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ARC Ref 3 Main Output Impedance Question

I need some advice on how to hook my ARC Ref 3 pre up to my self-powered Paradigm Signature Servo subwoofer. As further explained below, ARC recommends that I use a crossover/connector tdevice hat has a higher input impedance than the one I currently use and is set up for balanced operation.

The Servo's input impedance is 25KOhms in SE mode and 20KOhms in balanced mode. I use the Servo for low frequencies (10hz to 35hz) in mono mode. To do this, I sum the L/R outputs of the Ref 3 by using a Paradigm X-30 crossover unit. The X-30 has an input impedance of 20KOhms in SE mode. There is no provision for balanced interconnect.

My Ref 3 is connected to the rest of the system as follows. Main 1 outputs are connected directly to my ARC VS-115 in balanced mode. The VS-115 has an input impedance of 300KOhms in balanced mode. Main 2 L/R channels are connected to the X-30 unit in SE mode. The X-30 sums the channels into mono and connects to the Servo is SE mode.

Based on a call with ARC, I was advised that my current subwoofer set-up is compromising the sound quality of the Ref 3 because (a) the X-30 input impedance of 20KOhm is the bare bones minimum that is recommended and (b) I am running one Main output in balanced mode and the other Main output in SE mode. Further, I was told that the current set up is making the Ref 3 work harder than it should.

ARC recommend that I use a crossover/connector device that operates in balanced mode and has a higher impedance than 20KOhm. If I recall correctly, something north of 40-50KOhm would be much better.

Any suggestions would be appreciated. Thanks
bifwynne
bifwynne OP
2,206 posts
 
Atmasphere, do you think asymetrical loading would affect sonic quality? Not being an electronics techie, I of course have no view. All I can say is that ARC did not think it was a good way to load the Ref3.
hifigeek1
770 posts
 
Someone needs to do a test to settle this debate.I'm wondering if distortion products go up?
almarg's avatar
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almarg
9,670 posts
 
Do you think asymetrical loading would affect sonic quality?
Pending Ralph's response, my expectation would be that it could degrade the common mode noise rejection that would otherwise be provided by the balanced interface, at least a little. That would occur for two reasons. One is the imbalance that would be introduced in the impedance between each of the signals in the balanced pair and ground. The other is that the single-ended connection could introduce noise onto the balanced connection that is not common mode (i.e., that is not equal on the two signals in the balanced pair).

The higher the output impedance of the preamp, the greater I would expect both of those effects to be.
I'm wondering if distortion products go up?
I don't think that would occur as a result of the imbalance per se, but it might occur to some extent if the preamp has difficulties dealing with the lower impedance on the signal that is driving the two loads.

Regards,
-- Al
bifwynne OP
2,206 posts
 
Certainly an interesting discussion. However, in my case, now that I have been alerted to the issue, the problem is moot because Tom is making a custom buffer device with high impedance balanced inputs that will sum the channels for my sub. I am a little curious if I will detect any sonic improvements, albeit lower and tighter bass response, lower noise or cleaner output (i.e., less distortion products).

FWIW, just wondering out loud for a second, I wonder if the issue of asymetrical/low impedance loading is just an ARC issue or cuts across the board for all pre/line stages, regardless of type (i.e., SS or tube). If this is more of a ubiquitous issue, then Hifigeek's suggestion about an objective test might benefit other A'gon members, IMHO.
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almarg
9,670 posts
 
I wonder if the issue of asymetrical/low impedance loading is just an ARC issue or cuts across the board for all pre/line stages, regardless of type (i.e., SS or tube).
The two effects that I described above, related to degradation of common mode noise rejection, would assume significance that is roughly proportional to preamp output impedance, as I indicated. So I would expect them to be pretty much insignificant for most solid state preamps, but potentially significant for many tube preamps (that have relatively high output impedance), as well as being dependent on the noise environment, cable lengths, susceptibility of the particular components to ground-loop induced noise, and the amplifier input impedances.

Ralph's preamps, btw, although tube-based, have uncommonly low output impedance, and I would expect them to be essentially immune to these effects.

Regards,
-- Al
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