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-   -   How to wire up MC step up transformers (http://www.diyaudio.com/forums/analogue-source/192302-how-wire-up-mc-step-up-transformers.html)

webby 9th July 2011 12:18 PM

How to wire up MC step up transformers
 
3 Attachment(s)
Hi All,
I have just bought a matched pair of vintage Partridge 6:1 977 transformers, and would like to know how to connect them up?

I am using a Cambridge audio 640p phone stage, currently mc input, with at33ptg cartridge that has 0.5mV output.
So i want try the 977 connected to the mm input.

I have a metal boxe and some RCA's, cables etc, and want to know how to connect it all up?
I am unsure of the best earthing method and if i need to add a resister for loading?
The 977's have red and brown connectors, and a wire joining two others. Do i assume the red is signal positive, brown negative?
Do i also earth the transformer cases?

Do i wire the tone arm earth wire, to the step up earth and then the phono stage earth connector, so all three earths are together?

Thanks very much
Ian

kevinkr 10th July 2011 07:46 PM

The diagonal wire is the ground for input and output. I would need to check mine for the input and output pins, but I would NOT use the meter to determine which is which - misconnection will result in no or very little sound, but no harm either. Edit: Checking old photos I'm pretty sure the red pin is the output and the gold/yellow is the input.

Use the ground connection diagonal wire on each transformer just for the specific channel's cartridge ground and pre-amp ground, and that transformer's case - do not tie the grounds from both channels together! And the arm and your case ground can share the phono pre-amp chassis ground connection. The transformer should be isolated from your chassis box to prevent loops.

Do not overheat the connections, I recommend pre-tinning the wires and tack soldering them to the pins. I have used a number of these excellent transformers and have not run into one that is damaged, but it could happen I suppose if sufficiently overheated.

Dead silent with this implementation. Run straight into 47K without any additional load resistors sounds best to me with the DL-103D and Zu DL-103.

webby 11th July 2011 06:56 PM

1 Attachment(s)
Thanks Kevinkr!

I found the ebay seller making a boxed version, so have copied his. The outputs are on the left hand RCA's as you look at the picture, output on the left.
The transformers are touching the case, do you think i need to isolate them, i am having no hum issues at all?
The transformers sound great, even with the same phono stage. a worthwhile investment. These will be fitted to the phono stage i build next.
Any suggestions?
Many Thanks
Ian

kevinkr 11th July 2011 11:40 PM

Quote:

Originally Posted by webby (Post 2635923)
Thanks Kevinkr!

<snip>
The transformers are touching the case, do you think i need to isolate them, i am having no hum issues at all?
<snip>
Many Thanks
Ian

Looks just fine to me as implemented - no ground loops and it is quiet so no issues. I'm very pleased with the pair I own. :D

Note that they should not be placed near any transformers or motors - as well shielded as they are they will pick up fields from either.. (Noting that they are amongst the best shielded transformers I have)

webby 12th July 2011 07:43 PM

many thanks again kevinkr,
If i want to try different loading of these transformers, how do i interpret the following specs for the cartridge.

Coil impedance: 17 Ohms (1kHz)
Direct current resistance: 17 Ohms
Load resistance: 20 Ohms or more
Coil inductance: 70 mu H (1kHz)

I have looked at a few articles and calculators , but am not sure what is eleventh.
I am trying to find out for the valve based phono stage i build next?

Many Thanks
Ian

kevinkr 12th July 2011 08:06 PM

I've run them with 10 - 40 ohm cartridges so far and find the set up sounds best running straight into a 47K load. Noting that these are 1:6 that means the impedance reflected from the secondary load to the primary would be 47K/36 or 1.3K load reflected to the cartridge - which sounds way too high, but in my experience is just fine with this transformer.

If you want a specific R reflected to the primary then just multiply the desired load at the cartridge by 36 to get the resistor value to use instead of 47K. For example if you wanted a 250 ohm cartridge load 250*36 = 8.5K.

I just want to mention that I found no reason to do any of this with any of the cartridges I have tried with the 977, the sound quality did not improve but the gain was slightly reduced, and under more extreme loadings the dynamics were very slightly reduced as well.. Loading the cartridge directly just resulted in reduced gain and dynamics. These transformers are extraordinarily well behaved and don't seem to need any tweaking at all. (Other apparently poorer quality transformers did seem to benefit greatly playing these loading games, but even tweaked did not reach the out_of_the_box performance of the 977.)

webby 12th July 2011 09:26 PM

Wow thanks, they do sound very good.

The recommended loading for this cartridge is 100 ohms, so a 3.6K resistor.
Apparently on this cart below 100 you loose dynamics and a bit of output?

If trying to achieve 100 ohms loading on my current stage do i have to consider the existing 47k on the mm stage, ie the 53 ohms difference?

One phono stage upgrade option is a non diy stage, and i can specify the loading requirements free of charge, so for best results i might as well try and get them optimised.

thanks again for the help.
Cheers
Ian

kevinkr 13th July 2011 12:08 AM

Quote:

Originally Posted by webby (Post 2637096)
Wow thanks, they do sound very good.

The recommended loading for this cartridge is 100 ohms, so a 3.6K resistor.
Apparently on this cart below 100 you loose dynamics and a bit of output?

If trying to achieve 100 ohms loading on my current stage do i have to consider the existing 47k on the mm stage, ie the 53 ohms difference?

One phono stage upgrade option is a non diy stage, and i can specify the loading requirements free of charge, so for best results i might as well try and get them optimised.

thanks again for the help.
Cheers
Ian

The manufacturer's loading recommendation for most of the cartridges I own is 40 - 100 ohms, and I experimented a lot and found in the end that a plain old 47K gives the best results, I think you will probably as well. I just found that no matter what I did it made no audible improvement and beyond a certain point started to squash the dynamics a bit as well..

I don't understand your reference to 53 ohms. :D

Salas 13th July 2011 01:44 AM

Loading with SUT gives different subjective results than directly loading an MC cart with a resistor in my experience. If I pressed the secondary of a Dynavector silver wire SUT with low enough loading to reflect the cart's typical recommended load range to the primary, I could see the THD doubling for example. When the secondary was on 47K the THD dropped and the sound was still very well balanced tonally. Loading with a resistor x10-15 the cart's source resistance was generally optimum in straight input Jfet cascoded head amps that I had made, but with SUTs and tubes it depends. There are termination overshoots to check out on square waves, secondary impedance and current transfer capability that differ for each SUT, Miller of the MM stage, enough stuff. For one without gear like a properly attenuated multifunction HARDWARE generator, FFT, scope, I recommend to load the secondary with 47K, then 10K, and listen. I would expect the higher load to come out best for most carts, SUTs & preamps. Fine tuning can be done for proper square waves by shunting small capacitors for each secondary load trial if able to do such checks.

webby 13th July 2011 07:47 PM

Thanks gurus, but i only have my ears and a multimeter!

Other users of this cart have suggested that 100 ohm loading sounded best, so that is what i am trying to test.

Not sure if they meant 100 ohm loading into a 47k mm stage, primary or secondary?
Do i just put a resister in series on each output?
Cheers
Ian


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