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Tape Op SM57 Mod

Discussion in 'Microphones' started by DonnyThompson, Sep 23, 2014.

  1. DonnyThompson

    DonnyThompson Distinguished Member


    Has anyone tried this mod?

    It's a simple mod - basically, you're just removing the transformer from the XLR barrel housing of the mic by boiling (yes, I said boiling) the housing of the mic ( and of course you have removed the XLR jack and the capsule from the mic first...) which softens up the glue that holds the transformer in place.

    With the glue soft from the heat of boiling, you remove the transformer.

    After the mic has cooled, you connect the cables from the XLR jack directly to the capsule, with no transformer in between.

    I've read mostly positive reviews; the most common comments were that the modded mic had a much more pronounced low end, and a slight to moderate decreased mid range; the trade off is that the mic's output is attenuated by around - 20db, so you'll need a nice, beefy pre to gain it back up for recording (this mod isn't recommended for SM57's that are used for live performance).

    Most who have performed this process say that what you end up with tonally is essentially that of an SM7.

    I'm curious to see if anyone here has tried it, and if so, what your thoughts/observations are...?

    kmetal likes this.
  2. Torsten Borg

    Torsten Borg Active Member


    I have never tried it but it seems like an awesome mod.

    I looked around YouTube for some audio examples and found a few videos and this was probably the best one.

    I didn't find a video that did a comparison between the modded SM57 and an SM7. It would've been interesting to hear how close the modded SM57 got to it's big brother.

    Anyways, the only downside to this is if you would turn on the phantom power by mistake, because then it would probably break when there's no transformer, am i right?

  3. Boswell

    Boswell Moderator Distinguished Member

    I've never modded any of my own 57s/58s, but I have used a 57 that someone else had butchered. The owner lent it to me saying "this is a ribbon mic with a circular ribbon". Having tried it, I wouldn't put it quite like that, but it was certainly different from the stock 57s, not least in needing a lot of gain from the pre-amp. I would also not have said it sounded like an SM7. My feeling is that it was under-damped on transients, but I didn't have it long enough to try loading it with different damping resistors in the 30 - 60 Ohm region.

    Removing the transformer makes no difference to the phantom power tolerance. A transformer-free 57 is just like a standard 57, i.e. perfectly happy with standard phantom power present. There might be some differences if there were a fault, e.g. one of the signal conductors shorted to ground, but the 7mA short-circuit current is not going to burn out the coil any more than it would burn out the secondary winding of the transformer.

    One drawback of doing this mod is that I'm not sure that a pair of transformerless SM57s could be used as headphones in an emergency.
  4. DonnyThompson

    DonnyThompson Distinguished Member

    I've got more than a few old 57's laying around, I might sacrifice one of them and try this mod over the weekend. Stay tuned...
  5. Boswell

    Boswell Moderator Distinguished Member

    You mean wiring them up as headphones, of course.
  6. dvdhawk

    dvdhawk Well-Known Member

    Why not solder the capsule straight to an XLR cable and try it without going through the boiling phase? At least until you knew if you liked the results (or not).
  7. paulears

    paulears Well-Known Member

    The trouble with this mod is that the success of the project depends on the match each person's input stage has with the capsule. So there's no guarantee it's going to be good - dvdhawks suggestion is very sensible - give it a shot before destroying the mic, makes a huge amount of sense.
  8. Boswell

    Boswell Moderator Distinguished Member

    No pre-amp has an input impedance of 30 - 50 Ohms, which is the sort of thing that's needed to match a raw 57 capsule. If I'd had more time with the borrowed transformerless 57 I mentioned above, I would have wired it up with a 100 Ohm variable resistor across the pre-amp's input and tried the result on several different types of acoustic sources at several load resistances. Without that, it was sounding like a flappy diaphragm when feeding a standard pre-amp with an input impedance of around 2 KOhm.

    More interesting would have been to feed the capsule through a different transformer, for example, a type designed for quality ribbon microphones. The standard transformer in the 57/58 is obviously quite good, but given the sub-$100 list price of the mics, it can't be a very expensive one.
  9. DonnyThompson

    DonnyThompson Distinguished Member

    what transformer would you suggest?
  10. JohnTodd

    JohnTodd Well-Known Member

    I was going to put this one in my ribbon mic:

  11. DonnyThompson

    DonnyThompson Distinguished Member

    hey... now there's a thought... thanks for that link John
    JohnTodd likes this.
  12. Boswell

    Boswell Moderator Distinguished Member

    That looks a really good transformer for a ribbon, which is a bit like a moving-coil dynamic mic with only one turn on the coil. My feeling is that it has too high a turns ratio for a conventional moving-coil mic, as the transformer multiplies the impedance by the square of the turns ratio. It would be x1369 for this transformer, so a moving-coil mic like the SM57 with a coil impedance of around 13 Ohms would appear as 18KOhms to the pre-amp. This is too high for a standard microphone input, but would feed a DI input nicely. A turns ratio of 1:4 is the normal for this type of mic.

