SUT Problem - Help needed

[martinswimmer]

Hi everyone,

I've been having troubles with this home made SUT - I'm using 2 EMI BBC 1:30 input transformers with resistance of 12.47 KOhms and 42.4 Ohms but can't get any sound out of low output (0.2Mv) Entre 1 cartridge.

The cartridge is fine - I'm running it straight into gain adjustable phono stage and it plays great.
The transformers seem fine - I've used them in the past with Denon DL103 and AT-OC05 and 09 and just today I ran the sound out of cassette player into them and checked on multimeter and got correct voltage.

I can't get any measurable signal out of it when connecting this LOMC. Is there anything like not enough current from the cartridge to overpower the impedence of the coils of the SUT or any other aspect that I might be underestimating?

I've got a few other home made SUTs and they all work fine. I've found Tamura input transformers from Otari reel to reel to be particularly good.

Thanks! :)

[anubisgrau]

did you connect correctly?

[martinswimmer]

did you connect correctly?

Yes, connected properly and verified on multimeter that it's working fine with higher output from cassette player.
Uzgred, i ja sam iz Beograda, Dorcol.

[Barry]

Have you connected the output of the SUT to the disc input of an amplifier? Was there no audible output?

Assuming an input impedance of 47KOhm for the disc input, and with the figures you provide for the SUT, the impedance presented to the Entré MC cartridge will be 108.5Ohm. Given the 3.5Ohm coil impedance of the cartridge this loading impedance ought to be quite satisfactory (Entré recommend > 30Ohm).

Again given the coil winding resistances and the nominal turns-ratio of 1:30, the 0.2mV output from the cartridge will be boosted to 4.3mV: an acceptable figure.

How did you measure the winding resistances?

[martinswimmer]

Have you connected the output of the SUT to the disc input of an amplifier? Was there no audible output?

Assuming an input impedance of 47KOhm for the disc input, and with the figures you provide for the SUT, the impedance presented to the Entré MC cartridge will be 108.5Ohm. Given the 3.5Ohm coil impedance of the cartridge this loading impedance ought to be quite satisfactory (Entré recommend > 30Ohm).

Again given the coil winding resistances and the nominal turns-ratio of 1:30, the 0.2mV output from the cartridge will be boosted to 4.3mV: an acceptable figure.

How did you measure the winding resistances?

Hi Barry, I've got it connected to phono stage, but the input impedence is not 47Kohm - i've got the jumpers to present less of a load - around 100 ohms (if I'm correct). I measured winding resistances with a multimeter.
I'll try connecting it directly into an ampifier - phono stage might be tripping or something?

[Barry]

If you're loading the output of your SUT with 100Ohms, I'm not surprised you can't hear anything.

Change the input impedance of your phonostage back to 47KOhm.

[martinswimmer]

If you're loading the output of your SUT with 100Ohms, I'm not surprised you can't hear anything.

Change the input impedance of your phonostage back to 47KOhm.

LOL - thanks, I'll try that.

[disarmamant]

I measured winding resistances with a multimeter.

It’s never a good idea to measure the winding resistance with a multimeter, you could end up magnetising the core of the transformer.


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[RothwellAudio]

Hi Barry, I've got it connected to phono stage, but the input impedence is not 47Kohm - i've got the jumpers to present less of a load - around 100 ohms (if I'm correct).


If you're loading the output of your SUT with 100Ohms, I'm not surprised you can't hear anything.
Change the input impedance of your phonostage back to 47KOhm.
No, I'm not surprised either. 100 ohms on the secondary and a 1:30 turns ratio will present the cartridge with a load of just 0.1 ohms - near as damn it a dead short.


It’s never a good idea to measure the winding resistance with a multimeter, you could end up magnetising the core of the transformer.
Yes, that's true. It's tempting to do because it's an easy way of identifying windings and checking there are no breaks in the windings but it can magnetise the core. Fortunately it's easy to demagnetise the core with a signal generator.

[martinswimmer]

No, I'm not surprised either. 100 ohms on the secondary and a 1:30 turns ratio will present the cartridge with a load of just 0.1 ohms - near as damn it a dead short.


Yes, that's true. It's tempting to do because it's an easy way of identifying windings and checking there are no breaks in the windings but it can magnetise the core. Fortunately it's easy to demagnetise the core with a signal generator.

Thanks for this. Unfortunately, I need to measure resistance to find out what are these transformers.

[Barry]

Actually with the winding resistances you quote, the cartridge will be loaded with 56.5Ohm, so not a short circuit. However the voltage divider formed by the 100Ohm loading of the phonostage with the 12.47KOhm of the secondary winding will mean the 0.2mV output of the cartridge appears as 0.045mV at the phonostage input.

If you use a good quality digital multimeter having a high impedance, so draws very little current, then you might probably get away with using it to check winding resistance, but in general it is wise to be cautious. If you use something like an AVO then you could well easily magnetise the core. Most transformers used as MC cartridge SUTs explicitly state that no DC can be tolerated on the primary windings (Haufe, Lundahl or Cinemags for example).

How is it now behaving with the phonostage set to 47KOhm?

