The Xaudia Blog

STC 4033-ST upgrades

Updating an old STC 4033 microphone for use in a modern recording studio.

I have always loved the looks of the STC 4033A microphone, but as far as cardioid microphones go, it might be considered an evolutionary dead end. The 4033A is an example of a dual transducer mic, which was designed back in the 1930s when uni-directional (i.e. cardioid) patterns were needed for film set use, but most ribbon mics were figure-8 and most other microphones were omnidirectional. STC solved this problem by combining both of those elements. The front of the ribbon is in phase with the dynamic capsule and adds to it, whereas the rear of the ribbon is reverse phase and cancels. Add these together in the right proportions and we get a cardioid microphone.

Omni plus figure 8 combine to give a cardioid pattern
Omni plus figure 8 combine to give a cardioid pattern

At least that is the theory. In practice this works well enough in the mid range but veers away from cardioid at the high and low ends of the frequency range. To compensate, the 4033 has a filter network which rolls off the top end of the ribbon when combined with the capsule. A switch at the rear allows the engineer to select either the ribbon or capsule by itself, or the combined output. (BTW We recently built a few replica 4033 microphones at Extinct Audio – in this case we prioritised tone over pattern and omitted the filter network).

Wiring diagram for STC 4033A microphone
Wiring diagram for STC 4033A microphone

The Western Electric and Altec version of the 639 birdcage microphone, and the Western RA-1142 all used a similar approach to the 4033 , but these mics were soon superceded by other simpler cardioid dynamic and ribbon mics. The dual element cardioid mics were became obsolete curiosities, although they have developed a bit of a cult following in recent times.

I have recorded some great electric guitar sounds with a 4033, with the ribbon softening the tone, whilst the dynamic capsule can add some welcome bite at the top end. The low sensitivity and output level are well suited to louder sources and it doesn’t matter for this application if the mic has a little noise, as the amp noise will always dominate.

There are still some good working examples out there, but, as a jobbing microphone technician,  I see a lot of these which are in poor shape and need a lot of work. That is not surprising after 80 years or so – I expect I’ll need a service too when I get to that age!  They do tend to be a bit noisy, and they have a low impedance output, with the ribbon and dynamic elements having outputs around 25 ohms, and the combined mode is about 50 ohms. Often these are best used with a matching transformer.

Often the best way to approach a really old mic in poor condition is to do a complete rebuild, rather than trying to fix numerous small problems and then finding others. And that also gives the opportunity to make some changes, so that the microphone is more suitable for use in a modern studio setting. Common problems with 4033s are broken output pins, noisy, stretched or broken ribbons, flaky paint, erratic switches and bad capsules. Luckily the mic on my bench this week had a good capsule, and the other issues can be solved.

For this mic I decided to split the ribbon and dynamic outputs into different channels, bypassing the switch and filter section. Each channel gets its own transformer so that the outputs are both 300 ohms. The output impedances are now the same as a modern Coles 4038, and any good modern mic preamp will work nicely with a  300 ohms source. Adam at Extinct Audio re-finished the microphone for me and fitted new grill cloths, and I added a 5 pin XLR socket to carry the two outputs. Everything was re-wired and the ribbon replaced.

So now we have a modernised 4033 with two outputs, both at 300 ohms, which can be recorded onto separate channels and mixed together to taste. Each channel can be given separate equalisation or compression. I made a couple of recordings to show how this works.

Firstly, a little bit of speech to introduce the microphone and hopefully demonstrate how the two transducers sound, separately and together.

4033-ST Speaking:⬇️

And then some acoustic guitar:

4033-ST Ribbon only: ⬇️ The ribbon element is quite dark.

4033-ST Capsule only: ⬇️ You can hear that this is quite a bit brighter than the ribbon.

If we put those together and get a little creative it can sound like this:

4033-ST Ribbon and Capsule mixed:⬇️

The track above has a little bit of compression and bass cut on the ribbon element (to control my sloppy playing), and a plate type reverb on the dynamic capsule to give it a feeling of space and depth.

