Microphone of the Month - Philips Ribbon Mic

This old microphone by Philips came from a seller in Egypt - I have a vision of it being used back in the 1940s and 50's, broadcasting out in the desert, near the Pyramids and Sphinx....

The mic was in pretty bad shape and in need of a full restoration. The ribbon was broken, and it was missing a yoke and several other parts. However, it's a pretty interesting microphone and so gets to be our microphone of the month for December.

This microphone appears to be based closely on Harry F. Olson's drawings in early patents and presented in the Journal of the Society of Motion Picture Engineers, back in 1931.


The magnetic field is provided by one large permanent barrel magnet. This microphone had a measured field of about 1200 Gauss between the poles, with ribbon dimensions of 5.5 mm wide by 67 mm long.

The original ribbon - sadly very oxidised - was of the piston type, with corrugations at each end and a flat section in the middle region. On closer inspection, the ribbon appears to have been designed for in-field replacement: each end is terminated in a thicker, silver-plated fold of foil, with a hole drilled for 'easy' mounting (easy being a relative term in this case). The ribbon is held in place with two brass clamps, each mounted held in place with a singe screw. The disadvantage of using a single screw rather than a pair for the ribbon clamps is that the clamp has a tendency to rotate as it is tightened, which can distort or wreck the ribbon. The clamps are soldered to wires which run to the transformer primary, and these wires are doubled (or tripled) in each case, presumably to keep resistance noise to a minimum.

With a rewire and a new (corrugated) ribbon the microphone works and sounds rather full and rich. However, the output transformer is wound for high impedance output, and won't drive a standard mic preamp - so the microphone benefits from using an active buffer or an impedance matching transformer. Hum is also an issue with this, despite the massive brass housing.

I haven't seen another one like this - either in life or on the web. If you have any further information on this, I'd love to hear from you.

Fun with magnets and an Electrovoice V1 velocity ribbon mic

Here's an early Electrovoice velocity ribbon mic, model V1. These are great looking microphones, but the early versions are rather crudely made and this one, like many others, suffered from low output due to weakened magnets.

Bob Crowley has a few things to say about these mics - not all of them nice!

The motor of this model is based on a single cylindrical permanent magnet, clamped to a pair of metal plates which make up the pole pieces of the assembly. Because of the positioning of the magnet, the magnetic field is uneven, with a significant difference in field between the top and bottom of the motor assembly. In our example we found that the field varied from around 700 gauss at the bottom to 1000 gauss at the strongest point. This is very low for a ribbon mic, and, along with the oxidised ribbon is responsible for a low, noisy output.

Fortunately, we have sourced some very powerful cylindrical N42 neodynium magnets of a suitable size and shape, which are a perfect replacement for the original weak magnet.

With the new magnet the field is increased by a factor of around four, to about 3000-3200 gauss, a much healthier figure which should lead to an increased output and much improved signal-to-noise performance.

Now it's time to cut a new ribbon, reassemble the microphone, and do some listening tests. In the meantime, we made a rather attractive bracelet from some of the spare magnets.

Cat Mac Attack!

Pansy the cat has got it in for the MacBook, and launches surprise attacks like this, usually whilst I'm writing an important email, a blog entry, or leaving feedback on ebay.

At least it explains the spelling!

Winter is here! And so are Modern Day Chicane.

This was the scene at Xaudia studio yesterday - a beautiful cold, crisp and bright day in North Yorkshire, and it's times like this you really appreciate the peace & tranquility we get here. Ideal for creativity and recording. And this was the view from the studio over the surrounding fields.

We're a little isolated, not quite in the middle of nowhere, but certainly just down the road from the middle. They don't grit the road here, and sometimes the pipes freeze, but yesterday everything was perfect.

Modern Day Chicane were here to record some drum tracks for a future EP, in what turned out to be a productive, if somewhat frantic session. I arrived early to mic up the kit as drummer Pete was coming up from London on the train, which of course arrived a little late due to the weather, which left us about 4 hours to get the drum takes. We tracked at a blistering pace, cutting six live versions that may (or may not) see the light of day as bonus tracks, then we did five of the songs again to a click track, with Ryan and Nicola (pictured) in the control room providing guide vocals, guitar and bass.

Once we had a couple of decent takes of each drum track, we abandoned the editing and packed up early because it was getting cold and dark, and the talent had to get to a gig. It had already dropped to -4︒C by the time we had to scrape the ice from the cars. Brrr!

