Audio Research amps - balanced & non

Ideas/suggestions anyone?
I have an original ARC SP-10 pre-amp (perfect, re-tubing once in a blue moon but totally reliable after ~25 yrs so reluctant to replace) but I would like to replace my original D-115 amp with VTM 200's (i.e. balanced) or get a VT-130SE (also balanced) to bi-amp alongside the D-115 to drive the bass end, assuming that's even possible.

Question - can balanced power amps work with a 'pre-balanced' pre-amp like the SP-10, or even work in a bi-amp arrangement as the only balanced component? I suspect not but would like some user comments.
The VT-130SE does not have a phase splitter and requires a true balanced input to operate correctly. If the SP-10 is not a balanced design you can't run it directly into a VT-130 SE.

Good Luck

Sorry, I didn't see the bi-amp part of your post. Some active x-overs will output a true balanced signal even if the input is single ended. The NHT x-2 is an example. If you bi-amp as follows: Source -> ARC sp-10 -> NHT x-2 -> ARC vt-130 se/main speaker (via NHT balanced main out) + subwoofer (via NHT subwoofer out), you should be okay.

There is a way to create a quasi-balanced output from a single-ended source. Ashly (pro sound equipment) uses it, and it works.

From the single-ended signal ground connect a resistor equal to the output impedance of the signal to the (-) wire of a balanced interconnect. Since the (-) impedance matches the signal (+) impedance both wires will pick up the same noise, and this will be canceled in the balanced input circuitry of the power amp.
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By adding all this junk to the signal path, you significantly defeat the major strengths of the SP-10 and the VT-130.....and this is portrayal of space. These both were at the top of their class in this key area. Keep things simple and you will be rewarded by what these products can do.
"The VT-130SE does not have a phase splitter and requires a true balanced input to operate correctly."

It will operate "correctly" but not "optimally" in conjunction with a single-ended source. With an SE source and an RCA/XLR adaptor, one phase of balanced circuitry floats unused.

BTW, VT130 is a much better amp with Infinicap or similar upgraded coupling caps.

With a single ended input through an XLR adaptor the VT -130 SE will produce less than half the power output that it will produce with a true balanced input signal and that only with THD nearly an order of magnitude higher. I'm unsure of the precise reason for this result, but that was the position of the tech dept. at ARC when I posed this question to them some years back. I feel okay about describing such operation as "incorrect" rather than "sub-optimal", but I'm you're certainly entitled to feel differently about that.


With a single ended input through an XLR adaptor the VT -130 SE will produce less than half the power output that it will produce with a true balanced input signal ...
Really? That seems kinda odd. What does power out put have to do with it? The input signal will be half as the negative phase is essentially ground but that just means the line stage needs to provide for greater output for the amp to achieve full power.
Marty, You could be right, but the difference between balanced and unbalanced operation is more typically described in terms of loss of gain(typically 6db) and signal-to-noise ratio, due to sacrifice of common mode noise rejection inherent in balanced circuitry. I would be surprised if one single phase of VT130 used in SE mode has distortion greater than a comparable high-quality single-ended design. However, it is admittedly a waste not to utilize a fully-balanced amplifier to its full capability.

Just quoting ARC as best as I recall. They may have used imprecise language for my benefit, but the conversation definitely was in terms of power output and THD (or possibly just "distortion"). I'm pretty sure that the "less than half" and "order of magnitude" are verbatim from ARC.

Martykl you are correct! When driving an ARC balanced amp with the SP-10 preamp you will find that you are unable to get symmetrical clipping out of the VT-130. At about half the rated output of the amp, one half of the waveform will indeed softly clip and you will be unable to get anymore output out of the amp period. Since the waveform is asymmetrical, i.e. both halves of the waveform are not clipping at the same point, distortion products go way up because no distortion cancellation takes place. Since the amp puts out approx. 122 Wrms across 8 ohms,( i just looked at one) you would probably get about 55 Wrms. BTW if the SP-10 has not been looked at for 10-15 yrs., there are a few filter caps in the power supply that tend to fail. Look at the end gaskets on the caps with leads that come out of each end of the body of the cap (axial caps) and look for bloating or an actual hole where the cap may be venting. I believe they are located near the power transformer just to the right as you are facing the front face plate, and then look slightly below those caps as well. Hopefully, my memory serves me right. If any of the caps look like that, it's time for service. G
Will these problems occur with any fully balanced amp or preamp used with an XLR/RCA converter, or only ARC?
Can't answer that as question as I don't know other manufacturers circuit topology. Even with an XLR/RCA converter
you don't have the inverting half of the balanced input so unless the input circuit can compensate for that, your not going to get proper output. This is true with ARC.
The input sensitivity of both the VT130 & VTM 200 are much different than the D115. Level matching may be an issue. Why not just buy another D115? If you need more power, a D250 maybe a better option.
The quasi-balanced interface that I suggested (using a resistor to ground for the (-) wire) gets you the noise pickup reduction benefit of balanced lines. It is appropriate for equipment which is single ended internally, and useful (instead of an RCA/XLR adapter) if the power amp has only balanced input.

