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Moving Coil Transformer Question
On 27 Jul 2004 23:37:17 GMT, Isaac Wingfield wrote:
In article l21Nc.30404$eM2.8847@attbi_s51, (Fred) wrote: Hello; Several years ago I purchased a custom MC step-up transformer. The unit supplies 26 dB of gain to my 40 ohm cartridge and sounds amazing. Lately, I have been looking to try different cartridges. I notice that many have 3 ohms impedance. Is it possible to use the three ohm cartridge into my transformers? It seems like the cartridge seeing a slighyly higher impedance would not matter. Do the lower impedance cartridges require more gain than what I have available? I appreciate any information regarding this since I do not want to make an expensive purchase from, perhaps, out of my country to only have the cartridge not work with my setup. Trying it to see if it works may or may not be an option - depending where I purchase from. A transformer "looks" a lot like a bandpass filter -- can't pass very low frequencies, can't pass very high ones, passes ones in the middle. Also like a bandpass filter, the source and load impedances "scale" the bandpass corner frequencies. That is, the frequency response is only correct if the impedances are correct. Using it at "wrong" impedances by more than ten to one (3 to 40 ohms) may significantly alter the frequency response. It'll probably be flat in the middle, but the low and high cutoffs may not be what you expect. If you can afford the loss of signal, you could try placing a 33 or 37 ohm resistor in series with your 3 ohm cartridge to see if you can hear the difference in bass and treble response. The old business about "600 ohms for audio signals" was more about maintaining proper frequency response in all the coupling transformers than anything else. Certainly it wasn't to match the transmission line impedance. Isaac If you take a 3 ohm cartridge, and run it into your 40 ohm transformer (with secondary load), it should "see" 40 ohms. I am assuming that the secondary has a proper load resistor. If your cartridge is happy about that 40 ohms, then, as far as frequency response goes, the low frequency response would be improved. (this is by virtue of the hipass formed by the cartridge lower Thevenin resistance, and the primary "magnetizing" inductance.) The high frequencies would not be affected much, since the lopass filter formed by secondary load and leakage inductance is the same in either case. Your problem is not the transformer (in terms of frequence response). The manufacturer of your cartridge should specify a range of load resistances that the cartridge specs are guaranteed at. If 40 ohms is OK, then everything should be fine. My guess is that moving coil transducers have such poor magnetic/mechanical coupling (i.e., a very inefficient generator), that they will tolerate a wide range of load values without much effect. Cartridges that act as a more efficient generator, will be affected by load values. The mechanical response could have various degrees of damping depending on the load. Also, with higher impedance cartridges, the internal inductance can cause frequency response problems if the load is not correct. So you might ask, why not always use a 40 ohm transformer? If the impedance is 3 ohms, it implies a very low voltage from the cartridge. You need as much step-up as possible, and that you will get with a proper 3 ohm transformer. The 3 ohm transformer has about 3 times (+10 db) the voltage gain of the 40 ohm one (square root of 40/3). That will give you a possible improvement over the noise floor. -Paul .................................................. .............. Paul Guy Somewhere in the Nova Scotia fog |
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