The Xaudia Blog

Sony Condenser Microphone Evolution – C37, C38, C48 and friends

Sony made a series of excellent large diaphragm condenser microphones, some of which have become studio classics. The earliest production model was the C37A, a tube microphone which may or may not have been used by Frank Sinatra – he has certainly been pictured with one – and it was definitely used to record the voices of Bugs Bunny and Daffy Duck. It has become sufficiently iconic for at least two clones to appear, made by Mojave and Tonelux.

On test – Sony C37p, C38b, C48 and C450

The C37P is the FET version of the C37, and is claimed to be one of the earliest production transistorised condenser microphones, although that honour is generally given to the Syncron AU7a. The C37p was followed by the C37Fet, and the C38 models. All of these microphones feature an unusual capsule design with an internal vent, which could be moved with a screwdriver from the rear of the microphone, changing the pattern from cardioid to omnidirectional. A very similar capsule is now made by Josephson Engineering, which is used in some of their own models and also rumoured to be supplied to other brands.

Sony C37 capsule – front
I am a big fan of the C38p and for the past 20 years have almost always used one for recording on my own guitar amplifier, often alongside a Shure 57 or whatever else is free after setting up for the rest of the band. The Achilles’ heel of this design is that the mechanism can become difficult to turn, and many the C37s and C38s are stuck in the cardioid position. For many users that is not a problem.

Sony C37 capsule – rear view

The later C48 and C800 models use a different dual sided capsule with centre terminals, which looks similar to those used widely by Neumann (and almost everyone else.) The C48 is a versatile microphone with three available patterns, bass cut and pad switches. 

Both the C38 and C48 models can run on 9V batteries and use a DC converter inside the microphone to generate the polarisation voltage for the capsule. These converters are tricky to repair when they fail.

Sam Inglis & I recently had an opportunity to compare a few Sony condenser microphones to see how they had evolved over time. On the day of testing we had access to a pair of C37p, one C38b, a C48 and a C450. In addition we looked at a Josephson C705 for comparison.

By comparing the microphones we hope to discover how these Sony LDC mics compare and are the new ones better than the old ones (or vice versa)? Does the capsule hold the key to the Sony sound, if there is indeed such a thing? And are the new capsules from Josephson good copies of the old Sony C37/C38 capsules?

As usual, the frequency sweeps are good for comparison purposes but are run in a small booth and should not be considered absolute measurements. The reference microphone is omnidirectional, which can cause a few inaccuracies when testing cardioid transducers. The dip around 12K is an artefact. Measurements were made at 30 cm distance from a concentric speaker. Measurements were made with a swept sine measurement and recorded using Fuzzmeasure Pro.

1. Comparison of C37p, C38b and C48

Comparison of C37p (lowest), C38(middle) and C48 (top)

As one would expect, the major change between models is the increasing output level (and signal to noise). The C37p is a very early transistorised microphone and transistor technology changed rapidly in those early years. It has a nice sounding bass proximity boost, and the mic is equipped with four EQ settings marked M, M1, V1 and V2 to compensate. (M is the unfiltered output).  The two C37s that we tested were very close, which is reassuring.

I own a C38b which I use it for my own recordings, and my perception is of a warm sounding microphone, without a harsh or hyped top end. I was surprised to see that it has a bit more of a top end lift than its predecessor.  It carries the M, M1, V1 and V2 settings through from the C37 and also adds a high cut switch. The M1 position acts as a high pass filter whereas the ‘V’ or vocal positions are more like a long shallow shelf. 

C38b filter – M (red),  M1 (blue) and V1 (green)

And the C48 is louder again. This is a more conventional multi-pattern microphone with electronic switching between cardioid, omni and figure-8. It also has bass cut and pad switches which are accessible via  a sprung panel on the rear of the microphone.

In my experience the C48 is a top quality recording tool and makes a good alternative to a U87 – I know a few engineers who prefer it to the Neumann. Barkley McKay at Valleywood Studios said “one of the reasons I like the 48 is it’s gentle roll off before 16k – it’s a little like an enhanced ribbon.

2. C48 vs C450

Sony C450 capsule, front. 