The result of this is that we now have a microphone with a sufficently low noise floor to recording gentle acoustic fingerpicking, and the two outputs makes it versatile and opens some creative doorways, such as blending the sounds to taste ands addding EQ, dynamics or effects. One can also pay for a stereo effect. And the ribbon motor and dynamic elements themselves remain unchanged, so the sound is very close to that of a classic 4033A. Best of both worlds? Maybe.

I like this one so much that I am keeping it, but will be offering a few for sale soon as well as upgrades as a service.

PS the guitar music is an excerpt from “Kinkachoo I Love You” by Phillip Houghton.

Oktava ML19 recording acoustic guitar

Oktava ML19 recording acoustic guitar
Oktava ML19

The ML19 ribbon microphone from Oktava is still a little underrated and I think this is one of the best sounding end-address cardioid ribbon mics.

Based loosely on the RCA BK5, the ML19 has a wider ribbon and is a little warmer in tone with a good bass response, without being overly dark or woolly. It is great for upright bass and horns and any source where you need rear end rejection. It also sounds rather nice on acoustic guitar.

Here is a short acoustic guitar recording I made with a spaced pair of Oktava ML19 cardioid ribbon microphones.

The tune was played (by me!) on an old Levin flat top acoustic with silk-and-steel strings, and recorded into a UA Apollo 8 preamp with 57 dB of gain, using Logic Pro. There was no processing aside from normalising the file. I pointed one mic at the 12th fret and the other at the lower body. However I suspect a crossed pair might sound really good here.

These mics don’t age well and it is worthwhile getting them serviced if that not been done. Generally they need new cables, new ribbon and perhaps most importantly the internal foam should be cleaned out and replaced.

Oktava ML19
Oktava ML19

Lustraphone VR53 magnets (again)

Lustraphone VR53 magnet upgrades
Lustraphone VR53 magnet upgrades

The problem:

The Lustraphone VR53 was first made in the 1950s and would have been used with either an old reel to reel tape recorder or valve PA system, with lots of lovely background hiss.

The V53 has two (probably) ferrite horseshoe magnets and some rather hefty steel pole pieces,  The result is a weak field which conspire with the long ribbon to give a weak, flabby sounding signal.  It is a very dark sounding mic even for a 50s ribbon, and maybe for some recordings this is the sound that you need! But for most users the signal to noise and output are too low for our expectations of modern recordings, and the lack of top end limits its use to special effects duties or spiky sounds where you really want to kill the top.

However these microphones are very stylish and it would be nice if they sounded as good as they looked.  Fortunately, the VR53 is ripe for modification because…

(a) it is not a very good microphone, so there is a lot of scope for improvement,

(b) it is (still) relatively inexpensive, probable because of (a) and…

(c) it is common so we don’t need to worry about hacking a rare antique. I would not do this kind of thing to an old RCA 44bx

The limiting factor here is really the weak magnetic field. If the field can be increased then the output level and signal to noise will improve and the bottom end response will be tamed a little, giving a less flabby sound. I have posted previously about machining the old pole pieces to make space for some booster magnets. That worked well but milling the old steel parts proved to be very time consuming and so expensive to do, and sometimes the old parts would fall apart. I don’t really like machining ferrous materials here in our workshop because the combination of iron filings and strong magnets can cause trouble later.

Now we have a simpler magnet upgrade which works just as well, or better but is much quicker to fit. Adam at Extinct Audio and I designed some small steel holders to replace the old pole pieces. These are screwed in place and then new Neodymium magnets simply slide into these holders and the magnet upgrade job is done. I measured 6500 gauss in the ribbon gap which is a hefty improvement and is comparable in field strength to something like the Coles 4038. Then it just needs a new ribbon and putting back together – I generally replace any perished rubber parts at this stage.

