Transformer diagram question

CJ · Feb 10, 2012

power transformers are sometimes designed to have leakage inductance which limits there frequency response,

this cuts out a lot of high end stuff,

experiments are a quick way to determine where to ground the black wire,
build the circuit and try with or without the e shield,

easy to try different grounding points,

you can measure leakage with a bridge by shorting the sec and measuring the pri,

try this while shorting the e shield lead to either side of the primary while measuring the leakage and see if there is a slight change.

i was looking over some MC sites, trying to see if they play games with the shield wire, let me search some more,

i do not think that the grounding point will make That big a difference,

power transformers usually do not have a shield wire, so the shield, if it has one, will be grounded to where the transformer sits, which is ok,

besides cutting down on noise, the shield breaks up the e field of the capacitor formed by the pri-insl-sec, which improves response.

this capacitor reacts to the signal in the transformer at the time,

so if using the shield mainly for freq response, i would ground it at the audio ground,

but again, it won't make that much difference.

Jean Clochet · Feb 10, 2012

CJ said:
power transformers are sometimes designed to have leakage inductance which limits there frequency response,

this cuts out a lot of high end stuff,

Indeed. This can be a valid reason not to use a torroid for a power transformer given they're generally wide-band and can let through more noise from the mains supply.

CJ said:
if using the shield mainly for freq response, i would ground it at the audio ground,

Although I agree it's only slight, it's possible at times to see a measurable difference in response.

Anyway, thanks for your input CJ, I appreciate reading your findings here.
Sorry to Ethervalve for the thread diversion on my part.

Cheers.

CJ · Feb 10, 2012

use that black wire for experiments,

put a scope on the ungrounded shield with music playing thru the transformer,

never tried that experiment,

you might see noise, you might see the music, maybe both?

CJ · Feb 10, 2012

here is a site that has been around a while so you may have seen it aleady,
moving coil transformer options and reviews,

" Some transformers have a separate terminal for the shield in which case this should be connected to a binding post wherein the tonearm cable ground lead should also be attached. Another possible scenario is when the case itself is connected internally to the shield. In this situation you cannot use an all plastic chassis since the case has to be "grounded" onto the metal chassis itself as well to ensure proper shielding."

also a pic of his grounding scheme,

e shield tied to in and out grounds and core and case,

http://members.myactv.net/~je2a3/mic-mcstep-up.htm

here is some more info:

"The first rule of thumb is that most (some exceptions exist) MC cartridges like to see a load impedance of 3X to 6X their output impedance value. This is evident in the example above where the specification states "100 ohms with amplifier". This is most likely the rock bottom input impedance value at 2.5X the output impedance of the cartridge. In this example, our 40-ohm cartridge would have a desired input impedance range of 120 ohms to 240 ohms. This translates to a step-up transformer of approximately 20:1 to 14:1 respectively. This now provides a gauge to evaluate step-up transformers by - namely the step-up ratio (ratio of secondary windings/preamp input side to primary/MC cartridge output side). Be advised that the impedance is transformed by the square of the turns ratio. So a 20:1 step-up transformer provides a 400:1 impedance change. Thus for a 47K preamp input, impedance is transformed to approximately 120 ohms."

and

"Resistor loading or tweaking
Let's say based on all that you have read here so far, you have a particular pair of step-up transformers all picked out, you wired them up according to a suitable turns ratio and the sound is a little bass lean. Reverting back to our previous example, suppose that the step-up device you selected is a well-made 15:1 transformer. This translates (for a 47K preamp input impedance) to 208 ohms on the primary (cartridge output) side. Assuming that you are using the same 40-ohm impedance cartridge, you may wish to experiment with lowering the primary impedance value in order to witness what affect it has on the resultant sound from your system. This can be accomplished by using resistors in parallel across the secondary side of the transformer.

For instance, a 100K resistor placed effectively in parallel with the preamp's 47K input resistor yields a resultant phono input stage impedance of roughly 32K ohms. Using the same step-up transformer, the cartridge now sees 142 ohms instead of 208 ohms. This is referred to as resistor loading the transformer. CAUTION: resistor loading can cause ringing on the signal and may not be a long-term solution. It also does not provide any additional voltage gain and the type of resistors used can effect the overall sonic presentation. However, it does provide the user a much better idea of what that particular cartridge, with their preamp, wants to roughly see as an input impedance.

So if you now liked the sound much better at 142 Ohms then at 208 in your setup, then a step-up transformer of 18:1 would be more to your liking and provide slightly more voltage gain than your existing 15:1 transformer. Always remember the old adage "no resistor sounds as good as no resistor at all!"

So does that mean that your 15:1 step-up transformer is now audiophile junk? Not at all. All it means is that in your system, a higher ratio transformer may be a better fit. However, you may enjoy the sound you now have even with the resistors. In fact, you may even find an 18:1 step-up transformer and still prefer the 15:1 unit with resistors. Why? If the new 18:1 step-up transformer is not constructed and/or shielded as well as the 15:1, it may not produce the desired result in your system. However, it is a reasonable expectation that you will be able to find a quality made 18:1 step-up transformer (which doesn't require resistors) that provides superior results in your system.

Grounding
In general, most hum caused by the use of MC cartridges can be traced back to poor grounding techniques. Transformer step-up devices are to be wired such that the windings are in phase and the negative or ground sides of both primary and secondary windings are tied together along with the case and/or core. For convenience, the ground terminal on the step-up transformer box or case is on the output side - closest to the ground lug on the preamp.

Grounding should consist of a single wire going to a known ground point. I like to use a ring terminal lug that fits under the cover plate screw on the outlet box. I always check this connection with an ohmmeter to third wire ground on the outlet as well as other nearby electrical outlets to be certain that proper grounding has been achieved. I then use a star or fan-out technique at the far end of the grounding wire to connect the single point ground to preamp, step-up transformer and turntable."