The C450 is a less well known model which looks a lot like the C48 but with some cheesy 1980s stickers applied. However, the C450 has a smaller capsule, around 24mm diameter, and runs on a single 1.5V battery.  We have not discussed or measured the C500 here because I don’t have access to one, but I have a hunch that this is a similar size to the capsule in that model. Can anyone confirm that or share a picture of their C500?

Sony C450 capsule – rear

The C450 has an average output level around 11 dB lower than the C48, but is significantly brighter at the top end. It is equipped with high pass filter and -10dB pad switches, although you won’t need the pad very often! The noise level is also a bit disappointing. It probably has a role on the right sound source but I haven’t found it yet.

Frequency sweeps of C48 (top) and C450 (bottom)

3. Does the Josephson C705 sound like a better C37?

Josephson C705 in the testing booth at Xaudia

The Josephson C705 is an excellent microphone with a reassuringly solid feel, and does indeed sound much like a good C37p, albeit 12 dB louder and with an even better improvement in signal to noise. It is a transformerless circuit and is cardioid only (like a lot of C37 and C38 once the capsule becomes stuck!). It lacks the high pass filter options of the Sony microphones. The C705 it is not trying to be a clone or tribute in any kind of cosmetic sense. I admire Josephson because they do their own thing and try to make the best products they can. In this case they have done some clever things with the grill and acoustic environment around the capsule, moving the vertical support struts further back out of the way of the cardioid capsule. At around £2500 this is not a cheap option, but is a professional recording tool which will get you close to the Sony C37 sound.

Sony C37p (purple) and Josephson C705 (green)

One final thought – Whilst doing some background reading for this post, I found that the older Sony mics were not highly regarded on internet forums back in the early 2000s, compared to Neumann and AKG. Here is a certain Mr K,.H.’s informed opinion.. 

“I never liked the C37A all that much, and would put it into the category “Post War Japan makes good” (as in their tiny late 1950s sports cars patterned after Fiats.) The C37A always struck me as a poor cousin of a ….?? Neumann mic: pretty poor craftsmanship, akin to what the Russians did in the 1970s; not much personality, pretty bad tube choice, cathode follower circuit with its associated gain and dynamic problems, etc… …and then Sony went downhill from there with its FET mics… so I think.”

I disagree. And the first thing I will do when I die and go to heaven or hell or Valhalla will be to march up to Frank Sinatra and ask him if he really did use a C37a! 

Electrovoice PL10

The Electrovoice PL10 is a cardioid dynamic microphone which looks very much like a cut down version of the popular RE20.  It has no transformer or filter circuit but it still sounds excellent. These omissions presumably kept the price down.

Just like the RE20, the foam that holds the capsule in place can cause trouble. Over time this can degrade into a sticky pulp and then the capsule becomes loose and rattles around and may eventually destroy itself. This microphone needs new foam and a good clean right away!

The microphone body is in three parts – grill, body and base – which are screwed together and some kind of glue applied. I had to heat the threads and apply more force that I would like to break the glue and get the microphone apart. That was the hard part of the job, and once opened it is easy to remove the foam with a bit of isopropyl alcohol. As always, care must be taken around the diaphragm to avoid damage. The metal parts went into the ultrasonic bath and cleaned up nicely. 

Once back together, the PL10 is an excellent sounding dynamic microphone. The PL10 should be a cheap alternative to an RE20, but in fact they are scarce, and prices on eBay and Reverb may be higher than an RE20, which is a bit daft. 

Here are some comparison frequency sweeps of the two mics conducted at around 25cm from the source (as usual take with a pinch of salt.)

Frequency sweeps for RE20 (red) & PL10 (blue)

AKG D99c manual


Here is the German language manual for the AKG D99c, also known as Harry.

An English translation courtesy of Google…

Our first artificial heads for experimental stereo listening were made in 1949

The Physics Institute of the University of Budapest conducted experimental tests. Many years of well-founded knowledge of the physical and physiological relationships of human hearing and the experience as a specialist company for electroacoustics have enabled us to make a noteworthy contribution to the artificial head stereo microphone, which has become current again, with the series production of a recording head within a short time. 