These microphones were available in different impedances but the transformers were well made. If the microphone has the 200 or 600 ohm transformer then it can be left in place and the microphone will work nicely with modern equipment, although we can squeeze another 2 or 3 dB out with a new transformer. The result is a substantially higher output and signal to noise performance, and more controlled bass response which lets the top end shine through.  The new ribbon also helps with the highs.


For comparison, here are two short recordings of acoustic guitar made with the stock microphone with old magnets and ribbon


And here is the modified version at the same gain settings. Be warned this is much dB louder and you might want to turn your headphones down now!


Lustraphone VR53 magnet upgrades
Lustraphone VR53 magnet upgrades

AKG DX11 microphone… with reverb!

You can easily imagine the product development meeting at AKG back in nineteensixtysomething…

Or something like that. It is certainly true that AKG made some great microphones (too many to list) and a lot of nasty ones (also too many to list). The AKG BX10 and BX20 spring reverbs have become studio classics. And so if any company was well positioned to make a microphone with a built in reverb, it  had to be AKG. The result is the AGK BX11, which is a battery powered dynamic microphone with a built in single spring reverb and a battery powered circuit which uses five germanium transistors to amplify the signal and drive the reverb spring.

AKG DX11 echo microphone

There are some major shortcomings to this design. It is impossible to use this as a hand held microphone because the spring just rattles around. The microphone capsule is not great quality, battery life is short and the germanium transistor circuit is noisy. And although the microphone element is certainly not of hi-fi quality, and the spring is ‘boingy’ rather than lush, none of that matters because it is quirky and fun and offers something unique. 

AKG DX11 capsule and circuit board

The owner of this one had enquired about converting the mic to run on phantom power, but that was not trivial because the circuit has a positive ground, and I also had doubts over whether we could supply sufficient current with phantom. The output of the microphone of the DX11 is unbalanced 15K ohms or unbalanced 200 ohms. 

AKG DX11 spring reverb

I added a small Neutrik 10:1 transformer to drop the unbalanced 15K output down to a balanced 150 ohms*, which makes it a bit more compatible with mic preamps and mixers, and wired to a normal balanced XLR output. The transformer fitted in neatly in the cavity behind the capsule. (*You could use a 1;1 transformer from the 200 ohm output).

Schematic for AKG BX11 Echo mic
DX11 circuit hacks – Click on the image to see a bigger version.

After a bit of initial testing, I came to the conclusion that reverb is better than the microphone, and so, after a discussion with the owner, we decided to fit a line input to the reverb circuit. The microphone capsule itself has a measured output of 250 ohms, so it was easy to hack into the circuit at that point, via a switched mini-jack socket. Now one can run other things into the reverb, such as a better dynamic or ribbon mic, a quiet line input or a guitar via a DI box. 

Here is a picture of Adam from Extinct Audio test-driving the DX11 reverb (on the bench) with a Jazzmaster guitar…

I hope to share some sound clips soon.

Further reading

Vintage Microphone World

Hi-Fi Archive – advert for the DX11

Review of the DX11 in EQ magazine

Review of Extinct Audio microphones –

This week we had a couple of nice reviews of the Extinct Audio family of ribbons microphones. I was involved in the design of these microphones and the start up of the business and it is nice to see them gaining some traction. This review was published on the German language site Amazona. You can read the original version here, which contains lots of useful sound clips and pictures. Below is an English translation.


There are Englishmen that every child knows. Harry, Charles and William are currently the talk of the town, Ringo, Paul, George and John have written music history. Away from the limelight, there are four Brits, Stewart, Adam, Paul and Ant, who handcraft fine ribbon microphones in their small microphone forge Extinct Audio. Our author Raphael Tschernuth took a closer look at these fine British mics.

Ribbon mics have enjoyed unbroken popularity over the past two decades. While they were almost extinct until the early 1990s, they have been celebrating a resurgence at least since the Royer 121 and digital recording. The name Extinct Audio is an allusion to the fact that this type of microphone had almost disappeared from the face of the earth and was considered extinct. An old “Snake Stone” fossil, a local legend, serves as the company logo for the young English company.

