Added an SUT...not sure I understood this

I just added a Denon AU-320 step-up transformer in between my AT-OC9XML cart and my ARC SP-14 preamp.  I am glad that the (relatively quiet) hum that had been present before is now gone...and I mean gone...since that was what motivated me to add an SUT.


I sort of expected that I would also experience a noticeable increase in gain.  Specifically, using the 40-ohm (10X) tap, I would have expected maybe a 6-8 dB increase in volume, and more with the 3 ohm tap.  I am not hearing that, and in fact am getting the opposite effect.  This means I actually have to peg the volume control if I want to achieve 95 dB levels at my listening position, something I rarely, but still occasionally, do.

Also, I removed the 22-ohm loading resistor upon connecting the SUT.  I noticed previously that a 40-ohm loading still had the cart sounding pretty bright.  But with no loading and using the 40-ohm tap, things sound natural.  I sort of expected I was going to need to add a 40-ohm resistor (at the tonearm) to achieve the same loading.

All of this confuses me; I'm happy so far with the sound yet perplexed.  Perhaps some good Samaritan here will be able to explain why I am hearing what I am hearing.  in the meantime, I'm just going to enjoy my quieter background. 

Ag insider logo xs@2xwoofhaven1992

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?



@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.




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!


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.