2. analog
  3. 178923-132572

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

Showing 1 response by zazouswing

zazouswing
140 posts
 

Your tx is 1:10. Take what the input resistance is at the phono preamp - it should be 47k, and divide it by 100 (10 squared). That is 470 ohms if you have 47k. This number should be at least 10 times the internal resistance of the cartridge. I suspect the input resistance is less than 47k and you are pinching the current. Remember ohms law - current is a relationship between voltage and resistance. I have several SUTs and it is worth noting a that even when the correct resistance is applied, the apparent loudness does not follow the voltage. It’s misleading. 2.5mv and 5mv just sound the same in terms of loudness.
 

 If the goal here is to chase down a hum, and if the tx solved this, sounds like a grounding issue. Try reworking the grounds. Ground the TT to the phono only. Try a star ground. Try shielded cables. Try shielded cables with the shield only landed on 1 side and switch the side the shield is landed on between the TT and the phono pre. try to disconnect all grounds and use a loose ground wire to touch the outside of the rca connections. Ground hum finds a path of least resistance to ground and goes there. Usually this is whatever has the largest power transformers. 

More to discover