A few months ago I was trying to find some thread adapters for a customer with an old RCA junior ribbon microphone, who wanted to use it with a modern standard 5/8″ microphone stand. Most suppliers were charging around $25 dollars, plus post and the inevitable import and handling duties.
So we decided to make some.
These fit most of the ‘big’ vintage RCA microphones, including the 44, 74b and 77 ranges, and also are perfect for Sony professional mics, including the C38b, FV300 and C48 microphones. They also fit Syncron AU7A / Fairchild F22 microphones, and several other American and Japanese microphones with a large thread.
Update:
These are now for sale on the Xaudia website priced at £6.00.
This unusual Sony microphone (left) looks like the younger brother of a C38b (right) or maybe a C48 condenser mic, and shares the same high build quality and some hardware components with these mics. But in actual fact this is a high quality cardioid pattern dynamic microphone, model F-V300.
Inside, the microphone is very simple, with just a capsule in the top compartment, and the on/off switch and transformer wired to a printed circuit board below. The fixed grey output cable looks to be the same type as found on the C38b.
The capsule is suspended from 8 small springs to act as an internal shockmount, which seems to work well, in conjunction with the yoke, to acoustically decouple the microphone element.
This example has some traces of foam around the element, indicating that an internal windshield has at some time been removed.
The label states that the mic is nominally of 1kΩ impedance, although in practice seems lower than this and it has no problems driving standard low-Z microphone preamps. The output is strong and clear across the range, and the output is similar in level and detail to a healthy EV RE20, although with a more pronounced proximity effect.
As far as we know, this microphone was only marketed in Japan, but if you have any information to confirm or deny this, or in fact any information about this mic at all, please let us know!
UPDATE 22/2/11 This microphone has become a bit of a favourite for guitar cabs – it seems to have just the right amount of proximity boost for that application, giving clear and solid presence to the lower end. It also makes a decent tom mic.
Microphone ribbons are generally made from very thin metal foil, and aluminium is the ideal material as it is very light but also very conductive. The output of the microphone is inversely proportional to mass, and so a thicker, heavier ribbon will give a lower output, and a thin light ribbon will be more sensitive. Many manufacturers use something typically around 0.0001 inch or 2 microns in thickness. The ribbon is also typically corrugated either along the full length to prevent lateral motion, or at the ends to give a ‘piston’ style of ribbon. Well, that is how it should be.
However, ultra-thin aluminium is hard to get hold of, and the non-specialist may be tempted to make repairs using materials that are more readily available. Here are some things I have found inside microphones masquerading as ribbons – needless to say they were all replaced with good quality aluminium foil of an appropriate thickness!
1. Cigarette Paper.
This microphone actually worked, to an extent! It at least made a sound. The ribbon was made from an old fag packet.
Cigarette packs used to come lined with paper-backed foil – I’ve never been a smoker so I don’t really know why, but I imagine for freshness or something. The foil is thin and already textured – it just needs to be separated from the paper. Actually this last part seems to be optional, and sometimes bits of paper are still attached, making the ribbon heavy and noisy.
1.b I’ve heard that chewing gum wrappers were also used for redneck ribbons, if you want a minty fresh microphone.
1.c Here’s a lovely example of cigarette paper being used for a ribbon in an old GEC microphone.
2. Kitchen Foil
Kitchen foil is easy to handle, yet much too thick to make a decent ribbon. But that doesn’t stop people trying. This is a common ebay trick… the ribbon looks in good condition, but when the mic arrives the output is low and sounds crunchy.
3. Sweet Wrappers
Plastic coated foil or metallised plastic, like that found in sweet wrappers is an interesting innovation, but is generally too heavy and has too high a resistance to make a decent ribbon. Also the plastic doesn’t conduct. This microphone gave almost no signal, and it isn’t hard to see why.
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.
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.
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.
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.)
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.
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.
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.
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.
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.
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!
