The Xaudia Blog

11-01-2015

Shure Unidyne 545 transformer bypass switch mod.

I have read a few posts on internet forums wherein the transformer is removed from Shure mics such as the SM57. This will inevitably reduce the output level and also reduce the impedance to that of the capsule alone, and will of course remove any chance of overloading the transformer! Which means that the result is a quiet mic with low output, perfect for putting close to a drum head.

I decided to try it out. I had a Shure Unidyne 545 with a faulty transformer, so I took that out and wired the capsule directly to the output socket. The result was absolutely ideal for close-micing snare drum.

One of my regular customers asked about making this into a switchable feature. Well why not? With a double-pole double-throw switch it should be possible. The only challenge was finding a switch small enough. In the end, after trying several brands, I found that a spare switch for a Fender Jaguar would fit… but only just!

Firstly the capsule needs to be removed, unsoldered and new wires attached to the capsule. I also ran new wires to the XLR socket to be safe. The mic is filled with quite a lot of sticky brown gunk so that slows down the work.

The trick is to fit the switch from the outside of the microphone, and so the wires need to be fed out through the switch slot. It looks a mess in this state, so they are trimmed back to roughly equal length and soldered to the switch, and it all goes back together. I marked the transformer position with an X. (‘Trans’ in Latin can mean ‘cross’ or ‘across’, and X is a cross!).

Here is my sketch of the circuit, with the capsule on the left and the XLR on the right. Note that the black and white transformer wires are a high impedance winding and are not used in this circuit. Transformer wire colours may vary for other models and examples, and need to be checked before starting work.

The final job is to check the polarity of the mic against a known reference. If it is out of phase then the wires need to be swapped either at the capsule or the XLR socket.

With the transformer bypassed, the mic measures around 15 ohms at 1 Khz. The transformer brings this up to 200 ohms, and increases the output level by 10 to 12 dB. but also introduces a little bass cut. The plot below shows the difference the transformer makes. I am uncertain if the bump at around 800 Hz is real.

With the transformer bypassed, the mic can be placed close to loud sources and there is also a bass proximity effect, so the mic sounds really nice and fat!

Thanks to Lee Mouatt.

15-12-2014

Oktava ML16 Manual

Here is a manual for the ML16 ribbon microphone… in Russian!

30-10-2014

2-Track Mind’s Tannoy mic restoration

Here is a nice article by Seth at the 2-Track Mind blog, describing how he restored a very early Tannoy ribbon microphone. Seth went to great lengths, building his own corrugator and refinishing the mic, and the results look great. I played a tiny role here and was able to supply a little brass mount so that he could put it on a stand.

The mic itself is quite unusual, and the motor assembly (above) is different to most of the other Type 1 Tannoy ribbon mics that I have seen. Below is the more usual look of these mics inside, with a large cylindrical magnet and simple flat pole pieces:

It is becoming clear that Tannoy made many different variations on their ribbon mics. I feel a big Tannoy blog post coming on…

25-10-2014

Syncron AU7A schematic (again)

Many thanks to Dan Zellman, a former Syncron employee, who sent in this schematic for the AU7A condenser microphone. This was the ‘last issue’ diagram.

Syncron AU7A factory schematic

There are a few differences from my traced circuit that I posted in an earlier blog entry, most noticeably I had the 42V battery supply reversed. Also, I have worked on several of these mics, but have never seen one with the 470pF capacitor present.

SJT 25/10/14

30-09-2014

Mount-a-Mic assembly instructions

Here is a step-by-step guide to assemble an Xaudia Mount-a-Mic cable:

1. Make sure you have all the parts.

2. Strip and prepare the cable ends ready for soldering. Push through the Mount-a-Mic adapter

3. Solder the female XLR connector. We recommend connecting the barrel lug to pin 1.

4. Fit the plastic strain-relief

5. Slide the metal barrel over the plastic parts

6. Slowly screw the barrel into the adapter, allowing the cable to rotate freely as you go.

7. Fit your choice of connector to the other end, and enjoy!

Mount-A-Mic adapters from Xaudia

Have you ever struggled to find the right clip or mount to put a microphone on a stand? Here is a solution: the Mount-a-Mic adapter from Xaudia.

