The Xaudia Blog

21-09-2011

Syncron AU7a revisited – Phantom power

A while back I wrote about the Syncron AU7a FET condenser microphones, which are sometimes badged as the Fairchild F22.

In that post I had sketched out the schematic. I have since converted one for a customer to run on phantom power, and spotted a glaring error in the schematic. Here is the revised version…

The transistor is of course a P-channel JFET, and the battery polarity is reversed, giving a positive ground. The batteries are switched off when the plug is disconnected, and the routing through the plug makes tracing a little tricky – that was my excuse anyway.

All of this means that some small modifications are needed for phantom power use, because negative ground is by far easier to implement. Using an N-channel JFET makes things much more straightforward – something like this…

The ‘adjust’ resistor is tweaked for best response to a sine wave applied across the head amplifier, and in this case the result was around 1kΩ. JFETs can vary quite a lot, and it is sensible to adjust this individually for each mic.

I built a small breakout board to supply the required voltages from the phantom power. The board fits neatly in the battery compartment.

The “110K” is again adjusted on the bench to ensure that the voltage is correct under load.

There is one more thing to note – now we have switched to negative-ground and an n-channel device, the output cap needs to be flipped round.

Here’s a measured frequency response plot for the modified mic (the dips at around 150 Hz and 600 Hz are likely to be room modes)…

The microphone works perfectly, and it is nice to hear one brought back to life after all these years!

05-08-2011

MOTM: GEC BCS2370 & 2373 ribbon mics

GEC BCS 2373 and 2370 microphones

This pair of rather handsome ribbon microphones by General Electrical Company are our microphones of the month for July & August.

The mic on the left is labelled with the model number BCS2373, and was the ‘studio’ model, with a single layer mesh grill, and thumb screw terminals. Like many early studio ribbons, this one has a 30 ohm output impedance.

On the right is the 2370, with a more robust housing and a curved body. These were probably used as ‘lip’ microphones for sports commentary and broadcasting in noisier environments. Indeed, one of our customers sent in a very nice example that came complete with its original handle.

GEC 2370 with handle

These mics were available with different output transformers for different applications. I’ve seen two examples of the 2370 – one had a 10K ohm output, the other measured 600 ohms impedance.

Although they look rather different from the outside, they are twins under the skin – both have identical motor assemblies, with cylindrical pole pieces attached to a large horseshoe magnet, held in place by the magnetic field along The design of the microphone is described in this patent from 1947. All of the GECs that I’ve seen have held their magnetic field well over time. The only real differences between these mics are the output transformer and the body.

As usual, after all these years they benefit from a good clean and a new ribbon. Particularly as this one arrived with a ribbon made from a fag-packet! Both mics are now working well and should provide some good service for years to come.

* Thanks to Santiago Ramos for additional information.

Addenda – one of these came up recently for sale complete with original box. The owner was kind enough to share this photo of the label. 

30-06-2011

Coil winder 3 – guitar pickups

There is no ‘Mic of the Month’ article this month – normal service will resume in July. You can read the archive HERE.

Since the Meteor ME307 coil winder arrived, things have been rather hectic and we’ve been busy getting it working, and having some fun winding transformers, inductors and pickups. The friction wheel / clutch that drives the lateral action was worn, but we were able to get a good used replacement part from Poland, and everything is working well.

Hungry? P-p-p-pickup a P90…

James from Hungry Ghosts came down to the Xaudia workshop for an afternoon, which we spent rolling our own guitar pickups. James wanted a pair of P90s to replace the stacked humbuckers on his ‘Vintage’ guitar, and I was still playing about with options for my Reverend Slingshot.

P90s are pretty simple things. We made the base plates by drilling and tapping a bar of 5mm square mild steel, and two “Alnico-5” magnets were glued to this, side by side, like this.

We wound four bobbins, at 7000, 8000, 8500 and 9500 turns, reverse winding some so that they could combine to give a humbucking effect….

Dipping the magnets in hot wax prevents microphonics, and we rigged up a ‘redneck’ wax potting system using a £10 slow cooker from Tesco. This works really well and I’ll describe it in a separate blog post. That done the pickups could be assembled and wired.

