Tube Preamp/ McCormack Amp impedance matching

Emerson -- Apart from the issue of the powered subs, I think that your original question has been well answered (and we are very fortunate to have Steve participating here at Audiogon).

Re the powered subs, there are three key questions:

1)What is their input impedance?
2)Does the VTL 2.5 have separate buffer stages on the two sets of outputs, or are the two sets of jacks simply wired together inside the rear panel (which is probably how the majority of preamps with two sets of outputs provide them)?
3)How long will the interconnect cables be, and if you are intent on using specific interconnects, what is their capacitance?

Regards,
-- Al

Thanks Newbee and Al. Al,I'm using 20ft runs of Blue Jeans LC1 cables which I learned from you are low capacitance. I believe the input impedance on the sub amps are 6k ohms.

Forgot to say the subwoofer cables are Blue Jeans LC-1.

Emerson -- Yes, thanks for reminding me of our conversation about the Blue Jeans cables, which have an extremely low capacitance of 12.2 pf/ft. So I don't think you've got any problem in terms of capacitive loading. Even if you were to have three twenty foot cables loading the same preamp output, their combined capacitive impedance ("reactance") at 20KHz (the worst case frequency) would be around 11,000 ohms, according to my calculations.

However, if the subs really have a 6,000 ohm input impedance, and especially if the VTL 2.5 doesn't separately buffer the two outputs, you've definitely got a problem there.

If there are not separate buffers, the preamp would see a load impedance consisting of 6K, 6K, and 10K in parallel, which is about 2.3K (the reciprocal of the sum of the reciprocals of the three numbers). If there are separate buffers, and if you had one of them driving the two subs, that output would see a load of 3K. Or if you had one of the subs connected to the same output that drives the DNA-500, that output would see a load of 3.75K. All of these numbers are most likely too low to be acceptable.

Let us know the make and model number of the sub, and perhaps one of us will be able to find a definitive indication of its input impedance. And perhaps check with VTL to find out if the two outputs are separately buffered (or if convenient, remove a cover which would allow you to see if the two output jacks for each channel are simply wired together, or if they appear to be coming from separate circuit points).

Regards,
-- Al

Oops! I wasn't thinking too clearly when I wrote the last post. I assume the two subs are on separate channels, so if the two sets of preamp outputs are not separately buffered, they would see 6K in parallel with 10K, or 3.75K on each channel. Still too low for comfort.

If there are separate buffers, each channel of the pair of outputs driving the subs would see 6K, which may also be too low if the preamp output impedance rises significantly above the specified 200 ohms at low frequencies, which it probably does.

Regards,
-- Al