If the power amp has internally balanced circuitry, and performs poorly single ended, (as appears in this case) the transformer suggested by Bob_reynolds is the way to go. To describe such a transformer as "junk" reflects ignorance.
Sorry Eldartford, it's not ignorance, but rather observations with my system. I have played the game with trying to use transformers to achieve balanced signals out of DACs. And the result was a loss of low-level detail and truncation of harmonic overtones.

Once a device takes away detail, and I can survive without it, as far as I am concerned, it is junk. For high resolution system, transformers in the signal path absolutely affect the sound. I suspect this would not be observed with a system loaded with other bandaid devices such as equalizers.

Comparing advice from electronics professionals and their theories vs. observing the affects on such system configurations are very different worlds. And as much as a "corrector" may be a temptation because it looks good on paper, it often gets removed from the system. An example of this or me was the Rives PARC.

Rather than trying to tweak the circuit to accommodate a pairing of balance and single-ended devices, my advice is to use products of the same topology.

The advice to hunt down another D115 or D250 is good. Perhaps you might even find the rare and coveted M100s, a mono block version similar to the D115. The SP-10 and these amps are absolutely outstanding products. Don't piss away their musical magic through other attempts at "correction".

To be fair to Jafox, I don't think he was dissing the quality of the devices (including the one I suggested) being recommended for use between the pre and power amps when he used the term "junk". Rather, it seems to me that he was advocating the "simplest signal path" philosophy shared by many hobbyists on this site. Whether you agree or disagree with that position, I'd cut him some slack as I think he meant no disrespect.

Never mind my previous post - Jafox has responded in his own fashion.

Thank you Martykl, yes I am a big advocate of the "simple signal path". As mentioned above, I tried a Rives PARC, against the advice of two A'gon members I highly respect. Yes it did what it was supposed to do, but the "side effects" that they told me would occur, well, they were right on. We know we should listen to those with much experience but often we need to learn the hard way. And I am simply giving a different perspective for Westchr to consider before going down a path of complication vs. choosing the "appropriate" amp/preamp combination.
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Of course there are transformers and there are transformers. Galvanic isolation is a good thing, as is balanced operation--both enabled by coupling transformers. But design & quality vary.
The Jensen transformers are excellent, and priced accordingly.
Twenty seven years ago I was, with my business partner, designing a head amp. At that time I had a meeting with a man named Dene Jensen at his facility. I spoke with him for about an hour. He was the first person to actually design a computer program to simulate various transformer designs and how they would react in a given circuit. The man was brilliant.He was in a wheelchair at the time and was not a well man. He made an impact on me. He was passionate about his research into transformer design and materials. After our meeting I was unconvinced that a transformer could work better than an active circuit for stepping up small signals. I was doing research on moving coil cartridge pre-pre amps, and was considering using his step up transformers. For use as an impedance matching device it is far easier to use a transformer. However, both Dene and I knew the inherent problems associated with transformers and frequency extremes for my application. Although his transformers were revolutionary for what they were capable of doing, you can't defy the laws of physics and frankly I wanted to design a very wide bandwidth device. The point is , in some applications transformers work extremely well. In others...not so well. Everything you do is going to be a compromise. Since there is no such thing as a straight wire with gain the only thing you can do is try it and see if it works. I didn't work. Audio Research designed and sold a single ended to balanced converter that used an IC not a pair of transformers and that was their decision. Was it right or wrong? Who knows.
Hifigeek1... A step-up transformer, such as used with MC cartridges, is a lot different from a 1:1 transformer used for stage-to-stage coupling, which is what we are talking about here. A little circuit with an IC is a lot simpler and cheaper that a Jensen transformer, and no doubt was a good decision by AR. The one big advantage of the transformer is that it is power supply needed.
...And the one big disadvantage of a transformer is phase shift and frequency roll off at the extremes. If the transformer is feeding a poorly designed wide bandwidth circuit then in fact the transformer might actually improve the sound over all. And I do agree, an active circuit is more complicated. But I think you missed my point. The point is that basically to each his own. You can't say you don't like something until you have tried it and it fails in that particular application.
Hifigeek1...Before suggesting that Jensen signal transformers exhibit frequency roll off you really ought to check the specs. I know that high power audio output transformers exhibit roll off (usually corrected by ultralinear feedback) but these low level transformers are a different kettle of fish.

If you comment further, please report the specs you find.
As if these responses are not confusing enough, I'll add my own experiences with transformer coupling. I use a LF-frequency optimized Jensen Isomax between a balanced preamp and a sub plate amp-- which does a great job. On the other hand, the Jensen step-up transformers in my original BAT VK-P10 phono stage sounded mediocre. Similarly, in another high-gain balanced phono preamp, I found Cinemag step-ups were easily surpassed by gain achieved using matched bipolar transistors. Finally, the very expensive Audio Consulting Silver Rock step-ups that I use in the analog section of a modded CDP sound superb-- discernably better than other step ups I tried in that application.

I don't think generalizations about transformers hold up across applications.
Thank you all for your considered responses and suggestions.
Another option, of course, would be to replace the D-115 altogether with an SP-10-compatible amp such as VT 200SE or VM 220's that have singe-ended inputs (as it happens, each of these have both balanced/xlr and RCA inputs).

On that thread, I know the Ref 300 Mk II has balanced inputs only, but does the original Ref 300?
Any other Amp suggestions given I need 200wpc plus to feed MG 20.1's from an SP-10 and both phono & line/cd inputs?