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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
atmasphere's avatar
atmasphere
11,841 posts
 
Gmuffley, don't worry- as I pointed out, the question is does it matter? If you measure the unit at a low enough frequency you will detect the rise. More to the point is that as long as the maximum cutoff frequency of 2 Hz is observed by the designer, no audible losses will occur down to 10X the cutoff frequency. In your case, you should be good to about 1-2 Hz.
lewinskih01
909 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.
rrolack OP
54 posts
 
In the end, I did end up getting a Marchand XM44, and an ARC LS26 preamp; the bass is very good with this setup.
lewinskih01
909 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
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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