Distortion with ARC Ref 150 and Maggie 3.7

I agree with many of the others that the likeliest cause of the problem is related to cartridge installation and adjustment, including VTF. One thing that hasn't been mentioned is anti-skating. Take a look at the cartridge from the front, while a record is playing, and see if the cantilever is pointed approximately straight ahead, or more precisely, that it is at the same angle as when it is lifted off of the record. If there is a significant deflection of the cantilever to the right or the left while the stylus is in the groove of a rotating record, relative to its rest position, it means (IMO, at least) that anti-skating is significantly misadjusted.

However if further work with the cartridge-related adjustments does not resolve the problem, overloading, as mentioned by others, is also a strong possibility IMO. I say that despite the fact that the SL1 appears to have unusually good overload margin.

08-11-14: Pakwong
Answer to Jfrech:
"Do you have the Bob's devices SUT into the MC or the MM input on your CAT pre?"

CAT SL1 Ultimate MK2's phono stage are thought to be MM only. Is doesn't comes with MM/MC switch.

According to the specs shown here for your particular version of the SL1, its phono stage gain is 47 db. The lowest gain setting of your SUT (which I assume is the setting you are using) is 26 db. The line section of the preamp will add up to another 26 db depending on the setting of the volume control. That all seems very high even for a cartridge rated at 0.3 mv.

So an experiment that would seem to be very much in order would be to see if the distortion still occurs with the SUT removed from the system, even if that results in unacceptable noise levels.

Susceptibility to overload will be further increased (especially when high frequency energy is present at high amplitudes, consistent with your description of the problem) if the cable between your SUT and the preamp is long and/or has high capacitance per unit length. The capacitance of that cable, as seen by the cartridge, will be multiplied (not divided) by the square of the voltage step-up ratio of the SUT. In other words, by 400 times (for the 26 db gain setting). That will raise the amplitude and lower the frequency of the ultrasonic resonant peak in frequency response formed by the interaction of the inductance of the cartridge and the capacitance it is loaded with.

In addition to keeping the cable length between SUT and preamp short, you might try putting a load resistor across either the input or the output of the SUT. I see that the recommended load resistance for your cartridge is specified as ">30 ohms," while the 26 db gain setting of your SUT together with the 47K input impedance of the preamp will result in a load of 118 ohms. You might try something like 50 to 100 ohms across the primary (input) side of the SUT, or 20K to 40K across the secondary (output) side.

One final point: You mentioned that the problem remained when you tried a single-ended tube amp. If that amp provided an XLR input even though its internal signal path is unbalanced, and if you connected the preamp to that amp with the same RCA-to-XLR cable used with your ARC amp, the cable is not ruled out as a suspect. If the unused signal pin on the cable's XLR connector (generally pin 3, although possibly pin 2 in some countries) is not grounded (i.e., connected to pin 1), it could account for the problem. Proper connections within the cable can be easily verified with a multimeter.

Hope that helps. Regards,
-- Al

"It so happens that I have some AQ Columbia ICs with DBS but it just didn't work after I tried them this afternoon. I also tried with different taps with and without the autoformer and it didn't work either."

Is your Columbia balanced or single ended? I know I said to try both if you can, but a balanced cable with an adapter for the preamp connection will most likely be the fix (if the problem is actually cable related.).

Pakwong, using zeros will not help, and could possibly hurt. Tube amps like the ARC Ref 150 work best if the speakers impedance is perfectly flat as a function of frequency response.

Take a look at John Atkinson's bench test results of the Ref 150 and look at his measurements of output power and distortion as a function of speaker impedance. The amp's ability to produce clean power is attenuated if the load impedance deviates from the amp's nominal impedance at the relevant output tap.

This is not a big issue because as Al (Almarg) recently remarked, a rough rule of thumb is that an amp's power output demand is roughly 50/50 at the 300 Hz frequency. Stated differently, one would do better to match the speaker impedance in the low bass/midrange frequencies to the amp's nominal impedance at the relevant output tap.

An example might help. If your Maggies present a flat 4 ohm impedance load as a function of frequency response, then I surmise that the Ref 150 will deliver close to its rated 150 wpc channel with the least distortion ... if the 4 ohm taps are used. By contrast, if the Maggies impedance is between 3 and 5 ohms between 20 Hz and 500 Hz power region, then rises to 8 ohms thereafter, I surmise that the Ref will still do a pretty good job delivering clean power into that load if the 4 ohm taps are used.

OTOH, if the Maggies impedance is between 3 and 5 ohms between 20 Hz and 500 Hz, then rises to 8 and 15 ohms thereafter, I surmise that the Ref ability to deliver clean power may be a tad attenuated if the 8 ohm taps are used. The reason is the amp is being asked to drive a low impedance load in the power range when speaker and amp impedance is not well matched.

Hope this helps. In the end, most tubies like me say, go with what sounds best. Nevertheless, I am a little concerned as to whether the Ref 150 can bring out the best in the Maggies.

I think, you overload your phonsection. Distortion has nothing to do with Antiskate, Azimuth or wrong alignment when you only can hear it in the upper frequencies when the dynamics start to rise. "This" kind of distortion in Phono is normally the result when the SUT is too strong( or the cartridge is too strong for the SUT, you can see it this ways or the other way). 0.3mV MC run normally easy with 60-63dB, sometimes a bit more but not much. Try to loan a SUT with 10dB less, just for a check or when one of your friends owns a cartridge with 0.20mV, try to loan it. Next is the output power from your Preamp to your power amp. When the Pre drives the amp easily, this kind of distortion becomes much more present even at lower volume.

Sorry that I was lazy in my response above. I should have
stated what Al, (as usual, very highly detailed and correct.
We are really lucky to have Al on here. Yeah, I'm a fanboy)
did about the gain(s).

I did look up all the gain stages, but didn't really get
into it before. If you have 26 + 26 + 47 that's 99db of
gain which is WAY TOO HIGH! That really leads me to
determine that the distortion is coming from overloading. I
can almost guarantee that's your problem.

Try taking out the SUT and see what happens.