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DIY SUT for MM phono stage

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Was thinking of buying some partridge MC step up transformers and just sticking them in a little box with some RCA plugs as a cheap and out of sight SUT. Is it just a case of wiring them direct to the RCAs or do I need anything to for any impedance matching? I assume that if I can find a MC cart that requires 300 ohms I should be good to go?

The primary has 20 ohms DC resistance and impedance is 300 ohms.

The secondary has 250 ohms DC resistance and the impedance is 10K ohms.

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300:10k implies a step-up of 1:5.77. I guess it's one of their 1:6 transformers.

You'd probably want a cartridge with a healthy output, around 0.5mV.

The 300 Ohms primary rating is nominal, transformers don't have impedance as such, they simply reflect the impedance you present to the secondary on the primary, scaled by the turns ratio squared. (In this case, the impedance ratio is 1:33)

The transformer you mention will only present 300 Ohms if loaded by 10k, most MM stages present 47k, so your carriage will actually see 1.4k. This is well outside the ideal loading for most cartridges, so either you'll have to add additional resistors to the secondary or primary to get the impedance you want.

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Whilst Pete is technically correct my view is they are pretty good transformers for the money and behave better than expected.

I have been using these Partridge 1:6 transformers for a while now on various cartridges of around 0.5-0.65 mV into a valve phono stage with 47K I/P. Whilst the cartridge load may be more than ideal/normal they have always sounded pretty good and better than some more expensive and higher ratio 1:15 to 1:20 that would provide a better load on the cartridge. I didn't feel the need to add an extra load resistor.

At least you can always add a resistor to the secondary to tune it (which you can't if your load is too low to start with.)

It may be worth buying the ubiquitous croc-clip leads to try it out with different earthing strategies and extra load resistor (variable for trial purposes?)

As a guide I used an aluminium project case form maplin and some solid core silver wire and it sounds pretty good with mimimal hum pick-up.

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Super Wammer

I am mystified by the suggestion to load the secondary to get the load as seen by the cartridge correct. I would much prefer to load the primary, as loading the secondary also loads the transformer, which puts an additional and unnecessary strain on core saturation at LF.

Given that most transformers are designed to see a 47k load, loading the secondary just seems wrong to me.

Any reason why loading the secondary is preferred?

S.

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Super Wammer

Chris - I have a pair of the Partridge 977 1:6 transformers in a box with RCA's you are welcome borrow if you want to try them out. There is an extra set of RCA sockets wired to the primary so you can use loading plugs to match the cartridge.

I personally think they are much better with primary loading. I used them with a Koetsu Black Goldline which has quite a healthy output but needs loading at 100ohms or so. I used 120 ohm resistors in parallel with the reflected load of 1300 ohms which loads the cartridge with 109 ohms.

If you were to load the secondary for a reflected load of 100 ohms you would need a 15K resistor in parallel with your 47K MM stage input impedance resulting in the transformer working into a load of just over 3.5K ohms which IMO attenuates the output of the transformer too much.

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I am mystified by the suggestion to load the secondary to get the load as seen by the cartridge correct. I would much prefer to load the primary, as loading the secondary also loads the transformer, which puts an additional and unnecessary strain on core saturation at LF.

Given that most transformers are designed to see a 47k load, loading the secondary just seems wrong to me.

Any reason why loading the secondary is preferred?

S.

Only that some transformers may ring if the secondary doesn't have some kind of additional load. In general, though, I agree, just load the primary.

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Super Wammer
Only that some transformers may ring if the secondary doesn't have some kind of additional load. In general, though, I agree, just load the primary.

All the MC transformers I have tried ring to some extent when loaded with 47K ohms. Loading the primary to match the cartridge and then loading the secondary with an RC zobel network can tame most of the ringing. However I found that experimentation, and a great deal of patience, is required to optimse the RC network values.

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Super Wammer
All the MC transformers I have tried ring to some extent when loaded with 47K ohms. Loading the primary to match the cartridge and then loading the secondary with an RC zobel network can tame most of the ringing. However I found that experimentation, and a great deal of patience, is required to optimse the RC network values.

However, at what frequency was the ringing? Is ringing at, say, >35kHz really relevant to phono use which doesn't have much above 18kHz?

I agree that transformers need to be correctly terminated, but if the designer has done their job properly, they would design for any resonances to be well above the audio bandwidth.

S.

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Super Wammer

Serge

I never normally look outside the audio bandwidth. I feed the transformer primary with a 1Khz square wave from a 5 ohm source and tweak the zobel network for best response.

Sent from my iPhone using Tapatalk

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Super Wammer
Serge

I never normally look outside the audio bandwidth. I feed the transformer primary with a 1Khz square wave from a 5 ohm source and tweak the zobel network for best response.

Sent from my iPhone using Tapatalk

That's how I would do it, just bear in mind that if your signal generator is half-decent, the square wave rise time will take you well into the ultrasonic region, and your ringing will be well above audio frequencies if the transformer is a good one.

S

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Thanks for the offer but have decided to try a ready made one for now and maybe look at DIY later, just bought some little Ortofon T5's off eBay as they seem to work well with the dl-103 which I've had before and really like.

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Super Wammer
Thanks for the offer but have decided to try a ready made one for now and maybe look at DIY later, just bought some little Ortofon T5's off eBay as they seem to work well with the dl-103 which I've had before and really like.

OK, the offer still stands if you want to try the Partridges with your Denon. I can make some loading plugs to suit.

Is your Denon a DL103 or a DL103R? If it's the DL103 you might find the T5's over damp it a bit, IIRC the T5's have a 1:20 step up ratio so the reflected load may be a little too heavy at 117 ohms.

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Sorry to jump on this post - I've seen these on eBay and they seem good value and I was thinking of trying out a DIY SUT. What would I need for a cart that has an outpost of 0.3mv and a 20 ohm impedance load? Would I need a resistor on the primary winding?

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Super Wammer
Sorry to jump on this post - I've seen these on eBay and they seem good value and I was thinking of trying out a DIY SUT. What would I need for a cart that has an outpost of 0.3mv and a 20 ohm impedance load? Would I need a resistor on the primary winding?

For a 300uV cartridge, you need 10-15x ratio so the output is 3-4.5mV . That will load the following phono stage correctly, whilst still presefving a decent overload margin. I would prefer 10x to 15x for that reason.

A 10x step up will give the cartridge a 470ohm load which is fine for a 20ohm cartridge so no additional resistive loading should be necessary.

S

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