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

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)

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.

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.

Sennheiser MD409 foam failure

This seems to be a common issue with old Sennheiser MD409 and similar microphones.

Sennheiser MD409 foam failure

The capsule is held in place by a block of polyurethane foam, which slowly decomposes over the decades, especially if the microphone has been in use in a sweaty rock venue.

Sennheiser MD409 new foam

If left for too long, that capsule will rattle around and there is a risk of more serious damage. the foam can also become sticky and contaminate the capsule diaphragm. If things have not gone too far, it is an easy job to cut some new foam, clean the grills and then the mic is ready for another shift in the studio.

MD409 pair with new foam cleaned and ready to rock

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.

A mount for an STC 4017 dynamic mic

A customer asked me to make a mount for his STC 4017 dynamic mic without modifying the microphone itself. Often these come with a handle or a threaded stud for mounting, but this one came with nothing at all.

My first thought was to make some kind of ring clamp, but that would require a large diameter brass tube and was starting to look quite expensive and bulky. So I came up with this…

It is simply a folded strip of aluminium screwed to a threaded brass cylinder. The bass of the cylinder is threaded to fit a 5/8″ mic stand. The mic slides into a slot in the aluminium and is held by its own ground clamp.

Some shrink sleeve ensures that the signal outputs are not shorted by the new mount.


Reslo Dynamic microphones

Reslo are of course best known for their ribbon microphones, particularly the RV and RV models, However, it should not be forgotten that over the decades they made many dynamic microphones, and even some condensers too.

Reslo VMC with base station

The earliest Reslo dynamics seem to be the VMC and VMC2, with the initals standing for Velocity Moving Coil. These had a big heavy permanent magnet and a paper diaphragm, and are really rather lo-fi devices. These two models are more or less the same inside, although the VMC was hard-wired, and the VMC2 had a new grill and an output plug.

Reslo VMC2, with output connector.

In the late 1960s Reslo made a range of more modern light hand-held dynamic mics. The UD1 used a Japanese-made capsule which sounds rather good, of comparable quality to the Shure mics of the era. I have seen transformerless 200 ohm models, and also a dual output 30 & 600 ohm version with a transformer in the body to convert the impedance.

Reslo UD1 microphones

The UD1 was apparently used by Bob Dylan at the 1969 Isle of Wight festival!  Here is an advertisement scan from the Reslosound blogspot….

The advertising also mentions a high impedance model, although I haven’t come across one yet.

Some of the UD1 mics were hard-wired to the cable, and it was also supplied with a Reslo connector, and later a 3-pin din plug, with different impedance options available through the different pins. They are wired like this, with one side of the capsule and transformer primary wired to the output socket.

In addition to the UD1, there was a short stubby version that used the same components, and would often have been used with a gooseneck adapter. Pictured below is also a smaller dynamic mic for use with a tie clip.

These models appear to have been replaced in the 1970s by the Reslo Superstar range, which were essentially the UD1 with an updated body and grill. The Superstar was supplied with a DIN plug – I have converted this one to XLR.

Reslo Superstar 80 dynamic mic.

SJT, May 2014

Film Industries M5 dynamic microphone

Film Industries Ltd. are perhaps best known for their M8 ribbon microphone, but what about models M1 to M7? Well, here is 14.286 % of the answer: the M5 moving coil microphone.

Film Industries M5 moving coil microphone

This model was likely to be a competitor to the Reslosound VMC, and like the Reslo, the M5 features a paper diaphragm driving a moving coil in a magnetic field. The output is wired to a pair of screw terminals, for easy wiring without the need for a custom connector. It does not have a separate ground connection.

The large black rubber cylinder at the base is supposed to give some vibration damping, although perhaps not enough to make a major difference.

Although a 30 ohm mic, this one at least has a strong output, due to the large strong magnets.

And here’s a somewhat wiggly frequency sweep of the mic:

Lustraphone Lustrette – dynamic mic

Here is a little Lustrette LD/61 – a charming little egg-shaped desk microphone by Lustraphone.

I picked this one up with a couple of crystal microphones, and for a long time I had wrongly assumed this to be a crystal too. But when I opened it up I found a dynamic capsule and step-up transformer. I should have guessed from the model number – ‘D’ usually means dynamic.  In fact it features in a Lustraphone product catalogue that I posted earlier – so I have no excuses!

Lustraphone LD/61 inside

The mic comes in a stylish box, with a picture of the mic, so you know what you’re getting.

The dynamic element looks similar to the one found in the more common C51 model, and it is possibly the same mic in a different package.

Lustraphone desk mic or speaker?

Here’s a funny little thing from Lustraphone, found on ebay…..

This was sold as a desk microphone, but it may actually be a small speaker. Or both! It looks very stylish, with tolex covering and brushed nickel finish.