The interest of a large circle of committed tape fans and serious amateurs in an inexpensive one, which has recently been aroused by many press reports and extremely positively rated AKG demonstrations among trade visitors. We can now match the artificial head for stereo recordings with the AKG D99c stereo recording head, which is available now.

The material structure of the head and shape of the ear cups, in conjunction with a simplified but acoustically effective simulation of the ear canals in connection with the two special dynamic transducer systems integrated in the head, are tailored to an optimal recording [which is] analogous to human hearing.

When listening through high-quality headphones – regardless of whether closed or open earphones are used – the listener experiences an intensive acoustic sense of space that cannot be achieved when recording with two individual microphones. 

The artificial head as a physical structure itself is above all a prerequisite for the natural occurrence of the factors that determine the spatial impression, such as the difference in intensity, the difference in transit time and the frequency-dependent shadowing effect from one ear to the other. 

The AKG D99c stereo recording head, with its optimally designed frequency response (it must be viewed with different criteria than usual microphones), always ensures consistent objective recording properties and can be used as a “double”, so to speak, where you need to keep your own head free for directing.

The stereo artificial head recording is particularly interesting where moving sound events are involved. For example, playing, singing children, lively design of radio plays or reports; also outdoors, for example with passing cars, noises in the background and much more. 

A further aspect: when recording conferences, it is easily possible for the listening recording clerk to concentrate on a specific speaker and to record his words due to the acoustic-spatial localisation. The spatially lively playback effect of the AKG stereo recording head D99c can be achieved with any AKG stereo headphones of your choice, regardless of whether the recording comes to the listener live via an amplifier or, as a tape recording, is listened to afterwards.

Technical specifications:

Frequency Range: 50-12500Hz

Sensitivity: 2.0mV/Pa = 0.20mV/ubar/channel

Impedance: 600 ohms per channel

Channel deviation: <3 dB for the entire transmission range

Optimum recording height by using an AKG floor stand.

Harry the Head – modification to condenser mics

The AKG D99C is a binaural dummy head microphone, often known as ‘Harry the Head’ or just plain Harry. Like me, Harry was born in the 1970s and still looks good for his age, albeit with a slightly fuzzy finish (which presumably is to reduce reflections). Inside his head there are two rather basic dynamic microphone which point upwards towards the vents in his ears.Harry may look good but many of his kind sound disappointing and the sensitivity is low. My own D99C sounded particularly poor with a lack of low end response and a sort of crunchy mid range. 

Harry the Head, or AKG99C to his friends

And so here is an easy way to improve the sound and sensitivity – fitting two small omnidirectional condenser mics inside will make Harry much more useable. What is more, this modification is completely reversible. You can go back to the former crunchy lo-fi tones or retain the vintage value for a collector in the future. 

Audio Technica ES945. Remove the grills and mounting nuts

For the upgrade I used two Audio-Technica ES945* boundary condenser microphones, which can be found used for reasonable prices (I paid £33 each on ebay) and I made up two short and skinny XLR cables. You will also need some needle-nose pliers and a screwdriver.  Let’s go!

Microphones, cable and aluminium strain relief 

Carefully peel off the base of the head, using a scalpel if needed to remove the glue. You can see two screws that secure the cable strain relief. Undo these. Remove the two circlips which hold the microphones in place. Then pull out the whole assembly of cables and two dynamic microphones.

End view of the AKG dynamic mics.

Undo the three small screws that hold the grill of the boundary mic in place and remove the grill. Also take of the big nut and rubber mounts if they are still attached. The mic is a little too narrow in diameter and I put a layer of heat string around it, but you could use some tape here. This stops the mic from rattling and moving around.

Then fit the cables and push the microphones into the holes in the head. You will notice that the mics are at 90 degrees to the ear-holes.

New mic with shrink sleeve and XLR cable attached.

The next thing to note is that the position of the microphones is critical to the sound. If you push them too deep past the ear vents then the microphone is essentially blocked and sounds tinny and horrible. But too gar below the vent it will sound hollow. I placed them by ear (sorry for the pun), listening as I moved the mics up and down, and settling for an optimum position. just below the ear vent.

