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 undertow

undertow
1,033 posts
 
As long as you stay below 1000 ohm output you should be okay with any of them. However it is hard to find many tubes with that low. But there are a few, BAT makes some in the 500 to 700 range. And Conrad Johnson has some as low as 50 ohm output which would drive a super low 10k input on a crossover. However if you can use XLR balanced connection, with like a BAT or similar normally the crossover or amp driven will be for example 20k input on single ended, now 40k input on balanced connections, thats why most of these units are mostly for PRO audio use not really home use.

Yes your bass will suffer using single ended RCA connections with most of the Tube preamps that are 1.5k ohm and up outputs unless you can get an easy load of 20k or better on the crossover(or amp connected) to the other end of it in my experience. Highly suggest trying to go XLR in this case if you have to use a PRO unit in the crossover.

By the way there is a connection "Impedance" convertor device made by Rolls which is a pro company that can accept your RCA input off the Preamp, and convert it to the XLR output with an actual gain boosting circuit, I used this in a pseudo style Pro home theater setup once with good results feeding stereo subs, as it does have a right and left channel, also its pretty cheap around 40 bucks online. Again this is only if you must go this route, I don't suggest pushing any of this kind of setup if possible.
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