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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
pakwong OP
38 posts
 
Updates:
About one week after using the Jensen PI-2RX, the Ref 150 fuses blow twice. The second time one of the electronic part overheated. I wrote to ARC and here what they answer:

"The overheated part is the turn-on surge resistor, R67. This is a 21.5 ohm / 25W resistor that is designed to cushion the current in-rush at cold turn-on of the REF150. The reason the resistor has overheated is there is a fault that is preventing the RY1 AC power relay from closing after about 1.5 seconds and bypassing this resistor. If the AC relay does not close, R67 will very quickly overheat, as it is not meant to be in-circuit for more than a few seconds. A schematic is attached. Have your technician troubleshoot the soft-start circuit to determine the fault. Note that in 220-240VAC models, this resistor is 21.5 ohms, not the 5 ohms shown on the schematic."

I was asking is this cause by the transformer or due to tubes problem (power tubes about to change) but ARC didn't give any answer.

In the mean time, I got a copy of CAT SL1 Ultimate manual and noticed that there was some instruction on how to connect the preamp properly to a balanced amp as follow:

"When using amplifiers with balanced inputs, the negative leg of the balanced input should be shorted to ground. This prevents hum and noise problems caused by the floating inputs. While most amplifiers with balanced inputs provide a switch for this purpose, inserting a jumper in the XLR connector will accomplish the task. Alternately, an XLR connector can be prepared with this jumper soldered internally, and the connector then inserted into the XLR jack."

If the CAT's wired method for the RCA to XLR cable really works, I think perhaps it's better to stick to the conversion interconnect cable.

Any advice?

Thanks in advance!

almarg's avatar
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almarg
9,670 posts
 
Sorry to hear of this development, Pakwong.

However, I feel quite certain that a problem involving the AC power relay not closing within a few seconds after turn-on has nothing to do with the use of the Jensen signal transformer.

Also, the instructions you quoted from the CAT manual, while being applicable to most balanced amps, are not applicable to yours. As you had found when using the adapter cable, and as has been explained earlier in the thread, using the adapter cable (without the Jensen transformer) sacrifices approximately 75% of the amp's power capability, as well as having various sonic disadvantages.

In fact, although it is probably unlikely, and I'm not specifically familiar with the design, I wouldn't completely rule out the possibility that the amp became susceptible to this problem as a result of having frequently been driven into clipping (i.e., being required to exceed its greatly reduced power capability) when the adapter cable was being used for a prolonged period of time.

In any event, it sounds like either the relay or something in its associated circuit needs to be replaced.

Good luck. Regards,
-- Al
almarg's avatar
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almarg
9,670 posts
 
P.S: To clarify my previous comment about what is stated in the CAT manual, the adapter cable you had been using resulted in the amp being operated in exactly the same manner as what would occur if you were to do what is described in the paragraph you quoted from that manual.

Regards,
-- Al
mapman's avatar
mapman
21,851 posts
 
It may have been covered earlier but was the rating and description of the amp on agon when you bought it consistent with your findings since? If not, it would seem to memaybe you have a case with the seller to either return or seek financial compensation for repairs.

Just a thought.
atmasphere's avatar
atmasphere
11,590 posts
 
As you may have surmised by this time, the ARC amplifier cannot be used with a single-ended source! If you do so, the amp **will not make anywhere near full power** and it will be distorted.

This is because the amplifier has a very poor Common Mode Rejection Ratio figure, on account of its input circuit (which apparently is differential) is not in fact very differential. IOW, IMO/IME, ARC missed a bet here.

At any rate, the only way to use this amplifier with a single-ended source such as the CAT is to use a transformer like you are doing (The Wadia worked of course because it has a balanced output).

There are several problems with this. The Jensen, while excellent, does not load the CAT ideally nor does it have the bandwidth of the CAT, so you are not hearing everything it does. However, unless you sell the CAT and get a balanced preamp, this is the only game in town.

I have to admit I am puzzled why the amplifier is designed in this fashion, but in conversations with the good people at ARC (a good number of whom I have known for decades) this is very much the case (As a manufacturer of balanced line products, we had to sort out compatibility with single-ended equipment many years ago). If I were you I would be seeking a balanced line tube preamp, but that's me: I don't like transformers. Plan B: Sell the amp and get something that works with your CAT. Both units are capable of excellent performance; its really a matter of which you like more. Good Luck- keep us informed.
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