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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.

However:

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
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woofhaven1992 OP
65 posts
 

@rauliruegas Yes I have been using the SP14 with no SUT for some time, and the sound is great except for a touch of hum just at the level of the noise floor. I decided to fiddle with this SUT as a way of addressing that problem, which otherwise has been  persistent despite lots of attempts to address it, including having ARC work on the pre-amp twice.

You are correct that through the SUT I lose some resolution,  although I was blaming that on the crappy built-in output cable. I figured I could find a way to replace that but maybe I should, as you say, forget it.

mulveling's avatar
mulveling
1,963 posts
 

In my experience it will be a bad result relying on downstream gain (20dB line stage) to make up for the lack of a proper MC headamp or SUT. Such preamps, with a normal MM stage (e.g. 44dB in your case) plus enough line stage gain to "make up" for MC’s needs were very specific to that bygone era. These days having that much gain in a tube line stage (usually from 12AX7 tubes) is just asking for noise issues.

OP, you did the right thing seeking out a SUT to help your MC cartridge, unfortunately I suspect you’re right that something might be wrong with it.

lewm
13,269 posts
 

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.

lewm
13,269 posts
 

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.

vinylzone
288 posts
 

The lower the ohm rating on a tap the higher the gain.  As a rule of thumb, you should use the tap that most closely matches the internal resistance of your phono cartridge.

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