Extinct Audio was founded by none other than Stewart Tavener of, one of the reference addresses when it comes to ribbon microphone repair. In 2007 Stewart started to offer his service for ribbon microphones and since then the order book is bursting at the seams.

Some of you might be familiar with the microphone blog of Xaudia Elektrik, where you can find a lot of information about long gone microphone models of all kinds (links in the box below).

Since the founding of Xaudia, Stewart has had well over 10,000 ribbon microphones on his desk for repair. He knows them all – from great RCA classics to fancy designs from Italy or Denmark to modern China replicas. His experience over the last few years has finally led him to not only repair microphones, but to develop independent designs and bring ribbon microphones to the market himself.

Together with his friends Adam and Ant, he started Extinct Audio, with the goal of offering handmade ribbon microphones at a reasonable price. Stewart and Adam are musicians themselves and know how tough life can be with this profession. But not only did they want the mics to be affordable, they also wanted them to be responsibly made from local resources and meet the highest quality standards.

In fact, the company that builds the bodies is located just a few miles from the Extinct Audio workshop. The transformers required for a ribbon microphone are of their own manufacture. The wooden boxes of the microphones are made by a company that otherwise mainly produces wooden boxes for high-quality whiskey bottles. And yet the English somehow manage to keep the price in the €900 range, while other mics, such as the 121 from Royer or the Coles 4038, have seen huge price increases, especially in recent years.

The underlying design of the two Extinct Audio microphones under review is based on the Bang & Olufsen BM3 which was developed in Denmark in the late 50s. This microphone was at the time one of the smallest ribbon microphones, its form factor was only made possible by the development of an ever stronger magnetic materials.

Perhaps some of you are familiar with the really thick ribbon microphones from the 1930s, such as the Siemens M25 or the BBC Marconi, each weighing in at over 4 kilos. The high weight and the high mass were necessary at that time, because there were no magnetic alloys with which strong magnetic fields could be realized in a space-saving way, as today with modern neodymium variants.

The BM3 from Bang & Olufsen was a revolution in the 1950s and was and was an inspiration for the Royer R121 in the 90s.  The newly available neodymium magnet materials helped the Royer 121 achieve tremendous output. The Extinct Audio BM9 is also based on the concept of the BM3, even keeping the BM naming for “ribbon microphone.”

The BM9, nicknamed “Viking,” is a general-purpose ribbon microphone with a pure figure-of-eight characteristic. In terms of sound, it is clearly in the tradition of legendary ribbons such as the RCA 44 BX, Melodium 42b, etc. Its full-bodied bass range paired with brilliant highs gives voices and instruments an inimitable “Bigger Than Life Sound”. The proximity effect is, as typical for this type of microphone, very pronounced and therefore a little distance to the sound source is recommended.

No less than the great John Williams used various BM9s for the recording of his soundtrack to Star Wars for the orchestral recordings. And this for a production that can afford any microphone imaginable. This could almost be called an accolade for Extinct Audio (not to be overlooked from about 30 seconds on):

The Black Ops, on the other hand, is recommended for all applications where the microphone is to be positioned close to the sound source. It is particularly well protected against wind and air currents, and its specially adapted transformer ensures a well-balanced sound even at a very close distance from the sound source.

This allows the Black Ops to be positioned directly next to a guitar amp or snare drum, for example, without having to equalize the bass range by using an EQ. Like the BM9, the treble imaging is incredibly detailed and smooth for a ribbon microphone. The Black Ops was designed for harsh live use; among others, the English band Foals relied on various Black Ops for their recent live tours.

The “classic” BM9 Viking uses a 1.8 µ thick ribbon, the Black Ops uses a slightly thicker 2.5 µ. For comparison: the thread of a Spider’s web is about 6µ thick, a human hair even 50µ. Therefore, one should be as careful as possible with this type of microphone.

