I have a soft spot for Oktava microphones. They look cool, come from Russia, are fairly priced, and everyone has fun modifying them. That’s enough to make the ML52 microphone of the month for June.
Oktava have a long tradition of ribbon microphones dating back (at least) to the ML11 in the 1950s. The ML52 is one of the most recent models, with a slightly odd double-ribbon motor assembly. I had some in to service recently, and noticed something odd….
Here are two ML52s – can you see any differences?
The answer is “Yes”. The bottom mic has a logo and serial number, and an XLR that is machined into the bottom bell. It also has slotted screws rather than pozidrive. The grill spacing is also slightly different, but that is harder to see on the photos.
Under the grill, both have twin ribbons, but the motor assemblies are totally different. The one on the left is the same as the one found in Avantone microphones, the right hand one looks typically Russian, with silver contacts and big baffles to protect the ribbons.
The transformers are also very different. The right hand one is the mic with the logo, and it uses a toroidal transformer. The left hand transformer can looks typical of the China mics, and is connected to a (pointless) circuit board.
Sound-wise, the mics actually sound quite similar, which goes to show just how much the body and grill influence the sound of these mics. The Chinese one has a longer ribbon and stronger (neodynium) magnets, and has slightly stronger output.
Response plot for Russian (Red) and Chinese (Blue) ML52 mics
I don’t want to say the word “fake”, but if forced to jump, the obvious conclusion would be that one mic is from the Oktava factory in Tula, Russia, and the other is made in China – either under license, or perhaps not. 🙁
This transformer came out of a faulty Beyerdynamic M260.
Beyerdynamic M260 transformer
The mic was giving no output, but the ribbon was OK. The problem turned out to be a short in the primary winding, which gave an excuse to take a peek inside.
I was surprised how roughly the primary coil is wound – it really looks like it has been done by hand. Perhaps it is to minimise parasitic capacitance, although that is not usually such a big problem with low impedance windings as they only have a few turns.
In this case the short was located and the original transformer was repaired, but that is not always possible, and sometimes a rewind or replacement is required.
Some of the Beyerdynamic ribbon mics, such as the M500 and some models of M260 have a ‘built in’ high pass filter. This is done by manipulating the transformer inductance, allowing some of the bass frequencies to pass to ground. Some people like this, but others (including myself) prefer a full frequency response.
Xaudia B-series (top) and Beyer transformer (below)
They recommended a number of modifications, which included….
Swapping the transformer to one with higher primary inductance,
Conversion from 50 ohm to 300 ohm output,
Fitting a more open interior mesh,
Removing the cloth around the ribbon motor,
And reversing the motor assembly so that the ribbon element is at the front.
Xaudia have recreated these mods and added a few more of our own. “The Beeb” has an XLR output, and new 1.8 micrometer ribbon. We also ultrasonically clean these before re-assembly to remove 50 years of gunk. And we put a (removable) sticker on them, so you know that they are 300 ohm mics.
Here is my much abused Reverend Slingshot, now complete with a properly mounted stereo pickup.
The nice people at Reverend guitars sent me a NOS white scratch plate with routing for 3 P90s, which is perfect for mounting the new pickup alongside the two standard P90s. All I needed to do was drill out a hole for a second output jack.
The open chrome pickup cover was made by modifying a covered one, using a dremel cutting wheel.
Our new Reslo upgrade transformers are the result of this work. They are made here in our workshop in York, UK using high quality German laminations and teflon insulated silver plated lead-out wires.
They outperform the original Reslo units in terms of resistance noise and frequency response, giving less noise, an extended bottom end, and better impedance matching into modern equipment.
Typical Characteristics
Primary DC resistance 22 milliohms.
Primary Inductance >620 µH at 1KHz, >2.0 mH at 100Hz.
Frequency range <20Hz to >65 KHz.
Ratio 1:32 (available in 1:12 to 1:40, as required).
I have been thinking for some time about a stereo guitar that would spread out the signals from the different strings to a pair of amplifiers, or to stereo inputs for recording. So here is my first attempt at winding a stereo pickup.
In order to get some separation between the strings, I decided to make six mini-pickups and combine them into sets of three. Here are the parts – six bobbins, six steel pole pieces, an Alnico 5 bar magnet, and a home made top board.
Each was a coil of 5000 turns 42 AWG enamel wire, around a steel bar. Three of the coils were wound normally and three reverse-wound as they will sit on magnets of reverse polarity.
Winding mini-pickups on the Meteor winder
These were then arranged in two sets of three, magnetised by a bare of Alnico 5. The hard part is keeping track of the winding direction, the start and end of the wires, and the magnet polarity – so that everything is in phase! Each set of three coils were wired in series, and the E,D & B strings sent to one output, and A,G & E sent to the other. Once done, the parts were glued in place, soldered, and then dipped in the wax bath.