I went home and lit the fire!

Mic of the Month - RCA Junior ribbon mics

October was a bad month for blogging - I was busy with the haunted house sound installation, and this was compounded by a fault with my Macbook, which took the Apple repair centre three weeks to find and fix, a long time to track down a faulty cable. With a microphone, that would be the first thing to check! Amongst all the chaos I completely forgot to do the 'mic of the month' column.

Back in the real world, I have chosen the RCA Junior ribbon for November's Mic of the Month. This is because they seem popular at the moment, and we've seen four at the workshop for service or repair. The fun thing about this family of microphones is that they vary somewhat in construction, so it is possible to compare and contrast versions from different eras. They tend to be a bit more affordable than the bigger RCA 44 and 77 mics, but still have a good tone that is very usable in a modern studio, especially if the ribbon is in good condition and the transformer is healthy and wired correctly.

The 'Junior' was created as a budget version of the RCA44, with a similar motor assembly but smaller magnets and housing. The most commonly seen models are the 'black badge' and 'red badge' versions, and these are actually quite different inside - the black badge model has a 3.0 mm x 55 mm ribbon, whilst the red badge version I examined has a wider, 4.5 mm ribbon and a stronger magnetic field.

The output transformers on these microphones can be set for 50 Ohm, 250 Ohm or 10KOhm output impedance, and it is worth checking that the mic is wired correctly to get the best performance with modern studio equipment. Normally that will be the 250 ohm setting.

The earliest and rarest version, the MI-4010-A, is shown on the right in the picture below. It is slightly larger than the later versions, with a different ribbon assembly which has horseshoe style magnets around the back of the ribbon. The magnetic field in this example is weaker, and the output lower than the more modern versions, although the tone with a new 1.8 micrometer ribbon is very pleasing. 

Finally, some RCA mics were actually made in Europe, and it would seem that some appear under different names. The microphone on the left is badged as 'Magneti Marelli, Milano, Italy' but is almost identical to the black badge RCA 74b. The only difference is that the Magneti has an alternative transformer, but still with high and low impedance options. The sound is every bit as good.

We'd love to hear from anyone who knows more about the Magneti Marelli microphones and their relationship with RCA.

(Thanks to Jules at DADA Studios in Belgium and Jørn Christensen at Rodeløkka Studio in Norway.)

Drew Stephenson - A Driven Man is (Finally) Here

Almost two years ago, singer-songwriter Drew Stephenson and Cellist Hilary Jones started recording an album in our studio. Anyway, after what seems like a very long time, Drew's album "A Driven Man is Coming" will be released on November 19th at a free launch gig in York. The gig will be at 1331, on Grape Lane. More details can be found at Drew's website.

Support comes from Dan Webster.

Do come and join us if you can.

Sounds from the halloween maze

The big 'Hallowscream' event at York Maze is now over for the year, and the guys and girls are now hurridly getting ready for Guy Fawkes night. It's a quick turn around so good luck!

Several people asked to hear the sounds that we 'created' for the two Halloween haunted houses. I took most of the sounds from the Freesound Project, which is a very handy searchable library of sounds, most of which are available through the Creative Commons license scheme. Other sounds used were my own creations.

The creepy clown theme for the 'CarnEvil' haunted house was based on a loop from The Midnight Syndicate, who specialise in music for haunted house and similar attractions. Some of their tracks were also used for background music in the theatre area.

I'll leave these tracks up for a week or so, so grab them quickly. Enjoy!

Carnevil Haunted House

Main Clown Theme

Upside down room music

Meat Locker

Boiler Room

Barnegeddon

Toilet Lobby
(A mash-up of Beethoven, Mozart, and some C&W in the background!)

Steampunk 1

Steampunk 3

Halloween? Hallowscream

For a change, I have spent most of October fitting speakers and providing the sounds for the Halloween events at York Maze close to our studio, and now it's all up and running - we had over 1000 people through the gate last night and are expecting even more this evening. The two haunted houses are excellent scary fun, and the creepy clown music will worm its way into your brain and keep you up all night.

A couple of people have asked for clips of the music from the haunted houses, and I'll try to post those up tomorrow.

Microphone of the month - Old Czech tube mic: Tesla?

This is the first 'Microphone of the Month' blog, featuring classic or unusual microphones. Hopefully I'll manage to find time each month for this!

