ARC REF150 & WILSON SASHA 1

Bi Amp would be interesting but im not sure you can with sasha1

I’m doubtful that biamping these two amps would be worth pursuing.

For one thing, in a passive biamp arrangement (i.e., biamping without inserting an electronic crossover between the preamp and power amps), you would in effect be converting the Bryston amp that is capable of providing 900 watts into 4 ohms into not much more than a 150 watt amp.

That is because while a passive biamp arrangement relieves each amp of the need to provide current and power in part of the frequency spectrum, assuming the amps are properly gain matched in some manner both amps would still have to output essentially identical voltages corresponding to the full range signal. So turning the volume control up high enough to utilize much of the Bryston’s power capability would drive the ARC amp into clipping, and make those settings of the volume control unusable.

And using an electronic crossover ahead of the amps, in addition to the speaker’s internal crossover, would result in having two crossovers in the signal path which may very conceivably do more harm than good.

Finally, of course, having the speaker powered by two very different amps would raise concern about possible loss of coherence in the overall presentation.

Regards,
-- Al

Kalali, my understanding is that in general it is extremely unlikely that a 2:1 mismatch between nominal speaker impedance and output tap rating, in either direction, would cause any harm. And I’ve never seen any reports of experiences to the contrary. Also, in this specific case a lot of additional confidence is of course provided by the fact that Peter McGrath specifically suggested doing that, as well as by the comments in the Audio Beat article Gpgr4blu linked to.

I believe that in general a greater possibility of damage would arise if a speaker having a particularly high impedance, such as 16 ohms, were connected to a 4 ohm tap. That may result in "back-emf" from the speaker being applied to the tubes and circuitry on the primary side of the output transformer at voltage levels higher than the designer may have anticipated, since the transformer’s turns ratio for a 4 ohm tap is greater than for an 8 or 16 ohm tap.

Best regards,
-- Al

I find it interesting that the ARC amp’s 8 ohm tap is preferred given the Sasha’s 4 ohm nominal impedance and its characteristics.

Yes, that is somewhat surprising, Charles. And even more so because JA’s measurements of the amp indicate that THD performance and maximum power capability are both significantly compromised when speaker impedance is less than the nominal rating of the tap that is being used.

Schematics I’ve seen for other ARC balanced amps, such as can be seen near the bottom of the page here for the older VT100 model, show what I believe to be a somewhat unusual circuit configuration on the output transformer’s secondary (output) winding. With the 4 ohm terminal being connected to circuit ground, and presumably corresponding to a center tap of the winding. And with feedback into one half of the balanced signal path being taken from the "Common" terminal, that is connected to one end of the winding, and feedback into the other half of the balanced signal path being taken from the other end of the winding (that being connected to the 16 ohm terminal on some models, although the VT100 omits the 16 ohm terminal). So assuming the REF 150 is configured similarly in that respect the explanation might relate to the fact that the 8 ohm terminal is electrically "closer" than the 4 ohm terminal to the point on the winding from which feedback is taken into one half of the balanced signal path, resulting in a closer match with the characteristics of the other half of the balanced signal path that takes its feedback from the Com terminal. That benefit perhaps outweighing the adverse effects on THD and power capability which JA found to result from connecting a 4 ohm load to the 8 ohm tap.

In any event, as you said listening sorts out the net result of all of the factors that may be involved.

Best regards,
-- Al

From a technical standpoint I don't find it surprising that the REF 150 would do well with the Sasha.  In addition to the robustness of its power supply which was referred to in Gpgr4blu's post it has a particularly low output impedance for a tube amplifier, due in part to the use of greater amounts of negative feedback (14 db) than in many other high quality tube designs.  John Atkinson measured the output impedance of its 4 ohm tap as being only 0.55 ohms at middle and low frequencies (where the Sasha's impedance characteristics are most challenging), and 0.87 ohms at high frequencies.  While those impedance numbers aren't quite in solid state territory, they are a lot closer to it than in the case of many other high quality tube amps.

Such low values will minimize the tonal effects that would otherwise result from amplifier/speaker impedance interactions, and will result in better bass control than in the case of most other tube amps, and in conjunction with the robust power supply will enable it to supply more current into low impedances than in the case of most other tube amps.

So from a technical standpoint I don't find Gpgr4blu's experience with the combo to be surprising.  And I'll add that I always consider his posts to be among the most credible to be found here at Audiogon. 

Regards,
-- Al