step up transformers for moving coil cartridges

[Paul Joppa]

There was a recent discussion on the Seduction page, of how to obtain both the desired load impedance for a cartridge, and the desired voltage ratio, at the same time. In that discussion I posted a couple times but had not done a very thorough study of the problem.

Today I spend several hours buried in the theory and I've come to a few conclusions. The main one is that you will usually have to load the cartridge with a resistor, and then use a transformer which presents a higher load impedance to the cartridge. The alternative is to present the cartridge with a high (transformer) impedance. In all cases, the transformer secondary load pretty much has to be that recommended by the manufacturer so that the resonance will be properly damped. You can't get a reduced input impedance by putting a low resistance across the secondary, without giving up a lot of treble response.

I have been looking only at the frequency response fro this. Don't forget, if you load the cartridge with anything close to its own resistance you will reduce the output voltage and will want a higher stepup ratio - twice as great for a true matched resistance.

Here are a few more detailed notes:

If the source (cartridge) resistance is lower than the optimum matched impedance, then the corner frequency drops slightly but the Q becomes strongly dependent on the load resistance, being roughly proportional to the ratio of resistance to optimal resistance for very low source resistances. This is often the case with MC cartidges, so it is important to load the transformer properly.

Sometimes people will attempt to reflect a low impedance load to a cartridge by using a lower transformer load impedance. In the usual situation, the cartridge resistance is lower than the transformer optimum primary impedance, and loading down the secondary will reduce the Q more rapidly than it increases the corner frequency, resulting in a substantial loss of treble.

There are thus only two practical ways to use a low-resistance cartridge with a higher-impedance transformer:

1)Load the transformer properly, and accept that the load presented to the cartridge is higher and will not contribute to cartridge self-damping, or

2)Shunt the cartridge with an appropriate loading resistor, and again load the transformer properly.

In order to match the cartridge's impedance without using a shunt resistor at the transformer primary (as is often desired) you must find a transformer designed to present that impedance to the cartridge.

[xcortes]

Hi Paul,

Thanks the time you're putting into this. I really tried to follow your post but haven't really understood it well. Maybe a numerical example would be very helpful. So let me volunteer my data ;>)

I've been using an EMT TSD 15 cartridge with Peerless 4797 SUTs and a Seduction for close to four years very successfully.

The cartridge has a 24 ohm impedance. The SUTs have two discrete primary windings which are put in parallel for 50 ohms and in series for 200 ohms. Secondary impedance is 60k ohms. I'm almost 100% sure I'm using the 50 ohms primary.

My understanding is that the cartridge is seeing a 39 ohm load.

Would that be considered a good impedance match?

Thanks!

[Paul Joppa]

...
The cartridge has a 24 ohm impedance. The SUTs have two discrete primary windings which are put in parallel for 50 ohms and in series for 200 ohms. Secondary impedance is 60k ohms. I'm almost 100% sure I'm using the 50 ohms primary.

My understanding is that the cartridge is seeing a 39 ohm load.

OK, the transformer is designed for a 60K load and you are using 47K. That will reduce the Q but only slightly. Assuming (for lack of other evidence) that the design "Q" of the low pass response is 0.707 (a Butterworth filter), your load will slightly reduce the Q, maybe to something closer to a Bessel filter - which should give somewhat better transient response at the price of a rolloff that starts a bit sooner. I doubt you'll hear the difference.

The cartridge sees 50 times (47K/60K) which is 39 ohms. Its output will be reduced by 39/(24+39), a factor of 0.62. The stepup ratio is the square root of (60000/50) or 34.6, so the net stepup is 34.6*0.62 or about 21.5.

If you wanted the cartridge to see a lower impedance, perhaps 24 ohms, you would parallel a 62 ohm resistor across the transformer primary. This would further reduce the output, for a net step up of 17.3.

Does that help?

[xcortes]

Does that help?

You bet it does.

I will try those 62 ohms resistors and see what difference do they make.

Thanks Paul!

[Balle Clorin]

http://www.mennovanderveen.nl/mc10/mc-10-en.pdf
Balle

[Paul Joppa]

Balle, thanks for that link - I had not seen that paper before. Vandeveen is one of the very few who consistently puts up real data and real science.