This old Czech tube microphone - a recent ebay find - may well have been made by Tesla. The capsule is connected using a connector that can also be found on old Tesla and Phillips microphones. Some of the capacitors are also made by Tesla, who were a large state owned electronics company in communist Czechoslovakia.

 

The 'Tesla' looks very much like an imitation of the Neumann / Gefell CMV563 bottle mic. In fact the microphone is smaller in diameter than the CMV, has no output transformer and has an unbalanced output. Like the CMV, the capsule may be swapped, and presumably other polar patterns were available. This one is marked with a red circle, which probably means omnidirectional. (I have yet to test the capsule).

The amplifier is a very simple grounded cathode amplifier, based around a Soviet 6Ж1Л (6Z1P) tube, which is a small signal pentode similar to EF95. These are also found in some Lomo and Oktava microphones, including the Lomo 19a9 and Oktava MKL2500.

Without the original power supply we can only speculate on the operating voltages. However, a B+ supply of 90V would be a good place for experiments to start - this would give a voltage on the capsule of around 60, and a sensible current through the tube circuit.

Tube mic circuits - Connecting the capsule 2

Last week's technical article talked about different methods of connecting a condenser capsule to the grid of a tube amplifier, in order to build a tube mic. In this part we consider how to connect a capsule with two diaphragms in order to get a multi-pattern mic.

First let’s examine the different pickup patterns available. There are three extremes: Omni, where the microphone hears sounds equally in all directions.  Cardioid (heart shaped*), where sensitivity is greatest in the direction in which the microphone is pointing, falling off to a null point behind. And Figure-of-Eight, with equal (but opposite) pickup in front and behind, and null pickup to the sides.  To complicate things further, the pickup pattern may depend upon the frequency, and some mics will have good directionality at higher frequencies, but become less directional as the frequency drops.

But what if we want a microphone with selectable pattern? This can be achieved by arranging a pair of cardioid capsules back-to-back, and combining there signals in different ways. We’ll call these capsules front and back, although of course they could be pointing in any direction. If we require a cardioid signal, we just take the front capsule and for omnidirectional pickup, we mix both signals equally. If we want figure of eight, we subtract the output of the rear from the front: where the signals overlap at the sides of the microphone, they cancel each other out producing null points. Other patterns such as hyper-cardioid and super-cardioid may be considered as in-between positions of these extremes.

So, what is the best way to achieve this practically in our hypothetical tube microphone? Two of the earliest commercial mics with more than one pattern were the Neumann U47, which offered cardioid and omni, and the U48, with cardioid & fig. 8. Let’s look at the U47, as this is probably the simplest way to combine the two capsules.

In the U47 the front diaphragm is grounded through a 100 Meg grid resistor, and the backplate of the microphone’s dual diaphragm is polarised with about 60V, providing the potential difference required. The rear diaphragm is connected to a switch. When the switch is open, the rear capsule is left floating and only the front cardioid diaphragm is active. When the switch is closed, the rear capsule adds its contribution to the front, making an omnidirectional microphone.

What about the U48? We have seen above that if we require figure of 8 instead of omnidirectional, we must subtract, rather than add, the sounds from the rear. To do this we must invert the polarity of the rear diaphragm by reversing its relative charge. So, rather than grounding the rear diaphragm, we must raise the potential by 60V** above the backplate, and 120V above the front capsule! This is easily achieved by using the HT supply to the anode of the tube, but creates another problem. We can’t simply connect the two diaphragms because they are now at different potentials, and so a blocking capacitor must be used. The circuit looks like this:

Finally, to make the microphone have variable pattern, we simply need to make a supply that is adjustable from 0V to 120V, and apply that to the backplate. Alternatively, the signal may be taken from the backplate, through a capacitor to the tube grid.  The Neumann-Gefell UM57 does it exactly this way, with the pattern selector in the power supply.

*Really more kidney shaped, and in some languages this is the word used.
**  In fact the U48 operates around 50V / 100V.

Further reading: The G7 page at Gyraf.dk

Tube mic circuits - Connecting the capsule part 1.

Even in the simplest of tube microphone circuits, there are different approaches to connecting the microphone capsule to the tube. Let's use a single-sided microphone capsule as our starting point.

