Old STC Capacitor Mic - Hook-Up

[Rough Slope]

Hello everybody, grateful if I could pick the collective brain once again. I picked up an old STC capacitor microphone kit the other day - it is a very small 4126A model complete with power supply, dating from the mid-60s or so from what I can gather. I have no idea if it works yet! Information on-line about these mics is quite thin on the ground, and I wondered if anyone has come across one before and/or could offer some advice on hooking it up to give it a test.

Here are a couple of pictures.




All connectors on the mic and power supply are 5-pin DIN type. The kit included some 5-pin DIN cables and so I can hook up the mic to the power supply mic input. I also found an old advert from Wireless World which appears to give the mic pin connections (1, 5-signal and 3-earth). I have a few questions but the main one I have though is on the 'line output' and how I might hook it up to my audio set up - would anyone familiar with these mics (or the pin connection convention, if there is likely to be such a thing for a mic of this age) be able to provide some guidance on how to replace the existing 5-pin DIN output with say a 3-pin XLR connector? The cable is multicore and all pins are currently connected to the plugs as follows. (1-red; 4-black; 2-sleeve; 5-green; 3-brown).

I'm also unsure as to why the 5-pin connections were used. Although not indicated from the on-line information perhaps an impedance selection consideration?

If any more information is needed, just let me know - many thanks.

All the best,
Neil

[Kwackman]

Interesting box!
What's on the back of it?
Mains connector?

[Rough Slope]

Interesting box!
What's on the back of it?
Mains connector?

Hello - it is, yes! Just the mains connection and fuse at the back.

[Hugh Robjohns]

I picked up an old STC capacitor microphone kit the other day - it is a very small 4126A model complete with power supply, dating from the mid-60s or so from what I can gather.

A rare mic I think... I've never come across one before. But I think it might be llate 60s or even early 1970s, judging purely by the model number. The 4126 wasn't listed on a 1963 price list I've seen and the highest model number then was 4108.

I also found an old advert from Wireless World which appears to give the mic pin connections (1, 5-signal and 3-earth).

Do you have a link (or date) for the WW advert? That would help track down any reviews in audio magazines of that era.

The pin-out you describe would be unusual wiring for a DIN connection. The middle pin (2) is the default pin for ground on all DIN usage I've seen. And signal is more typically either 1/3 (opposite outer pins) or 1/4 outer and inner pins on same side.

So, the safest option would be to have a look inside the power unit to see how the connectors are actually wired. It will be pretty obvious which pins carry signal and cable shield.

BEWARE: if it's a valve mic the PSU will be provide an HT Anode voltage of anywhere between 100-300Vdc, and that voltage could persist on smoothing capacitors for quite some time after being switched off, so be extremely careful if you're poking around inside the box. Don't open the PSU box if you not competent to handle valve electronics!

Also, bear in mind that the output may not be balanced. It was common to provide unbalanced outputs with different source impedances in the 60s — at least with dynamic mics. Don't know if that was also common with capacitor mics.

I have a few questions but the main one I have though is on the 'line output' ...

Capacitor mics have a much higher output than dynamic mics, but I'd be surprised if that box provides line-level outputs.

...provide some guidance on how to replace the existing 5-pin DIN output with say a 3-pin XLR connector?

I wouldn't do that. It would ruin the authenticity of the power unit... and all you actually need is a bespoke DIN to XLR adapter cable.

The cable is multicore and all pins are currently connected to the plugs as follows. (1-red; 4-black; 2-sleeve; 5-green; 3-brown).

That will be a standard hi-fi DIN cable (as used by Quad, Naim, Revox, Philips and others). Typically, left/right record signals would be on pins 1/4 and replay on pins 3/5 with a shared ground on 2. Obviously, the pin usage will be different for the mic, but a standard cable would be convenient.

I'm also unsure as to why the 5-pin connections were used. Although not indicated from the on-line information perhaps an impedance selection consideration?

Two pins for a balanced output (or different impedances), if it has a valve impedance converter then two pins for HT and heater supplies, plus a common ground. That's 5 pins used....

If you're in the UK (or even if you're not), it would be worth contacting Stewart from Xaudia / Extinct Audio as he may have specific knowledge of the mic, but might be able to advise on getting it working. He specialises in ribbon mics and doesn't routinely deal with capacitor mics... but I know he has an interest in vintage gear and he can always say no....

[Pippacairn]

Would I be correct that the item with 300 Ohms written on it plugs into the bottom of the microphone?
John H

[Rough Slope]

Hi Hugh - thank you for the comprehensive reply! Much appreciated.

I picked up an old STC capacitor microphone kit the other day - it is a very small 4126A model complete with power supply, dating from the mid-60s or so from what I can gather.

A rare mic I think... I've never come across one before. But I think it might be llate 60s or even early 1970s, judging purely by the model number. The 4126 wasn't listed on a 1963 price list I've seen and the highest model number then was 4108.