Approximate position of the microphone inside Harry’s head.

Once the mics are in position, thread the cables through the channels in the bottom of the head. You can replace the cable clamp if you wish, although that will require unsoldering the cables. (I used a hacksaw here to liberate the cables!). Then stick the base of the mic back on and the job is done. Now it is time to go out and record something.

Routing of the XLR wires under Harry

Personally I think that the AKG D99c, with it’s cubist styling, is the best looking binaural head microphone, although the Neumann KU100 at £6800 GBP must surely be a better microphone!  

Whether binaural recordings sound better or worse than spaced pairs or every other kind of stereo recording is a discussion for another day. 

* I am sure that other models of boundary mics would work here just as well or better. I can imagine using a couple of Oktava MK012s with omni capsules, or even a pair of 451s if you want to stay with the AKG brand. Perhaps we should consider Harry to be a microphone holder, rather than just a microphone.

Here is the German language manual for Harry, along with a translation (by google, sorry).

You can read more about Harry at Vintage Microphone World.

Update 26 August – this post has been called ‘Sacrilege’ by Heinbach… “I call sacrilege. The grainy tone of Harry is part of its Charm.” 

Xaudia Active Dynamic Microphone (ADM)

The ADM is my take on the ‘Speaker as microphone’ concept. You may have come across this idea elsewhere, in certain vintage microphones, perhaps using a larger speaker as a sub-mic for kick drum, or in some boutique models, some of which are a bit low fi, but can be surprisingly good. 

A small speaker and a dynamic moving coil microphone operate on the same principles, with the differences being in the details such as mass of the coil and diaphragm. Ideally a speaker should be robust and handle some power, whereas a microphone element might be as light and sensitive as possible. Headphone speakers are generally small and light and can make decent microphones. One classic example is the Beyer M380 which uses the same element as (older) DT770 headphones.

The ADM uses a genuine new-old-stock Sennheiser headphone speaker which sounds very nice when reversed and used as a microphone. Like the Beyer M380, this has a figure-8 pickup which means that it also has a decent proximity effect and good side-rejection. The impedance of this speaker is a little higher than most microphones, and so I have fitted a phantom powered balanced buffer circuit to lower the impedance, reduce the noise floor and increase the common mode rejection, as well as increasing the output level. Overall it works very nicely.

I have a limited supply of parts and so this will inevitably be a limited run. 

Further details and sound clips to follow soon. Available from September 2022 at £199 plus postage.

Calrec 6-series Microphone Upgrades

These Calrec small diaphragm condenser microphones have been kicking around in the workshop for several years. They sound nice but the noise floor was always too high to be of much use, and they also need a specific power supply rather than standard 48V phantom. And they are difficult to service because the high impedance section of the circuit board is potted in what looks like epoxy resin – including an electrolytic capacitor. If anything goes bad in there then it is near impossible to replace.

Consequently these never got used and nobody wanted to buy them.
I decided to hack them and my approach to these was to do a full rebuild using a new circuit board from Russell Technologies. The new circuit is based on designs by Okatava and Schoeps, and is a simple two stage transformerless design. It works well with the Calrec capsules and sounds nice and full without being overly bright.
A couple of things to note. The connection to the capsule is made by a copper spring, This needs to be removed from the old circuit board, cleaned and soldered onto the new board. The rear of the capsule should also be cleaned to remove any oxidation.
Top – New circuit board with spring and locking ring..
Secondly, there is an internal locking ring which holds the circuit in place. This is expanded using a grub screw. This ring needs to be re-used here to ensure a little compression of the spring and good contact with the capsule back plate.
Xaudia can offer this as a repair upgrade service. Please get in touch if you need further information.

Swapping the motor in Beyer ribbon microphone.

Beyer sell replacement motors which allow repair of some of their microphones although the cost in the uk is rather high. Swapping the motor is a bit fiddly but can be done with patience and care. This is more or less how I do it, although the models do vary a bit and you may need to improvise.

Beyer M160 motor with rounded edges (left).

