There's nothing per se wrong with having those 100 ohm load resistors on an MC input, most of the time. So, I would not see the point of removing them except if one wanted to load an MC cartridge with a higher resistance. The problem seems to have been connecting a SUT to a pair of MC inputs in the first place, unless I have missed something. I finally broke down and looked up the specs of the SP14. It seems to have only an MC or high gain (66db) pair of phono inputs. The nominal load is said to have been 47K ohms, but the reviewer (in TAS) says his unit was factory loaded with 100 ohms, like the OP's unit. So, no SUTs allowed or needed here, hum or no hum. Especially since the linestage section is said to add 20db of additional gain.
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.
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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! |
@mulveling : " and resume using it with the SUT! "
With all respect certainly you don't know what you are talking about in this OP specific issue.
R. |
mulveling, Perhaps you know this unit much better than I do. My info was taken from the "specifications" section of a 1990 review of the SP14, published in TAS. Based on those data, it appeared to me that the phono section alone has a gain of 66db and that the linestage per se adds 20db of gain. But I certainly could be wrong. This is from a second source:
Apparently, if you wanted to use this unit with an MM cartridge, you could do so via a selector on the face plate that selects for various levels of attenuation of its phono gain or perhaps of its linestage gain. Kind of strange. Hence the inclusion of another selector that allows the user to add phono input capacitance. So if you really really must have a SUT in the signal path with an MC cartridge, you would need the 47K ohm load resistors, not 100 ohms, and you could select for attenuation of phono gain on the front panel. That's a minefield for the novice. |
Again: 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.
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