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).

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.

Voila!

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!