Mic of the Month – Shure 508 Stratoliner

Here is our first Microphone of the Month for 2015. This Shure Stratoliner looks like a Zeppelin airship or perhaps Flash Gordon’s space ship. But is it a crystal, ribbon or dynamic microphone?

I had always thought that the Shure Stratoliner series were crystal microphones, and indeed the 708 model does have a crystal element. But although this 508C uses the same body, it is quite a different microphone inside.

This one says ‘dynamic’ on the label but it is really a rather strange ribbon microphone. Here is the business end of the mic with some kind of flat ribbon in place, possibly made from kitchen foil.

Once we remove the ribbon and clean the motor, we can see a series of slots right behind the ribbon. These make an acoustic cavity and help to control the directional pattern.

The motor design is quite basic, with two cylinder magnets behind the ribbon, covered in felt. The transformer is mounted behind the magnets.

This construction and slots behind the ribbon also do odd things to the frequency response. Here is the published response chart from the Shure 508a user manual:

Even the published chart is pretty bumpy, and it seems that Shure were willing to sacrifice fidelity for directionality. And the mic does sounds peaky – definitely one for the vintage / special effects shelf in your microphone locker.

HMV 2350 ribbon microphone documents

These HMV and EMI ribbon microphones show up quite often for repair. The models are identical, just branded differently, and very often they come complete with wooden box, which survives decades in a damp shed or attic better than cardboard.

It is rare to find any paperwork with these but I was lucky enough to come across an HMV 2350 complete with brochure and manual

The manual confirms that they were designed to drive a 20 ohm line. I have found that most of these mics measure around 15 ohms and have really low output. A suitable matching transformer makes a big difference to the usability of these microphones.

HMV 2350 brochure

HMV 2350 manual

PS, some of you might recognise this mic from Marvel’s Captain America movie – it was used by Howard Stark in the scene where Steve gets his treatment!

Review of the Reslo RB by F.C. Judd

Here is a scanned review of the Reslo RB miniature ribbon microphone,

The review is written by Fred C. Judd, who was an early champion of both electronic music and DIY audio electronics. You can hear and buy some of Judd’s creations at this bandcamp page.

This article arrived with a microphone but I am unsure which magazine it was originally published in. Perhaps “Amateur Tape Recording”, of which he was technical editor. If you know more, please let me know.

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.