The ‘device’ measures 600 ohms at 1KHz, and 50 ohms at 100 Hz. Inside, the dynamic element looks more like a small speaker than a mic element, although of course the two things are fundamentally the same technology. When wired as a mic it is pretty lo-fi with a narrow bandwidth. And it does transmit as a speaker too. My guess is that it was part of an intercom system and serves both purposes.

Whatever the intended purpose, one can easily imagine this on a gentleman’s desk, so that he could talk to his secretary whilst smoking a pipe and considering important, worldly matters… like this chap!

Reslosound VMC2 Velocity Moving Coil

Here is a nice looking Reslosound VMC2 microphone, successor to the VMC, which (probably) stands for Velocity Moving Coil.
Reslosound VMC2
As far as I can tell, the only real difference between the original VMC and this VMC2 is that the newer model has a slightly different grill, and a three pin Reslo output socket instead of a fixed cable. Inside, both mics have the same heavy paper diaphragm and coil.
Reslosound VMC2

The badge proudly declares that this specimen is a low impedance 15 ohm model.

Reslosound used a fibrous paper to manufacture the diaphragm, and the coil is simply glued to the paper tube at the rear. In fact the technology looks very similar to early speakers.

Reslo VMC2 coil and diaphragm.

When I plugged it in, the mic gave a very lo-fi sound – even more distorted than one would expected for this era. On investigation, there were some metallic particles sticking to the diaphragm, preventing it from moving freely. The paper cone had also become detached from the diaphragm.

Diaphragm from a Reslo VMC2

The remedy in this case was to carefully unsolder and lift the diaphragm and coil. Then I used a magnet and paint brush to flick and drag the dirt away, both from the diaphragm and from the magnet below.

Reslosound VMC2 magnets
Once the obstructions have been removed, the cone was then lightly glued back using some clear nail varnish, and then replaced in the magnet gap, ensuring that it was centred and free to move. This greatly reduced the distortion.

In use, the mic is slightly boxy sounding, with a steep roll off at the bottom end and some dramatic peaks and dips in the response. It very obviously sounds like an early dynamic mic!

Reslo VMC with base station

For comparison, here is a Reslo VMC with announcer’s base station. Note the difference in the grill, with five horizontal slats, as opposed to three vertical.

Huge heavy Meico dynamic mic

Here is another big old dynamic microphone, this time made by Meico.

Meico Dynamic microphone

The big ferrous magnet inside the mic bring the weight up to nearly 1.5 Kg!

This one is certainly a heavyweight!

The badge is very cool though, and the star and wings look like it might be inspired by Soviet artwork of the era!

Like Mexico, but without the X….

According to this eBay seller, the mics were made in Congleton, and they were used as announcer’s mics in boxing matches. I have found nothing to confirm or deny this, so as always if you know more than I do, please get in touch!

But if it fell on your head, then it would certainly be…. a knockout.


Short, stubby and dynamic – The Reslo PGD

Reslos are best known for their ribbon mics, but they made some dynamics too. 

Short, stubby and dynamic – The Reslo PGD
This early model is labeled ‘Dynamic – PGD’, which one assumes stands for Pressure Gradient Dynamic. It proudly says ‘Reslo’, on the badge –  the later RB mics were mostly labeled ‘Reslosound Ltd’.

The PGD appears to be made of leftover parts from the RV ribbon mics. The base of the mic is the same, complete with swivel mechanism, and the grill looks like a cut down version of the RB too. As usual it uses the annoying Reslo plug.

The head on the RGB could be tilted for best pickup of sound.
There is space in the base of the mic for an output transformer, although this 30 ohm example doesn’t need one. I don’t yet know if they were produced with other output impedances, but it would not be surprising, as later models like this pencil mic came with switchable outputs.
Reslo PGD – aluminium diaphragm

Like many early dynamics, it has a pressed aluminium diaphragm, which is heavy and stiff compared to later polymer film designs. Consequently has a quite lumpy response. Here is a frequency plot for one mic – other examples may differ!

Frequency sweep for Reslo PGD mic.
Thanks to Sean Davenport.

Massive Old Dynamic Microphone – GEC?

We’ve had some website issues this weekend, but everything has been sorted out, so let’s get back on track with another vintage microphone curiosity.

This gigantic bronze dynamic mic has no badge or makers name on it, but arrived in a box of GEC microphones. Connection to the outside world is made via two terminal posts at the rear. From the size and style it probably dates to the 1930s or even earlier. 
It is around 11.5 cm across and 8.5 cm deep, weighs nearly 2 kg, and would originally have been mounted on springs within a hoop. Two of the suspension mounts are missing – it will be a quick job on the lathe to turn new ones from a bit of brass. 
Inside the mic is very much like a speaker in reverse, with a paper and fabric cone driving a coil into the field of an enormous magnet.
On initial testing, the mic wasn’t picking up well as the cylinder in the centre of the diaphragm was scraping against something as it moved. Some careful cleaning to remove the dirt eased the movement, and the mic sounds OK, if rather peaky. The addition of some foam inside the body helped to dampen some of the ringing from the cavity.
Even with the foam, the mic is far from flat in response with an enormous bump at 200Hz. So if you want to give something an EQ boost at 200Hz, this is your mic!
Frequency plot for big bronze dynamic microphone!
Other uses include door stop, paperweight, shot-put and burglar deterrent (ouch!).
If you recognise this one or have information on any of our other unidentified mics, please get in touch.