First note that Beyer motors may have round or square edges at the rear of the magnets. For many microphones this won’t matter, but the M160 grill will only accept motors with rounded magnets. In the picture above it looks rather crude like it was rasped down with a file! It is probably possible to make a square edged motor fit an M160 but the magnetic filings would surely wreck the ribbon,

Also worth mentioning is that Beyer M160 and 130 have two ribbons whereas M260s have a single ribbon.

To exchange the motor, first one needs to remove the socket and unsolder the connector and transformer.  The socket may be glued, screwed and/or pinned in place. Remove the screws or pin. If the socket does not slide out then it is glued and will need heating until the glue fails. Do this at your own risk! 

Then unsolder the socket, remove any rubber grommets and unsolder and remove the transformer.

Beyer M260 with socket and transformer removed

The next job is to pull the motor through the acoustic labyrinth but keep in mind that you will need to reverse the process in a few minutes. I usually solder an extra piece of wire into the end of the motor leads which will let me pull the new motor wires back into place.

Remember to solder on a guide wire before you remove the motor

Once this is done you can pull the motor out and then unsolder, leaving the new wire in place.

Pull the motor and guide wire through the body

Unsolder the old motor and then reverse the process. Solder the new motor wires onto the guide wire, pull through the labyrinth and then unsolder. That parts is easier said than done because you are pulling some stiff wires around a bend that you can’t see. Reattach the transformer and socket. 

The only job left to do is to check the polarity of everything. Beyer do not always colour code their wires so you have to guess. Compare the microphone to a good modern mic. If your repaired Beyer is out of phase then simply reverse the wires at the XLR or din plug.

As a final warning, I have found that Beyerdynamic parts vary a lot and they do love to glue stuff together – why spend money on a screw or two when a tube of glue will do?  Be prepared for a certain amount of frustration and keep the swear-box to hand.

Synchron / Vega S10 Microphones

The Syncron Corporation are generally credited with launching the first transistorised capacitor microphone back in 1964 – the AU-7a, which was priced at $169.50 USD. In their own marketing, the manufacturer claimed to have built “the first microphone to successfully employ the Field Effect Transistor.” At the time, competitors such as Neumann were selling microphones with tube circuits such as the KM64 and U67 which required a separate power supply. The Syncron mics ran on batteries which saved lugging around an extra box and cable. I know we all love and revere the old Neumann tube microphones, but few would argue that they take longer to set up and warm up than a solid state mic.

Syncron-Vega S10 microphons

Following the AU7a, Syncron launched their second and final microphone, which was a rather nice small diaphragm condenser known as the S-10. By this time, the brand was owned by ‘Vega Electronics Corporation’ and had the address on their documents had moved from Connecticut to Santa Clara in California. The price for new microphones was $260.  

I was sent three S-10 microphones to clean, service and convert to phantom power for a customer. These mics were badged ‘Vega’./ Like the AU7As, they were designed to run on two obsolete batteries, and so being able to run on standard +48V would be make them much more useable.

Vega S-10 original circuit and transformer, 

The mics were a bit of a mess. They had been stored damp at some point and showed corrosion inside and out. I wanted to be sympathetic to the original simple circuit  They are a one-transistor circuit with an output transformer, much like a Neumann KM84, and so for the phantom conversion I decided to re-build with a KM84 style circuit, keeping the original transformers. 

KM84 type circuit – Russell Technology board

With a bit of improvisation I was able to rebuild the circuits using circuit boards from Russell Technologies, utilising the space freed by removing the batteries to house the phantom circuit and the transformer. The circuit boards were originally designed as an upgrade for the AKG C480B, and uses a smaller transformer than the Vega. To accommodate the larger transformer I hacked off the end of the board and wired the transformer directly.

PCB from Russell Technologies – rear

S-10 with new PCB and transformer in place.

These microphones used a 4-pin XLR output. The fourth pin was used to make a connection to the battery within the plug of the connecting cable, which means that the batteries would not go flat so long as the microphone was unplugged between sessions. I swapped these for standard 3 pin XLRs of course.

Is four better than three? No.

