Impedance Matching Preamp/Amp Assiance Pls?

You need to know the OUTPUT impedance of the preamp, not the input impedance, to match it to the amp.

Kal

Nick,

I think what matters is the output impedance, which is 400 ohms for the Cary SLP-05. Given the Spectron's 50K input impedance, you should be fine. The rule is 10:1.

Thanks. Can you explain the 10:1 rule? 10 X 400=4000 ohms and the input impedance of the Spectron it 50,000 phms. So I'm not sure I understand why everything is ok.

Also, can you provide me an example where impedances don't match well and the consequences?

Thanks again.

Nick , the rule means that the input impedance of the amp should be 'at least' ten times greater than the output impedance of the preamp . More is fine .

Happy tunes .

All of the preceding responses are correct. A theoretically ideal audio amplifier or preamplifier will have zero output impedance and infinite input impedance. That is obviously not practical for many reasons. The numbers cited above (several hundred ohms output impedance, or perhaps even one or two thousand ohms output impedance, and 50K or 100K input impedance) are typical for preamp inputs and outputs and power amp inputs, and are completely compatible.

Power amp output impedances have to be far lower (a fraction of an ohm) in order to drive speaker impedances, which are usually less than 8 ohms.

If an output impedance is not much lower than the input impedance of the device it is driving, a consequence is that a fraction of the voltage put out by the driving device will be wasted across its own output impedance, instead of appearing across the load device. The fraction lost would equal the output impedance of the driving device divided by the sum of that output impedance and the input impedance of the driven (load) device.

That assumes that stray capacitances and inductances in the circuit are negligible. That is not always the case, and if it is not the situation can be much worse. If interconnect cable capacitance or inductance, or the input capacitance of the load device, are significant, and the driving device's output impedance is too high, than the loss across that output impedance will be frequency dependent, which will cause the system's overall frequency response to be non-flat.

All of this applies only to audio frequency devices. At radio frequencies (such as antenna inputs to FM tuners, or high speed digital signals) completely different considerations apply, which usually require source, cable, and load impedances to be matched. Different considerations can also apply in certain specific situations involving audio frequencies, such as differential balanced outputs of professional microphones, which are often specified to work into load impedances that essentially match their output impedances.

Regards,
-- Al

All the above, are typical answers. Check the archives for some different points of view.

It is not unusual for tube preamps to greatly exceed their reported (at 1K hz) output impedance at lower frequencies. This can be heard as a bass roll off with some match-ups, usually into SS amps with input impedance below about 20K. John Atkinson provides useful information regarding impedance matching in the "Measurements" section of Stereophile reviews, and here is what he said about your SLP 05, in Sept of 2006:
"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 1500–1600 ohms in the midrange and treble, rising to 3400 ohms at 20Hz, with similar if slightly lower figures for the balanced output. The Cary preamp needs to be used with power amplifiers having input impedances of 10k ohms or greater if the bass is not to sound a little lean."

I have the SLP-05 and my amps input imp is 100k but I also use a Ashly 3.24 crossover and it is 18k input. Is this ok?

TIA

Mitch2 ; very useful info on the varying impedances . I learned something today , thank you .

Happy tunes .

Saki70, you are welcome. I am glad to help with things I have learned, as many others here have helped me along the way. To Nick778, I am a little surprised at JA's comment about using the Cary with amps having input impedance of at least 10K ohms. I consider the 1:10 ratio recommended by many to be a minimum at the lowest frequency where your speakers provide useful output. Personally I prefer a ratio of about 1:20 or better when possible. Since the 05 was measured to have output impedance of 3,400 ohms at 20hz, I would probably look for an amp having input impedance of at least 47K ohms (seems to be a common input impedance) to use with the 05, and I believe your Spectron amp at 50K ohms should work fine.

I had the Cary SLP 05 for about a week with my Pass Labs X 350.5 Amp using balanced interconnects.
Not only was the bass rolled off but the treble was weak.
Tvad set me straight on this one considering impedence mismatch.
Best advise, try before you buy. (if you can )