You absolutely need to ensure you’re loading a SUT at 47K. Any less load resistance than this, and you’ll start throwing away gain, as well as encouraging other distortions. Your talk of 40 ohm, 22 ohm resistors makes me nervous you’re adding a heavy load to the SUT. If the SUT ratio is chosen properly for your cartridge, then 9 times out of 10 the natural reflected load from the SUT directly into a 47K ohm MM input works out "fine", with no need for additional loading on its primary or secondary.
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.
Showing 7 responses by mulveling
Make sure your inputs and outputs to the SUT are not reversed! I've done this before. I've also accidentally stacked two SUTs, which actually goes backwards in gain (due to really bad loading). Double and triple check all connections. In your setup, that SUT should give you an extra 20dB of gain which is HUGE and absolutely noticeable. There's no way you should be pegging the volume unless something is seriously wrong. That line stage has 20dB of gain! That's an awful lot. You should be having the opposite problem - you should be able to hit 95dB relatively low on that volume control - like below 12 o'clock, for sure. |
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. |
The EAR MC-3 and MC-4 also use that "tap / coil ohms" labeling, and I hate it lol. There is no "standard" for what actual dB gain or ratio these ohm labels correspond to. And they generally assume a traditional iron coil former (efficient), but cartridges with the same coil impedance can have wildly different output levels (or vice versa) depending on design - look at Benz’s LPS (ruby plate former) with 38 ohm coils providing 0.34mV output, versus My Sonic Labs with 1.4 ohm coils for 0.5mV output! This kind of taps labeling works better for tube speaker amps, because almost everyone has either 4 ohm or 8 ohm speakers, and gain matching isn’t as crucial there (adjust your preamp volume accordingly, to an extent). For a SUT, labeling either the gain ratios OR dBs is much more useful. On my EAR MC-3, the taps: 4 ohms, 12 ohms, 40 ohms correspond to (I think, roughly): 4 ohms = 28x (+29dB), 12 ohms = 18x (+25dB), and 40 ohms = 10x (+20dB). I think this tap labeling was used only to help its users avoid the most minimal amount of math calculations: match your cartridge's coil spec to the tap, and hope the SUT designer had your cartridge's relative design / efficiency in mind. |
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! |
Glad we got that mystery solved! As for the MC vs. MM - perhaps semantics but I wouldn’t consider the SP14 as having an MC stage. An MC stage would have a ~ +20dB headamp or SUT up front *before* the RIAA stage. This SP14 has a 44dB RIAA stage up front, and then a high gain 20dB line stage AFTER this. Maybe on paper that’s fine, but personally I haven’t found good results feeding too low a signal level into an MM RIAA stage and then "making up" for it with line stage gain. This suboptimal arrangement may partially explain why OP liked it with such a low loading impedance (20 ohms, or even less if in parallel with 100 ohms), in an effort to change the sound. I disagree with ARC’s decision to solder 100 ohm resistors on the phono input. That absolutely kills any flexibility. With 47K fixed input, a user could always get a loading kit (like from DB Systems) if they needed to change loading for MC direct-input. Also agree with others - a 12 ohm coil like this AT cart should be loaded at LEAST 100 ohms or greater. As long as this 100 ohm resistor is fixed on the phono input, that means absolutely no SUT, and no additional loading should be used. You’ll start losing dBs and modifying frequency response as you move lower. Personally I'd clip out those 100 ohm resistors and resume using it with the SUT! |
@lewm From specs listed in the SP-14 Sterephile review:
I think the clue is that they list phono stage gain: "66dB (to main out)" BUT only "46 dB (to tape-out)". To me, that indicates they’re adding the line stage’s hearty +20dB gain to get that 66dB figure. If the phono stage were doing 66dB before its line stage, then that should have applied equally to the tape-out. It seems like it was a thing for C-J and ARC full-function preamps of this era: standard MM phono stage, but with GOBS of downstream line-stage gain to cover users who wanted to run MC cartridges anyways. I think I recall seeing spec’d phono stage gain on some of these preamps worded like "including line stage gain". It was a weird era. Fortunately this practice has fallen out of favor, as it’s suboptimal. The extra gain for MC carts should be before the RIAA stage, where it belongs - and not in the line stage, where it just increases noise floor for modern digital sources that are 2V - 4V or more. And I agree with @atmasphere , the hum in direct connection is concerning, and should be looked into! Also the capacitor age, though op did mention it was recently serviced... |