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Author Topic: mic input transformer for passive gain  (Read 2237 times)

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Offline weroflu

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mic input transformer for passive gain
« on: August 16, 2014, 07:14:08 PM »
I'm interested in going from CMBI mics (500 ohms) unbalanced to a mic input transformer and then obtaining an unbalanced signal  from the transformer to go into a pcm-m10 or other unbalanced recorder.



Is this possible to do with no other components

Offline DSatz

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Re: mic input transformer for passive gain
« Reply #1 on: August 16, 2014, 09:04:43 PM »
Yes, with careful selection of a transformer you could feed the CMBIs' signals to the line input of the recorder. But what do you hope to gain (pun not especially intended) from it? I don't immediately see any advantage in such an arrangement.

edited later to add: Input transformers are most useful when connecting balanced microphones to unbalanced inputs. If you had CMC amplifiers (which work with the same capsules as your CMBIs) and you wanted to connect those microphones to the line inputs of a recorder, I would understand that better. But your CMBI microphones have unbalanced outputs, so any connection from them to a pair of transformers would be unbalanced anyway.

It sounds as if you just want a voltage step-up so that you can feed the line inputs rather than the mike inputs of your recorder. But I don't think that will give you much, if any, actual improvement in signal-to-noise ratio; it could even make things worse. And the Sony's mike inputs aren't noisy or hard to drive.

So again I am wondering what this is supposed to be for.

--best regards
« Last Edit: November 26, 2014, 04:27:15 PM by DSatz »
music > microphones > a recorder of some sort

Offline TSNéa

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Re: mic input transformer for passive gain
« Reply #2 on: August 17, 2014, 06:51:38 AM »

Offline weroflu

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Re: mic input transformer for passive gain
« Reply #3 on: August 17, 2014, 10:35:13 AM »
That's how I plan to run them most of the time - as they were designed. And thanks for posting that picture I think I will make a similar cable.

The transformer route has to due with tinkering curiosity, plus I may wind up getting some new preamps in the future and this will allow me to audition different transformer sounds, and yes it is for hopefully slightly higher quality gain than the m10 preamps. It's my understanding that the first gain stage determines the quality of the amplified signal even though it will be hitting the m10 line amp afterward. Noise - not so concerned about it.

So back to the original posting - it's fine to just use one leg of both the primary and secondary of a mic transformer even though they are designed for balanced input/output? and I won't need another stage before the line M10 line input?

1:10 ratio is perfect since I need exactly 20db gain.

Offline DSatz

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Re: mic input transformer for passive gain
« Reply #4 on: August 21, 2014, 10:43:52 PM »
One more thing--kind of a cautionary note. Weroflu is talking about using input transformers to provide a substantial voltage step-up. It should be noted that the greater the step-up, the more critical the impedances on both sides of the transformers become.

1:1 transformers provide no voltage step-up, and they present the microphone with essentially the same load impedance that they themselves are "looking into." Now keep in mind that a typical condenser microphone is designed to drive an input impedance of 1 kOhm or (preferably) greater. If Mr. Flu takes a line-level input with an impedance of, say, 20 kOhms and connects a microphone to it via an input transformer with a 1:10 turns ratio, he'll get a voltage step-up equivalent to 20 dB--but the impedance translation is the square of the turns ratio, i.e. 1:100, so the microphone will then be driving a 200 Ohm load. Depending on the microphones, this will almost certainly cause premature overload--the maximum SPL will be significantly reduced.

Another issue is frequency response and freedom from "ringing". When the transformer's turns ratio is high, the microphone + cable + input transformer part of the circuit becomes very touchy about how it is loaded. It's common for substantial peaks (or occasionally losses) to occur especially at the upper end of the frequency range. It depends to some extent on the precise output impedance of the microphones as well as various inductances and capacitances in the circuit that most of us aren't normally aware of. Some people might mistake this for being "the sound of the transformer," but it is actually the result of creating an inadvertent resonant circuit. -- This can often be compensated for--especially if the result is a peak rather than a rolloff--but first you have to know that it's occurring, which takes careful measurement. And the frequency of the resonance will vary somewhat, depending on the different driving (output) impedances of different types of microphones. So a preamp that once was suitable for many types of microphones may start sounding better, or worse, with some types of microphone than others.

It is very important to keep the cable on the secondary (output) side of the transformers as short as possible. You can't use a three- or five-foot cable there, or you'll start having audible high-frequency losses and the possibility of greatly increased distortion from your microphones, particularly if they themselves contain output transformers.

Finally, the greater the voltage step-up of an input transformer, (all other things being equal) the easier it is to overload and saturate the transformer, particularly at low frequencies. So again a preamp that might have been suitable for use with many different types of microphones before, starts to be less and less appropriate for use with some microphones.

So the turns ratio of input transformers should be no higher than necessary. All in all, people who imagine the voltage gain of a step-up transformer to be some kind of "free lunch" often find that it's not such a good lunch after all, and that it's far from free.

--best regards
« Last Edit: November 26, 2014, 04:38:17 PM by DSatz »
music > microphones > a recorder of some sort

 

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