Step Up Transformers and Loading?

[Spur07]

Hi all,

I recently purchased my first MC cart and I'm looking at SUT's.

I'm gradually starting to understand the theory behind the concept, and from what I've read correct loading is very important to getting good sound, but i was just wondering what people here thought of the idea advocated by Dave Slagle (Intact Audio) that no loading (in the secondary phase) is most preferable? He seems to argue for a bespoke approach where:

"..the basic design procedure takes into account the internal impedance and the output voltage of your cartridge and sizes the turns ratio to match needs of your overall gain structure."

I take it most people using SUT's will be loading them, so I was wondering if anyone has any opinions. Does secondary loading really degrade the sound?

(Apologies to Dave and anyone else if I've grossly misunderstood the theory)

Paul

[Barry]

By altering the turns ratio (that is, the voltage step up ratio) you only have one variable to play with. The impedance the cartridge will "see" will thus be fixed and equal to the input impedance of the preamplifier, or phonostage, (usually 47kOhm) divided by the square of the turns ratio.

Thus, for example, if the turns ratio is 10:1, the output of the cartridge will be increased 10x, and the cartridge will be loaded with 47000/102 = 470 Ohm (ignoring the resistance of the primary and secondary windings). In general cartridges are viewed as voltage sources, so ought to be loaded with at least 10x their internal impedance (i.e. their coil resistance; the cartridge coil inductance is so low it can be ignored). Most MC cartridges have a coil impedance of around 10 Ohm or less (though there are some notable exceptions), so a 470 Ohm loading impedance is too high.

Too high a loading impedance can cause the treble to sound a bit too lively and uncontrolled, too low a loading impedance and the cartridge is damped too heavily and will sound dull.

One aspect of using SUTs, which is rarely mentioned, is the need to locate them close to the preamplifier, and not close to the turntable and arm. This is because it is important that the capacitance of the cable connecting the SUT to the preamp be low (and thus short in length), since it can resonate with the high inductance of the secondary winding. If the cable is short and the capacitance is low the resonant frequency will lay well above the audible frequency range.

[Spur07]

Thanks Barry. The Coil Impedance of my cart is indeed 10 ohms (on the spec sheet) so I guess I'm looking to load to approx 100 ohms? My amp internal loading is 100K so I'll use an online calculator to find the appropriate value in parallel.

Have you ever heard the K&K audio SUT's?

[Barry]

K&K Audio use the well-regarded Lundahl transformers.

Are you sure the imput impedance of the disc input of your preamplifier is 100K ? That is high - most disc inputs with the necessary RIAA equalisation, have an input impedance of ~ 50K Ohm.

Assuming yours does indeed have an input impedance of 100K, then with a 10:1 SUT, for your cartridge to 'see' 100 Ohm at the primary, the secondary needs to loaded with 11.1K Ohm. Call this either 10kOhm or 12kOhm; making the primary impedance 90.9 Ohm or 107.1 Ohm respectively. In practice the actual primary impedance will be a little higher, as the resistances of both the primary and secondary windings needs to be included.

But I wouldn't worry too much about getting the loading impedance to be exactly 10x the cartridge coil resistance. It's only a rule of thumb - and like a lot of things in audio is not a hard and fast rule. Experiment with the loading: you won't cause any damage and you may find a preferred value

To give you a couple of examples: the Denon 103 cartridge has a coil resistance of 40 Ohm, yet works well into 100 Ohm. I'm using a 103 at the moment and have it running into a phonostage presenting a loading impedance of 390 Ohm (that is, ~ 10x the 40 Ohm coil impedance), and it too sounds fine, though I may experiment with reducing this impedance down to ~ 200 Ohm. The second example is the well-regarded EMT cartridge. This has a coil resistance of 24 Ohm, yet EMT's own SUT (the STX20) has a secondary loading resistor of 22K, so that with a preamplifier of input impedance 47K, the cartridge loading is 168 Ohm (including the winding resistances), and the EMT equalisation amplifiers present a (switchable) loading impedance of 246 Ohm or 181.2 Ohm with the EMT 155 amplifier, and 180 Ohm with the EMT 153 amplifier.

So my advice would be for you to use a 10:1 SUT and load the secondary windings with 12K Ohm resistors. After that you can always experiment with other, higher values.

By the way, what exactly is the MC cartridge you will be using?