Assembled six-coil pickup with magnet
And then it was mounted in the test guitar – my trusty Reverend Slingshot, with the pickup simply taped in place for a test run. The next job is to mount it properly and put a cover on to hide the mess – the hard bit was finding an undrilled P90 cover, but ebay came to the rescue.
Reverend Slingshot with prototype stereo pickup
So, how does it sound? Here is a little run on the guitar and the screenshot from protools. The recording was made into the instrument input of an API A2D, using the digital output at 24bit, 96 kHz into protools. Then no further processing, other than bouncing to 44.1 kHz, 16 bit.
[The stereo image of laptop speakers is usually pretty awful, so best checked out on headphones!]
The isolation between the two sets of strings is actually very good – you can hear and see on the screen shot that the two channels are very different, following the two sets of strings. It gives a nice interesting stereo spread with the run on the B-string being panned to the right, and the lower notes played on the A string going left.
I imagine this would be good for double tracked guitars with big Pete Townsend style chords – using two amps. Although this is wired for stereo, it could easily be made into a hexaphonic* pickup. I’m not sure what use that would be in the real world, but it would certainly be fun to find out.
Footnote
*The concept of a hexaphonic pickup is by no means new. The earliest patent reference that I could find to the word ‘hexaphonic’ is in this application by Gibson filed in 1990. This refers to midi pickups and states “The first category usually makes use of a special pickup which is mounted on a normal guitar as is the case in the present invention It is deemed important to retain the guitarness or feel of the instrument and a retrofittable hexaphonic magnetic pickup is utilized….” So the idea of hexaphonic pickups was well understood by that point, and in fact must have been obvious even to the very first pickup winders.
It is time again for microphone of the month, and for May we have these lovely Grampian ribbons.
Grampian ribbon mics
Grampian mics come in a variety of model names and numbers which we will try to decypher. These were made in the 1960s and 70s by Grampian Reproducers Ltd, of Feltham in Middlesex, England.
Grampian GR1/L and GR2/L ribbon mics
The mics are labeled GR (“Grampian Ribbon”), followed by a number and a letter. The GR1 designation was used for the semi-cardioid version, and the GR2 is the ‘normal’ figure-8 pattern. The letter shows the impedance of the mic, set by the output transformer winding. They come in Low, Medium, High, and ‘X’ flavours – the letter codes are as follows:
GR1/L and GR2/L … 30 ohms
GR1/X and GR2/X … 200 ohms
GR1/M and GR2/M … 600 ohms
GR1/H and GR2/H … 50K ohms
Here is a rather wobbly scan of the original Grampian data sheet.
The GR1 usually has a silver grill at the front, and a black one at the rear to show its asymmetry, whereas the GR2 has two silver grills. However, inside there is very little real difference between the two models, and any cardioidness* is imposed by additional foam padding around the rear of the ribbon. This foam usually depolymerises over the 40 years or so since manufacture, and the mics will typically fall apart when opened. If the mics have been stored for a long time they will require careful cleaning and new foam suspension before being put back into use.
The mics have a small thin ribbon, held in a removable plastic frame for ease of service. In that respect they are similar to the Reslos and Film Industries mics with which they undoubtedly competed. Unlike the Reslo and FI mics, the symmetrical magnet arrangement means that the GR2 has a true figure-8 response, and are suitable for Blumlein pair or a side mic in a mid-side array.
The internal transformers have a tendency to break, particularly on the H models which have very thin wires. If the break is in the right place the transformer can be repaired, but a rewind may be necessary.
Grampian plugs and connectors are a bit of a pain – they are getting scarce now, and rely on the barrel to make the ground connection, which can lead to hum if the connector is worn or oxidised. I have blogged previously about XLR conversions for these.
Grampian plugs
External matching transformers were also available, in case one needed to connect to an input of a different impedance. These are still handy for getting a bit more level out of the 30 ohm models.
These were mid-price microphones when they were made. I have a Grampian price list from June 1976, and the GR1 & 2 microphones were priced at £32.05 (plus tax) in a wooden instrument case, and £27.55 in a cardboard box. This would translate today as £180 to £334 plus tax, which would buy one of the better Chinese ribbons or a Beyer M260.
Some Reslo RB mics have a transformer with a split secondary that gives the owner the choice of either a 30 ohm or high impedance output. Often these mics get dismantled and rewired, so here is where the internal transformer wires go….
For a 30 ohm output, yellow is the ‘hot’ output, and should eventually end up at XLR pin 2. White is cold, and goes to pin 3.
The green wire should not be grounded, but this sometimes happens by accident if the plug uses one of the pins for ground. The result is a high impedance path to ground from the output, which can act as a filter and give a weak output and weird frequency response.
Update May 2021
One reader reported that his microphone was reverse polarity after following this wiring diagram.
It is always a good idea to check polarity after any repair or modification. The polarity can be corrected by simply swapping over the two transformer output wires.