Yes, I think you might be right. Some more idle Googling has thrown up some more references including a couple of old BBC R&D reports that appear to date a 2nd version of the mic (which looks like mine) at 1966+. The STC logo on the mic grille is also in blue which I think (but am not quite sure) might date it a little later/into the 70s.

I also found an old advert from Wireless World which appears to give the mic pin connections (1, 5-signal and 3-earth).

Do you have a link (or date) for the WW advert? That would help track down any reviews in audio magazines of that era.

Here is the WW advert link, plus links to the two BBC R&D reports.
https://worldradiohistory.com/hd2/IDX-U ... DX-112.pdf

http://downloads.bbc.co.uk/rd/pubs/reports/1966-22.pdf

http://downloads.bbc.co.uk/rd/pubs/reports/1966-36.pdf

The pin-out you describe would be unusual wiring for a DIN connection. The middle pin (2) is the default pin for ground on all DIN usage I've seen. And signal is more typically either 1/3 (opposite outer pins) or 1/4 outer and inner pins on same side.

So, the safest option would be to have a look inside the power unit to see how the connectors are actually wired. It will be pretty obvious which pins carry signal and cable shield.

BEWARE: if it's a valve mic the PSU will be provide an HT Anode voltage of anywhere between 100-300Vdc, and that voltage could persist on smoothing capacitors for quite some time after being switched off, so be extremely careful if you're poking around inside the box. Don't open the PSU box if you not competent to handle valve electronics!

Also, bear in mind that the output may not be balanced. It was common to provide unbalanced outputs with different source impedances in the 60s — at least with dynamic mics. Don't know if that was also common with capacitor mics.

From the BBC report it appears it is not a valve mic but employs a FET instead. In any event, I'm a novice and so will limit my fiddling to careful inspection inside the PSU, at the very most.

...provide some guidance on how to replace the existing 5-pin DIN output with say a 3-pin XLR connector?

I wouldn't do that. It would ruin the authenticity of the power unit... and all you actually need is a bespoke DIN to XLR adapter cable.

Sorry, I should have been clearer. I wasn't planning to mess about with the PSU, only replace the DIN connector on one of the cables that came with it. I can make up a DIN to XLR cable so that would be a neater solution.

The cable is multicore and all pins are currently connected to the plugs as follows. (1-red; 4-black; 2-sleeve; 5-green; 3-brown).

That will be a standard hi-fi DIN cable (as used by Quad, Naim, Revox, Philips and others). Typically, left/right record signals would be on pins 1/4 and replay on pins 3/5 with a shared ground on 2. Obviously, the pin usage will be different for the mic, but a standard cable would be convenient.

I'm also unsure as to why the 5-pin connections were used. Although not indicated from the on-line information perhaps an impedance selection consideration?

Two pins for a balanced output (or different impedances), if it has a valve impedance converter then two pins for HT and heater supplies, plus a common ground. That's 5 pins used....

If you're in the UK (or even if you're not), it would be worth contacting Stewart from Xaudia / Extinct Audio as he may have specific knowledge of the mic, but might be able to advise on getting it working. He specialises in ribbon mics and doesn't routinely deal with capacitor mics... but I know he has an interest in vintage gear and he can always say no....

A good idea. I'm UK-based and know Stewart as he has kindly helped out with ribbon mic-related questions in the past. I'll drop him a line!

Thanks again,
Neil

[Rough Slope]

Would I be correct that the item with 300 Ohms written on it plugs into the bottom of the microphone?
John H

Hi John - a couple of these came with the mic, with the other badged 30 ohms. I presumed these were matching transformers but am not 100% sure on that, or where they would be hooked in. If I was forced to choose I would have said on the output side.

[Philbo King]

I don't know if this info is helpful, but more info is always good. (Link to a .pdf file detailing BBC testing on a few samples of thos mic)

https://www.bbc.co.uk/rd/publications/rdreport_1966_36

[Pippacairn]

Hi Rough Slope, the pdf posted by Philbo King shows the matching transformer in-line, which would mean 3 DIN cables being required, assuming that the transformer is DIN on both ends.

John H

[Hugh Robjohns]

Thanks for those reports, and the advert. The output wiring does appeR to be that unusual arrangement.

I'll have a rummage in my old Tape Recorder magazines to see what I can find...

The Beeb report mentions the output transformer unit with 30 and 300 Ohm taps. And yes, an early FET impedance converter but still with both low and high voltage power rails (dual battery or mains PSU).

[Rough Slope]

Thank you all for the information/help - I'll keep you posted on any developments!

All the best,
Neil

[Hugh Robjohns]

I found this interesting snippet in International Broadcast Engineer magazine, December 1966.

A recent 'world first' from S.T.C. Harlow is the new 4126 miniature transistorised capacitor micro-phone. The microphone and head amplifier unit is 7/8 inch in diameter by 2 inches long and can operate from small long-life batteries in a separate container, or from a mains unit. It is a high quality cardioid which does not depend on complicated RF circuitry. A full range of special accessories is available. One of the interest-ing features of this microphone is its versatility. It is a miniature unit suitable for all studio purposes. It's unobtrusive appearance makes it suitable for 'in-shot' use and it can be used on all normal types of stands and booms. It performs well as a lavalier or personal microphone when used with a special electrical equaliser and can be used as a high quality feed for a radio microphone system.