[Spur07]

Thanks. Just about getting to grips with it now. I guess it's a case of experimentation with loading - have you any preference for brand/type of resistor Barry. Hifi collective have a big selection.

It's an Audio Technica AT-33 PGT/II

Output is 0.3mV

Sounds nice into my active pre, but thought I might check out the world of SUT

[Barry]

Sorry Paul, I have only just noticed your question.

I would first find out what value of shunt resistor you need. And to do that, I would use some inexpensive metal film resistors. Once you have the optimum value (as far you ears are concerned), then you are free to experiment with different resistor types and from different manufacturers. Don't however use wire-wound types; they will be inductive, and I would avoid bulk carbon rod types; they are too noisy.

I wouldn't presume to suggest any parlicular one, but there are some enthusiasts who are adamant some resistors sound better than others.

Some you might like to consider are those made by Holco, Resista/Roederstein, Shinkoh and Vishay bulk-foil. The latter are very expensive but are considered by many to be the best. I have used Z-film types, which are similar to the Vishay, but you will have to choose.

Hi Fi Collective sell some of the types I have listed. http://www.hificollective.co.uk/components/resistors.html

[awkwardbydesign]

To give you a couple of examples: the Denon 103 cartridge has a coil resistance of 40 Ohm, yet works well into 100 Ohm. I'm using a 103 at the moment and have it running into a phonostage presenting a loading impedance of 390 Ohm (that is, ~ 10x the 40 Ohm coil impedance), and it too sounds fine, though I may experiment with reducing this impedance down to ~ 200 Ohm. The second example is the well-regarded EMT cartridge. This has a coil resistance of 24 Ohm, yet EMT's own SUT (the STX20) has a secondary loading resistor of 22K, so that with a preamplifier of input impedance 47K, the cartridge loading is 168 Ohm (including the winding resistances), and the EMT equalisation amplifiers present a (switchable) loading impedance of 246 Ohm or 181.2 Ohm with the EMT 155 amplifier, and 180 Ohm with the EMT 153 amplifier.

So my advice would be for you to use a 10:1 SUT and load the secondary windings with 12K Ohm resistors. After that you can always experiment with other, higher values.

By the way, what exactly is the MC cartridge you will be using?
Ah, this is helpful. I have an EMT STX20, and want to use it with an SPU. I can't remember offhand which model (I would have to dig it out and check), but it was "repaired" by Van den Hul many years ago. He cut the cantilever and inserted a boron wand with one of his tips! The only time I tried it, it sounded very bright and thin. I seem to remember SPUs like a low loading, so I guess I'll have to check that out now.

[Barry]

Ah, this is helpful. I have an EMT STX20, and want to use it with an SPU. I can't remember offhand which model (I would have to dig it out and check), but it was "repaired" by Van den Hul many years ago. He cut the cantilever and inserted a boron wand with one of his tips! The only time I tried it, it sounded very bright and thin. I seem to remember SPUs like a low loading, so I guess I'll have to check that out now.

Hello Richard,

The early SPUs had a very low coil resistance of ~ 2 Ohm and a very low output. I believe the later designs now have both a higher coil resistance and output.

I would agree with your statement that SPUs like a low loading. Their 2-15K 'in-line' transformer, was designed to match the 2 Ohm cartridge coil resistance to a nominal 15K of a preamp. If we replace the '15K' with one of 47K, then the cartridge will 'see' a load of ~6.3 Ohm. I would have thought 20 Ohm or a little higher would be better.

[Spur07]

Thinking I might try these:

http://www.diyhifisupply.com/opcart/index.php?route=product/product&path=179_120_127&product_id=357

[anubisgrau]

Thanks Barry. The Coil Impedance of my cart is indeed 10 ohms (on the spec sheet) so I guess I'm looking to load to approx 100 ohms? My amp internal loading is 100K so I'll use an online calculator to find the appropriate value in parallel.

Have you ever heard the K&K audio SUT's?

your phono preamp has 100k impedance? that's very strange and unusual, given that 47k is considered to be a standard for phono input.

as for K&K SUTs, they are based around lundahl input transformers. their 19xx transformers are truly magnificent and out there with the best regardless of cost (and these are quite reasonable). i don't know which exactly would suit your AT, but for example LL1943 utterly demolished recently a much hyped dedicated SPU transformer on a test with my SPU spirit (won't mention names here....:)) so if you are handy with solder you can order just transformers (either from K&K or lundahl depending where you live) and box them on your own.

