Impedance Matching

Ralph, thanks for chiming in. I had brought this issue up in another of the OP's current threads. The preamp in question is a Hovland HP-100 with the moving coil option. Its output impedance, per JA's measurements in Stereophile, is 2.4K (presumably at 1 kHz), and 4.3K at 20 Hz.

The power amp he is considering is a Bryston 28BSST2, which is specified as having an input impedance of either 15K or 16.5K, depending on what document is looked at. He is using Maggie 20.7 speakers.

The opinion I expressed is that while that combo may work reasonably well with many recordings, especially those having limited deep bass content, I see no point in introducing what is at best a known marginality into the system, when other alternatives are available that would avoid the issue.

The output impedance at 20Hz will tell you. If it is the same value then you have no worries. If it is a lot higher at 20Hz they you might have a problem.

This is an excellent point that I think warrants emphasizing. What matters most is not the relation between amp input impedance and the nominal output impedance of the preamp, but the relation between amp input impedance and the range of VARIATION of preamp output impedance as a function of frequency. That is why, for example, some people report that resistance-based passive preamps, having VERY high output impedances, still work fairly well into amps having input impedances such that the 10x guideline does not come close to being met. The output impedance of that kind of preamp will have very little variation as a function of frequency.

Regards,
-- Al

If you can audition the amp with the preamp then sure. But other than that, I would not go that route, especially if there was a potential for losing money in a re-sale on the used market.

Unfortunately I cannot 'try before I buy' but the price is very, very, good and hard to pass up. Magnepan voices and runs their speakers with Bryston gear because (1) it sounds good to them, (2) the Brystons NEVER break driving the DIFFICULT Maggies.

At the bargain price I almost wonder if it would be worth picking up a good SS pre to drive them and just keeping the Hovland too!

At the bargain price I almost wonder if it would be worth picking up a good SS pre to drive them and just keeping the Hovland too!

Another possible approach, although it would result in having an additional active stage in the signal path, would be to buy the Bryston, and if the Bryston/Hovland combo proves to be unsatisfactory purchasing and installing a separate active buffer stage in between them.

I'm not sure if the Burson AB-160 buffer is still being made, but if you can find one new or used that would probably be a good choice. Also, A'gon member Bifwynne (Bruce) has had a custom buffer stage made for him at a modest price by Tom Tutay of Transition Audio Design, I believe in Florida. Tom is reported to be an outstanding person to work with, and Bruce reported excellent results with the buffer stage.

Either of those devices would provide a very high input impedance and a very low output impedance, just what you need.

Regards,
-- Al

Stickman ..., Al is correct. Tom Tutay built a custom impedance buffer for my subwoofer that sums the L and R channels and also ties into the balanced outputs of my linestage.

I have a caveat that is best addressed by Al and/or Ralph. My impedance buffer only ties in the subwoofer, which is self powered. There are no artifacts between my linestage and my main amplifier. I only use the sub to supplement the bottom octave or two (below 50 or 60 Hz) where my main speakers roll off.

So, perhaps Al and/or Ralph can speak to whether inserting an impedance buffer between the pre and amp could compromise the signal.

Bruce (bifwynne)