The frequency range of both mics is between 30 Hz – 15 kHz and sonically the foreground and rear are absolutely identical. While the BM9 has an impedance of 300 ohms, the Black Ops has an impedance of 250 ohms – in practice this is irrelevant, as both mics with these values will harmonize perfectly with current preamps. The sensitivity of the BM9 is 2.23 mV/Pa, a high value that does not demand too much from the microphone preamp. Thus, the BM9 delivers a higher signal than many a moving coil candidate. Nevertheless, it should be mentioned at this point that ribbon mics often benefit sonically from stand-alone, external preamps. For entry-level or mid-range audio interfaces, an inline amplifier is a good choice, and the market is now teeming with them. Alternatively, one can also fall back on an active variant.

While testing the two mics described above, the manufacturer posted on Instagram that development of the active variant was complete. Without further ado, I made an effort to get hold of one of the first series models for the test and I was sent the first available stereo set. The active circuit increases the output signal by about 22 dB of noise-free gain. This significantly relieves the preamp and the recorded signal is comparable to that of condenser microphones. We will also consider this model in the practical test.

Extinct Audio offers manufacturing character par excellence with its microphones. When unpacking the microphones, the testers can’t help but be amazed at how much love and attention to detail has gone into the work.

The wooden cases are, as already mentioned, not cheap barware from the Far East, but extremely solid and excellently crafted. They offer perfect protection for one or two microphones. A serial number is emblazoned on a metal plaque on the outside, and inside the casket a signed certificate provides information about the day on which the microphone was manufactured and which employee took care of which production steps. The microphones themselves are beyond reproach in terms of workmanship.

This is pure perfection, without compromise. Metal workmanship and are excellent, logo and metal gauze perfectly attached, all edges rounded, the serial number engraved on the back – that’s how it should be. With their 355 grams, the Black Ops and BM9 feel good in the hand and radiate their very high value. Due to their light weight, they can be securely positioned even with simple microphone stands and are easy to set up. A screw-in mount was also included with the review sample, but ordinary medium-sized microphone clamps can also be used.

If there is one application that one immediately associates with the Royer 121, it is the miking of a guitar amp. This ribbon mic has earned legendary status since its introduction in the late nineties. Compared to many classic ribbon mics, it delivers a somewhat thinned-out bass image and can be positioned relatively close to the amp. With an old, venerable Coles 4038, for example, the immense proximity effect would absolutely require the use of a high-pass filter. So with ribbon mics, it’s always a matter of finding the sweet spot and using it profitably in your work.

In the following audio sample, you can first hear the mentioned Coles 4038 at a distance of only 13 centimeters. The recording is much too bass-heavy due to the proximity effect. After applying a high pass filter (100 Hz, 24 dB/oct) the tide turns and the sound suddenly becomes usable, possessing a “creaminess” typical of ribbons:

In comparison, you can hear the sound of the Shure SM57 dynamic moving coil microphone, which is considered the “industry standard” for amp recording. This is much more “grainy” and garish to work.

In terms of sound, the Extinct Audio BM9 Viking ribbon microphone is on the “classic” side of power. Similar to the Coles 4038, it therefore requires the use of a high-pass filter to deliver a balanced signal at a distance of only 13 centimeters:

The Extinct Audio Black Ops, on the other hand, does not require an additional filter, since the bass increase was compensated for by the transformer used. As a reference, you will also hear the Royer 121 afterwards, unprocessed at a distance of 13 centimeters from the speaker. Again, an HP filter would suit the signal well. With Black Ops, on the other hand, the signal is immediately “mix-ready”.

