Adding shielding to old ribbon mic transformers

[geedy]

Title self explanatory. I recently got a 74B for the logo vibes and holy cow does the thing ever hum, at almost same level as my voice!

Obviously cannot put a mu metal can around it. What are my other options, other than replacing the transformer?

 

[emrr]

Change with orientation? Sound ok otherwise? Doesn’t sound right.

 

[shabtek]

hiZ version?

 

[Tubetec]

I just looked at the 74B , seems like one side of the ribbon is tied to ground and back to the transformer via the casework , maybe this contributes to noise pickup .

 

[emrr]

lots of people use 74's, there's something wrong

 

[Tubetec]

Geisnote do a small mumetal transformer screening can , not sure if its suitable size wise though .
If noise is as loud as signal Id suspect the ground connection is at fault .

 

[geedy]

I've read in many, many places that 74Bs are notoriously bad with electromagnetic noise. Lots of home recorded stuff on YouTube using a 74B has a similar hum (sounds like guitar feedback), just a little quieter.

That said, I buy that this isn't the problem in this case. While I'm still interested in the original question, I pulled the mic apart and I can see a few things that might be hints towards problems

1. The outer shell of the transformer is rusty/corroded, as it looks like the inside layers may be, too.
2. The wires all look original, and when I move the disassembled mic around while plugged into an audio interface, the signal sometimes cuts out leaving just the noise, or the audio leaving the mic cuts altogether. Reassembling the mic tends to make it work, with the original noise.

Thoughts?

 

[Tubetec]

I think I saw that the magnet/motor assembly in the 74b have some form of grommets for acoustic isolation .
Its possible these rubber gromets are no longer doing their job and causing some kind of grounding issue .

 

[jhaell]

Something has to be broken. My 74b is dead quiet with the original transformer.

 

[Tubetec]

http://coutant.org/rca74b/
maybe you have the wrong impedence connection like Shaba suggested .

 

[fum]

I would echo what some others have said. Check that you are wired correctly for 250 ohms.

Check that you have a good pin1 connectivity to the body, and that the ribbon is not damaged.

Once the ribbon starts to go, you get lower output, which sometimes people hear as higher noise (having to crank it up more).

I repair a lot of 74B's, and a proper working one sound great.

Regards

Jon

 

[geedy]

I've confirmed the transformer is wired for 250 ohms:

But that transformer looks pretty corroded. Could that be a problem? Here is a side view:



I'm curious as to a couple more things. As mentioned above, the signal will cut out sometimes when I've got the mic disassembled from body/grille. Sometimes the noise remains only, and other times the signal drops completely. I suspect old/frail wiring, does that sound reasonable?

If so, any suggestions on where to buy a good replacement cable, and how I should affix a new wire going from the ribbon lead into the bottom of the transformer?

 

[fum]

Transformer looks fine to me (tranformers don't go bad, but in rare instances), the shield is firmly soldered to the body where it should be.

If it's intermittent, that typically is in the wiring. Wire doesn't typically go bad unless subjected to mechanical stress, and the stress points are at connections. You could try resoldering the wire to the lower tab.

The body of the mic acts as the other "wire" to the transformer, so the various mic parts have to be making electrical contact. Listening to the mic "pulled apart" could be problematic.

Aside from that, you've got the typical dance of moving the various parts around to see if you can isolate what causes the intermittent.

Regards

Jon

 

[geedy]

After a lot of experimentation, I've concluded the following:

1. The Transformer must be attached proper for the mic to reliably work.
2. The loud "hum" only occurs when I touch the microphone.
3. Reassembling the microphone is a HUGE pain in the rear, to get case / body /transformer aligned.
4. The ribbon almost certainly needs to be replaced.

I think once I reassemble it and reribbon it it'll be good as new.

 

[Tubetec]

So the downside to the case forming the return wire from one side of the ribbon is the mic body is suceptable to interference by touch , maybe seperating the return from case/ground would be a good idea . Not sure how feasible that is , it may well have been a cost cutting/ constructional decision and might require extra insulation where the magnet assembly is mounted to the frame . It seems almost certain if the ribbon was feeding the transformer in a balanced fashion and not with one side connected to case , it would provide much better rejection of external noise .

As you can see the lower end of the ribbon is insulated from the frame , the top end isnt ,
If your having it re-ribboned anyway you could arrange the same insulation at the top of the motor , although drilling and cuting the frame would be required , the geometry of the magnets is held by the threaded bar at the top , so it shouldnt make any difference structurally .

Of course these are valuable mics and any destructive mods ,even if it gives better useabillity and lower noise might detract from the re-sale value .

What do you think Doug ? the frame provides the lowest possible resistance path to the transformer ,but a mic thats buzzes when touched is not what you want under any circumstances .
Did they cut a corner on the construction , surely if this mic is mounted on a metal stand the whole apparatus turns into an aerial when touched .

 

[fum]

I've seen the grounded primary approach used in several production ribbon microphones, and theoretics aside, it hasn't proved problematic.

Making these types of modifications on inexpensive microphones, I'm certainly down for, if you have an itch to scratch.

That said, I've seen people take information in threads like this and do horrible things to classic microphones (most recently an RCA 44BX), so please people, take these comments with a grain of salt, and perform modifications on the $100 mic.

$.02

Jon