The Xaudia Blog

02-12-2011

Stellar RM3 upgrades

This Stellar RM3 ribbon microphone recently came into the Xaudia workshop for a transformer upgrade and new ribbon, which of course gives us a chance to poke around inside and take some photos.

Stellar RM3 with new Xaudia transformer and ribbon. The old transformer in in the foreground

The design closely follows the Speiden / B&O / Royer style of microphones – possibly a little too closely for comfort! Like the B&O mics, the ribbon frame slides out of the top, along with the transformer and lots of synthetic woolly packing. The body forms part of the magnetic return circuit, so it was good to see that this one was made of a magnetic steel, rather than brass or aluminium.  In my opinion this is one of the better made of the budget ribbon mics.

As is commonly the case with budget microphones, the design is let down a little by the transformer, which measured 47 mΩ (milliohms) on the primary winding, and 90 Ω for the secondary, with a turns ratio of 1:54. Overall the mic gave a measured output impedance of about 650 Ω, of which over 200 ohms is pure DC resistance from the transformer. And in ribbon mics, resistance equals noise.

The new Xaudia 595/40 transformer (in this case a 1:40 ratio) has DC resistances of 0.02 Ω  for the primary and 16 ohms for the secondary – much lower than the stock trannie. Although the lower transformer ratio would be expected drop the output level of -2.6 dB, the thinner, lighter ribbon gives an almost equal but opposite effect, and the output levels before and after are very similar. However, the lower DC resistance of the replacement transformer gives much lower noise, and the larger core allows higher sound pressure levels before the onset of saturation.

30-11-2011

MOTM – Lustraphone VR53 (a.k.a. Grundig) ribbon mic.

November’s microphone of the month is a British long-format ribbon that was sold under the names “Lustraphone” and ‘Grundig”.

A Lustraphone-badged ribbon microphone in glorious brushed stainless steel finish

This mic was available in at least three different impedances, and I have come across 30, 200, and high impedance models. Unfortunately the badge often falls off so you don’t always know what you are getting! Most of the models I have seen are finished in a bronze hammerite colour, although there is a deluxe low impedance model which has a gorgeous brushed stainless steel finish. Despite the different badges and finishes, the mics are exactly the same on the inside (transformer aside).

Lustraphone ribbon microphones on the bench

One of the ribbon clamps sits on a spring-tensioned screw thread, which allows fine adjustment of the ribbon tension. This makes tuning the ribbon very straight-forward, and allows the owner a little bit of grace if the ribbon becomes a little stretched over time. This feature should probably be mandatory on all ribbonmics!

Grundig badged lustraphone mic, opened up for service

The magnetic field is supplied by a pair of horseshoe magnets (which unfortunately sometimes age with time, losing their strength). Connection to the rest of the world is made by a balanced three-pin paxolin plug, which are hard to come by now. The middle pin is ground, with the audio on the outer two pins.

Rear connector and original plug.

Fortunately, a male XLR connector can be modified to fit by slicing off part of the barrel.

XLR connector modified to fit the mic.

The long ribbon and motor design gives these mics a full bottom end and a pronounced proximity effect. Here are the frequency plots for three of these mics that we have serviced recently:

Lustraphone ribbon mic frequency plots.

(Thanks to Mark Stevens for additional information).

Update 23/1/12. These microphones were also sold under the brand Pamphonic. One appeared recently on ebay:

Update 29/2/12. And here is one with a Mimco badge!
30-09-2011

MOTM – Cadenza Rocket ribbon mic

Sometimes I see patterns or trends in what arrives on the bench. A year ago it was RCA 74s, and in spring 2011 it was Electrovoice ribbon mics. And then the summer brought Melodium 42bs. Of course these are just statistical anomalies or ‘blips’ in the random noise of what my customers send me to repair, but it does at least suggest ideas for the regular ‘Microphone of the Month’ column.

