Added an SUT...not sure I understood this

Why the loading resistor? Plug the SUT into the MM input.

Yes, the standard 47K mm input on the SP14 should work fine.

@noromance the SP14 does not have an MC setting.  The expectation was that one would either just crank up the gain or use SUT or headamp.

@jasonbourne52 I wish I had someone else in my orbit with an SUT to borrow, but my audio clan is very anti-vinyl, so no joy there.  It's good to know my expectations were not crazy.

The AU-320 has a built-in cable for output, and maybe I can find someone who can solder in a set of jacks instead, since the cable is pretty generic.

@dweller the SP14 has a built-in phono stage.

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.

What mulveling said.  There is only one correct way to terminate the SUT, with a 47K ohm load (or higher in some rare cases for special reasons) across the secondaries and input to an MM phono stage.  With a SUT that has a 1:10 turns ratio (like yours), this will have the cartridge "see" a 470 ohm load, which is OK for 99% of LOMC cartridges.  It's really simple.  Don't overthink it.

The cartridge likes to see a load impedance that is at least 10 times its internal impedance or greater.  Your cartridge has an internal impedance of 12 ohms, so you are fine with a 470 ohm load.  (Greater than 10X ratio is acceptable.)  The load seen through a SUT is equal to the value of the phono load resistance (47K ohms) divided by the square of the turns ratio (10-squared = 100).  Hence the cartridge sees 470 ohms (47,000/100).  Your cartridge puts out 0.4mV at the standard stylus velocity.  So the MM phono stage will see 4mV, which is perfect for driving any typical MM phono stage.

Well, as I said, I removed the load resistor when I installed the SUT.  Thus, the SUT is driving my preamp's 47k ohm input impedance without modification.  The pre-amp was just serviced and spec'd out by ARC, so I know there's no chance its impedance or gain are other than as designed.

It sounds like everyone agrees with me on what adding the SUT should result in, both in terms of gain and tonality.  It's just that is not what is happening.  

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. 

@mulveling Thanks.  Clearly there is something wrong with this SUT.  There's no possibility of wiring it backwards since it has a built-in output cable and two sets of RCA jacks for inputs.

You are right about the gain on my line stage.  When playing from a digital source, my light fixtures start rattling before I reach the 10 o'clock position.

Dear @woofhaven1992  : For that cartridge you don't need to use any SUT. The SP 14 spec MC sensitivity is 0.25mv and your cartridge has an output 0.4mv, repeat you don't need any SUT and obviously not that low quality Denon that was a vintage entre level Denon line.

You need to read the SP 14 very carefully to make the rigth set up/gain for your SP 14 stand alone and check for every system link to avoid any hum/noise developed.

You are in the wrong path with a SUT, jus forgeret and focus on your whole system with that cartridge.

Regards and enjoy the MUSIC NOT DISTORTIONS,

R.

@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.

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.

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.

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.

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.

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. 

@woofhaven1992 

Here is what you need to know -

Your Audiotechnica AT-OC9XML has an internal impedance of 12ohms and voltage output of 0.4mv

Therefore it needs to see a load higher than 100 ohms ( as per specs ).

When you ran the OC9 straight into the SP14 with a 40ohm loading resistor you would have strangled the sound.

Now the AU320 step up.

The 3ohms and 40ohms on the front panel refer to the cartridges internal resistance - NOT the loading.

With the 3ohm input, the gain is 36x which means the cartridge will see a load of 36ohms. The SP14 phono will see 14mv. The sound should be awful because the cartridge at 12ohms cannot drive 36ohms, and the FET based front end of your phono is probably overloading.

With the 40ohm input on the AU320 the gain is 10x which means the cartridge will see a load of 470ohms which is fine, the phono input will see 4mv which is perfect.

Adding loading to transformers can be a minefield, therefore I would suggest you run the AU320 into the SP14 phono at 47k without any additional loading resistors .

What I would suggest is get a pair of 470ohm resistors and try running the OC9 straight into your SP14 loaded at 470ohms. This will give you a direct comparison with and without the step up with the same loading. You might find the SP14 phono with the OC9 at the correct loading may be fine. Of course the additional step uphill add gain if you need it.

