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 4 responses by lewinskih01

lewinskih01
907 posts
 
Lamm LL2 has an output impedance of 250 ohm nominal, 245 ohm for mid/trebble, and 3300 ohm at 20 Hz as noted in the Lamm website. So this would confirm your point. Since Lamm posts this on its website, you might want to ask them directly. Clearly they aren't trying to hide it.

Regarding active crossovers, the Marchand XM44 has an input impedance of 30 kohm (per their website), so might allow you to circumvent this issue.

FWIW, I'm also looking into this same subject, so please keep posting your findings.

Just my 2 cents.
lewinskih01
907 posts
 
Great!! You made my day. Now I can go thinking I did help someone, at least once! This forum has been great for me, but I've been taking more than I am able to give.

BTW, as mentioned above I'm considering a crossover as well. How are you implementing your setup?
Source->LS26-> XM44-╤->amp-> speakers
..................................... └-> self powered sub

Is this how you have it set up? What crossover frequency are you using? Which amp?

BTW, thank for posting a follow up with what you decided to do. Not done as often.

Cheers,
Horacio
lewinskih01
907 posts
 
which sub are you using?

My REL Storm III doesn't like being crossed over above 55 Hz...which has me thinking of using a SS for bass or switching subs. My main amp is tubed (MC275).

Not sure how much relief would the 275 get from a crossover at 80 Hz, though, but I guess my speakers would also appreciate not being forced to go low as they are not really full range.

Ah, so many possible paths!!
lewinskih01
907 posts
 
Nemesis,

This one is not balanced, but it is what I'm moving to now: Lamm LL2. Has 250 ohm nominal output impedance for mids/trebble, and goes up to 3300 ohm at 20 Hz. You can check out their website for the info, or see Stereophile measurements. They run used in the $2.5-3.3k range.

Check it out, though, as its ergonomics aren't for everybody. No bells and whistles whatsoever.

Horacio
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