My friends in The Savoy Ballroom needed some stage DI boxes for their numerous acoustic instruments. They wanted to be able to mute the signal whilst tuning up or switching instruments, so I build these nice little boxes for them.
They are based on Bo Hansen’s easy to make design, but with added bells and whistles – well, a switch, a lamp and an extra output.
Job done – now back to recording!
This week we have been building lots of little transformer moxes to match vintage 30 ohm ribbon mics to modern mic preamps. Correct impedance matching can deliver a +12 dB increase in level without noise penalty or loss of frequency response, which is welcome for many older microphones. Here is how a Reslo RV microphone behaves with and without the transformer:
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| Frequency response plots for Reslo mic with and without an impedance matching transformer |
They are suitable for many old microphones including the following:
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| Evolving Melodium 42b – serial no. in the 1700s, 4300s and 6600s (L to R) |
We have been lucky enough to have four of these through the workshop, and have had the opportunity to observe some for the finer details that have evolved during the production of these beauties. Over the years the grill of the 42b has been refined, with the holes becoming larger, and a solid unperforated band appearing across the bottom of the microphone. Later microphones have a three pin connector at the rear, whist early models are hard-wired.
The magnet structure has also changed over time – earlier models have U-shaped magnets above and below, whereas the later microphones have four block magnets, arranged in pairs and connected by metal plates to complete the magnetic circuit.
The photo on the left shows the later style of magnets. The transformer and inductor are in the circular metal can below. Some models have a rectangular can, but the transformer and inductor inside are similar, at least in the ones we studied. The transformer has a ratio of 1:14, giving an output impedance of 50 ohms**
There is also a facility to adjust the ribbon tension, which should be mandatory on all ribbon mics!
The 42b has a three-position switch on the front which is used to select “Speaker”, “Voix”, and “Musique” modes. In the first two positions an inductor is switched in parallel with the output transformer which causes the lower frequencies to be rolled off.
* It is worth noting that the magnets on these microphones can fade with time – a healthy 42b should have a magentic field between the pole pieces of around 4000 Gauss. One of our microphones measured at just 1500 gauss and it was necessary to replace the magnets with suitable modern alternatives. If you have a Melodium with a weak output it may be worth having the field measured and the ribbon checked. Xaudia can of course help with all those things.
** For optimum results with modern equipment a matching transformer is recommended.
April’s MOTM was a nice old Framez ribbon mic, and its chunky Meazzi sister also made an appearance. Here’s the article… and here are the microphones:
I’ve had the opportunity to compare these in the anechoic box – here’s how they look (click to enlarge).
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| Frequency plots for Meazzi and Framez mics |
The Meazzi has an overall flatter frequency response, but in both cases there is a significant peak around 4KHz, which should add attack to recorded percussion and clarity & presence to vocals. The Framez is slightly more sensitive in the mid range, but falls away somewhat below 150 Hz.
Whilst revisiting these mics, it is worth looking at the Meazzi’s perforated backplate resonator, which contributes to the pickup pattern and frequency response.
(Nb. Both microphones were fitted with 1.8 micrometer ribbons and wired for low impedance.)
