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.
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. |
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. |