A Schiit Mani 2 ($150) with its mc gain setting probably will sound better than the SP14 phono/SUT combination.

Vinylzone, I think you’re correct about the labeling of the SUT inputs, but in this case the best choice is the 40 ohm inputs despite the fact that 12 ohms ( the internal R of the cartridge) is closer to 3 than to 40. That labeling custom can be very misleading. “40 ohms” results in a 1:10 step up, which is what is needed here. The 1:10 voltage step up will give you an input Z of 470 ohms. That would be about 10X the internal R of 40ohms. So the label really only means that pair of inputs is ok for cartridges that have a 40 ohm R or lower than 40 ohms. I am glad that “logic” is no longer used.

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.

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.

Sounds like something is wrong with your AU-320. There is a strong probability that the switches are in a need of cleaning, not an easy job.

This will give you an idea of what's inside.

BillWojo

@woofhaven1992

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!

@woofhaven1992 

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.

@atmasphere

I’ve already explained to the guy the 3ohm tap on the AU320 has a gain ratio of 36 and the 12ohm ATOC9 will see a load of 36ohms. Furthermore the FET based phono on the SP14 will see 14 volts - this would explain his results - cartridge cant drive 36ohms and the SP14 is probably overloading with 14 volts into a fet. It will sound like c**p.

I doubt there is anything wrong with the AU320 - it is behaving as I would have expected running into the SP14 FET based phono.

Previously he was running the 12ohm ATOC9 into the SP14 loaded down to 40ohms which is too low and probably killing the sound.

With the 40ohm input on the AU320 the gain is 10, the phono sees 4volts which is pretty much bang on where he needs to be.

@mulveling 

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 

I’ve already explained to the guy the 3ohm tap on the AU320 has a gain ratio of 36 and the 12ohm ATOC9 will see a load of 36ohms. Furthermore the FET based phono on the SP14 will see 14 volts - this would explain his results - cartridge cant drive 36ohms and the SP14 is probably overloading with 14 volts into a fet. It will sound like c**p.

I doubt there is anything wrong with the AU320 - it is behaving as I would have expected running into the SP14 FET based phono.

Previously he was running the 12ohm ATOC9 into the SP14 loaded down to 40ohms which is too low and probably killing the sound.

With the 40ohm input on the AU320 the gain is 10, the phono sees 4volts which is pretty much bang on where he needs to be.

@billwojo 

Sounds like something is wrong with your AU-320. There is a strong probability that the switches are in a need of cleaning, not an easy job.

I’ve already explained to the guy the 3ohm tap on the AU320 has a gain ratio of 36 and the 12ohm ATOC9 will see a load of 36ohms. Furthermore the FET based phono on the SP14 will see 14 volts - this would explain his results - cartridge cant drive 36ohms and the SP14 is probably overloading with 14 volts into a fet. It will sound like c**p.

I doubt there is anything wrong with the AU320 - it is behaving as I would have expected running into the SP14 FET based phono.

Previously he was running the 12ohm ATOC9 into the SP14 loaded down to 40ohms which is too low and probably killing the sound.

With the 40ohm input on the AU320 the gain is 10, the phono sees 4volts which is pretty much bang on where he needs to be.

@woofhaven1992

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.

I’ve explained to you above why running into the 3 ohm tap doesn’t work - the load is too low for the cartridge and the high gain likely overloads the SP14 phono.

The 40 ohm input on the AU320 should work - your results suggest there could be a dicky joint with your cables or the AU320 input. This is unlikely because both channels would have to have the same fault to explain your results.

You need to check your SP14 measures 47k on the phono input and that you have no other loading resistors in the chain when using the SUT.

The other question I would ask is there any evidence some moron hasn't tried to modify the AU320 - these are generally very reliable units - I have never seen one fail.

Dear friends: Sometimes happens things as what is happening here where several of you keep postng about that SUT when the OP NEEDS NOT any SUT to handle his cartridge.

Even if that vintage entry level SUT been in good shape it only can degrades the cartridge signal with out any quality improvement for the LP sound reproduction.

Well, go on on that " party ".

R.

