30/50 ohm mics, how to match to medium impedance
I have several very old mics (ribbon and dynamic) which are 30 or 50 ohm impedance. Most of the mics dont give much output, loss of magnet strength, low eff trannys etc so when connected to 200-600 ohm inputs I have low output and noise. I know they could be improved by re-magnetisation, better trannys etc but cost is an issue. I have some 30ohm to 50K trannys intended for tube circuits which I could build but then noise is a problem OR I could build some FET preamps to match to 50k. I also have some 30/600 ohm to 100k which I could use in the same way or maybe use just the primary windings as an auto transformer 30 in 600 out. I guess I am not the first to have this problem, any ideas welcome. microx
It's not possible to give a full answer to this question without knowing the type of pre-amp you are hoping to use with these low-impedance microphones. You mention "200-600 Ohm inputs", but you don't say whether this is the actual input impedance of the pre-amp or if it's the range of microphone impedance that the pre-amp manufacturer expects his design to be used with. Pre-amps of this type typically have an input impedance of 2K - 5K Ohm, unless as part of the design they have switchable lower-value resistors across the input for applying a damping load to dynamic mics (this includes ribbons). Medium-ratio transformers are the preferred method of connecting low-impedance microphones to this type of input.
However, it is perhaps not that well known that most quality dynamic mics such as the Shure SM57/SM58 already contain internal transformers. The function is to raise the output e.m.f. of the coil or ribbon by the transformer turns ratio at the expense of raising the output impedance (and hence the source Johnson noise) by the square of the turns ratio.
Many pre-amps have a "Hi-Z" input intended for piezo guitar pickups, and, despite usually being single-ended (unbalanced), these can make good inputs for mics fed through high-ratio transformers. These transformers were originally meant for feeding the high input impedance of valve (tube) circuits, so can work well into the Hi-Z inputs provided care is taken to avoid losses at high audio frequencies due to capacitance. This means siting the transformer as close to the Hi-Z input as possible and connecting the two by a very short length of low-capacitance screened cable. The old Reslo "lump in a cord" transformer used to be a common sight at the jack plug end of the cables from their ribbon mics. Of course, a FET buffer sited at the transformer secondary winding would reduce the capacitance problems, but may introduce an unacceptable amount of noise.
Without knowledge of your pre-amps, my advice would be to try the high-ratio transformers feeding the DI or Hi-Z inputs of the pre-amp to see if you get enough gain without losing too much at the top end of the frequency range. If this does not produce an acceptable result, then the 30/600 Ohm autotransformer idea feeding the standard mic inputs would be worth trying, although you might get problems with hum due to unbalancing the circuit.
Thanks for your reply Boswell, when I stated 200-600 ohm input I meant what is said to be suitable by the preamp's maker (Maudio Omni studio) rather than knowing it's true input impedance. I also have a Gyraf G9 which has a Hi-Z input I can use with the transformers, I will try it out. With regard to the auto transformer idea they are 600-30-0-30-600 primary so I could preserve the balanced status. If you can remember Reslo in lines you are maybe my sort of vintage. I sold lots of mics and amps to groups in the Birmingham area in the late 60's, come to think of it I used to make up some boxes with two or three transformers so they could connect multiple mics to the input of the "Watkins Dominator" they all seemed to have! this also avoided the 200kc/s light programme coming over the speakers, happy days. I will try out both options and post my findings.