Added an SUT...not sure I understood this

@mulveling 

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 

I’ve already explained to the guy the 3ohm tap on the AU320 has a gain ratio of 36 and the 12ohm ATOC9 will see a load of 36ohms. Furthermore the FET based phono on the SP14 will see 14 volts - this would explain his results - cartridge cant drive 36ohms and the SP14 is probably overloading with 14 volts into a fet. It will sound like c**p.

I doubt there is anything wrong with the AU320 - it is behaving as I would have expected running into the SP14 FET based phono.

Previously he was running the 12ohm ATOC9 into the SP14 loaded down to 40ohms which is too low and probably killing the sound.

With the 40ohm input on the AU320 the gain is 10, the phono sees 4volts which is pretty much bang on where he needs to be.

@billwojo 

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.

I’ve already explained to the guy the 3ohm tap on the AU320 has a gain ratio of 36 and the 12ohm ATOC9 will see a load of 36ohms. Furthermore the FET based phono on the SP14 will see 14 volts - this would explain his results - cartridge cant drive 36ohms and the SP14 is probably overloading with 14 volts into a fet. It will sound like c**p.

I doubt there is anything wrong with the AU320 - it is behaving as I would have expected running into the SP14 FET based phono.

Previously he was running the 12ohm ATOC9 into the SP14 loaded down to 40ohms which is too low and probably killing the sound.

With the 40ohm input on the AU320 the gain is 10, the phono sees 4volts which is pretty much bang on where he needs to be.

@woofhaven1992

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.

I’ve explained to you above why running into the 3 ohm tap doesn’t work - the load is too low for the cartridge and the high gain likely overloads the SP14 phono.

The 40 ohm input on the AU320 should work - your results suggest there could be a dicky joint with your cables or the AU320 input. This is unlikely because both channels would have to have the same fault to explain your results.

You need to check your SP14 measures 47k on the phono input and that you have no other loading resistors in the chain when using the SUT.

The other question I would ask is there any evidence some moron hasn't tried to modify the AU320 - these are generally very reliable units - I have never seen one fail.

Dear friends: Sometimes happens things as what is happening here where several of you keep postng about that SUT when the OP NEEDS NOT any SUT to handle his cartridge.

Even if that vintage entry level SUT been in good shape it only can degrades the cartridge signal with out any quality improvement for the LP sound reproduction.

Well, go on on that " party ".

R.

Well this has been interesting, and now the mystery is solved.  The prize goes to @mulveling for correctly deducing that when ARC last serviced the SP-14, they soldered 100-ohm load resistors in there.  They did know I was using an MC cartridge, but wow it would have been nice if they had said something.

Anyway, this explains a lot.  Not only does it explain why the SUT behaves as it has, it also explains why when not using the SUT, the cart sounds better when I load the tonearm with 22 ohms than when I load it at 20 ohms, which is the spec for this cartridge.

I am definitely not going anywhere near that circuit board with a soldering iron, so there will be no more experimenting with this SUT for me.   I'm going to try 25 ohms on the back of my arm, and if I like that sound, start tracing everything I can to find a way to eliminate that last remaining hum.  Thanks to everyone who contributed.