IIt is usually accepted that the ratio be 1 to 10
Yes, I see a problem with this. As indicated in John Atkinson's measurements of the VK-51se, while its output impedance is 410 ohms across most of the audio band, it rises to 4700 ohms at 20 Hz. That kind of impedance rise at deep bass frequencies is not uncommon, especially among tube preamps, due to the coupling capacitor most of them use at their outputs.
The widely cited 10:1 minimum ratio for optimal impedance compatibility should be applied to the **highest** output impedance of the preamp at any audible frequency. Which says that the power amp's input impedance should ideally be 47K or more, although a bit less than that would most likely be fine also.
Some people report good results with ratios that are much less than 10:1, but my expectation is that in most of those situations the **variation** of impedance as a function of frequency is much less than in this case. Most of the issues that would result from non-conformance to the 10:1 guideline result from the **combination** of substantial impedance variation as a function of frequency and the low ratio. Also, how objectionable the adverse effects will be of low ratios at deep bass frequencies will depend on the deep bass extension of the speakers.
Al's right Cturnem.
But a simple solution for you if you want to go this way and use the Mc602, is to get a technician to change the coupling cap in the output of the Vk 51se and replace it with one of the same quality but say 4 x larger in uF (microfarads).
This would then take the high 4700ohm impedance from the -3db at 20hz region down to -3db at 5hz, where it should have much less effect on the audible bass frequencies.
Good stuff, as usual.
Its good to know detailed impedance measurements when available, but in lieu of that a good rule of thumb seems to be to use an amp with 60Kohm or higher input impedance to be safe with most tube amps. Slightly higher than the 10:1 minimum (as applied to worst case measurement at particular frequency) usually cited is a good and safe insurance policy.
FWIW, you will find most amps that explicitly mention or support use with tube pre-amps tend to have at least 50-60Kohm input impedance spec, often 100kohm unbalanced and twice that even balanced, which is usually preferred for the best and most consistent results.
Cturnem, sounds like you want to keep your Bat preamp, and that Mac looks to be a nice amp, if your keen on owning it.
Your Bat preamp is a low 410ohms output impedance at most frequencies and into the Mac it has a good ratio of 1.24, the problem is in the bass, and as I said above this is an easy fix if you get a tech to change the Bat's output coupling capacitors to 4 x larger in uF. Then it will be a perfect 1:24 ratio at all frequencies.
This job to do should be around $100-$200 then your Bat will also be a match for any amp you purchase in the future as well.
Georgelofi. I believe what you are saying, but am not sure I want to make the VK 51se non stock( due to resale value) I am running a Classe CA 400 with the Bat. I think it is 70k input impedance. I am running B&W 800D speakers. I have no reason to want the MAC, except from reading all of the reviews, and it is more powerful. I have also considered the BAT VK 600Se. I have never owned anything MAC. What do you run?
I like big solid state, biased hard into class A with huge power supplies, and that has bi-polar output for great current delivery.
The Classe CA400 you have is no slouch either, I would be quite happy owning that even though they are Mosfet output.
Your Vk 51se should be happy into that 75kohm input impedance.
The B&W 800D's are a bit of a bitch to drive, if you can understand the test here.
I would be more likely to go the big class A bi-polars for these 800's like what I use. Things like the big high class A biased Krells, Mark Levinson's and such that use bi-polar output stages, for good current delivery for the 800's.
Changing the coupling caps in the preamp may not be the best move. The bigger the coupling cap, the more coloration it can impose at frequencies well away from the cutoff frequency. For this reason coupling caps are often made as small as possible while still retaining some ability to drive lower impedances.
We got around the problem by having a direct coupled output.