[Hugh Robjohns]

And this in Wireless World, April 1968.. along with a photo of the capsule affixed to a gooseneck as a talkback mic.

It is a pleasure to be able to record that
the first microphone employing f.e.ts was
made in this country by Standard Telephones and Cables, and this microphone, the
type 4126, also has the claim of being the
smallest on the market. The output is com-
parable with that of dynamic microphones. A
later version, type 4136, has a higher output
and uses a cable with only two conductors
instead of the multicore cable required for
the earlier model.

[Hugh Robjohns]

There's a DIY capacitor mic construction article in Studio Sound /Tape Recorder magazine January 1970 which uses the STC 4126 capsule which might provide some insight...

[Rough Slope]

And this in Wireless World, April 1968.. along with a photo of the capsule affixed to a gooseneck as a talkback mic.

It is a pleasure to be able to record that
the first microphone employing f.e.ts was
made in this country by Standard Telephones and Cables, and this microphone, the
type 4126, also has the claim of being the
smallest on the market. The output is com-
parable with that of dynamic microphones. A
later version, type 4136, has a higher output
and uses a cable with only two conductors
instead of the multicore cable required for
the earlier model.

Thanks Hugh - these magazine articles (and those BBC R&D reports) are a great resource!

I was wondering why this mic doesn't have much of a 'footprint', so to speak and was perhaps not so popular/widely taken up in the end. Maybe as much to do with the fate of the company as the quality/usability (or otherwise) of the mic? I also see in that WW article a reference to the AKG C451 entering the field, and so maybe they were just trumped by the alternatives ...

[Hugh Robjohns]

I was wondering why this mic doesn't have much of a 'footprint', so to speak...

It appears it was the first solid-state DC-biased capacitor mic and FETs were relatively noisy and inconsistent. And STC evidently replaced the 4126 with the better performing 4136 pretty quickly.

Also as you say, other manufacturers were also quickly releasing their own solid-state mics. Sennheiser introduced their MKH RF-bias models, and AKG the C45x range — the latter quickly becoming a broadcasting and studio standard, not least because of its amazing versatility and optional accessories.

Also, STC got out of the mic business around then, selling out to Coles (Colin and Les) who only continued the 4038 ribbons and 4104 and related lip-mics.

[Kwackman]

Coles (Colin and Les)

That's a fun fact to know.

[Hugh Robjohns]

Don't know their surnames but Colin and Les were toolmakers working for STC. Around 1964 they left and set up their own toolmaking /engineering workshop which they called Coles Engineering. They still did some contracting work for STC, and when that company decided to cease mic manufacturing in the early 70s Coles acquired the IP, tools and staff to continue manufacturing the 4038 and 4104.

[Rough Slope]

Stewart at Xaudia has very kindly agreed to take a look at this one when time/ribbon mic commitments allow. I'll post an update to the forum when I have one.

Best,
Neil

[Drew Stephenson]

Stewart at Xaudia has very kindly agreed to take a look at this one when time/ribbon mic commitments allow.

He is good people.

[Hugh Robjohns]

Excellent. I look forward to hearing more in the fullness of time.

[Rough Slope]

Hello again - resurrecting an old thread with an update ...

Stewart at Xaudia very kindly took a look at that old STC 4126A, and I'm pleased to report it works! No work was required to the mic itself, but the PSU needed a bit of attention - replacement of the electrolytic capacitors and some diodes in the power supply, and re-work to a lot of the soldering.

In action, the mic works fine if somewhat noisy - also susceptible to a bit of rumble on speech tests, suspected to be evidence of leakage in the capsule. Here's a link to the mic recording my abysmal acoustic guitar playing.

https://soundcloud.com/user-805545299/s ... al_sharing

https://imgur.com/a/VUc9UKt

All in all, it's been a fun digging into this one. I doubt very much the mic will get much use in my small home studio set-up and is probably more of a museum piece, but an interesting bit of mic history all the same.

Thanks again for all your inputs.

Cheers,
Neil

[Rough Slope]

Edited to fix image links - JP

[James Perrett]

In action, the mic works fine if somewhat noisy - also susceptible to a bit of rumble on speech tests, suspected to be evidence of leakage in the capsule.

Yes, that's typical of a damp or dirty capsule or possibly leakage in the high impedance part of the electronics.

[ajay_m]

Early enthusiasm for jfets was rapidly tempered by the realisation that unlike bipolar transistors or indeed valves, the diffusion process by which they are made results in significant differences between devices for their transfer curves and particularly gate cutoff voltage.
This means that a design that worked with a particular device might then require that dozens of devices had to be screened to find similar units. I can certainly remember going through a couple of dozen 2n5485s to find two that were reasonably matched for a differential pair, for example.