Added an SUT...not sure I understood this

The EAR MC-3 and MC-4 also use that "tap / coil ohms" labeling, and I hate it lol. There is no "standard" for what actual dB gain or ratio these ohm labels correspond to. And they generally assume a traditional iron coil former (efficient), but cartridges with the same coil impedance can have wildly different output levels (or vice versa) depending on design - look at Benz’s LPS (ruby plate former) with 38 ohm coils providing 0.34mV output, versus My Sonic Labs with 1.4 ohm coils for 0.5mV output!

This kind of taps labeling works better for tube speaker amps, because almost everyone has either 4 ohm or 8 ohm speakers, and gain matching isn’t as crucial there (adjust your preamp volume accordingly, to an extent).

For a SUT, labeling either the gain ratios OR dBs is much more useful. On my EAR MC-3, the taps: 4 ohms, 12 ohms, 40 ohms correspond to (I think, roughly): 4 ohms = 28x (+29dB), 12 ohms = 18x (+25dB), and 40 ohms = 10x (+20dB).

I think this tap labeling was used only to help its users avoid the most minimal amount of math calculations: match your cartridge's coil spec to the tap, and hope the SUT designer had your cartridge's relative design / efficiency in mind.

getting back to the OP.... He seems to state that he expects 6-8dB of gain in place of the nearly 20dB that a 1:10 suggests.  He then says he is getting the opposite effect which I take as less gain.  

for the OP.

In sticking with just the 40Ω (1:10) tap.  Does the gain increase, decrease or stay the same compared to the same cartridge directly into the MM input?

dave

@intactaudio Connecting to the 40 ohm tap results in less gain that connecting the cartridge directly to the MM input.  On the front panel of the SUT, there is a selector to bypass the transformer, to use the 40-ohm tap, or to use the 3-ohm tap, and the  gain achieved by each of these is exactly the opposite of what you would expect:

Bypass - loudest, 40-ohm tap - quieter, 3-ohm tap - quieter still.

Sounds like something is wrong with your AU-320. There is a strong probability that the switches are in a need of cleaning, not an easy job.

This will give you an idea of what's inside.

BillWojo

@woofhaven1992

Your observed behavior is entirely consistent with the MM phono input load being much lower than 47K ohms. I know you said it was recently serviced by ARC and therefore should not deviate for spec, but mistakes do happen - it’s possible they sometimes add say a 100 - 500 ohm resistor in there for users who want to direct-connect MC cartridges? Triple-check that your tonearm is free of all the loading resistors you’ve used before.

Perhaps also measure the phono input with a multimeter (make sure your cartridge is disconnected from the chain when measuring). I know we all suspect the SUT is bad, but I’m not sure how a SUT can even go bad such that this negative-gain behavior (with no other significant distortions) is demonstrated. If you can borrow another SUT, that would be a good sanity check right now!