Although we started with three microphones, one capsule was bad and one transformer was open circuit, so we have two nice condenser microphones, which are well balanced and sound good. The sound is a little less bright and a touch warmer than a KM84. I liked these microphones and was sorry to see them go home.

Syncron-Vega S-10 microphones


 Jason at Crunch studios kindly shared the photo below and sound clip of his Vega S-10 in action.

Recording drums with the S-10 at Crunch Studios

Sound clip of the S10 in action:

You can read more about Syncron microphones at the Coutant website. 

Thanks to Jason Baldock at Crunch Studios

Testing the Sennheiser MD409 family

I recently had the opportunity to test a bunch of Sennheiser MD409 and related microphones. These small dynamic microphones command eye-watering prices because they were used by a couple of famous rock bands in the 1970s. 

Devices under test – five MD409s, BF509, MD609 and another.

The prices seem high for such a simple device – the microphone consists only of two grills, stem with an XLR socket, frame, and capsule, along with some foam and a couple of screws to hold everything in place. There are no transformers in these microphones, and the output runs straight from the capsule to the XLR socket.

Inside the MD409. Photos by Lester Smith at Abbey Road.

All of those parts except from the capsule are also found in the current e609 model, which is not revered in the same way and can be found new or used for less than the price of a Shure SM57 .

So the magic must be in that capsule…. right?  Similar looking capsules were used in various other Sennheiser and re-branded models, although there can be subtle differences especially in the baffle at the front and the vents in the rear. The 402/3 capsule is often mentioned in internet chat rooms as a close alternative, and so I tested one for comparison, wired into the body of a similar looking Chinese mic.

Sennheiser 402/3 capsule

For the test I had access to five MD409-U3s, one BF509 and one e609, as well as a 403/3 capsule in a replica body. 

The questions I wanted to ask are as follows….

Firstly, do MD409s age well and remain consistent? In other words, if we have a conversation about “the 409 sound”, are we even talking about the same sound? 

Secondly, is the BF509 really the same microphone?

And thirdly, (for those of us without deep pockets), how close can you get with either an e609 or a reportedly similar alternative like the 402/3 capsule?

Tests were performed in the booth at Extinct Audio, with a swept sine wave method. Here’s what we found.

1. Frequency sweeps of five MD409-U3 microphones.

1. The MD409s are very consistent. We tested five used MD409s and four of them were very close to each other, with a fifth mic showing just a little less bass. The signature sound of these capsules is a 5dB presence bump in the 100 to 200 Hz region, with a dip below 100Hz.

2 & 3. MD409, BF509 and e609 microphones compared.

2. The BF509 sounds similar and the sweep and falls within the range of the five MD409 that we tested. 

3. The e609 is another nice microphone but is a little different to its predecessors. It also has a low end bump, a bit broader than the MD409, and has a broad lift around 5KHz. The top end rolls off a bit earlier than the older mics, which you may notice if you haven’t stood in front of a guitar amp for a couple of decades. Your dog will know. 

4. MD409 compared with a 402/3 capsule

4. The 402/3 capsule tested lacks the low end bump but shows a very flat response. It is nice but not the same.

In summary, the MD409s were consistent and the BF509 seems to be the same microphone. The e609 is a little different, but with a touch of EQ it makes a good substitute for those who can’t afford the current vintage prices, or who don’t want to take their precious artefact on tour.

Thanks to Sam Inglis for help with this article.

We have a Reverb store!

We have opened a store on for Extinct Audio parts and accessories.

The motivation behind this is the recent changes to import laws from the UK into Europe.

For orders of new goods under €150, we can ship direct without any additional taxes to be paid on import to the EU. The customer still pays the tax but it is collected by Reverb at the point of sale. This means that there are no unexpected bills to pay when you receive the order.  Consequently we would encourage our friends in the EU to buy accessories though our Reverb store.

The store sells our thread adapters, transform boxes and splitters.

This only works for lower value orders and so the shop will just be for parts and accessories. If you wish to order Extinct Audio microphones then you will still need to either buy direct from the Extinct Audio web shop, or from one of our international dealers.