SUT Problem - Help needed

[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.

DSC05384.jpg

[Barry]

DSC05384.jpg

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.