The different proximity has a direct impact on the use. Positioning all microphones at the same distance in front of a sound source would therefore be extremely bad for a meaningful test report, since every microphone deals differently with the bass boost and has its own sweet spot. The BM9 is full-bodied, with the Royer 121 the proximity effect is somewhat reduced and with the Black Ops it is somewhat less than with the Royer. On the acoustic guitar I therefore decide on three different distances

Extinct Audio Black Ops: Distance 30 centimetres

Royer 121: distance 40 centimetres

Extinct Audio BM9 Viking: distance 50 centimeters

Many thanks at this point to the really impressive singer/songwriter Fabian Holland, who recorded the acoustic guitar for this test.

The test shows that the Extinct microphones can easily compete with the Royer 121 in terms of sound quality. This is impressive when you consider the price difference of almost 1000 €. Ribbon microphones are also excellent for imaging percussive instruments. One reason for this lies in the natural mapping of the transients, which are mapped very quickly and vividly.

In the following example you hear the drummer Achim Färber. The BM9 stands a few meters away from the drums. It is a mono recording without any effects. The Black Ops was positioned on the snare, also without any change in signal:

During the test period, the Black Ops has become one of my favorite microphones for the snare drum. The figure-8 characteristic makes it excellent for masking out adjacent toms or hi-hats. The strong attenuation on the 90- or 180-degree axis can be used excellently.

Last but not least, here’s a short sample I recorded myself with two BM9s as overheads and a subkick. As before: no EQ, no compressor… nüschte, as the Berliner says.

During the test period, I had the chance to use the Extinct Audio Mics on many other sound sources. They delivered excellent results on cello, as a stereo Blumlein set on piano, or on vocals with soft S sounds. The stereo pair was also perfectly matched before delivery, and the two measurement curves were absolutely identical.

By the way, the active version sounds exactly like a passive BM9, only louder. Sure, you could also loop in an inline preamp like the Fethead, but that makes the construction a bit unwieldy on the one hand, and on the other hand the shielding falls by the wayside. Unfortunately, inline preamps are prone to turning cell phone radiation into audible tones. When the network builds up, beeping and whistling can therefore be picked up unintentionally. The active version is well shielded in this respect and does not transfer any noise to the audio signal. And by the way, it looks much better…

Final Thoughts

Chapeau! The small microphone workshop Extinct Audio, delivers with the BM9 Viking and the Black Ops two ribbon microphones that leave nothing to be desired. The workmanship was done by hand with great attention to detail, the materials used are high quality, locally “sourced” and guarantee a long life. Each microphone is individually measured before delivery and stereo pairs as well as active +48 variants are also available. The latter brings the already high output to the level of a condenser microphone. Sonically, the BM9 is in the tradition of legendary ribbon microphones, while the Black Ops is predestined for close miking. Both reproduce percussive instruments with tangy transients, deliver vocals with smooth S-sounds, and are predestined for a wide range of applications and sound sources. In addition, they possess a remarkable top end for microphones of this type. Combined with a very good price/performance ratio, this leaves an unclouded picture that clearly stands out from many Asian competitors due to the high quality.


Excellent workmanship

Very good stereo matching, measured values

High output

Very versatile

Great, classic ribbon sound (BM9)

Balanced sound when positioned close to the sound source (Black Ops)

Durable design, handcrafted, made from local resources

Beyer M500. A.K.A. Why does my microphone sound like a kazoo?

OK, this might be a little bit controversial: the M500 is my least favourite ribbon microphone from Beyerdynamic. This is for a number of reasons. I’ll list them.

  1. The transformer rolls off a lot of bass. More so than an M260.80.
  2. The grill has a dense cloth screen which also reduces the bass response.
  3. The internal baffle has a plastic plate in front of the ribbon which robs yet more bass and causes other problems, as I will explain in a moment.
Beyer M500 and a Kazoo. Can you tell the difference?

These three points are all intentional parts of the design. The M500 was intended as a vocal microphone and designed to be used at point blank range. The severe bass cut compensates for the proximity of the ribbon, and the extra screening and baffle protect the ribbon from plosives and other blasts of air. So to some extent you have to take your hat off to Beyer and say ‘chapeau‘ for making a ribbon that can compete with an SM58.