    It's an interesting area which has already a fair amount of history. Shure themselves experimented with non-transformer SM57/SM58, and brought these models out as the SM77 and SM78. To make up for the loss of output relative to the transformer models, they upped the number of turns on the voice coil, but had to go to aluminium voice coil wire rather than copper to avoid a large change in moving mass. The output was still several dB less, but, coupled with a different body material, they were substantially lighter than the 5x range, and they thought this would appeal to a certain class of singers who hand-held their mics.

    The transformer company Cinemag makes a replacement for the stock transformer inside the SM57/58. Here is the data sheet. They seem a bit confused as to whether their product is a CM-1050 or a CM-1057, but I think that trying this type would be a better way to go than transformers designed for ribbon mics.
  13. DonnyThompson

    DonnyThompson Distinguished Member

    Actually, my interest on the transformer that John mentioned was that it might be a nice replacement for my MXL 860 Ribbon.

    Certainly it's no Royer, but it still serves my purposes (mostly M-S on acoustic and guitar amp miking, although not bad for my vocals ) but if I could improve the sound by spending a mere $30 on a transformer replacement, then this sounds very interesting to me. Now, I understand that there's more to a ribbon mic's sound than just the transformer... there's the ribbon itself, along with the basket resonance... but the transformer is still going to be a major part of the ultimate sound, right?
  14. JohnTodd

    JohnTodd Well-Known Member

    I've read that that particular xformer (the one I mentioned) makes a dramatic diff on lo-end ribbons like my Nady RSM-4. I guess the lo-end mics go skimpy on the xformers.

    Don't know if the 860 has this problem, but the RSM4 had an extra windscreen inside to "protect" the ribbon from inexperienced users (saves a lot of bad will since it's not covered under warranty). This interferes with the sound, so I took the inner windscreen out. Opened it right up! Sounds like it costs $200 more. See if that can be done to your 860.

    Sorry for the hijack.
  15. MarkF48

    MarkF48 Active Member

    I have an SM57 that I've been considering modding by removal of the transformer, but before I do I may see if I can get hold of an SM48 which is transformerless and generally can be found pretty cheap. I'm not too keen on possibly trashing a good mic as an experiment. The cartridges the SM57 and SM48 use are different, but the SM48 without the transformer purportedly has similar characteristics of the SM57 with the transformer removed, that of a crisper high end. The output of the SM48 is 1.3 mV/Pa, so it's down around what the SM7B's output is and would likely benefit from an inline FetHead or Cloudlifter. I suspect the transformerless SM57 would be pretty low output as well.
  16. pcrecord

    pcrecord Don't you want the best recording like I do ? Well-Known Member

    I'm not really in Mods but wouldn't it be a good Idea to add a switch to bypass the transformer when you want to ?
    (I know it's a more extensive mod since you need to drill down the casing.)
    May be Worth exploring.. ;)
  17. Boswell

    Boswell Moderator Distinguished Member

    One of the mods on the original Tape Op board used the switched version of the SM58 (SM58S) for exactly that function. It involved changing the type of switch to a DPDT and getting more wires past the transformer, but otherwise sounded reasonably straightforward. That said, I never like giving a singer a microphone with a switch on it - it's a recipe for disaster.
    pcrecord likes this.
  18. JohnTodd

    JohnTodd Well-Known Member

    Check 1, 2, check 1, 2 "Let me flip the switch" Check 1, 2, check 1, 2! SQUEAL!
  19. pcrecord

    pcrecord Don't you want the best recording like I do ? Well-Known Member

    :LOL: good thinking Bowell. It's even more true for live situations ;)
  20. paulears

    paulears Well-Known Member

    I can't quite, looking back, really see the point. A 57 or a 58 is a known quantity - everyone knows what they sound like and how they perform. If you do the mod, then you have a different mic, and there are plenty of alternatives. It's a bit of fun, a bit of a novelty so if you have a spare one you can destroy, then why not? At best you get a mic that's a bit different sounding - so maybe you could just have used something else, because it's no longer a 57.

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