[Spur07]

your phono preamp has 100k impedance? that's very strange and unusual, given that 47k is considered to be a standard for phono input.

as for K&K SUTs, they are based around lundahl input transformers. their 19xx transformers are truly magnificent and out there with the best regardless of cost (and these are quite reasonable). i don't know which exactly would suit your AT, but for example LL1943 utterly demolished recently a much hyped dedicated SPU transformer on a test with my SPU spirit (won't mention names here....:)) so if you are handy with solder you can order just transformers (either from K&K or lundahl depending where you live) and box them on your own.

Thanks Gordan.

I requested the 100k internal resistance in my amp so I could play about with loading above 47K when I was using MM carts.

Does the LL1943 come shielded, or will I need something to box it in? Is there a basic schematic for SUT builds knocking around. I'm guessing something like to the schematic in my link above - in/out/ground/and a resistor after the secondary winding.

[awkwardbydesign]

Hello Richard,

The early SPUs had a very low coil resistance of ~ 2 Ohm and a very low output. I believe the later designs now have both a higher coil resistance and output.

I would agree with your statement that SPUs like a low loading. Their 2-15K 'in-line' transformer, was designed to match the 2 Ohm cartridge coil resistance to a nominal 15K of a preamp. If we replace the '15K' with one of 47K, then the cartridge will 'see' a load of ~6.3 Ohm. I would have thought 20 Ohm or a little higher would be better.
Well, I think my SPU is a GE, 2ohm coil. So with the EMT STX20 (10:1) into a 47k phono stage, what loading would suit?

[anubisgrau]

Thanks Gordan.

I requested the 100k internal resistance in my amp so I could play about with loading above 47K when I was using MM carts.

Does the LL1943 come shielded, or will I need something to box it in? Is there a basic schematic for SUT builds knocking around. I'm guessing something like to the schematic in my link above - in/out/ground/and a resistor after the secondary winding.

yes it's shielded, it comes in a mu metal box. you do need to place them in a box with input and output connectors but it's as prone to hum as possible - one of the most quiet SUTs i tried recently.

you also get connection schematics from lundahl.

you need to examine which of their SUTs suits your situation the best, they have different models. this one is 1:16 and 1:32 ratio, while LL1933 (i think) is 1:8 and 1:16. that may be better for you due to a higher primary impedance.

i didn't hear their amorph core models but frankly i prefer permalloy (like 1943 and 1933), it has more life and joy.

[Spur07]

yes it's shielded, it comes in a mu metal box. you do need to place them in a box with input and output connectors but it's as prone to hum as possible - one of the most quiet SUTs i tried recently.

you also get connection schematics from lundahl.

you need to examine which of their SUTs suits your situation the best, they have different models. this one is 1:16 and 1:32 ratio, while LL1933 (i think) is 1:8 and 1:16. that may be better for you due to a higher primary impedance.

i didn't hear their amorph core models but frankly i prefer permalloy (like 1943 and 1933), it has more life and joy.

Cool. Did you buy direct from Lundahl. I'll have to contact them for a quote. Yeah, I think 1:10 or 1:8 may be better for me. The top line K&K SUT kit is something like $650, I expect I could do it cheaper.

[Barry]

Thinking I might try these:

http://www.diyhifisupply.com/opcart/index.php?route=product/product&path=179_120_127&product_id=357

I haven't heard of Silk transformers and know nothing about them. However the 20:1 version, presenting a load of 25 Ohm would seem to be ideal.

I would recommend you look at Lundhal, Cinemag and Haufe transformers as well. All of these are well regarded.

[anubisgrau]

I haven't heard of Silk transformers and know nothing about them. However the 20:1 version, presenting a load of 25 Ohm would seem to be ideal.