The capsule behaves as a variable capacitor, changing its capacitance in response to changes in air pressure (i.e. sound). In order to generate a signal, the capsule needs to be polarised by some voltage, creating a difference in potential between the diaphragm and the back plate. This is the first decision that needs to be made - should the polarising voltage be applied to the diaphragm or the capsule backplate?

In the circuit shown on the left, the backplate of the microphone is polarised at 60V, which is obtained from the B+ supply, via a resistive divider and a small capacitor to stabilise and filter the polarising voltage. The membrane is connected directly to the tube grid, and a high value resistor (Rg, typically 100 kΩ to 1000 kΩ) connects both the grid and the membrane to ground. We have our potential difference across the membrane, and the sensitivity of the mic may be adjusted by increasing and decreasing the polarisation voltage. As the capacitance of the capsule changes in response to sound, a tiny current will flow through Rg, and this signal is amplified by the tube.

An example of this arrangement may be seen in the Neumann Gefell M582.


In some cases the grid resistor may be omitted. In the circuit below, which appeared in an article in Tape Op magazine by Dave Royer, the capsule diaphragm is grounded by grid leakage rather than a 'real' resistor. It works perfectly.

This simple arrangement is not possible when the capsule backplate is mechanically (and electrically) connected to the body of the microphone. In this case the diaphragm must be polarised directly.

However, having a voltage of around 60V on the tube grid this would adversely change the operating points of the tube circuit, and so a capacitor must be used to block the DC voltage (left). Some listeners claim to hear the difference between different types of capacitors, and so normally a very high quality type should be used in this position. An additional high value polarising resistor is also required, otherwise the high impedance audio signal would be attenuated through the stabilisation cap.

An example of this method of connection is the Neumann-Gefell CMV563, which is designed to be used with bayonette style capsules such as the M7, M8 and M9.

Sometimes it is the membrane which is connected directly to the body, such as in this Teladi K120. The approach is the same as the circuit above.

In my next post I'll consider how to connect mic capsules with two membranes, and how to combine them to generate different polar patterns.

Unknown German Prototype Tube Microphone - 'The Unbekannt"

Unknown German tube microphone

Here is a recent Ebay find. It's an unbranded tube microphone, and judging from the components probably from the 1960s, in what was formerly West Germany. We've called it the 'Unbekannt', which is simply German for 'unknown'. The amplifier circuit is a 3 stage unbalanced transformerless design, using EF40 pentode and an ECC81 twin triode. The final stage is a cathode follower.

The schematic is here.

As is so often the case, the microphone has been separated from the original power supply, so it is not possible to say what the exact operating voltages would have been. However, the voltage divider for the capsule polarisation may give us a clue - 2 Meg and 400K would be a simple way of using a 240V supply to put 40V on the capsule.

The metalwork is nicely done, and is comparable to Reissmann, Thiele and Teladi microphones of similar age. It seems too well constructed to be a DIY mic, but the oddball range of parts makes us think that it is some prototype from one of the microphone makers of the era.

The capsule is quite unusual, but sadly is missing a diaphragm at the moment. It uses springs and screws to adjust the tension and space to the back plate, so this can be adjusted after installation. We'll try to get that up and running very soon so we can see what it sounds like!

Lomo 19a9 tube mic manual

Here is a scan of the original Russian manual for the LOMO 19a19 tube microphone.

The full schematics for the 19a9 have been hard to find, particularly the power supply. I’ve saved those schematics as separate files. Click here to see them. I’ve combined the key components for the microphone circuit in the figure below.


There is a discussion about the microphone here at Group DIY.

Thiele microphone brochure

Thiele M5 tube microphone

Here’s a scan of a short advertising document for Thiele M4 and M5 (photo to the left) tube microphones. 

Theile sales document

Note how expensive the microphones were at the time - 500 and 600 Deutchmarks. For reference, between 1950 and 1960, 4 Deutchmarks approximately equalled 1 US dollar.

Syncron AU7A microphones Part 2

Last time I wrote about a pair of Syncron AU7A microphones. The capsules were in good condition, but the batteries had leaked, causing corrosion and damage to the circuit. For one of these mics I decided to fit a tube circuit based on a 6205 subminiature tube (5840* would do just as well or better)**.

Tube modified Syncron microphone circuit, 6V regulated heater supply omitted.

