2. amps-preamps
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Distortion with ARC Ref 150 and Maggie 3.7

I have this problem that drive me nuts for quite a while. I purchased a like new fully balanced ARC Ref 150 tubes amp through Audiogon for my single ended only CAT SL1 Ultimate preamp and connected both with a RCA to XLR interconnect. It sounded okay with most recording but has awful distortion with certain recording specifically piano and vocal. Some of this recording happens almost on entire record but some only on certain musical passage. Most of the time with higher pitch or peak of music or higher volume.

For your information I listen to vinyl only most of the time and more on Jazz music. Other component listed as follow:

Turntable: Sota Nova, Tonearm: Origin Live Illustrious, Cartridge: Dynavector XV1-S, Step up transformer: Bob's Device CineMag 1131 (Blue) feeding directly to CAT's own phonostage, Speaker: Magneplanar Magnepan 3.7. Power cords, ICs, Speaker cable, Autoformer: Paul Speltz Anti-Cable.

Trouble shooting which has been done includes: checking preamp tubes condition and checking power amp bias. Since ARC claims their Ref 150 was design for balanced preamp only so I also tested by replacing it with single ended tubes amp but the distortion remain. As for the cartridge I believe I have done the alignment pretty accurate with the Mint's Best Tractor but not very sure with the azimuth.

While tested with my other 2 pair of speakers, one which has higher spec show the same problem while the lower spec one seems get rid of distortion. So I suspected the issue probably was with the new Maggie. Called the dealer and he performed a test with his transistor amp with no distortion at all. So he assumed my Maggie is okay. Is it true that the Maggie only good with transistor amps?

By now it leaves me with total confusion! Sincerely hope fellow audiophile here could give me some advice and save me from this endless misery !

Thanks very much in advance!
pakwong
almarg's avatar
almarg
9,670 posts
 
ZD, yes, very uncharacteristically you are missing something :-)

Yes, there will be a gain difference. But that is not the problem. The problem is that the **output stage** of the amp will not be able to deliver much more than 1/2 of the voltage that it must deliver to reach its maximum rated output power without clipping, and therefore the amp won't be able to deliver much more than 1/4 of its rated power. The reason being that it is designed to be driven by a balanced pair of signals, having equal amplitudes but opposite polarities, but the design of the amp apparently is such that when a single-ended signal is provided to its balanced input, one of those polarities ends up being MIA (missing in action) at the output stage.

BTW, given all of that some may wonder why the OP reported that the problem remained when he substituted a single-ended tube amp for the Ref 150. There are two possibilities that occur to me. Either a second problem has been present which resulted in similar symptoms (as I speculated in my 8-12-14 post), or the substitute tube amp was clipping simply because it was not powerful enough for the particular speakers at the particular volume level.

Best regards,
-- Al
almarg's avatar
almarg
9,670 posts
 
P.S. to my previous post. Think of it this way: As you (ZD) are certainly aware, a fully balanced amp puts out signal voltages on both its + and - output terminals, relative to the amp's ground, those voltages being of equal amplitude and opposite polarity. (In saying that, and in saying what follows, I'm oversimplifying a bit by not addressing the fact that an output transformer is present in the case of this and most other tube amps). The design of the amp in question is apparently such that when it is provided with a single-ended input, one of those two output terminals will be at zero volts relative to the amp's ground (reflecting the fact that zero volts has been substituted for one of the two input signals that would normally be present in the balanced input signal pair). Resulting in the voltage difference between those output terminals being half of what it should be. Resulting in the maximum possible voltage difference between those output terminals (i.e., the clipping point) being half of what it should be, or perhaps a bit more than that due to the lessened amount of current being demanded of its power supply.

Put simply, half of the output stage for each channel isn't being used. Visualize it (conceptually) as two amplifiers for each channel, one for the + and one for the -, with one of them receiving no input, and therefore providing no output.

Best regards,
-- Al
martykl
3,578 posts
 
To rephrase my post:

The OP may or may not have several issues with his system. However, if he's trying to feed a single ended signal into an ARC amp designed for balanced inputs he WILL have an issue very much like the one he describes. Per ARC, he should not drive his amp with anything but a balanced signal.

As a less technical type, I can only gather that the ARC design is unusual. However, I can assure everyone that the relevant ARC amps do, in fact, behave in precisely this fashion.
jtimothya
612 posts
 
I didna read the entire thread but any chance the cable could be wired wrong? RalphK once warned me certain XLR/RCA adaptors are wired incorrectly, eg Cardas leave pin 3 floating.
pakwong OP
38 posts
 
Thanks Al for the insight of single ended and balanced design. I'm sure learn a lot from you. What you've mentioned about substituted amp I used was correct. It was a pair of VTL MB125 which rated 55-watts and 100-watts into 5Ω in triode and tetrode modes respectively. It do sounded louder with the Maggie at lower volume in comparison with the Ref 150 but have the same distortion perhaps due to clipping as you suggested.

For your information I've wrote to ARC, Jensen and SMc. We'll see what the answer's like.

Regards
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