Audio Research amps - balanced & non

Dgarretson,

With a single ended input through an XLR adaptor the VT -130 SE will produce less than half the power output that it will produce with a true balanced input signal and that only with THD nearly an order of magnitude higher. I'm unsure of the precise reason for this result, but that was the position of the tech dept. at ARC when I posed this question to them some years back. I feel okay about describing such operation as "incorrect" rather than "sub-optimal", but I'm you're certainly entitled to feel differently about that.

Marty

Marty

With a single ended input through an XLR adaptor the VT -130 SE will produce less than half the power output that it will produce with a true balanced input signal ...

Really? That seems kinda odd. What does power out put have to do with it? The input signal will be half as the negative phase is essentially ground but that just means the line stage needs to provide for greater output for the amp to achieve full power.

Marty, You could be right, but the difference between balanced and unbalanced operation is more typically described in terms of loss of gain(typically 6db) and signal-to-noise ratio, due to sacrifice of common mode noise rejection inherent in balanced circuitry. I would be surprised if one single phase of VT130 used in SE mode has distortion greater than a comparable high-quality single-ended design. However, it is admittedly a waste not to utilize a fully-balanced amplifier to its full capability.

Guys,

Just quoting ARC as best as I recall. They may have used imprecise language for my benefit, but the conversation definitely was in terms of power output and THD (or possibly just "distortion"). I'm pretty sure that the "less than half" and "order of magnitude" are verbatim from ARC.

Marty

Martykl you are correct! When driving an ARC balanced amp with the SP-10 preamp you will find that you are unable to get symmetrical clipping out of the VT-130. At about half the rated output of the amp, one half of the waveform will indeed softly clip and you will be unable to get anymore output out of the amp period. Since the waveform is asymmetrical, i.e. both halves of the waveform are not clipping at the same point, distortion products go way up because no distortion cancellation takes place. Since the amp puts out approx. 122 Wrms across 8 ohms,( i just looked at one) you would probably get about 55 Wrms. BTW if the SP-10 has not been looked at for 10-15 yrs., there are a few filter caps in the power supply that tend to fail. Look at the end gaskets on the caps with leads that come out of each end of the body of the cap (axial caps) and look for bloating or an actual hole where the cap may be venting. I believe they are located near the power transformer just to the right as you are facing the front face plate, and then look slightly below those caps as well. Hopefully, my memory serves me right. If any of the caps look like that, it's time for service. G