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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
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
11,837 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 :)
bifwynne OP
2,206 posts
 
Thanks Ralph: I believe the amp, as you suspect, is balanced a AND differential, whatever that means. I clipped an excerpt from the ARCDB website, which describes the amp:

"Drawing upon the successful REF110 input stage design, the VS115 uses a FET input with two 6H30 triodes in each channel for gain and as a cathode follower-driver for the 6550C-equipped output stage. This allows both balanced and single-ended connections in a true differential topology yielding a much wider power bandwidth (110kHz in contrast to 40kHz for the VS110)."

So does that mean that the asymetrical loading of the Ref3's main outputs with a combined load that may have lower impedance than recommended will NOT affect the sonic quality of the power amp's output? I think both Tom Tutay and ARC thought there could be some rolloff on the low frequency side of the spectrum in such cases. I don't know at what frequency that would occur, but here again, this could be a big whoop too because I can adjust bass loudness.

Bottom line here: does all of this mean the issue which is the subject of the OP does NOT impact sonics, but perhpas just tube life??
atmasphere's avatar
atmasphere
11,837 posts
 
Bifwynne, what this means is that the preamp will drive a solid state amp the same way it would as if the tube amp was not even there.

ARC is pretty careful to make sure their preamps will drive the 10K input impedance of a solid state amp. So if its OK playing bass into that right now, then adding the tube amp will be of no consequence as its input impedance is about 10 times higher. If you work out the math, in engineering terms the higher impedance can be ignored.

Here's the math: 10K times 100K divided by 10K + 100K.

That works out to 9.09K ohms as opposed to 10K. I think you'll be OK :)
bifwynne OP
2,206 posts
 
Thanks Ralph, as stated in the OP, "[b]ased 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. . . . ARC recommend that I use a crossover/connector device that operates in balanced mode and has a higher impedance than 20KOhm."

Using your formula, the combined impedance is 20K x 300K, divided by 20K + 300K, or 18.75K, which is below the minimum recomended threshold. Further, ARC commented that running the pre with "one Main output in balanced mode and the other Main output in SE mode" is not an optimal way to load load the outputs.

Thanks again for your comments.
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