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.

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.
The Quicksilver full pre has an output impedance of 1.5 ohms. It should drive anything out there.
Second the Quicksilver Full preamp. It will actually drive K-Horns without a power amp--not that I am recommending that, but it will drive any load you connect to it.
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.
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.
Sorry to disagree with Atmasphere, but the Quicksilver Full preamp does not suffer the usaul rise in impedance at low frequencies. The output coupling cap is 160 mfd and there is 18 db of feedback from the totem pole cathode follower to the preceding gain stage. Yes, yes that evil feedback! But in this case it is around a single gain stage and it is not limited in the low frequency range given the large value coupling cap. All I know is that it works and works well. The imaging of the preamp is phenomenal, which I attribute, in part, to the -3db point being well below .1 HZ. A very stiff power supply and high quality parts also contribute to the result.
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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.
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.
In the end, I did end up getting a Marchand XM44, and an ARC LS26 preamp; the bass is very good with this setup.
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.


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.

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.
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!!
What are some other good tube preamps with low output impedance at low frequencies, preferably balanced? I know the Audio Research SP16 is very low, but unfortunately it is not balanced. Price limit is $3000 new/used.
I believe most subs should have a -3db rating on the high end of their frequency range (e.g. -3db @ 150 Hz), this might help in finding an appropriate unit.
Look for something that's transformer coupled at the output. My Supratek Chenin has no problems driving Gilmore Raptor amps with 8K input impedance. Plenty of good tight bass.

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.