I would recommend you look at Lundhal, Cinemag and Haufe transformers as well. All of these are well regarded.

i owned both cinemag and haufe and lundahl 19xx is seriously step above. i had red cinemags (3440 or something like that) and the only haufe MC SUT you can order from the factory, can't remember the markings. no contest there, lundahl 1943 wins big time.

yes it was purchased direct from lundahl, i think a pair comes to something like 260e + VAT a pair

[AlfaGTV]

I'd recommend the K&K pcb's, even if you dont want to use their complete kit.
Its a lot easier to change ratios and loading. I personally use K&K's pcb and have a Lundahl LL1678 strapped for 1:16 ratio. This works perfect for my Benz Micro Wood SL and Lyra Delos. It also works very well for my Denon DL-103.
I have a two pole, three way switch, where i can select between 85Ohm, 120Ohm and 180Ohm loading.
Now i just need to find a respectable box to put it in! (Right now in a chocolate box!!)
It has got Gotham coaxial shielded cable in very short lengths for connecting to the MM Phono. No hum, excellent audio, good slam and dynamics with excellent sound staging nakes for a very good stepup!
ATB Mike

[albertan]

I also run an Audio Technica AT33PTG11 cartridge. I use an Audio Technica step up transformer. The AT3000T model. The newer AT2000T is lighter and less expensive, but offers the same sonic performance. One of the keys to sut's is to match impedances. This is why I chose the Audio Technica model. It was designed specifically for AT moving coil cartridges. Another over looked facet of sut operation is the importance of the phono lead between the step up transformer and the moving magnet phono stage. It must be a phono lead. It will be handling tiny currents with extreme frequency requirements, and shielding from RFI and other sources of hum is essential. I chose a Furutech model. There are many others but when I hear of nightmarish sut stories this is often the culprit. The Audio Technica step up transformers are quiet and reliable. Well reviewed as well. An excellent .5 metre ground cable is included with Audio Technica AT2000T step up transformers from the factory. Used sut's often have these handy little gems pinched. At least that is what happens when I buy used stuff.

I thought long and hard before I took the plunge. My pre amplifier has two phono inputs. A full featured moving coil set up, and a basic moving magnet section. On paper, the pre amps moving coil section's 66 dB of boost should be louder than my step up (with 24 dB of gain) hooked into the 40 dB gain of my moving magnet section. This is not the case. Because the step up is very quiet , and with the improved dynamics, the 64 dB of gain of my step up hooked into the moving magnet input seems louder and clearer. The 33PTG is not a super expensive cartridge; but it is an excellent cartridge worthy of top shelf tables, tone arms, cables, and step up transformers. It is cheaper to do it right the first time. You are already half way there.

[Spur07]

I also run an Audio Technica AT33PTG11 cartridge. I use an Audio Technica step up transformer. The AT3000T model. The newer AT2000T is lighter and less expensive, but offers the same sonic performance. One of the keys to sut's is to match impedances. This is why I chose the Audio Technica model. It was designed specifically for AT moving coil cartridges. Another over looked facet of sut operation is the importance of the phono lead between the step up transformer and the moving magnet phono stage. It must be a phono lead. It will be handling tiny currents with extreme frequency requirements, and shielding from RFI and other sources of hum is essential. I chose a Furutech model. There are many others but when I hear of nightmarish sut stories this is often the culprit. The Audio Technica step up transformers are quiet and reliable. Well reviewed as well. An excellent .5 metre ground cable is included with Audio Technica AT2000T step up transformers from the factory. Used sut's often have these handy little gems pinched. At least that is what happens when I buy used stuff.

I thought long and hard before I took the plunge. My pre amplifier has two phono inputs. A full featured moving coil set up, and a basic moving magnet section. On paper, the pre amps moving coil section's 66 dB of boost should be louder than my step up (with 24 dB of gain) hooked into the 40 dB gain of my moving magnet section. This is not the case. Because the step up is very quiet , and with the improved dynamics, the 64 dB of gain of my step up hooked into the moving magnet input seems louder and clearer. The 33PTG is not a super expensive cartridge; but it is an excellent cartridge worthy of top shelf tables, tone arms, cables, and step up transformers. It is cheaper to do it right the first time. You are already half way there.

Walter, I'm just curious, what value are you loading your PGT to?

[albertan]

When I am using the pre amplifiers loading resistors I use a 100 ohm load. I could and should play with this, but I am lazy. It works and it's not broken. The load presented to the cartridge from the Audio Technica step up is 185 ohms or so. Using the rule of 10 times the cartridges' resistance gives us 100 ohms. Audio Technica states that a resistance load of at least 100 ohms is required. The technical specifications are somewhat nebulous as I have read from one source that this step up presents a 137.5 ohm load, and another source states that is presents a 187 ohm load to the cartridge. It is more than one hundred ohms, but less than 200 ohms.

I chuckle when I read of these golden ear phono buffs who make a tiny move with resistance loading (or some other existential tweak) and wax poetic about this revelation. An ohm here or there is rarely an earth shattering event.