Selecting a 1:4 transformer for dynamic mics.

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MaxDM

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Apart from size and humbucking or not, what should be the ideal characteristics of a 1:4 step up to boost a voice coil to modern mic input?
 
Depends on what you plan to feed it into but generally high primary inductance, low leakage inductance and low inter-winding capacitance preferably achieved with a Farady screen all housed in mu metal.

Cheers

Ian

Cheers

Ian
 
Depends on what you plan to feed it into but generally high primary inductance, low leakage inductance and low inter-winding capacitance preferably achieved with a Farady screen all housed in mu metal.

Cheers

Ian

Cheers

Ian
what value range would you consider high primary inductance?

I plan to feed the voice coil of an SM58 into it
 
Apart from size and humbucking or not, what should be the ideal characteristics of a 1:4 step up to boost a voice coil to modern mic input?
The SM58 has an impedance of 300 ohms. With a 1:4 xfmr, it turns into 4.8 kohms.
If you connect it to a typical mic input, the input impedance of about 2 kilohms will choke the mic.
You would need a mic preamp with an input impedance of 10 kilohms minimum.
Even with a 1:2 xfmr, the reflected impedance of 1.2kilohms will struggle.
 
what value range would you consider high primary inductance?
Primary inductance is what limits LF response so you want it to be high enough not to load the microphone at the lowest frequency of interest. As Abbey has said, an SM58 has an approximate impedance of 300 ohms. If you want the response to be no more than 3dB down at 20Hz then the reactive impedance of the primary (2 x PI x f x L) needs to be at least 300 ohms. This means L must be at least 300 / (2 x PI x f) = 2.4 Henries. In practice, good quality microphone transformers will have a primary inductance of 6 Henries or more which will have a 3dB point at close to 8Hz.

Cheers

Ian
 
I am referring to the voice coil of the mic.

I want to substitute the internal transformer.
 
The tranny in there is 1:4 I think. I can measure it.

SM58 used to have a dual impedance model for 50 ohm inputs as well. What does that translate to as source impedance?
 
AMI has the T58 replacement transformer for Shures, they say the capsule coil impedance is 10 - 20 ohms (or that's what the tranny is for at least)
Ah, so I was misinformed.
That would make more sense. With a 1:4 xfmr, the capsule's impedance is multiplied by 16, so 10-20 reflets as 160-320 ohms.
320 ohms is close enough to the specified 300.
 
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So, would a normal 1:4 mic preamp step-up work, or would the low source impedance affect performance negatively?
 
Was there a reason you needed to replace the original Max or are you just chasing better performance
 
So, would a normal 1:4 mic preamp step-up work, or would the low source impedance affect performance negatively?
A standard 1:4 xfmr would have a nominal secondary imedance of 2400-3200 ohms. If you connect it to a standard mic preamp with a nominal impedance of 150-200 ohms, which actual input impedance is about 1500-2000 ohms, there would be about 3-6dB loss.
 
Say we dumped the internal transformer in a 58 , instead used a 48v powered fet inline 20db booster ,would we end up better or worse into the nominal 3kohm mic input ?
 
Say we dumped the internal transformer in a 58 , instead used a 48v powered fet inline 20db booster ,would we end up better or worse into the nominal 3kohm mic input ?
The frequency response may be a little more extended, at both HF and LF, but the overall noise factor may be poorer. It depends on the quality of the mic pre. If it is very quiet, the booster will be noisier.
 
Was there a reason you needed to replace the original Max or are you just chasing better performance
the original makes the mic sound distorted. I didn't try loading it with a 600 ohm resistor, which some people say makes the transformer sound better. I hadn't thought about loading it, before I removed the transformer, but I'm not sure that would help.

To me it sounds crappy.

Without transformer it sounds a lot more usable to me.

So I thought maybe a different TX would improve the situation.
 
the original makes the mic sound distorted.
Are you sure about distortion? The SM58 is known for its aggressive frequency response, that helps it cut through a busy live rig, but tends to emphasize nasals and fricative, in a way that may be perceived sa distortion. The SM58 is not a studio mic.
I didn't try loading it with a 600 ohm resistor, which some people say makes the transformer sound better. I hadn't thought about loading it, before I removed the transformer, but I'm not sure that would help.
Loading takes off some of the high midrange but will not turn it in a contender for a Sennheise 441 or even an SM7.
To me it sounds crappy.
That's what many people think, and what has made it a staple of any live situation... :)
Without transformer it sounds a lot more usable to me.
It may be the case, not everybody would agree. It will never be a good mic for picking up acoustic instruments, transformer or not.
But you may want to check your mic against another 58; maybe yours is defective?
So I thought maybe a different TX would improve the situation.
Considering the cost a a new decent xfmr, you'd be better off buying a good specialized mic.
 
Are you sure about distortion? The SM58 is known for its aggressive frequency response, that helps it cut through a busy live rig, but tends to emphasize nasals and fricative, in a way that may be perceived sa distortion. The SM58 is not a studio mic.

Loading takes off some of the high midrange but will not turn it in a contender for a Sennheise 441 or even an SM7.

That's what many people think, and what has made it a staple of any live situation... :)

It may be the case, not everybody would agree. It will never be a good mic for picking up acoustic instruments, transformer or not.
But you may want to check your mic against another 58; maybe yours is defective?

Considering the cost a a new decent xfmr, you'd be better off buying a good specialized mic.

By distorted, I don't mean heavy distortion, I mean the kind that you get from cheap transformers.. soft, tizzy top, lack of bass etc.

I have different kinds of 1 to 4 transformers, and I will check them out.

My question was related to the fact that they are all meant for 150-200 ohm input, and I suppose in theory 30 ohms would create a top-end rise and maybe a hump in the bottom of the freq. response? but, I just was curious if there would be any major tech issues.

By the way, all SM58's sound similar to me, at least the modern ones.

The sound does not get as good as a sm7, but it gets closer, IMO..

 

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