Jerry,
For a direct coupled amp, there is no way for the output power to be stored. In a cap coupled amp, the capacitors must first be charged then uncharged. If connected in bridge mode, essentially they act in series and theoretically double the voltage available.
Sure there is, DH! Power is stored in the large caps in the power supply. It's actually easier to see what happens in a bridged dc coupled amp. Let's assume we have rails of +43 and -43 volts.
Unbridged the maximum voltage applied to a speaker would be ... what? +40 volts and ground = 40 volts.
When bridged we'd have +40 volts and -40 volts = 80 volts.
Problem is we have not changed the load, but have double both the voltage and current and all that current must flow through the output transistors.
Unless the heat sinks were designed with this in mind, I'll bet there would be a problem. I mean there is just going to be too much heat.
I really don't think those coupling caps do anything but insulate the speakers from DC.
The power supply may be an issue. But I think the issue is not so much with the rectifier but with the current limiting scheme. It may be necessary to take out the current limiting resistors and the diode in the feedback loop.
Well, any messing with the feedback scheme and distortion will go to hell in a hand basket.
Since the amps are bridged, the two transformers are working in parallel - so power capability is not too much of an issue.
What two transformers?? I can only see one!! I thought you were talking about bridging the two amps within a single ST-120.
The distortion would be a product of unbalanced currents between the two amps. Assuming that the two amps have the same gain, etc. distortion will not increase much, if at all.
Bingo!! That's just the problem. There is no chance in hell, that the two amps will have EXACTLY the same gain
at all frequencies. Just look at the tolerance of the components.
Unequal gain (over the entire frequency range) = IM distortion!
Now, would you hear this if used to power a sub-woofer? Probably not, but you sure would if used to power full range, high quality speakers.
Finally, we also use a 120 for a PA amp driving two ohm loads in my wife's band.
HERE
DH, I have to plead ignorance when it comes to PA amps as I have little understanding of the spectral power requirements. For example, is much in the way of low frequencies (like a bass line) going through this amp?
We sometimes use a smaller ST-80 for the monitor speakers or even for the PA in smaller venues. This amp - this ACTUAL amp - is very cool in that it was also used as a PA for Perry Como and Cab Calloway.
DH
That is cool!! How did you happen to acquire that amp?
Regards,
Jerry