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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
 
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
bifwynne OP
2,206 posts
 
Well Al, . . . if one of you techies think this could potentially affect a significant swath of owners of tube pre/ equipment, other than the Atmasphere OTL type of course, perhaps someone with the equipment and know-how will run some tests as Hifigeek mentions above to see whether this issue is just hype or real.

As I already stated, the point is moot for me now, but it might be useful for other A'gon members. However, as soon as I hook up my impedance buffer with the balanced inputs, I'll A/B my set up to see if there are any noticeable sound improvements and report back. Right now, I'm still getting used to the exchange of my old SED 6550 tubes for new Tung Sol KT-120s, which as I said in another post are quite impressive. In any case, it may be that the only improvement I realize will be longer tube life rather than significant sound improvement. We'll see.

Cheers.
hifigeek1
770 posts
 
Well I can tell you that I have looked at driving a balanced input of an ARC amp with a single ended input while looking at it on my scope. This was done with a 250W Dale non-inductive resistor loading the amps 8 ohm tap. This was on an amp that only had balanced inputs. The results were not pretty. It does tend to shift the DC operating points a bit and the amp would not clip at rated output. In fact you give up about half the amps output by doing this because there's no inverted input to drive that section of the amp. In my shop since I do not have a balanced output signal generator, I use one of ARC's IC based balanced converters. Signal goes in single ended and comes out balanced. Pretty nifty as I didn't want to spend the time to build one myself. How that relates to this issue is hard to tell although I would tend to think, at least with ARC products, it's best to present the balanced input of the amp with the same impedance on both inverting and non-inverting inputs.
atmasphere's avatar
atmasphere
11,834 posts
 
Bifwynne, the issue lies with the input of your amplifier. If it is differential with a high Common Mode Rejection Ratio, you will be fine even if the loading of the preamp is unequal (the amp will do fine even if one input has 1/4 the input voltage of the other input). But if the amplifier is balanced but not differential, the CMRR will be comparatively rather poor, and then it will make an audible difference.

Being the 21st century and all, I really would not expect too many amps out there that are balanced but not differential, but maybe my expectations are not that realistic :)
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