Transformerless SM58

antichef

antichef

pornk rock
I have a Shure SM58 from which I removed the transformer, attaching the capsule leads directly to the XLR pins. This was my first mic mod ever, and it came out well, which was encouraging.

It sounds kind of cool; it's no SM7b, but it's nice to have around.

Here are my two related questions:

1) will phantom power now hurt the mic, since there's no transformer in between the current and the diaphragm?

2) should it be considered unsafe for live use? It occurs to me that one function of the transformer was to block DC in the event there was a mis-wiring, but obviously that won't happen now either.
 
antichef said:
I have a Shure SM58 from which I removed the transformer, attaching the capsule leads directly to the XLR pins. This was my first mic mod ever, and it came out well, which was encouraging.

It sounds kind of cool; it's no SM7b, but it's nice to have around.

Here are my two related questions:

1) will phantom power now hurt the mic, since there's no transformer in between the current and the diaphragm?

2) should it be considered unsafe for live use? It occurs to me that one function of the transformer was to block DC in the event there was a mis-wiring, but obviously that won't happen now either.
Click to expand...

Yes and yes. You might consider replacing the transformer with a better one.
 
antichef said:
I have a Shure SM58 from which I removed the transformer, attaching the capsule leads directly to the XLR pins. This was my first mic mod ever, and it came out well, which was encouraging.

It sounds kind of cool; it's no SM7b, but it's nice to have around.

Here are my two related questions:

1) will phantom power now hurt the mic, since there's no transformer in between the current and the diaphragm?

2) should it be considered unsafe for live use? It occurs to me that one function of the transformer was to block DC in the event there was a mis-wiring, but obviously that won't happen now either.
Click to expand...
1) NO.
If you wired the 2 wires from the moving coil to pins 2 and 3 of the XLR-3 male and wired the chassis of the microphone to pin 1 of the XLR-3 male. The phantom power is sent to the mic as such:
Pin 3 +48V
Pin 2 +48V
Pin 1 ground.

No current can flow through the moving coil as the potential difference across the wires is zero ie +48 minus +48 =0 volts

2) NO.
It is not unsafe.
 
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I can see that the mic is going to sound "different" sans transformer but since it has a turns ratio of about 1:4 you are loosing 12dB of signal dropping the sensitivity to a depressing -68dB you are almost certainly going to need a Cloudlifter type inline booster for anything other than a rip snorting amp or very loud rock vocalists.

As well as the voltage lift a mic transformer also plays a part in correcting the frequency response of the mic capsule when loaded by 'real world' inputs which, for a 57/58 if often said to be 600 Ohms but that figure is debated.
Input transformers of all sorts also play a big role in keeping RFI out. We have come a long way in proofing transformerless mic inputs but, in extreme cases, a transformer will be better.

Dave.
 
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ecc83 said:
I can see that the mic is going to sound "different" sans transformer but since it has a turns ratio of about 1:4 you are loosing 12dB of signal dropping the sensitivity to a depressing -68dB you are almost certainly going to need a Cloudlifter type inline booster for anything other than a rip snorting amp or very loud rock vocalists.

As well as the voltage lift a mic transformer also plays a part in correcting the frequency response of the mic capsule when loaded by 'real world' inputs which, for a 57/58 if often said to be 600 Ohms but that figure is debated.
Input transformers of all sorts also play a big role in keeping RFI out. We have come a long way in proofing transformerless mic inputs but, in extreme cases, a transformer will be better.

Dave.
Click to expand...
People much smarter than us designed the sm58, and that mic has a decades long positive track record.

It’s my opinion to leave well enough alone and to use it as intended.
 
Dave, you just don't understand that any tweak that you do to a standard microphone will instantly improve it. Anyone can immediately hear the improvements, which probably can't be measured, and even if they measure worse, the measurements are wrong. Change the capsule, and it simply sounds better, swap a capacitor and the harshness instantly vanishes, reform the screen and the heavens part and the light shines through. Adding a transformer is only good if the manufacturer didn't use one in the first place!

Manufacturers ALWAYS use substandard crap so you must change things to make them better.

Its true... I read it on the internet!
 
I sold some of the mics I reviewed and really hated, or were even faulty. It’s fun when the buyers tell you what they’ve done to them and made them wonderful. The nasty sounding capsule is still in there!
 
antichef said:
I have a Shure SM58 from which I removed the transformer, attaching the capsule leads directly to the XLR pins. This was my first mic mod ever, and it came out well, which was encouraging.

It sounds kind of cool; it's no SM7b, but it's nice to have around.

Here are my two related questions:

1) will phantom power now hurt the mic, since there's no transformer in between the current and the diaphragm?

2) should it be considered unsafe for live use? It occurs to me that one function of the transformer was to block DC in the event there was a mis-wiring, but obviously that won't happen now either.
Click to expand...

So basically what you did is turn it into a PG58 which is the same thing without a mic transformer.
Nothing is going to happen when you put a dynamic mic on phantom. Because the circuit is across pins 2&3 on a dynamic mic and pin 1 is just shielding.
Its going to be the same on stage as a PG58.
The only design issues with their capsules is source resistance, so you might want to play with putting a 470-1K resistor across the mic output (pins 2&3) if you want to play with mid-range boosting.
 
TalismanRich said:
Manufacturers ALWAYS use substandard crap so you must change things to make them better.
Click to expand...
Its common for a lot of mass produced corporate stuff. The only problem with shure mics are design based. The SM7 for it to be correct, it needs a dynamic mic step up transformer added. (Which was the common mod way before those cloudlifters were invented. Which the cloudlifter adds noise and another stage to the circuit) The other dynamic mic capsules, have poor source resistance stability which I shared the shunt resistor mod in post number 9.
 
"So basically what you did is turn it into a PG58 which is the same thing without a mic transformer."
Not really because the PG58 uses a 300 Ohm capsule and has a sensitivity of -53dBV/Pa (2.2mV) which is slightly hotter than the specc' usually given for the SM58.
I would also expect the higher impedance of the capsule to cause some response differences and the PG to be slightly more sensitive to pre amp loading?

Dave.
 
If you do this you end up with a 58 that no longer sounds like all the other 58s. If that works for you, it's a useful extra tool in the mic box. I've fiddled with genuine and fake 57's - and created some monsters and some gems. If I put a real one on a snare drum (as I usually do) and it sounds a bit dull, or too cutting - I know exactly which one to swap it for. (two have labels - dull and bright).
 
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