What Shall I Do?

[MicMaven]

Here is your chance to get in on the ground floor and be an "influencer" as I consider modifying a vintage microphone:

I recently acquired a lovely vintage Vanco MD-353 dual impedance dynamic microphone with its original cable, specification sheet, purchase receipt dated December,1974, and box. It looks brand new with no blemishes or marks. A diminutive low impedance transformer is concealed behind the ON/OFF switch. It mounts on a standard 5/8"-27 thread microphone stand. Its hollow body has a 38mm diameter opening where the cartridge is mounted. The head basket-grill is 30 mm deep and has multiple 18 mm side vents. It was made in Japan, and is roughly patterned after the popular ElectroVoice Model 664.

While I believe it sounds as it should, it is not as clear or articulate as my loyal 1968 vintage SHURE PE-53 Sphere-O-Dyne high impedance dynamic microphone with a HOSA in-line impedance transformer. Consequently, I contemplate various modifications to improve it. I may use it with an amateur radio transceiver and/or with computer speech recognition applications, depending on how it turns out. I believe there is precedence for modding this type of microphone; for example, on their last tour, ZZ Top used multiple modified ElectroVoice 664 microphones retrofitted with RE510 capsules. I contemplate a much lower cost replacement, and have the following

Preliminary Inquiries:

1) Does it have sufficient grill side venting to accommodate a modern dynamic replacement cartridge? (I think it does.)

2) If yes, is it so, would it worthwhile replacing the cartridge? (I have replacement cartridges on hand, but do not want to waste time attempting to improve its sound, if there are reasons against it. I recently replaced the original cartridge in a Turner Model 253 communications microphone with rather disappointing results, so I am not keen on repeating that sort of experience. I think the original cartridge sounds thick and muddy with microphonics from the body ... what can I expect from a typical dynamic replacement cartridge?)

3) I plan to bypass the original transformer as replacement cartridges are typically between 300-600 ohms. Ahem ... Right?

4) Are there any special considerations on installing a balanced transmission line? I presume the following Pin assignments ... um ... I guess:

* Map original Pin 2 (cartridge + /signal) to XLR Pin 2 (Signal)
* Map original Pin 1 (cartridge - /return/shield) to XLR Pin 3 (Return)
* Map cable shield to mic body and XLR Pin 1 and connector shell

OR is there a better plan?

5) Are there any clever suggestions on how to mount a typical 28mm dia. replacement cartridge in a gaping 38mm diameter handle?

6) Should I line or fill the body with vibration damping material? (I think yes because it already sounds resonant and microphonic, but then it might not depending on how I mount the replacement cartridge... or ... um ... what?)

7) What am I missing? What have I failed to mention or consider, and should?

Any traction? Is this a worthwhile cause, or should I relegate it to visual trophy status and let it look good on a shelf with other historic models? I will consider all sorts of ideas and suggestions, so let me have it. I would like to get it right on the first go, if possible.

Thanks in advance. James


View attachment HD353 vs PE53 vs 3835 compared 1.mp3

 

[Voyager10]

It's currently an omni so there's no acoustic path to the rear of the capsule. You would need that if you're fitting a cardioid capsule and intended to have a cardioid mic.

Not sure where you'd get an omni dynamic capsule of decent quality - EV 635 or Beyer M58 spares? Probably way too expensive.

One possibility might be a small cardioid electret capsule mounted close to the front so there's a path round the back of it. Could be held in place with stiff wires, perhaps attached to some stripboard or FR4 cut into a circle and glued in place.

 

[MicMaven]

It's currently an omni so there's no acoustic path to the rear of the capsule. You would need that if you're fitting a cardioid capsule and intended to have a cardioid mic.

Right! Exactly what I am thinking ... and yet, there ARE side vents in the grill piece that might allow enough sound in to match the side vents on these capsules (below) one of which with minimal venting, and another with (apparently) no vents at all, which I understand does have a cardiod pattern. In any case, I wonder whether any of them will sound better than the original ...

Do you suppose the side vents on the grill are sufficient? If so, which do you think is the best candidate?

[I hesitate to disassemble the original microphone only because I prefer to test a reasonable hypothesis, opposed to just trying stuff to see what sticks.]

Thanks - I appreciate the input. / James

 

[Voyager10]

I'd imagine the one with no vents would be the best bet, but I'm no expert on the actual physics here.

You could perhaps try mounting a candidate capsule in end of a piece of plastic pipe, to see the effect of obscuring the vents. (I remember doing this a long time ago to test various cheap dynamic capsules, but never found anything worth keeping. Hopefully modern ones are better).

 

[MicMaven]

Good Day Mr. @Voyager10 -

Thanks for sticking with on this! Here is my thinking, and perhaps I was not sufficiently clear previously: I sorta think the vents on the cartridge may line up "well enough" although not ideally with those cut in the grill piece. If so , maybe I am good to go with this type of cartridge.

I DO appreciate your point about using the cartridge which appears to lack venting altogether - that makes sense. I would prefer to use the one in the photo below, because, notwithstanding your logic, it sounds better in a typical, normal performance stile microphone handle.

I hope the attached photo conveys my thoughts on matching the vents on the cartridge to those on the grill and thinking they may be enough to let the cartridge work as designed. Whaddayasay ?

Thanks again for playing along - truly appreciate the uptake. James / K8JHR