The Syncron capsule operates happily between about 40V and 60V, and a simple voltage divider was used to supply the backplate with a suitable polarising voltage. As the capsule is cardioid only, the circuit can be made as simple as possible, and there is no need for a capacitor between the diaphragm and the tube grid.

With a little creative hacking I was able to reuse the circuit board to construct a valve circuit, which avoids damaging the microphone further. Although physically larger, the tube sits where the transistor was (I even used the same PCB pads as the FET), and there is room on the underside of the board for a couple of capacitors. An added bonus is that the original transformer is quite suitable for use in a tube circuit, and was rewired in 10:1 configuration. The rest of the circuit - 5 resistors and another cap - fit on the 'wrong' side of the board in the cavity below the capsule housing. Then it is a simple case of wiring the connector to the circuit and connecting the capsule, taking care not to damage the diaphragm.

One thing to look out for with this arrangement is that the amphenol cable plug & connector on the microphone are the reverse of the normal gender, which means that there can be 110V DC on the exposed pins. Consequently care must be taken to connect the microphone before the power supply is turned on, otherwise a short sharp shock can happen. Of course this isn't really an acceptable acceptable solution from a safety point of view.

In practice the microphone works very nicely and is suitably quiet for recording vocals. We tracked some female vocals with it yesterday and it performed very well in that application.

Meanwhile, I have managed to track down some 22V batteries from Farnell, which should be suitable for the capsule polarisation, so I'll attempt to restore the second mic to its original state. More on that soon.

** With hindsight the 5840 may be a better bet as there is an internal connection between the cathode and grid 3. This allows you to cut off two of the leads, which means using up one less precious pad on the circuit board inside - space is tight!

**Readers familiar with the 'Royer' tube circuit will recognise the topology, although a few of the component values are different.

Xaudia blog post on phantom power for these mics.

Syncron AU7A (Fairchild F/22) microphones Part 1

I was lucky enough to come across a pair of Syncron AU7A microphones (aka Fairchild F/22) on ebay. On arrival from the US I found that all the foam lining in the boxes had decomposed and spread black dust everywhere. Luckily the capsules appeared to be in fine condition, and the mics came with the original cables, so the should be a good chance of getting them back to working condition.

That's easier said than done! The mics run on 4 batteries - 2 x 4.2V for the amplifier and 2 x 21V for the polarisation. Unfortunately, our microphones came complete with the original vintage batteries inside, which had inevitably leaked and caused corrosion throughout. The batteries are now pretty much unobtainable, so I used a bench voltage supply to simulate the batteries. Microphone number one gave a very weak and noisy (hiss) signal - I suspected the FET had somehow become contaminated by the battery acids. Mic 2 was slightly better, but certainly not something you could use as a serious recording tool.

These are reported to be the first commercially available FET microphone, and searching the internet didn't throw up any schematics so I traced out the circuit, which is very very simple - capsule -> field effect transistor -> DC blocking cap -> transformer.

EDIT 21/9/2011 : please note that the schematic posted here contained errors. A revised version is here!

The transformer may be wired either for 200 or 50 ohms, and measurement showed it has a voltage ratio of 5:1 in series or 10:1 in parallel mode.

At this stage I needed to make a decision on how to get the best out of the microphone. More on that very soon, but for now here are some web links to Syncron information - there's not a lot of it about!

http://www.coutant.org/syncron/index.html

http://www.prodigy-pro.com/diy/index.php?topic=30014.0

http://recforums.prosoundweb.com/index.php/m/191308/0/

Neumann Gefell UM57 experiments

Lately I've had the opportunity to play around with several vintage Neumann Gefell tube microphones - a CMV563 (below with UM70 capsule) , a M582 and a pair of UM57s.

These all have broadly similar circuits, with a EC92 tube and transformer coupled feedback. The UM57 is configured for different polar patterns, whereas with the CMV563 and M582 you have to swap the capsule. There are other differences - the schematics are shown here.

One particularly common fault with examples of these microphones is that the original electrolytic output capacitor can dry out with age. This is by no means always the case, and the capacitor in the UM57 on the left above was in perfect condition after nearly 50 years!  The one on the right has been replaced with an orange modern metalised film capacitor.

