ST-70 Mono Configuration and Quadaptors

Here's a problem that may be academic in nature, but I'd like to know the answer. I‘m currently using a VTA boarded ST-70 amp and PAS-3X preamp. I also have a second ST-70 that is in stock condition. My system is wired into a Dynaco Quadaptor which I use to power a pair of Boston Acoustics A200 speakers in the front and a pair of Boston A150s in the rear. I usually don't go for gimickry, but I bought a used Quadaptor last summer and found out I really liked how it worked when running four speakers. Now for the problem. If I were to put a VTA board in my second ST-70 and run the pair as mono amps, what would be the best way to wire them through the Quadaptor so I can retain my current speaker configuration? Should I bridge the amps into mono configuration or bi-amp my speakers - front and back? If the latter, then what would the resistance be - 4, 8 or 16 ohms - and would I need a second Quadaptor? Finally, should I just sell my second amp, forget about this potential monster and be happy with the way my system sounds right now - which I think is pretty darn good. Thanks much for your help!

If by "bridge the amps into mono configuration" you mean to invert phase on one channel and connect the speakers between the two hot output connections on each Stereo 70...you can not use the Quadaptor because the Quadaptor requires that the two amplifier channels to be referenced against a common ground, and the bridged-mono connection can not be operated common-grounded.

If you intend to parallel the two channels in each Stereo 70 to produce a higher-powered mono output, don't bother. That doesn't work well and never did. Dynaco should never have provided that function. Perhaps the only good thing that could be said about it is that it can be used with the Quadaptor.

You could use four amplifier channels to biamp your front speakers and use two Quadaptors to "bi-drive" the rear speakers, if they have that capability.

Thanks - it sounds like it can't be done.

If biamping amp A will power the low frequencies and amp B the mids to highs. Just use the Quadapter with amp B.

So in this configuration the mid and high range drivers would be used on both the front and rear (ambience recovery) speakers and only the bass drivers would be used on the front speakers. Would there be a tendency for the amp that’s powering the mid and high drivers to overpower the amp that’s powering the bass drivers? Many thanks for your response!

Hmm... using you speaker's bi-wired is using the internal crossover to seperate the signal coming from the amps. So both amps would still be amplifying the full signal but will be wired to either the upper or lower binding posts of the speaker. Good for the Quadapter. But...It would be better to use an active crossover between the Pas and the amps to have only bass going to amp A and only say 200Hz and up going to amp B. This way amp A only has to deal with amplifying say 20-200Hz while amp B handles 200-20,000. This will take the transient load off the amps and should give more detail and cleaner presentaion, IMO. I have to look for my Quadapter at my parents sometime soon. That said I don't remember how it wires up, so I can't comment on the possible setup.

Listens2tubes wrote:But...It would be better to use an active crossover between the Pas and the amps...

And if you do that, you must watch out for too low an input impedance to the active crossover, since in all likelihood it's going to be solid-state. Remember that a PAS running into too low an impedance will exhibit a bass rolloff.

Could the crossover be passive if the preamp was a SP-6 or SP-8?

Yes, Vandersteen's 2Wq sub uses a simple high pass filter in much the same way. Only other consideration is volume balance, which may not be a problem with matching amps. Though I'm not sure being the amps are doing opposite duties.