Tuesday, 19 May 2020

DIY Parabolic mic part II - the mic

Initial mic tests
I made initial tests of the rig with a few mics. Ideally, a mic should be sensitive with low self-noise while being light and small enough not to unbalance the dish. I had high hopes for my LineAudio CM3, one of the smallest and nicest sounding cardioids around, but it didn't sound right inside the tubular mount and with the xlr plug attached was still a touch big and heavy. The best was one of my EM172 'clippy' lav style mics which weighed nothing and fitted neatly in the tube even with its bumblebee furry on. However the cable was a bit short and I didn't buy a Sound Devices MixPre-3 recorder to just run mics on plug-in power. A bit of googling turned up a simple circuit to convert EM172s to run on 48 volt 'Phantom' power, and the possibility of wiring two or three in parallel to improve the signal to noise ratio.

triple capsule viewed through handle

Capsule circuitry and assembly
I bought three capsules from micbooster.com with short cables already soldered on (safer, as the capsules are easily killed by excessive heat when soldering). I didn't bother with matched capsules as I figured any differences would be evened out as the outputs are summed by the parallel wiring. Once the capsules arrived I rigged up a breadboard and 1meg potentiometer to get my head around the 'simple p48' circuit to step the 48 volts of phantom power provided by the recorder down to the right voltage for the capsules.

Although there is plenty of info around online this post is about the best summary, with plenty more info in the comments too:
From reading it seemed about 7.5-8V was ideal, but I couldn't find any info on resistor values with multiple capsules. Using the breadboard and pot it worked out that to deliver ~7.5V across three capsules I needed a 10K resistor. Using the the MixPre-3 to deliver the power while wearing headphones was instructive; at lower voltages I could hear the sensitivity drop off, whilst at 10V and above (I got up to about 14V at one point, not intentionally!) the capsules seemed to get hyper sensitive and shrill, with the noise floor jumping up.

simple p48 components

With the components sorted I could build the cable and the mic. The 4.7uF electrolytic capacitor and 10K resistor fit in an XLR plug, I then added a bit more work by adding a 3.5mm jack and socket halfway to sit in the parabolic handle and avoid always having a cable dangling from it. Alternatively, a potentially neater but heavier option would be to put the electronics in an XLR socket installed inside the handle. For the mic itself, I first covered each capsule in heatshrink and then used sticky copper sheet to both hold them together and shield them from interference. A bit of electrical tape and heatshrink then beefed it up.

Initial tests were very encouraging. I was concerned there might not be an obvious difference, but it was clear as day. With hindsight, I was combining three improvements in one hit. Firstly, providing 7.5V to each capsule meant a higher quality signal - improved sensitivity and lower noise. Then by running three capsules in parallel I was summing three of these improved signals. Finally, the p48 XLR conversion meant I was now using the class-leading Kashmir preamps on the MixPre-3. Coupled with the physical signal boost from the parabola the whole rig was starting to sound like a very capable instrument.

Mic body
The mic needs to be supported at the focus point of the dish with a clear 'view' of the dish. After trialling a couple of pieces of PVC plumbing pipes with various shaped apertures I bought a piece of aluminium tubing and laboriously drilled and filed it to the shape seen above - giving each capsule a big 'window'. The challenge is to have as little material as possible near the caspules while providing a strong support. The ends of the tube were then plugged with foam with a socket cut for the mic capsules. Layers of electrical tape shimmed the tube to a snug fit in the dish holder.

Wind protection
Rycote sell kits of their acoustic baffle and fur for DIY wind protection. Sewing the two layers into a simple tube was all that was required, with an o-ring to prevent it sliding off. The final item was a spandex cover from parabolicmicrophone.co.uk. This is a bit tight but hopefully the elastic will loosen over time. The combination is very effective and in use the limits are more to do with the wind physically shaking the dish.

Handling noise
In my experience parabolics are very sensitive to handling noise - the Telinga I've used was just as bad as mine. Even if the mic is well isolated from the handle, the smallest vibrations from the handle pass into the dish which then amplifies them like a loudspeaker cone. Soft foam on the handle helps but the best trick is finding a comfortable grip and not to move your hand at all - easier said than done. As generally with nature recording with sensitive handheld mics it also helps if you can breathe silently and control your tummy rumbles!

Sound quality
I've got a few mics in my drawer now, but this one is by far the best performer for bird song. On-axis sounds get a significant volume boost with off-axis sounds not unduly distorted. Compared to my Audio-Technica AT4073A shotgun mic I get a much stronger and more directional signal with lower noise, at the expense of some dish 'colour'. Even professional models are reputed to 'colour' the off-axis sound in this way as the waves are reflected off the dish, with Schoeps providing an eq plugin to help correct the sound. Without direct A-B comparisons I can't say if mine is any better or worse than commercial offerings but the colour is definitely detectable if you're listening for it.

Sample recordings - coming soon

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