So what is the effect of ageing of this capacitor? As the electrolyte dries out, the absolute value of the capacitance drops, which will affect the frequency response of the valve amplifier inside the microphone. To simulate this, a capacitance decade box was wired in place of the output capacitor (C3), and the chart below shows how the frequency response changes as the capacitance decreases in 0.2 uF steps.*

Part of the circuit is shown inset within the chart. Although intuitively we expect the smaller capacitor to give us less bottom end, the network of the capacitor, transformer primary winding and resistor acts as a resonant filter, producing a peak in the bass region just above a sharp drop off. The human ear can perceive this as more bass - although not necessarily in a good way: the microphone may seem muddy or lack clarity.

So, having a good quality capacitor here is vital, and the value of this can be used to tweak the bass response if desired. Of course this analysis is just for the tube circuit inside the mic and does not consider the effects of ageing on the capsule itself - that's a story for another day.

SJT Feb 2010

* Measured using a swept-sine wave from 1Hz to 48KHz.

Beautiful girls in Milton, Wisconsin

This is the label on a reverb pan found inside a vintage Roland Space Echo SE555. “Manufactured by beautiful girls in Milton, Wisconsin, under controlled atmosphere conditions." Right! And you thought they were made in Japan by robots, didn’t you?

Recording with the Josephson C720

Back in August we were lucky enough to get our hands on a special edition Josephson C720 microphone for the studio, one of only 20 made in the first production run, and having lived with this for nearly half a year it’s probably about time we reviewed it.

The first thing you notice about the C720 is its size - this is one big mic, U47-big, and makes a big bold statement in the live room or vocal booth - it’s built like a tank and it definitely has that ‘wow’ factor which gets the artists talking.

The next thing that grabs your attention is its radical aluminium metal-foam headbasket, which looks like something you could scrub your pots and pans with. The non-periodic headbasket is actually designed to eliminate internal acoustic reflections and refraction of soundwaves, avoiding comb-filtering effects and so giving a more realistic recording.

The third thing you notice is that it has two XLR outputs. Like many condenser microphones, the capsule has dual diaphragms which can be combined in different combinations to give cardioid, omni, figure of eight patterns. Unlike most microphones, the two transducers have separate head amplifiers and separate outputs which means that the signals from opposite sides of the microphone can be recorded separately and mixed together, either in phase or with the polarity reversed, at a later stage. What this innovation means in practice is that you can pick the polar pattern after recording, which might be used to eliminate unwanted sound sources by rotating the null point of the microphone. Also, and more importantly in our studio use, with close-mic’ed sources such as vocals you can dial in more or less proximity effect, making a singer seem bigger, darker or lighter and brighter.

So what does it sound like? Possibly because of the lack of head basket comb filtering, and the facility to tune the pattern and proximity effect, this is a very versatile microphone. The one word that sprang to my mind in describing the sound, whichever pattern you pick, is ‘solid’.*

Another thing we’ve noticed is it sounds pretty damned good on pretty much anything you can throw at it. Because of it’s ability to handle high sound pressure levels and become a go-to mic for low end stuff, and is rock-solid on bass guitar and front of kick drum, but it’s just as handy on both male and female vocals - especially those with a tendency to get loud when they belt it out. Vocal recordings are up front, sound as they should, but without the top end brightness of, say, our TLM49.

Furthermore, it can be used for more radical effects by compressing or reverberating one side of the capsule only. For example, I had some interesting results on a male rock vocal by compressing the signal from the front capsule, mixing together with the phase reversed rear capsule, then compressing the sum of these. The effect was that as the vocals push harder, the relative amount of rear capsule included becomes greater giving it more of a hypercardioid pattern, balancing the tendency of the vocalist to step back from the mic when belting it out. With a bit of creativity the possibilities are endless.

Overall this is a brilliant, radical piece of thinking and and one of those bits of gear that comes into your life and is there to stay.

*After writing this I read another review of this mic which used exactly the same word - ‘solid’ - to sum up the sound.

Beyer dynamic transformer data sheet

Gardners Transformers data sheet

Gretsch Atkins SuperAxe circuit

New Toy!

Here is our new pride and joy - a 1960's AVO mkIII valve characteristic meter. It's built like a tank, with wonderful vintage knobs and dials. And comes in handy for getting those amps and mics up and running.

Teladi tube mic

Teladi K120 tube microphone

And here's something altogether nicer. It's an old Teladi end-address tube mic, probably from the 1950s. Construction wise, it feels very Gefell, with a small metal (nickel?) diaphragm and a U47 style connector.

I sketched out the schematic here....

Teladi K120 traced schematic