Added an SUT...not sure I understood this

I thought it was ok to mention current driven phono stages after the OP himself mentioned it.

what we found was that the SP14 had a 100 ohm resistance across its phono inputs. That coupled with a 1:10 SUT resulted in the cartridge seeing a 1 (one) ohm load. Which is clearly a bad thing and explains the bad SQ.

Per Dover’s point about the two different AT cartridges, I’ve never understood why the minimum load can be acceptably different for the same cartridge with vs without a SUT in the circuit. (Maybe not applicable here if indeed we’re talking about two different cartridges.) But you see this among those who use for example the DL103.

This is a second take on woofhaven’s post, nothing to do with what Intactaudio posted above, or only tangentially. Woof, you seem to ignore the fact that none of the available current driven phono stages actually presents a "perfect" zero impedance load to the cartridge. Each of the different units has a finite input impedance that is above zero and usually less than 10 ohms. Case in point is my BMC MCCI that I bought recently to experiment with current drive in my home system. (Otherwise, I would have to depend upon others’ opinions, which I don’t find helpful to do.) The MCCI has an input Z stated by the maker to be <3 ohms. I guess that’s why Intactaudio uses that value in his thinking process. In other words, none of these devices is "perfect" when it comes to current drive, because they really cannot be perfect; zero ohms is a short circuit to ground. My experience with two or three very good LOMC cartridges driving the MCCI is that you cannot predict in advance what you’re going to get out of the phono stage in terms of either gain or tonal balance. I’ve had to think of the cartridge and the MCCI as a single device where a major determinant of the output SQ and db is taking place at the interface between the cartridge and the MCCI I/V stage. The cartridge internal impedance, its voltage output (because that affects current output), and probably its inductance along with the ~3ohm input Z and the current amplifying devices of the MCCi would seem to be the important variables. So, there’s no free lunch, and there is nothing inherently superior about current drive vs voltage drive, at least so far in my experience. This is not to say that the results cannot be superb.

In a 30 year old unit, hum is not surprising and it need not have anything to do with the subject. Your logic makes some sense regarding phono stage gain. Good point about phono gain at tape outputs being 46db. The fact remains that a SUT is not needed.

mulveling, Perhaps you know this unit much better than I do.  My info was taken from the "specifications" section of a 1990 review of the SP14, published in TAS. Based on those data, it appeared to me that the phono section alone has a gain of 66db and that the linestage per se adds 20db of gain.  But I certainly could be wrong.  This is from a second source:

• Preamp Output: 2V (50V max)
• Frequency Response: 5Hz to 50kHz
• Distortion: 0.01%
• Gain: 66dB (mm), 20dB (line)
• Input Sensitivity:
• Signal to Noise Ratio: 78dB (mm), 98dB (line)
• Line Output:
• Dimensions: 480 x 134 x 260 mm
• Weight: 5.5 kg
• Year: 1989

Apparently, if you wanted to use this unit with an MM cartridge, you could do so via a selector on the face plate that selects for various levels of attenuation of its phono gain or perhaps of its linestage gain.  Kind of strange. Hence the inclusion of another selector that allows the user to add phono input capacitance.  So if you really really must have a SUT in the signal path with an MC cartridge, you would need the 47K ohm load resistors, not 100 ohms, and you could select for attenuation of phono gain on the front panel. That's a minefield for the novice.

There's nothing per se wrong with having those 100 ohm load resistors on an MC input, most of the time.  So, I would not see the point of removing them except if one wanted to load an MC cartridge with a higher resistance.  The problem seems to have been connecting a SUT to a pair of MC inputs in the first place, unless I have missed something.  I finally broke down and looked up the specs of the SP14. It seems to have only an MC or high gain (66db) pair of phono inputs.  The nominal load is said to have been 47K ohms, but the reviewer (in TAS) says his unit was factory loaded with 100 ohms, like the OP's unit.  So, no SUTs allowed or needed here, hum or no hum.  Especially since the linestage section is said to add 20db of additional gain.

By the way, when we speak of MM inputs vs MC inputs, those are euphemisms for low phono gain (usually 40 to 50db) vs high phono gain (usually >60db). Also with your SUT set at its 40 ohm mode(1:10 ratio), and into those 100 ohm resistors, the load seen by the cartridge is 1 ohm! 

You came here for help, and several knowledgeable people have tried to help. You ought to be open minded about accepting advice, since you clearly do not have an in depth understanding of what you’re doing. There’s nothing wrong with that, unless you continue to insist on many of your misconceptions. Dover pasted the spec sheet here, so you can see that the minimum load R is 100 ohms. At 20 or 22 ohms the cartridge ought to sound weak and dull, because a large fraction of its output would be lost to ground and high frequencies would be rolled off. The fact that you found 100 ohm load resistance in your unit proves you’ve been connecting the SUT to its MC inputs, because there’s no way ARC would have installed 100 ohms at MM inputs. So if you put your SUT away, just directly drive your MC inputs and sayonara. Like Raul said.

Vinylzone, I think you’re correct about the labeling of the SUT inputs, but in this case the best choice is the 40 ohm inputs despite the fact that 12 ohms ( the internal R of the cartridge) is closer to 3 than to 40. That labeling custom can be very misleading. “40 ohms” results in a 1:10 step up, which is what is needed here. The 1:10 voltage step up will give you an input Z of 470 ohms. That would be about 10X the internal R of 40ohms. So the label really only means that pair of inputs is ok for cartridges that have a 40 ohm R or lower than 40 ohms. I am glad that “logic” is no longer used.

Please omit “gel reader “ from my last post. I have no idea where that came from. Should read “47K or higher” load resistor. Nearly all MM phono stages use 47K standard.

Raul makes a good point, but if you’re determined to use a SUT then you must use a low gain phono stage (40 to <50db) with a 47K or gel reader load resistor.

What mulveling said.  There is only one correct way to terminate the SUT, with a 47K ohm load (or higher in some rare cases for special reasons) across the secondaries and input to an MM phono stage.  With a SUT that has a 1:10 turns ratio (like yours), this will have the cartridge "see" a 470 ohm load, which is OK for 99% of LOMC cartridges.  It's really simple.  Don't overthink it.

The cartridge likes to see a load impedance that is at least 10 times its internal impedance or greater.  Your cartridge has an internal impedance of 12 ohms, so you are fine with a 470 ohm load.  (Greater than 10X ratio is acceptable.)  The load seen through a SUT is equal to the value of the phono load resistance (47K ohms) divided by the square of the turns ratio (10-squared = 100).  Hence the cartridge sees 470 ohms (47,000/100).  Your cartridge puts out 0.4mV at the standard stylus velocity.  So the MM phono stage will see 4mV, which is perfect for driving any typical MM phono stage.