Cadenza microphone set with box, documents and stand

This month these little Cadenza ribbon microphones are in vogue – these were made by Simon SoundService Ltd in London, and were designed by Eric Tomson, Stanley Kelly Peter Bell. The mics have a very 1950s styling, and are often called ‘rocket’ mics.  However, having spoken to a couple of customers, the consensus was that many of these microphones no longer sound as good as they should – or at least as good as they look. The complaints are that they are noisy, with low output. 

The Cadenzas have dual impedance outputs, and can be wired for either 30 ohm or high impedance  (80 KΩ) output by changing the wiring at the connector. Neither option is really ideal for modern studio use, where something between 200 and 600 ohms is much more common for a mic output.

The ribbon itself is slightly unusual in that it has a fixed or nodal point half way along, where the ribbon is glued to an insulated support. This may well have been designed to reduce the likelihood of the ribbon being stretched, and one can imagine this either as two ribbons in series, or like a guitar string where the octave harmonic has been struck.
Cadenza mic transformer under the knife

From a technician’s viewpoint (i.e., my opinion :p), there are a some weak points in the design which all relate to the transformer. Firstly, the ribbon clamps are connected to the transformer simply by winding the wire under a nut and tightening it – really not a reliable long term approach. Secondly, the transformer wires are extremely thin, and half a century later the insulation becomes brittle and tends to break, with disastrous results. It is more common practice to use thick gauge wire for the fly-leads to the primary, to keep resistance and noise to a minimum. And finally, as these thin wires become old and oxidised, the mics become noisy.

This all points to a transformer upgrade or rewind. However, it seems that Simon Sound Services Ltd. did a better job of encapsulating the transformer than they did of connecting it to the ribbon. The tranny is glued inside a mu-metal can with a hard, clear resin that resists removal. It was necessary cut the metal shell in half and then to soak the resin in dichloromethane for 48 hours to remove the resin. This allows removal of the laminations and a better inspection of the transformer windings.
Cadenza windings – primary (left),
and 30 ohm output (right)

The outer high impedance winding is a very fine gauge (approx. 0.07 mm), below which is a single layer of 0.4 mm wire which makes up the primary. The innermost winding is the 30 ohm output, which is approximately 0.2 mm in diameter. Unfortunately the solvent also damaged the bobbin, and and a replacement was found to make a new transformer using the vintage laminations. 

The new transformer has a single 600 ohm output, with thicker gauge wires to give lower resistance.  It took some effort but the result is a higher output with a lower noise floor, and the mic is much more usable. I’ll post back with a ‘before’ and ‘after’ frequency plot for comparison.
28-09-2011

Reslo transformers vs the WEE monster

This little dinosaur sculpture was made out of waste winding coils, stripped mostly from old Reslo and other microphone transformers. 🙂

So why are we stripping transformer coils?

Some older ribbon mics were originally wound for 30 or 50 ohm output impedance, and tend to give a low output level when connected to modern recording equipment.

Many of these low impedance mics, including Reslo and Tannoy ribbons,  can be rewound for a modern 200 or 600 ohm input, raising the output to a more useable level, and avoiding noise from having to crank up the preamps.

Bobbin from Reslo transformer with secondary winding removed.

The old Relso 30/50 ohm transformers have an inner (primary) winding consisting of just 12 or 13 turns of thick (0.8 mm) enamelled wire, and a secondary winding of 152 turns of 0.4 mm wire. The thick wire of the inner winding ensures that the primary resistance is low, which keeps noise to a minimum.

Reslo transformer rewound for 600 ohm output

Re-winding the transformer involves removing the outer winding from the original transformer and replacing it with sufficient turns of a thinner gauge to reach the desired turns ratio and output impedance. Usually the original primary winding can be kept in place. The transformer is then reassembled and dipped in wax to fix the windings and lams in place.

This makes the microphone much more usable in a modern studio – transformers can be would for 250Ω, 600Ω or any other desired output impedance.

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.