[martinswimmer]

Actually with the winding resistances you quote, the cartridge will be loaded with 56.5Ohm, so not a short circuit. However the voltage divider formed by the 100Ohm loading of the phonostage with the 12.47KOhm of the secondary winding will mean the 0.2mV output of the cartridge appears as 0.045mV at the phonostage input.

If you use a good quality digital multimeter having a high impedance, so draws very little current, then you might probably get away with using it to check winding resistance, but in general it is wise to be cautious. If you use something like an AVO then you could well easily magnetise the core. Most transformers used as MC cartridge SUTs explicitly state that no DC can be tolerated on the primary windings (Haufe, Lundahl or Cinemags for example).

How is it now behaving with the phonostage set to 47KOhm?
Just tried it and works great ;-)
When I said great - I meant it gives me the right gain - I've got terrible ground hum on these transformers. They don't have a grounding wire - but I normally take the ground lead from the 2 screws that are holding the top on. I wonder is there a better way? All other SUTs that I have are as quiet as can possibly be.

[Barry]

How have you wired them up? Is there an 'earth' wire for the transformer cores? Are all the signal ground connections common?

Some photos showing the wiring would be helpful.

[martinswimmer]

How have you wired them up? Is there an 'earth' wire for the transformer cores? Are all the signal ground connections common?

The EMI BBC transformers don't have earth wire. I've just tried the Otari transformer and it's completely quiet, so the hum is from the transformers.

[Barry]

Some photos showing the wiring arrangement would be useful.

[martinswimmer]

Some photos showing the wiring arrangement would be useful.22593

[Barry]

22593

How have you wired the 'flying' leads to the input and output connectors? Do you use them unshielded? Twisting the input and output pairs tightly might help, but ideally the SUTs should be mounted in, or on, a metal box so the wiring is screened/shielded from external RFI, with the metal cans of the SUTs earthed to the metalwork of your phonostage along with the earth wire from your tonearm.

Are there any markings/model number of the EMI units to help us find out what they were designed for?

[martinswimmer]

How have you wired the 'flying' leads to the input and output connectors? Do you use them unshielded? Twisting the input and output pairs tightly might help, but ideally the SUTs should be mounted in, or on, a metal box so the wiring is screened/shielded from external RFI, with the metal cans of the SUTs earthed to the metalwork of your phonostage along with the earth wire from your tonearm.

Are there any markings/model number of the EMI units to help us find out what they were designed for?

The markings are T/5346/P. Maybe not EMI at all? I've mounted them onto a thick piece of wood and I've got RCA connectors and twisted pairs. Separately, I've got 2 wires going from the screws on the chassis of the transformer to the grounding tag on the phono stage. Maybe I should install them into a metal box?

[RothwellAudio]

Actually with the winding resistances you quote, the cartridge will be loaded with 56.5Ohm, so not a short circuit. However the voltage divider formed by the 100Ohm loading of the phonostage with the 12.47KOhm of the secondary winding will mean the 0.2mV output of the cartridge appears as 0.045mV at the phonostage input.

Interesting. How did you arrive at those figures? As a first approximation I ignored the winding resistance but when I include it I get even worse numbers than that.
BTW, those winding resistances seem quite high. Maybe these transformers aren't ideal for use with LOMCs.

[Barry]

Interesting. How did you arrive at those figures? As a first approximation I ignored the winding resistance but when I include it I get even worse numbers than that.
BTW, those winding resistances seem quite high. Maybe these transformers aren't ideal for use with LOMCs.

Yes your right. The actual voltage into the phonostage is lower than I stated, as I failed to take into account the modified source impedance as seen by the phonostage.

So to recap, the load as seen by the cartridge is (100 + 12470)/900 + 42.5 = 56.47Ohm. A potential divider is formed with the 3.5Ohm coil impedance of the cartridge, so the input to the primary of the SUT is 0.2 x 56.47/(56.47 + 3.5) = 0.188mV.

The source impedance as seen by the phonostage is (3.5 + 42.5) x 900 + 12,470 = 53,870 Ohm, and this forms a potential divider with the 100Ohm input impedance of the phonostage. So the voltage at the input of the phonostage is 0.188mV x 30 x 100/(100 + 53,870) = 0.01mV.

I can find no information for any small signal transformer marked as "T/5346/P" (either as an EMI manufactured device or as one made by Partridge (who use a similar designation system)), so cannot verify the values of the winding resistances; but they do seem high. However Martin reports that the SUT worked with his Denon 103 (0.4mV output with 40Ohm coil resistance) and now works with the correct input impedance setting on his phonostage, so the SUT would appear to be adequate for the job, if less than ideal.

[martinswimmer]

Yes your right. The actual voltage into the phonostage is lower than I stated, as I failed to take into account the modified source impedance as seen by the phonostage.

So to recap, the load as seen by the cartridge is (100 + 12470)/900 + 42.5 = 56.47Ohm. A potential divider is formed with the 3.5Ohm coil impedance of the cartridge, so the input to the primary of the SUT is 0.2 x 56.47/(56.47 + 3.5) = 0.188mV.

