I'd like to see the equations that determine the voltage increases of transformers of various ratios and what they do to the impedance seen by the cartridge. Any help with that is appreciated. So does a 1:10 transformer make a 0.3mv into a 3mv signal and a 1:20 transformer turn that signal into 6mv?
Yes. Multiply the turns ratio by the cartridge output to get the resulting voltage. The result should fall between 2-6 mV, the happy place for most MM phono sections.
Yes, multiplying the signal voltage by the turns ratio gives the output voltage, but only if the cartridge impedance is near zero and the turns ratio quite low. To get an accurate figure for all cases you need to take into consideration the source impedance of the cartridge and the reflected load impedance which become significant when the source impedance is high and/or the turns ratio is high. For example a transformer with a 1:10 turns ratio and a 5 ohm cartridge will output 9.9 times the cartridge voltage, ie
very nearly 10 times. On the other hand a transformer with a 1:30 turns ratio and a 40 ohm cartridge will not output 30 times the cartridge voltage. The actual output will be only 17 times the cartridge voltage.
I thought using a 1:10 transformer resulted in the cartridge being loaded at 470 ohms when using a 47k ohm MM preamp. Similarly a 1:20 transformer would yield a cartridge load of 235 ohms. If that is the case, where in the circuit are resistors applied to adjust the loading to meet a specific cartridge's needs?
Yes, a 1:10 transformer will present a 470 ohm load. No, a 1:20 transformer will present a 117 ohm load, not a 235 ohm load. The load seen by the cartridge is the load on the secondary divided by the square of the turns ratio.
Adding resistors to manipulate the load seen by the cartridge should be done with care because you can't raise the load impedance, only lower it, and you can't lower it without losing some of the signal and affecting the frequency response. The idea that the cartridge must see a specific load impedance isn't correct.
The Denon DL-S1 that I am using needs a SUT.
The Denon DL-S1 has an output of:
- Output voltage: 0.15mV
- Recommended load impedance: Over 100 ohms
Not many SUT to choose from in today's market.
If you take the recommended load impedance of >100 ohms as gospel you are limited to a maximum turns ratio of 1:20 (or 1:21.7 if you want to be more precise). Unfortunately the DL-S1 has a source impedance of 40 ohms so you won't get 20 times its output from the transformer, only 15 times. That gives you 2.2mV into the MM phonostage. That's just about enough to be ok.
To get the absolute maximum signal voltage you would require a turns ratio of 1:34. However, that still only gives you 17 times the cartridge voltage, so the signal into the MM phonostage would be 2.55mV, so hardly any more than the 1:20 turns ratio would give you. In addition, the recommended load impedance requirement would not be met because the load seen by the cartridge would be only 40 ohms, not 100 ohms.
The only alternative would be to use a headamp instead of a transformer. The Headspace would be one possibility:
http://www.rothwellaudioproducts.co.uk/html/headspace_mc_headamp.html