I found some nice deep headed screws which made perfect pole pieces for the pickups. Here is the finished P90…

And the new pickups imbued James’ guitar with a spooky pink glow!

OK, we made the last bit up but they do sound astonishingly good. 9500 is quite a lot of turns for a P90, but the pickups all had plenty of clarity and no shortage of bottom end either.
29-06-2011

Redneck wax potting bath

We needed a better way of wax potting our pickups and transformers. Here’s what we did on a budget…

Redneck wax potting bath for guitar pickups

It’s a hacked 300 Watt slow cooker, acquired from Tesco for the grand sum of £10, which contains an inner ceramic liner and holds about 3 litres. The probe is simply a cook’s thermometer (£14 from Barnitts of York).

It takes a little while to warm up, but sits at about 90 degrees all day, which is just about perfect for potting. The temperature can be further tweaked by running the bath on a variac.
Here’s a newly dipped transformer coming out of the soup – a mix of bees wax and parafin wax…

Update: 1 December 2011
The batteries in the cook’s thermometer died, and we really wanted to regulate the temperature and avoid the possibility of overheating the wax, so I built a little temperature controller into an old power supply box. The digital controller was ‘new old stock’ at £30 from ebay, and a 5 amp solid state relay and resistance thermometer came from Farnell to complete the setup. It works very well!
12-06-2011

Coil winding part 2

The new ME307 coil winder is up and running, and we’re jumping straight in to winding some guitar pickups. The idea was that this should be an easy place to start as they can be made with a single winding, and bobbins, wire etc. are all readily available. There is a big pickup winding community in interweb-land, so help and advice should be available.

The winder in action, with the pickup mounted on a metal plate that we made for the job…

Pickups typically use circa 42 AWG wire (0.0633 mm), which is a very fine gauge and requires care whilst winding and soldering to avoid breakages.

The wire tensioners that came with the winder were a little out of calibration, but with a little practice at setting the correct tension and winding speed, we could easily get 7000 turns onto a P90 bobbin with room to spare. It’s probably possible to get 8000 or 9000 turns with practice, to wind a really hot pickup. Here’s the first winding from the Meteor….

Looks nice! The next step will be to rewind some of the broken microphone transformers that we have accumulated over the past few years. But before that we need to mend the clutch plate, which is slipping.

07-06-2011

Our New Coil Winder – Meteor ME307

The ‘new’* Meteor ME307 coil winder has just arrived at the Xaudia workshop, after a 500 mile round trip to collect it…

We are very excited about this as it will expand our repair abilities and let us rebuild and rewind broken microphone transformers and guitar pickups. It has a counter to set the number of turns, foot actuated motor control, and of course the all important set of tensioners to allow the wire to be spooled at the correct tension.

The coil winder was probably built sometime in the 80’s and is built like a tank, with some beautiful Swiss engineering inside. Everything needed a little lubrication and a good clean and, although the machine has clearly seen some years of use, everything is still working perfectly.

ME307 internal gears and drive train.

We hope to be doing our first windings later this week…

28-05-2011

Direct Inject!

My friends in The Savoy Ballroom needed some stage DI boxes for their numerous acoustic instruments. They wanted to be able to mute the signal whilst tuning up or switching instruments, so I build these nice little boxes for them.

They are based on Bo Hansen’s easy to make design, but with added bells and whistles – well, a switch, a lamp and an extra output.

Job done – now back to recording!

22-05-2011

Transformer assembly line!

This week we have been building lots of little transformer moxes to match vintage 30 ohm ribbon mics to modern mic preamps. Correct impedance matching can deliver a +12 dB increase in level without noise penalty or loss of frequency response, which is welcome for many older microphones. Here is how a Reslo RV microphone behaves with and without the transformer:

Frequency response plots for Reslo mic with and without an impedance matching transformer

They are suitable for many old microphones including the following:

  • Reslo RBL and RB 30 ohm models
  • Melodium 42b
  • Grampian ribbon mics (including GR1/L and GR2/L)
  • Cadenza ribbon microphones (wired for low-Z use)
  • RSA & Selmer ribbon mics
  • STC 4033
  • Altec and Western Electric ribbon microphones

More information.