impedance match of preamp and main amp.

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.

Regards,
-- Al

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.

Cheers George

Thanks guys for all your responses. This helps out A lot.

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.

Thanks for all your help. I think I will look for an amp that matches my preamp. Maybe one of the BAT amps.