2. amps-preamps
  3. 1387725945

Impedance mismatch question

Amp is 23k ohm input impedance. Preamp is 1k ohm output impedance. I have been told that I need at least 50k input impedance on amp or sound will suffer. Can anyone explain why? How will sound be degraded?
2out2sea
georgehifi's avatar
georgehifi
10,312 posts
 
Al sorry I should have mentioned at the end of the day a Cymer tube pre with 2k capacitor coupled output impedance was subsituted for the Lightspeed and similar ratio results were observed. Of course as you say being tube tube and capacitor coupled it's output impedance will change a bit according to frequency. All impedance ratio changes were done on the fly on the input of a Cymer tube poweramp using a 32 position switch resistor loading to ground.

Cheers George
bifwynne
2,206 posts
 
2out2sea ... is your OP an academic question or an actual concern based on equipment you own or are thinking of buying? If this is an actual concern, based on Al's post, you may be close. Perhaps a call to the preamp manufacturer is in order.

Btw, I've been there and done this already. I bought a custom designed active impedance buffer for a very similar issue, but in my case only involving the sub-woofer. Check my threads. I doubt my solution would be a good fix if you needed an active impedance buffer like mine to drive your amp. I suspect even if it solved an impedance match issue with the pre, overall sonics might be degraded because you would be inserting an active artifact between the pre and the amp.
almarg's avatar
almarg
9,670 posts
 
Thanks, Georgelofi. As I said earlier, not meeting the 10x guideline (properly defined as described in my post) does not necessarily mean that there will be a problem. But meeting that guideline assures that there won't be a problem.

And as I'm sure you realize, the specific value of the coupling capacitor (which affects the amount of variation of output impedance as a function of frequency), and the deep bass extension of the speakers, and the deep bass content of the source material, are all relevant variables. So I would suggest caution to anyone who might be tempted to apply the results of your experiments in a universal manner.

For preamps that have been reviewed by Stereophile, the measurements and associated comments that John Atkinson usually provides in conjunction with the reviews are a valuable resource in assessing this issue.

Regards,
-- Al
charles1dad's avatar
charles1dad
10,250 posts
 
Hi Al,
Regarding tube preamps would an output trandformer in place of the capacitor eliminate or reduce this potential fluctuating impedance mismatch ?
Charles,
almarg's avatar
almarg
9,670 posts
 
Hi Charles,

Yes. In general I'd expect output impedance variation as a function of frequency to be much smaller in the case of a tube preamp having a transformer coupled output than in the case of a tube preamp having a capacitor coupled output. Assuming reasonably good design of the transformer and the output stage, of course.

Best regards,
-- Al
More to discover