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 2 responses by atmasphere

atmasphere's avatar
atmasphere
11,575 posts
 
The output impedance of a preamp at low frequencies has a close relationship to the size of the coupling capacitor at the output of the preamp. Many tube preamps including the Quicksilver (which has a very low output impedance otherwise) have a rising impedance at low frequencies due to the output coupling cap.

Whether this is actually a problem is another matter. The interaction of the coupling cap and the input impedance of what you are trying to drive follows a formula:f= 1,000,000/C x R x 6.28 where
C= the value of the coupling cap in microfarads
R= the input impedance in ohms
f= minus 3db point in cycles per second

So if you have a 5 uf cap and a 10K input impedance the cutoff frequency is 3.16 cycles per second. Keep in mind that the rolloff will manifest at a frequency 10 times higher, so you will notice a loss beginning at about 30Hz. This is why it is so important to get subsonic bass response from a preamp!

There is a tension between the bass cutoff and the overall sound of the preamp. The bigger you make the cap, the better the low end, but at a sacrifice of transparency.

There are tube preamps that bypass this issue by use of a direct-coupled output.
atmasphere's avatar
atmasphere
11,575 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.
More to discover