ElectroVoice EV RE20 vs RE320

Everyone who has worked with me will know that I am a big fan of the EV RE20 / PL20 microphones.  Although perhaps most famous as a radio presenter’s mic, they are used throughout studio-land for kick drum, horns and vocals – Thom Yorke from Radiohead is one famous user.  In our studio, they are the go-to dynamics for pretty much everything that makes a loud-ish sound –  kick drum, toms, bass, guitars, horns, vocals, percussion, organs, science experiments, and so on. We have three, and I could happily use more.

From top. EV RE20, PL20, RE320 and another RE20.

I don’t really like severe EQ’ing, particularly with digital EQ, and so the idea of having an RE20 that is voiced for kick is particularly seductive. So, I was very eager to get my hands on the new EV RE320, which is an RE20-shaped thing that is specifically designed for kick drum. But wait! It is also specifically designed for vocals and instruments. How does that work?

EV RE320 switch with kick drum (left) and ‘flat’ settings.

Whereas the original RE20 has a switch which operates a high pass filter circuit, the switch on the RE320 gives different voices for different applications, giving access to ‘flat’ and ‘notched’ settings.

So, what are the differences between an RE20 and an RE320? The most obvious thing is that the RE320 is very black…. supermassive-black-hole black. in fact. And it comes in a zipped reinforced thing that is a hybrid between a box and a bag. This is a big improvement on the crappy plastic boxes that the RE20 came with, which tend to snap at the first opportunity. The RE320 is also about £150 cheaper than the RE20, and is ‘assembled in China’, whereas the RE20 is ‘made in the USA’.

RE320 snug in its little box-bag

More importantly, what about the sound? Here is a frequency plot of a ‘normal’ RE20, recorded in the Xaudia test chamber at 20 cm from the source. The blue line is the flat setting, and the green is with the HPF switched in.

RE20 response (blue) and with the HPF (green)

I had hoped that the flat setting of the RE320 would be be the same as the RE20, but in fact the new model is brighter and louder. Here is the RE320 in ‘flat’ (red) and ‘kick’ (green) modes, along with the RE20 in blue as a reference….

RE20 (blue), RE320 in flat (red) and kick (green) modes.

The RE320 has a higher output in both positions, and also has a peak around 4 to 6 kHz, which may brighten up some vocals. And there is other stuff going on too! A more revealing way to look at the behaviour of the ‘kick’ position is as a difference plot…

RE320 – difference between the flat and kick drum settings.

This plot shows a complex filter network being applied, with a low shelf boost, a -4dB cut at around 350 Hz, and a +5dB boost at 3.5 kHz.

Despite the differences, the RE320 does sound like it belongs to the same family – in fact it sounds rather similar to an RE20 with a +5dB boost around 4-5 KHz. I guess that makes sense!

Only time will tell if I will love this as much as the RE20, but it is a decent dynamic mic and will certainly find uses. I will report back once I have used these on a real recording session.

Stewart, Xaudia

Mic Of The Month: Sony F-V300 Dynamic microphone

Sony F-V300 and C38b microphones

This unusual Sony microphone (left) looks like the younger brother of a C38b (right) or maybe a C48 condenser mic, and shares the same high build quality and some hardware components with these mics. But in actual fact this is a high quality cardioid pattern dynamic microphone, model F-V300.

Inside, the microphone is very simple, with just a capsule in the top compartment, and the on/off switch and transformer wired to a printed circuit board below. The fixed grey output cable looks to be the same type as found on the C38b.

The capsule is suspended from 8 small springs to act as an internal shockmount, which seems to work well, in conjunction with the yoke, to acoustically decouple the microphone element.
This example has some traces of foam around the element, indicating that an internal windshield has at some time been removed.

The label states that the mic is nominally of 1kΩ impedance, although in practice seems lower than this and it has no problems driving standard low-Z microphone preamps. The output is strong and clear across the range, and the output is similar in level and detail to a healthy EV RE20, although with a more pronounced proximity effect.

As far as we know, this microphone was only marketed in Japan, but if you have any information to confirm or deny this, or in fact any information about this mic at all, please let us know!

UPDATE 22/2/11
This microphone has become a bit of a favourite for guitar cabs – it seems to have just the right amount of   proximity boost for that application, giving clear and solid presence to the lower end. It also makes a decent tom mic.