The source impedance as seen by the phonostage is (3.5 + 42.5) x 900 + 12,470 = 53,870 Ohm, and this forms a potential divider with the 100Ohm input impedance of the phonostage. So the voltage at the input of the phonostage is 0.188mV x 30 x 100/(100 + 53,870) = 0.01mV.

I can find no information for any small signal transformer marked as "T/5346/P" (either as an EMI manufactured device or as one made by Partridge (who use a similar designation system)), so cannot verify the values of the winding resistances; but they do seem high. However Martin reports that the SUT worked with his Denon 103 (0.4mV output with 40Ohm coil resistance) and now works with the correct input impedance setting on his phonostage, so the SUT would appear to be adequate for the job, if less than ideal.


Thanks for this Barry - really helpful. Now another question. Is there any benefit in using a SUT and reducing the gain in the phono stage as opposed to using just the phono stage which has enough gain in max configuration settings? Is there a potential for a further impedence match/mismatch? More calculations to be done?

[Barry]

Thanks for this Barry - really helpful. Now another question. Is there any benefit in using a SUT and reducing the gain in the phono stage as opposed to using just the phono stage which has enough gain in max configuration settings? Is there a potential for a further impedence match/mismatch? More calculations to be done?

Now you're opening a can of worms there Martin. This topic has been discussed many times and caused much heat and division of opinion.

Without wishing to side-step the issue, I would say it come down to individual preference as much does in audio (analogue v. digital sources; valve v. solid state; passive v. active volume control etc.). Why not listen to your Entré both via the SUT and the phonostage set to 47K impedance and something like 3mV sensitivity, and then through the phonostage set to 100Ohm and 0.3mV sensitivity and report back with your findings.

At the end of the day it's your ears.

[martinswimmer]

Fair enough ;-)

[RothwellAudio]

Is there any benefit in using a SUT and reducing the gain in the phono stage as opposed to using just the phono stage which has enough gain in max configuration settings? Is there a potential for a further impedence match/mismatch?


Now you're opening a can of worms there Martin. This topic has been discussed many times and caused much heat and division of opinion.
Without wishing to side-step the issue, I would say it come down to individual preference as much does in audio (analogue v. digital sources; valve v. solid state; passive v. active volume control etc.). Why not listen to your Entré both via the SUT and the phonostage set to 47K impedance and something like 3mV sensitivity, and then through the phonostage set to 100Ohm and 0.3mV sensitivity and report back with your findings.
At the end of the day it's your ears.

Yes, in my experience the difference between a good step-up transformer and a good headamp or solid state mc phonostage is fairly subtle, though I'm sure there are some people who will claim the difference is night and day.
However, in this instance I'm not optimistic that the transformers used are well-suited enough for LOMC cartridges to give a truly hi-fi performance. The resistance of the primary and secondary windings are effectively parasitic elements and in an ideal transformer would be zero, ie the impedance would be all inductance and no resistance. In the real world that isn't possible but the resistance of the primary winding is usually only one or two ohms, not the 42.5 ohms of these transformers. Similarly the resistance of the secondary windings is usually a few hundred ohms whereas these transformers have 12470 ohms. As Barry's calculations show, those resistances merely waste voltage that should be going to the phonostage and in this case they're wasting a lot, even when terminated with the correct 47k rather than 100 ohms. Furthermore, the high resistances and high turns ratio suggest there are a lot of turns on the windings which will have a lot of capacitance, and that isn't good for a broad bandwidth free from ringing.
In short, although these transformers will work I doubt very much that they will be anything like approaching state-of-the-art. It would be interesting to compare the SUT+phonostage set to mm (with 47k input impedance, not 100 ohms) and the phonostage for mc operation. Let us know your findings if you do.

[martinswimmer]

What stops me from experimenting with these further is the ground noise - from memory I was able to reduce that to completely manageable with a careful placement and grounding. I don't have time at the moment to do it properly, so what I did instead is wired up a 1:10 SUT (Tamura TF21019A - Primary DC Resistance = 17 ohms, Secondary D.C. Resistance = 1,700 ohms) - but they are clearly not a good match and I don't have enough gain. I'm running them into a 4W single ended, so back to using MC option on the phono stage. I generally have good experience with SUTs and I'm using them in 2 of my setups, but this is the lowest output MC that I've got and it needs something else.

[RothwellAudio]

A turns ratio of 1:10 would be ok for that cartridge but something a bit higher, up to 1:20, would be better. Above 1:20 you're getting into other problems, so I'd avoid higher turns ratios if possible. Unfortunately, the DC resistances of the windings won't tell you what the turns ratio is.
The can't-go-wrong way to wire up the transformers is the have the L input earth and L output earths connected together, the R input earth and R output earth connected together, and the surrounding metalwork/casework/shield connected to a separate earth post (not connect to either the L or R earths) which goes to the earth point on the phonostage. Ok, maybe it's not quite can't-go-wrong because there will always be unusual setups and exceptions to the rule, but I think that gives you the best chance of trouble-free performance.

[martinswimmer]

Thanks Andrew, much appreciated!