I have too much respect for Nelson Pass to contribute advice to those that buy Pass "clones".
Have I mismatched my preamp/amp?
Hey all; I recently purchased a Cary SLP-50B which I will be mating to a Rawson clone of the Pass Aleph 2. When I started doing the math, I know I'm slow, I looked up the output impedance of the Cary and its Aleph 2 from the Pass site and it's 2.2 K Ohms I checked the Pass site for the Aleph 2, being I don't have the exact spec on the Rawson, and the input impedance is 10 K Ohms unbalanced and 25 K Ohms balanced differential. The Cary only has unbalanced outputs. So, how mismatched is this? I understand the 20:1 rule and I'm way off. My point was to use the Cary to warm up the sound a bit of the Rawson, its brutally sharp and I thought warming it up was a good idea. Advice/Comments are welcome. Jack
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- 31 posts total
I composed the following before seeing your last post, about 100K/47K. I think you would have probably been ok even if it were 10K: The usual rule of thumb guideline is that power amp input impedance should be at least 10x greater than the output impedance of the preamp, at the frequency within the audible range for which preamp output impedance is highest. If only a nominal output impedance is known, which presumably occurs at a mid-range frequency such as 1 kHz, a much larger factor should generally be used. With a tube preamp, the worst case output impedance commonly occurs at 20Hz, due to the impedance rise that occurs at low frequencies as a result of the coupling capacitor that is usually (but not always) used at the output. If the 10x ratio is not satisfied at that frequency, perceptible rolloff and/or phase shifts can occur in the bottom octave. In this case, however, as indicated on page 2 of the manual, the output coupling capacitor is 5uf, which is a relatively large value, and which I suspect is large enough to reduce those effects to insignificance with a 10K load. Assuming that the output impedance doesn't rise to a major degree at any higher frequencies within the audible range (which would be unusual although not unheard of), I think you're fine. Do try to avoid long interconnect cable lengths, though, and/or use cables having low capacitance per unit length. Otherwise a small but perceptible degree of upper treble rolloff could result from interaction of the high output impedance with cable capacitance. Regards, -- Al |
My rule of thumb is to look for 40Kohm input impedance minimum or higher as a general indicator to expect good performance with a tube pre-amp. I view it as an insurance policy to help achieve the kind of sound that I like, ie tight, clean, detailed, with good dynamics. Based on that the ops gear does not appear to be a mismatch. |
- 31 posts total