Monoblock power amps same dedicated outlet?


Looking for input whether putting both monoblock power cables into same dedicated line outlet receptacle is ill advised.

I would rather not get a dedicated line for each monoblocks ....

But if big differences...I could....

I have pass labs 260.8 monoblocks..
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I agree with georgehifi and the advice from Michael Fremer in the video from millercarbon, the amps should be on the same ground plane to avoid a ground loop hum.  Speaking from experience here.
The guys at Reno HiFi told me that Pass Lab amplifiers do not employ a ground reference at the speaker terminal which makes them "less" susceptible to ground loops.  Therefore, if you are running the big Pass mono block amplifiers, they recommend separate dedicated lines.  I get the argument for one dedicated line for the whole system, but some amplifiers may over tax the circuit. 
You should certainly try it both ways—amps in single outlet vs separate outlets—-and reach your own conclusions.  Each system may react a little differently. 
In my system, using tube monoblocks, the sound with separate outlets is slightly brighter, slightly leaner, slightly bigger and more spacious, slightly recessed in the mids, and slightly more exciting.  With both amps plugged into the same outlet, the tonal balance is more linear, more coherent, but also slightly smaller and less exciting.  
In past years with some different tube amps and different speakers, I preferred the sound with separate outlets.  With my present setup I hear the same types of differences noted above but overall I now prefer the single outlet.
I agree with Jafox that you should consult with Pass Labs. But FWIW let’s look at the specifics:

Gs5556 makes a good point in referring to the significance of the speaker’s impedance characteristics. And I see in one of your other recent posts that you are using Harbeth 40.2s. In terms of relevant specs the 40.2 appears to not be greatly different than the 40.1, for which Stereophile measured a sensitivity of 84.5 db/2.83 volts/1 meter, and stated that:

... the Harbeth is easy to drive in terms of its demand for current. The impedance drops below 6 ohms only in the middle of the midrange, and then only for a short while; and while the electrical phase angle occasionally reaches extreme values, this is only when the impedance magnitude is high, ameliorating the drive difficulty.

The relevant specs of the amp are:

Max power (8 ohms): 260 watts
Max power (4 ohms): 520 watts
Leaves class A at: 34 watts peak
Power consumption at idle: 3.1 amps, 375 watts

The 375 watt figure is low enough that I would have high confidence that turn-on surges will be reasonable and won’t trip the breaker, especially given that both amps will not be switched on at the same instant.

If we assume a 6 ohm speaker impedance the amp’s maximum power capability with these speakers can be calculated to be 347 watts. Given that the amps would be operating mainly in class AB at power levels approaching that amount I suspect that a safe estimate of the maximum amount of AC power each amp would ever draw with your speakers would be in the range of 800 to 1000 watts. So the total worst case draw of the two amps plus a preamp and say a couple of source components should fit within the 20 amps x 120 volts = 2400 watt capability of a single dedicated line, although without a great deal of margin if and when the amps are ever running close to full power.

From a sonic standpoint, though, the problem may be that since the amps will draw most of that AC power during just a small fraction of a given 60 Hz period, the total current drawn during those brief instants may be significantly more than 20 amps. Not enough to trip the breaker, given the short duration, but conceivably the resulting drop in line voltage during those instants could affect dynamics or have other sonic consequences, especially if the length of the run is long and/or if the gauge is not over-specified.

So the bottom line seems to be that you would probably be ok installing just a single dedicated line, which as has been mentioned would minimize the risk of ground loop issues, but it’s somewhat marginal and might not be sonically optimal, especially if you listen at high volume levels. Again, check with Pass.

Good luck. Regards,
-- Al

First of all thanks for all the detailed input.

I should have added more detail though line preamp and sep phono preamp and turntable are all on a second dedicated line plugged into a synergistic power conditioner.

My third dedicated line is just for the pair of REL subwoofers.

Late last nite I changed it so each monoblock on different dedicated line...but second outlet for each monoblock has REL sub....i figured less current draw more headroom for monoblock albeit not ideal same circuit as sub.

The sounds seems more open and bass still fantastic....maybe faster bass ?

I did reach out to Pass and Kent already emailed me back stating that one dedicated circuit is fine for both monoblocks that the power consumption is not that much ....unless you aee meters fluctuating a lot you not taxing amps nor line. He gave no opinion about sound but they claim power cables have little affect on sound and that is not my experience.

I was not thinking could blow something...maybe i should have....but just whether at loud volume would get more lively sound....seems to be the case but might be perception bias....will see over time over more records it still prefer this way...

Anyone think bad idea sonically to mix one dedicated outlet for left monoblock amp and left sub...I have REL set at low output and a low crossover so sub are very modestly involved.

I have another non dedicated outlet that could get another dedicated line but feel 3 dedicated lines enuf for SR atmosphere tweak goes into non dedicated line...

Lastly I did try running the subs into the power conditioner but felt somehow sound waa a little less big and live....the monos def prefer not going through power conditioner....SR says no power limits and my other system the stereo amp sounds better into their power conditioner but its a far less powerful amp