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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

Showing 5 responses by atmasphere

atmasphere's avatar
atmasphere
11,597 posts
 
Normally if you have a balanced circuit, 20K should be no problem. Balanced circuits **should** be able to drive 600 ohms- so 20K should be like a walk in the park. But many high end audio products don't support the balanced standard, and ARC is one of them.

So here is something you can do. Jensen makes a transformer that can perform the operation of going balanced to single-ended, and the transformer I have in mind is optimized for subwoofer operation, so it won't compromise the bass.Their part number is JT-112P-2HPC. you need two for stereo. If you set it up right, it would load the preamp at about 47K (or 39K, which ever you want). This would allow you to run balanced or single-ended. It would be a very simple matter to add a summing network at their outputs, bypassing the need for your summing crossover.

Tom Tutay is also an excellent resource.

FWIW, (disclaimer: manufacturer) we make a preamp that does support 600 ohm balanced operation so you could bypass the need for the summing amplifier- it can drive both the 20K and 300K input impedances in parallel in the balanced mode.
atmasphere's avatar
atmasphere
11,597 posts
 
Hifigeek1, we've built tube buffer versions of our preamp. They have a direct-coupled output. IME what you run into with tube buffers is voltage loss that is more noticeable than with semiconductors. As long as your sources have enough voltage that is not a problem.

I'm not buying the idea that the asymmetrical loading of the ARC would have any effect on tube life. Most tube preamps can be connected to a dead short without the tube life being affected...

Now the asymmetrical loading issue is another matter, but the way I have seen ARC preamps designed, that should not be a big problem if one half is loaded differently from the other.
atmasphere's avatar
atmasphere
11,597 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 :)
atmasphere's avatar
atmasphere
11,597 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 :)
atmasphere's avatar
atmasphere
11,597 posts
 
Bifwynne, sorry, seems I have been shooting my mount off; upon looking at the specs of the preamp, I have to agree. It did not occur to me that the preamp was not designed to drive 10K as a minimum.
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