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.