Xaudia – Mount-a-Mic assembled cable

This simple yet cunning gizmo is designed to convert XLR plugs so that they can be screwed onto a 5/8” mic stand.

Mount-a-Mic adapter from Xaudia

The Mount-A-Mic adapter simply replaces the cable gland on Neutrik XLR plugs. The cable now exits through the side, and the assembly can be attached to the mic stand.

Then just plug the mic in, and go! You can now put almost any microphone on your stand, so long as it has an XLR plug. Great for Geffell….

And a Neumann is no problem!

Here are some assembly instructions for those who wish to make their own cables.

19-09-2014

New RCA thread adapters

We have some shiny new thread adapters for RCA microphones. These thread adapters fit most of the ‘big’ vintage RCA microphones, including the 44, 74b, and 77 ranges, and also are perfect for Sony professional mics, including the C38b and C48 microphones.

We have been selling thread adapters for a while but the latest batch are a step up in quality. They are machined from solid brass and have a standard 3/8″ female thread that will fit most modern mic stands. They can also be supplied with 5/8″ on request.

It fits this RCA77B perfectly.

These fit the following microphones….
Most RCA 44, PB140, PB144, 74b, 77B, 77DX, MI-4010-A, BK-11.
Some (but not all) models of the RCA Varacoustic.
Sony C38, C48, FV300, Some C37s.
Many Aiwa, Toshiba and Matsushita ribbon mics
Syncron / Fairchild AU7a
Some older Amperite mics

Please ask about your microphone.

21-08-2014

Electrovoice V2 revisited

These old Elctrovoice ribbon mics look great but sadly the sound rarely lives up to expectations.

I have worked on a few of these over the years and there are several things that let these microphones down – happily they can all be sorted out with a bit of thought.

EV V2 clamp with through-ribbon bolt!

Firstly, and perhaps most annoying are the ribbon clamps. EV used thin plastic clamps to insulate the ribbon from the body of the mic, and the ribbon is secured by a central screw that goes straight through the ribbon. The old EV ribbons are quite thick and were supplied with (even thicker) copper end terminals, so that they could just be screwed in place. One problem is that tightening the screw twists the ribbon, pulling it into the side walls of the motor. It also does not make very good electrical contact, especially with a new thin aluminium ribbon. And the clamps are flexible too so the ribbon is not held securely.

New ribbon silver clamp

A better solution is to make new stiff metal clamps. These could be made from plated brass but I tend to use solid sterling silver to avoid the need for a plating process. One clamp must still be insulated from the body of the microphone to avoid shorting out the ribbon.

Rusty old magnet.

The next problem is the weak magnetic field. Replacing the old magnet with a new neodynium one can boost the field around five-fold, which gives a corresponding increase in output. With stronger magnets the steel screws can jump into the ribbon gap and wreck a new ribbon, so it is best to swap the screws out to some brass or stainless ones.

EV V2 transformer.

The old transformers are not great and lack bass but that’s an easy thing to sort out! And finally the old plugs are getting hard to find so we can put an XLR on the rear.

XLR fitted to EV ribbon mic

Voila! With new clamps, ribbon and magnet we have a healthy signal with plenty of bottom end!

20-08-2014

The world’s smallest ribbon mic?

Is this the world’s smallest ribbon microphone mechanism?

The Beyer M560n(c) has a motor measuring just 33 mm long and 10 mm deep, which fits into the mouthpiece of a headset.

The tiny transformer is mounted at the other end of the microphone. The mic is clearly designed for close speech and the transformer has a very low inductance (35 µH at 1 kHz), which gives a bass roll off to compensate proximity effect.