Intriguing Transformer

Intriguing Transformer

In the June 1979 Hands-On Report, Jim Ford and John Murphy reviewed the Otari MX-5050-B. I was intrigued by their comment about using an in-line transformer for the mic inputs (page 78). I have an Otari and was wondering what type of transformer they used to get a noise result of 47 dB? Also, where was the transformer placed in the chain? Was it at the mic end or at the deck end of the cord? Does it matter? Could I use it with my Pioneer 1020 reel-to-reel as well?

—Frederick Evans
Allston, Mass.

When the MX-5050-B was reviewed, we found that the noise performance with a low-impedance microphone input was not very good, the noise being only about 33 dB below 0 VU for "conversation level" speech at about six inches from a typical low-impedance mic (a Shure SM58). The reason for this poor level of performance is that the input impedance of the Otari is 50 K ohms, which is definitely a high-impedance input. Otari correctly states that the mic inputs can be used with either high- or low-impedance mics. However, the price you pay for using a low-impedance mic is poor noise performance.

This situation can be greatly improved by using an in-line, impedance-matching transformer, such as the A-95 series or the A-97 series from Shure. These are just small, in-line transformers that you plug your mic line directly into and then plug the transformer directly into the tape machine.

When we originally tested the Otari, we used the A-97, which yields about 13 dB of signal gain, which increased the signal-to-noise ratio of the Otari 13 dB. I think, however, that you could get even better performance by using the A-95 series, which gives you 22 dB of gain. This would change the Otari's noise level from 33 dB to 55 dB which is a more respectable figure. What the transformer is doing essentially is boosting the signal level, which could improve your Pioneer's performance in a similar manner.

As to your questions regarding the placement of the transformer, it should be placed at the deck end of the line. This allows you to run a long, balanced, low-impedance line and convert that balanced line to a high-impedance line at the tape machine.

We would, however, like you to look again at our original assessment of the Otari MX-5050-B. We feel that a machine of this caliber should be teamed with a high-quality mixer, and the use of the mic preamps avoided if at all possible.

—John Murphy
Technical Editor
Modern Recording Magazine