Friday, September 01, 2006

RCA BK5a Inductors

While we're on the subject of ribbon microphone transformers, here's a shot of the inductor set found in the RCA BK5a ribbon microphone.

The can on the right is the actual step up transformer and you can see the larger primary wires that extend through the mic body to the motor unit and its ribbon. To the left, the low pass filter reactor can be seen. This is routed to a switch mounted in the base which can be turned with a screwdriver to switch in a bass cutting shunt reactance. The transformer is the bottleneck in ribbon microphones. An inefficient, poorly designed transformer will lack bass, lack definition, and develop a weak output that is easily loaded down. Ohmic loss can be very high, relatively speaking, in small transformers such as the one shown above, which contributes to the low output that many old school ribbon mics suffer.

RCA was right to encase these in cans, but failed to provide a low Z path to signal ground, which would have reduced RF pickup significantly. Also, the orientation of the transformer in the microphone case is important: A ferrous microphone case should have the transformer mounted 90 degrees from the long axis for best hum rejection in any direction.

Weak bottoms

The materials used in the transformer are critical for good signal preservation. The core material (if used) must not contribute to loss, nor should the inductance represented by the windings and the primary to secondary, and turn to turn capacitance be permitted to be uncontrolled variables. The transformer must be properly matched to the ribbon. This is not nearly as simple as it seems - the usual "less than an ohm" assumed ribbon Z is not very pertinent to the real dynamic Z at various frequencies, nor is it very useful to calculating the critical damping factor of the primary-ribbon system. Many old school makers used to apply loudspeaker resonance tests to transformer coupled ribbons. This practice is obsolete, as it assumes that there is a significant acoustic response correlation with the electrical resonance point, often in the range of 30-50 hz. Attempts may be made to tune transformer inductance so that the electrical resonance occurs at some set point, which further exacerbates the loss problems, often resulting in weakness in the bottom end.

We have found only a weak correlation between the electrical resonance point in transformer- coupled ribbon microphones and the actual acoustic response around that electrical resonance point. In other words, factors such as the local acoustics in the near field, ribbon motor geometry, and realized impedance at the ribbon-primary junction are far more important than measured electrical resonance as a function of impedance. A strong, clear bass signal requires efficient transduction of acoustic energy, which is largely governed by accurate physical ribbon motion inside the motor unit. Once this motion is developed and converted into a signal, the role of the transformer must be to preserve the signal throughout the frequency range without excessive loss.

Here is some more about the BK5a.

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