August 2022 – Xaudia

29-08-2022

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)

27-08-2022

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.

26-08-2022

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.”

21-08-2022

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.

05-08-2022

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.