Sometimes I see patterns or trends in what arrives on the bench. A year ago it was RCA 74s, and in spring 2011 it was Electrovoice ribbon mics. And then the summer brought Melodium 42bs. Of course these are just statistical anomalies or ‘blips’ in the random noise of what my customers send me to repair, but it does at least suggest ideas for the regular ‘Microphone of the Month’ column.
|Cadenza microphone set with box, documents and stand|
This month these little Cadenza ribbon microphones are in vogue – these were made by Simon SoundService Ltd in London, and were designed by Eric Tomson, Stanley Kelly Peter Bell. The mics have a very 1950s styling, and are often called ‘rocket’ mics. However, having spoken to a couple of customers, the consensus was that many of these microphones no longer sound as good as they should – or at least as good as they look. The complaints are that they are noisy, with low output.
|Cadenza mic transformer under the knife|
From a technician’s viewpoint (i.e., my opinion :p), there are a some weak points in the design which all relate to the transformer. Firstly, the ribbon clamps are connected to the transformer simply by winding the wire under a nut and tightening it – really not a reliable long term approach. Secondly, the transformer wires are extremely thin, and half a century later the insulation becomes brittle and tends to break, with disastrous results. It is more common practice to use thick gauge wire for the fly-leads to the primary, to keep resistance and noise to a minimum. And finally, as these thin wires become old and oxidised, the mics become noisy.
|Cadenza windings – primary (left),
and 30 ohm output (right)
The outer high impedance winding is a very fine gauge (approx. 0.07 mm), below which is a single layer of 0.4 mm wire which makes up the primary. The innermost winding is the 30 ohm output, which is approximately 0.2 mm in diameter. Unfortunately the solvent also damaged the bobbin, and and a replacement was found to make a new transformer using the vintage laminations.
The new transformer has a single 600 ohm output, with thicker gauge wires to give lower resistance. It took some effort but the result is a higher output with a lower noise floor, and the mic is much more usable. I’ll post back with a ‘before’ and ‘after’ frequency plot for comparison.