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Why didn't this biamp strategy work?

I purchased a second Dynaco ST-70 to complement another that I have. I used one to run the highs on my Vandersteen 2ces and one to run the lows. I thought that the doubling of the power would appreciably increase performance. However when I hooked it all up, the soundstage deepened dramatically...instead of Paul Simon singing in my lap, I was now on row 20. Also, the voices were a bit more murky and the bass was not at all improved as I had imagined it would be. I left it this way for awhile hoping to eventually hear the advantages, however finally I just hooked up the one ST-70 and everything was wonderful again...better midrange AND better bass despite halving the power. I was not using a crossover as the amps have the same power curve and my preamp (an Aragon 24k) has dual outs. So there were no added ICs. What happend here? Will vertically biamping help?
issabre
plato
1,543 posts
 
My best guess is that you hooked up the two sections of each speaker in opposing polarity. If you had reversed the + and - leads of one set of amps so the midrange and highs were in opposite polarity relative to the bass sections you would probably get the effect you've described.
sean
6,231 posts
 
It's quite possible that the two different amps have different overall gains, sonic qualities, measurements, input / output impedances, etc... This could be due to TONS of various reasons. One of them is age. ANY tube based product will alter with age. Due to the heat generated, the support components in the circuitry will tend to wander and slowly change value. Secondly, even though you might have matching brands and models of tubes in each amp for all the different stages, their conductivity and gain curves might be significantly different from one another. The more tubes in each amp, the greater the potential for variances. You would really need some like Kevin Deal ( or equivalent ) to come up with a completely matched set of tubes and THEN make sure that the rest of both amps are up to snuff. Even after all of this, running two amps in parallel alters the impedance that the preamp is seeing and trying to load into. In theory, you've about halved the impedance that the preamp is loading into when compared to only using one of these power amps. You can try playing with cables (again), move into an electronic crossover or even just try putting resistors in series with the inputs of each amp. This would APPROXIMATE the input impedance of just one amp if done properly. Keep in mind that there are lethal voltages in your amps, so be careful. Vertical biamping might solve some of the problems, but it could also open up a whole different can of worms. Sean >
plato
1,543 posts
 
I don't think that the impedance issue would give the results that Issabre cited -- however, if the amps' input sensitivity differs, that could make for such a scenario. There are many driver stage modifications for the St-70, so this is also a possibility.
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plato
1,543 posts
 
Elizabeth, I know you mean well, but your theory is totally wrong -- things just don't work that way. You're not giving "power" to the amps, merely a voltage signal. Since the input stages of the Dyna 70's were very high impedance (at least 100k, if not more) halving that value should not have any deleterious effects with virtually any preamp I can think of. The volume control would not have to be advanced to twice the setting. If the input sensitivity is the same for the two pairs of amps, then the volume setting should be likewise.
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