2. amps-preamps
  3. 1265005525

Tube preamp output impedance at low frequencies

I'm looking to use a tube preamp with an active crossover (so as to send low frequencies to a sub), and unfortunately, most active crossovers have rather low input impedances (10k-20k). The only exception seems to be the Pass Labs XVR1, which unfortunately is out of my price range.

This being the case, I want to get a preamp with sufficiently low output impedance, to preserve the bass in my system. I know most manufacturers quote a single number for the output impedance, often at 1000 Hz, and this can differ greatly from the output impedance at 20 Hz. An example is this unit, which has Zout of 415 ohms at 1000 Hz, and 4.8k ohms at 20 Hz.

http://www.stereophile.com/tubepreamps/208bat/index4.html

I assume it would be a very bad idea to use this tube pre with one of the crossovers I'm considering. Other than Stereophile, is there any other source for tube preamp output impedances across the whole frequency domain? If I email a manufacturer directly, do they usually have this info on hand (and if so, do they have it for discontinued models)?

Has anyone else also faced this problem? If so, I'd love to hear about your experiences.
rrolack

Showing 1 response by dob

dob
577 posts
 
AthmaShere,

As always your explanation is greatly appreciated. Thank you.
Using your formulae, one can estimate -3dB cut off point knowing both capacitance of output capacitors and input impedance of downstream device.

However, audio manufacturers states in their specs not capacitance of their output capacitors but output impedance.

It has be shown that this value varies with frequency. So my question is how to "exploit" knowledge of nominal output impedance in estimation of bass performance???????

My naive logic suggests that input impedance also vary with frequency and in similar (but not identical) fashion as output impedance. If so I can take their ratio: say 1000 Ohms to 10000 Ohms = 10% and approximate that bandwidth lost about 10% here.

Thank you in advance.

Rafael
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