Just wanted to reiterate that if designer lays out his signal path to be
more direct to the XLR and the gain is set higher to that output it
will always sound better which is the intention, if the opposite is done
to the RCA out, it will always sound better as well. This statement is false. Balanced and single-ended (RCA) operations are inherently incompatible. So if RCAs are used its not balanced, and if the XLR outputs are used (and the preamp is properly balanced) then the use of the RCA connections will result in a buzz. IOW its one or the other and never both, unless additional active circuitry is used. The gain has nothing to do with it whatsoever. That is saying that to make something sound better, you just make it louder. |
So Ralph, based on your engineering knowledge, why would ARC make those design choices? Can you hazard a guess as to what the most likely reasons would be? My experience tells me that there must be trade-off considerations-at this level of audio, there always are. You are correct. There are several ways to do balanced operation with tubes. If you want to support the balanced standard though, your options become limited because of the low input impedances the standard requires you to be able to drive, and also there is that issue with ignoring ground as I stated in my first post.
In the old days of tubes, an output transformer was employed. That is how my Ampex recorders (which are single-ended internally) drive balanced lines. When transistors came along, and in particular solid state opamps, it became possible to direct-couple the output. But even with solid state, transformers are still in common use even today.
We developed a third means, which is a direct coupled balanced vacuum tube output, for which we also developed a patent. I'm pretty sure ARC didn't have any interest in infringing the patent, using output transformers or a solid state output, so they used the only means left to them, which was to not support the balanced standard. They knew they had to do something because balanced operation offers too many advantages to ignore!
As a result, you can easily hear differences in balanced cables while using their equipment. This is entirely because the balanced standards are not being observed.
So if I understand you correctly, unlike Vladmir Lamm's open admission to having psuedo-balanced XLR inputs on his amps, ARC's Ref Series amps, including my Ref 150se is truly balanced, but deviates from the convention most likely for the reasons you stated. I will leave it for another day and discussion as to whether you are correct that as long as the convention for true balanced is observed, all functioning XLR IC's will sound identical. I just know that with my Ref 6 preamp and Ref 150se amp, the choice of XLR makes a profound difference-I tried Mogami Gold, a custom-made Chris Sommovigo AirWave, a Morrow MA-6, a Harmonic Tech, and then Cardas Clear Beyond and each was markedly different. Only the Cardas gave me repeated goose bumps and chills down my neck and only the Cardas made my speakers disappear in my room. I am not touting Cardas as best, only that Cardas was the best I tried in MY system. |
Its probably not the amp that is causing you to hear the cables so much as the preamp. But there is a recent period of ARC amps that had me scratching my head. I'm not sure which models they are. They were out when Kalvin Dahl (with whom I went to school) was still at ARC (about 2-3 years ago). Apparently the amp has a very low CMRR (Common Mode Rejection Ratio) so it only has balanced inputs. Apparently also if you try to run it single-ended the power goes down and the distortion goes up.
I can't think of a good reason for a low CMRR in a differential amplifier (which is what these amps use). You wind up leaving performance on the table (I've been designing differential circuits since the mid 1980s).
If that kind of amp is used with a preamp of fairly high output impedance as per any of the ARC Ref series, the result will be that cable differences will be heard.
That last bit is the part I don't get- why defeat the purpose of balanced line? At that point you might as well run single-ended, as the ability to run long cables is lost as well.
It is things like this which is (IMO) why the single-ended/balanced debate continues! Think about this for a moment- from whom do you most often hear that saw that 'cables don't make a difference'? Usually its someone with an audio engineering background. Well, most of those people work in pro audio, where balanced lines are used, the balanced standard is observed and cables thus really don't make a difference. But in high end audio, for some reason (my guess is the difficulty) the balanced standards have been largely ignored, so this conversation continues...
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Atmasphere 2-28-2018
Its probably not the amp that is causing you to hear the cables so much as the preamp. But there is a recent period of ARC amps that had me scratching my head. I’m not sure which models they are. They were out when Kalvin Dahl (with whom I went to school) was still at ARC (about 2-3 years ago). Apparently the amp has a very low CMRR (Common Mode Rejection Ratio) so it only has balanced inputs. Apparently also if you try to run it single-ended the power goes down and the distortion goes up.
I can’t think of a good reason for a low CMRR in a differential amplifier (which is what these amps use). You wind up leaving performance on the table (I’ve been designing differential circuits since the mid 1980s). Ralph, I’ve never been able to find a schematic for any of ARC’s amps which only provide balanced inputs, such as the recent Reference series amps, but I’ve had the suspicion that instead of using differential stages they basically have a separate signal path through the amp (up to the output transformer primary) for each of the two signals in the balanced signal pair they receive for each channel. That would be consistent with a very dramatic reduction of power capability as well as an increase in distortion if the amp were to be provided with unbalanced inputs via RCA-to-XLR adapters or adapter cables, as was found to be the case with the Ref 150 used by the OP in this thread we had participated in some time ago. I believe it would also be consistent with low CMRR, due to the gain and other characteristics of the two paths not matching precisely. I have no idea why they might have chosen that kind of approach, and I can’t say for sure that they did, but if I am correct in suspecting that they did so it would seem to explain some or all of the things you mentioned about their amps that you referred to above. Best regards, -- Al |
It depends on who you believe |