But that’s my point really. Ribbon mics naturally have a big rich proximity effect, which is one of the things that we love about them.  In the M500 a lot of work has gone into removing that sound. To my mind, the M500 is good for one job only (close up vocals), and there are other microphones that do that job better. If you like to chew your microphone then you might as well use an SM58 (or better still a Beta 58), and use a ribbon for those places where it excels. 

These microphones are no longer made but change hands on eBay and Reverb at inflated prices. I think the biggest issue with them is expectations. Some buyers expect them to sound close to, say, an M260, and are disappointed when they don’t. In particular they sound very thin when used on instruments at a normal recording distance. At Xaudia we often get asked to modify these mics to have more bass and have more of a classic ribbon sound.

The motor and ribbon are more or less the same between the two mics, but everything else is different.  It is possible to get a better bass response from the M500 by swapping out the transformer for a full range model*, which helps a lot. But the mic can still sound odd because of what is in front of the ribbon.

So let’s get back to that baffle that I mentioned in item number 3. Here at Xaudia we call this the ‘Kazoo Flap‘. Let’s take a look at it.

The M500 “Kazoo Flap”. This bit of perspex causes trouble.

You can see that there is a flat piece of clear plastic in front of the ribbon. The problem here is that is can vibrate and come loose over time. I often see these come unstuck at one end and it can vibrate like the reed on a clarinet or saxaophone. It sounds like a kazoo solo has been added to every recording!

Luckily it easy to fix. The baffle can be re-glued or removed completely. I often do the latter and replace it with some fresh acoustic fabric and steel mesh. 

M500 baffle modifications.

Swapping the transformers and the baffle gets the M500 as close to an M260 as it can be. The grill construction and the acoustic chamber behind the ribbon are different between the two microphones so they can never be quite the same, but the changes improve the bass response by huge amount and, to my ears, make it sound like a proper ribbon microphone.

Beyer M500 – designed for close up use on vocals.


* We make a suitable full range transformer that will fit inside an M500.

AGA-Baltic T2 ribbon microphone

Xaudia has been running for about 15 years and we get a lot of repair work which now seems routine. We see hundreds of Reslos, B&O mics and Beyers in particular. When I started out, everything was new but it is now a rare occurrence to see a ribbon microphone that I have not worked on before. Sometimes I think I have seen it all and then something new comes along. These moments are exciting and a new challenge is always appreciated.

AGA-Baltic T2 Ribbon Microphone

This rare Swedish beauty came in for repair recently. Not only had I never worked one one before, I had never even heard of it! This was made by AGA-Baltic, who (according to Swedish Wikipedia) came into existence in 1928 after a merger between AB-Baltic and AGA companies. They made some rather elegant radio sets and, later, TV equipment. They clearly made microphones too.

Motor and transformer assembly from AGA microphone

From the magnet and transformer technology, I would date this as an early 1930s microphone. The motor comprises two large steel pole pieces sandwiched between four large horseshoe magnets, and it uses a massive toroidal transformer to raise the output voltage and impedance to 30 ohms. The holes around the ribbon are most likely designed to increase the high frequency response of the microphone. The only comparably sized toroidal transformer that I have seen is in the Siemens / Telefunken M201, which dates back to around 1928. Transformer technology got smaller quickly!

AGA motor after corrosion removed

The magnets in this microphone have held their field well and I measured about 2500 Gauss in the ribbon gap, which is fairly respectable for a microphone of this age. For reference, a good Melodium 42B would measure about 3000 Gauss. However, the whole motor assembly was corroded, so I stripped it down, treated with Rust-Off and then re-painted to prevent further corrosion. Everything else was cleaned and new grill cloth and mesh fitted. With old microphones that have been out of service for decades it is often best to do a complete rebuild.

AGA hardware parts being cleaned.

