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
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
rrolack OP
54 posts
 
Lewinskih01, I have the system set up exactly as you've said.

As for the crossover frequency, I've tried both 80 Hz and 100 Hz, both at 24 db/octave. 80 Hz was easily superior; too much stereo separation was lost at 100 Hz. I should probably give 60 Hz a try as well, but I'll probably keep things as is for now.

My amp is a Parasound A21, though after hearing what tubes can do in the pre, I'm considering switching to a tube amp at some point.
lewinskih01
909 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!!
rrolack OP
54 posts
 
I'm using an old SVS PC 20-39.
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