specified 1K output impedance of the solid state Pass preamps does not rise significantly at 20 Hz (or other) frequencies, so it too should have no problem working into a 30K load. Regards, -- Al |
Hi Ralph, Yes, I seem to recall that being true of some of their later preamps and/or phono stages. For the Ref 3, though, the specs shown here, and also in the Stereophile review I linked to earlier, indicate 20K. Although the wording leaves it unclear whether that applies to the balanced or unbalanced outputs, or both. In any case, given the 30K impedance of the balanced inputs on the Pass, and the measured worst case (20 Hz) output impedance of 1437 ohms for the Ref 3 (which it should be noted is only a bit more than a factor of 2 greater than the midrange output impedance), it would appear that there shouldn't be any frequency or phase response issues. And I'd be surprised if there were any audible distortion issues either, although you can speak to that possibility more knowledgeably than I can. Best regards, -- Al |