With a new ribbon in place the sound was weak and disappointing. The old toroidal transformer has not aged well and the transmission of high frequencies in particular was attenuated giving a dark woolly tone with a lot of noise. It also seemed to be a good aerial for picking up hum too.

Impedance matching didn’t help so we were faced with the dilemma of keeping it all original or switching out the transformer and having a useful working microphone, but potentially losing some of the value to collectors.

AGA Baltic T2 – Half way back to working condition.

This situation is always a difficult choice, but after consultation with the owner we decided that the microphone should be a working studio tool, and the most sympathetic repair would be to fit a new transformer from a Coles 4038. This replacement is also a toroidal transformer so we keep the ‘toroidal sound’, if there is one*. (*There probably isn’t.) The old transformer will of course be returned to the owner and can be re-installed in the future for collectors value.

And the results were good! The new transformers has a much better high end response, lower noise floor and does not pick up hum. The microphone is quite useable and still sounds very much like a vintage ribbon microphone. A well designed transformer should be there for impedance matching purposes and should not be contributing too much to the sound anyway.

Restored AGA-Baltic ribbon microphone

I wonder what other rare and unusual microphones are out there waiting for me? Surprises and challenges are welcome!

Thanks to Raphael Tschernuth of Gear And Sound.  

Further Reading. There is not much information on the internet about these microphones but this does get mentioned briefly at the AGA museum

Decca Ribbon Tweeters part 2

Last week I posted about repairing some vintage Decca London ribbon tweeters. I was confident that I had done a good job but was unable to do listening tests in context as I only had the tweeters to play with. These are now back with Andy, their owner, who kindly sent in his impressions and some photos. Overall I think we have done a good job and it’s a nice feeling to have helped get these back up and running.

Decca Volt speaker with ribbon tweeter

“The [new] ribbon sounds smooth, crisp and clear with no tizz or distortion. I ran it for a while at different volumes and with different music and it sounds excellent with everything I gave it. It is very close in sound to the original in the other speaker with the soundstage being in the middle and correct. I had to try hard to notice a difference, Bob Marley’s Is This Love has some high frequency cow bell drum noises and you could hear they were slightly crisper and louder from the new ribbon. The crossover point in the Volt’s is about 1200hz so any imbalance between the two tweeters is easily heard as the singer tends to move off centre.

New ribbon installation at Xaudia

“I think the differences could be classed as being between an old and new ribbon the sound from them is so close. I assume as the new ribbon breaks in it will soften in sound a bit. I am tempted to switch the other new ribbon in as well.

Andy’s impressive hi-fi setup.

“Considering the lack of info on the ribbons I thought getting a close match sonically was not good but it’s turned out to be very close indeed. It is a huge relief to know that if a ribbon gets damaged again I can get help.

“I am not sure but you might get a lot of interest, when I was trying to find someone to repair my ribbons I came across a huge amount of forum posts on the same issue dating back about 5 years. Nobody had managed to find anyone so in theory there are a lot of ribbons to repair out there.”

Thanks to Andy Mcgregor.

STC 4136 revisited

STC 4136 microphone

A long time ago I posted about the STC 4136 condenser microphone and how to modify it to work on standard 48V phantom power, using a small circuit built on perfboard.

STC 4136 microphone, in pieces.

I had another opportunity to work on one of these microphones. This one was a challenge as it had no circuit inside at all, just the capsule and bodywork, although that also gave free scope to start from scratch.

STC 4136 microphone capsule.

Space is tight in this microphone, but with a careful layout and small components everything will fit neatly. My solution for this one was to fit a small KM84 circuit and Neutrik NTE10/3  transformer. This transformer performs well, does not break the bank, and can be squeezed into tiny spaces where nothing else will go. 

New circuit board and transformer

This time I commissioned a printed circuit board rather than working on perfboard, which does save time and look more professional. The board will be useful for other projects too – I have a handful of other small microphones which would benefit from updated, lower noise circuits.

Made in England

Thanks to Robert at Russell Technologies for the board layout and advice.