Well this has been interesting, and now the mystery is solved.  The prize goes to @mulveling for correctly deducing that when ARC last serviced the SP-14, they soldered 100-ohm load resistors in there.  They did know I was using an MC cartridge, but wow it would have been nice if they had said something.

Anyway, this explains a lot.  Not only does it explain why the SUT behaves as it has, it also explains why when not using the SUT, the cart sounds better when I load the tonearm with 22 ohms than when I load it at 20 ohms, which is the spec for this cartridge.

I am definitely not going anywhere near that circuit board with a soldering iron, so there will be no more experimenting with this SUT for me.   I'm going to try 25 ohms on the back of my arm, and if I like that sound, start tracing everything I can to find a way to eliminate that last remaining hum.  Thanks to everyone who contributed.

@woofhaven1992

the cart sounds better when I load the tonearm with 22 ohms than when I load it at 20 ohms, which is the spec for this cartridge.

NO

How many times do I have to post. That is not the spec.

The recommended loading for your cartridge is Min 100 ohms.

Min means minimum

It is not 20 ohms

It is not 22 ohms

It is not 40 ohms

Look at the Audio Technics website if you don’t believe me

@dover The sheet that came with my cart says 20 ohms. AT has not been consistent with their specs on the OC9 series.  More importantly, it sounds better around 20 ohms than at 100.  Plenty of people agree with me on this.  I'm not going to load it at 100 just because you won't stop yelling at me.  I'm going to load it at whatever achieves the best sound in my system. 

@woofhaven1992 

The sheet that came with my cart says 20 ohms.

I don't believe this. All OC9's are recommended 100ohm minimum. I used to sell them. If it sounds better at 20 ohms, there must be something wrong with your system.

Perhaps you could post a picture of the sheet you are referring to.

You came here for help, and several knowledgeable people have tried to help. You ought to be open minded about accepting advice, since you clearly do not have an in depth understanding of what you’re doing. There’s nothing wrong with that, unless you continue to insist on many of your misconceptions. Dover pasted the spec sheet here, so you can see that the minimum load R is 100 ohms. At 20 or 22 ohms the cartridge ought to sound weak and dull, because a large fraction of its output would be lost to ground and high frequencies would be rolled off. The fact that you found 100 ohm load resistance in your unit proves you’ve been connecting the SUT to its MC inputs, because there’s no way ARC would have installed 100 ohms at MM inputs. So if you put your SUT away, just directly drive your MC inputs and sayonara. Like Raul said.

By the way, when we speak of MM inputs vs MC inputs, those are euphemisms for low phono gain (usually 40 to 50db) vs high phono gain (usually >60db). Also with your SUT set at its 40 ohm mode(1:10 ratio), and into those 100 ohm resistors, the load seen by the cartridge is 1 ohm! 

@lewm is right and so is @dover . At this point IMO the thing to do would be to get someone to remove those 100 Ohm resistors. SUTs are usually designed to drive 47KOhms (although Jensen makes one designed to drive opamps that expects to see 6.8K).

Dear friends: This is what posted the OP:

"" @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, ....""

So, he already knows that in that way the QS is excellent but that hum. Nothing is perfect and always with trade-offs but that SUT the OP choosed is a bs of SUT, it's a wrong path not only for that cartridge but for any cartridge in any audio system.

In the other side if that hum is at the system noise floor then it's not really a problem to die for.

R.

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.

@woofhaven1992

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:

• Preamp Output: 2V (50V max)
• Frequency Response: 5Hz to 50kHz
• Distortion: 0.01%
• Gain: 66dB (mm), 20dB (line)
• Input Sensitivity:
• Signal to Noise Ratio: 78dB (mm), 98dB (line)
• Line Output:
• Dimensions: 480 x 134 x 260 mm
• Weight: 5.5 kg
• Year: 1989

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.

@lewm From specs listed in the SP-14 Sterephile review:

Phono stage gain: 66dB (to main out), 46dB (to tape-out). Phono sensitivity: 0.25mV at 1kHz for 0.5V output. Line stage gain: 20dB (to main out), 0dB (to tape out). Line sensitivity: 50mV RMS at 1kHz for 0.5V output.

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...