Added an SUT...not sure I understood this

the industry standard of 47kΩ was adopted from the MM cartridge world and applied to the MC realm because is first and foremost "does no harm".... until you throw a SUT into the mix that is.  Once a SUT is added the load the cartridge can see can get vanishingly low to the point where you actually start losing gain as you increase turns ratio.

Most SUTs are meant to drive a 47,000 Ohm load since they are also built to work with a specific cartridge. Generic SUTs (like Jensens) might also be built to drive 47K although with some additional loading to allow it to express the turns ratio correctly with different cartridges (which have different source impedances). Some Jensen transformers are meant to drive lower impedances but IIRC that's still about 6.8KOhms.

Lets get back to the simple question I asked....  If going well below a cartridge manufacturers load is not recommended, how can we reconcile the use of transimpedance amplification or in your case a load that is 1.7X the cartridge internal impedance?

A transimpedance input is a special beast. It requires an opamp and when you set up an opamp circuit with feedback, you get something called a 'virtual ground' where the input signal meets the feedback signal. Its not actually ground ('virtual' means 'almost or nearly as described, but not completely or according to strict definition.'). This is why "virtually 100%" is not in fact 100%...

Anyway, in a transimpedance amp the cartridge is substituted for the input resistor. The significance here is that if the virtual ground were actually ground, the signal going thru the input resistor (or coming from the cartridge) would be snubbed at the virtual ground- the signal would stop dead. But it doesn't!

The lesson here is that virtual ground isn't in fact ground. Thus while its impedance might appear to be quite low, it does not function as if its impedance is actually that low. IOW it does not have all the qualities of an actual ground!

Its confusing yes, but in actuality the cartridge isn't driving an impedance that is at or near ground (which would be 0 Ohms or very nearly that). Its actually driving something quite a lot higher.

The gain of an opamp circuit is defined by the input resistor vs the feedback resistor. For example if the input is 100 Ohms and the feedback is 1000 Ohms, the gain will be 10. Since the cartridge is a variable source impedance from cartridge to cartridge, the gain of the circuit will vary from cartridge to cartridge too- more gain with a lower source impedance, less gain with a higher source impedance. This means the output will not vary that much as you change out the cartridge, but it also means that the circuit inherently has limits (due to the amount of feedback applied) and won't work with every cartridge.

Do you not think the cartridge designer and manufacturer knows what they are doing when they recommend a minimum of 100ohms ?

They know that the designer of the phono preamp may not have taken the RFI generated by the cartridge and tonearm cable into account. The 100 Ohm resistor detunes the resonance that they create and so prevents RFI from messing with the preamp.

If I had to guess that spec is set to assure near maximum output of the cartridge. 

The maximum output of the cartridge will be with little or no load. 47K is the industry standard in this regard and qualifies in this regard.

Again:

getting to the bottom of the hum thing might be a good idea too. SP-14s are older ARC preamps if memory serves (and a quick search says it was introduced in 1989...) - when was the last time the filter capacitors in it were replaced?

If the answer is 'never' then it would be a good idea to get it serviced out. Its old enough that filter caps (including those in the DC filament supply) can be failing. Failing filter caps have a way of eating power transformers in older gear and you really don't want that happening! It smells terrible and you can expect a transformer like that to be really expensive if you can even find it.

@lewm is right and so is @dover . At this point IMO the thing to do would be to get someone to remove those 100 Ohm resistors. SUTs are usually designed to drive 47KOhms (although Jensen makes one designed to drive opamps that expects to see 6.8K).

@woofhaven1992 

Sounds to me as if the SUT is wired backwards. If it were me to verify this I would grab a few adapters and hook it up 'backwards' and see if the gain increases. If it does, I'd send the unit back for repair and if not in warranty, have a local tech fix it.

OTOH getting to the bottom of the hum thing might be a good idea too. SP-14s are older ARC preamps if memory serves (and a quick search says it was introduced in 1989...) - when was the last time the filter capacitors in it were replaced?

If the answer is 'never' then it would be a good idea to get it serviced out. Its old enough that filter caps (including those in the DC filament supply) can be failing. Failing filter caps have a way of eating power transformers in older gear and you really don't want that happening! It smells terrible and you can expect a transformer like that to be really expensive if you can even find it.