TF: What are the advantages and disadvantages of mics with output transformers?

GM: I don't like them for any reason. Even in live situations where one might reason that transformers would reduce interference over long distances, I avoid them where possible.

JH: A transformer at the output of a mic can step the impedance of the mic up or down as required. It also blocks the +48V phantom supply from getting into the circuitry or capsule of the mic where damage could occur. It is a similar situation at the input of a mic preamp. If there is no transformer at the input of a mic preamp, capacitors are usually used to keep the +48V phantom supply voltage from traveling forward into the active circuitry of the preamp where it could cause damage. Capacitors can cause phase shift at low frequencies. They can also smear the audio signal because of a problem known as dielectric absorption. Some capacitors are much better than others in this regard.

Transformers have their own potential problems, including phase shift and ringing at high frequencies, and core saturation at high signal levels and/or low frequencies. A well-designed transformer minimizes these problems.

On the positive side, a transformer coupled mic preamp has a much higher common mode impedance and a much higher breakdown voltage than most transformerless mic preamps. This results in the potential for a much higher common mode rejection ratio, and the ability to handle and reject much higher common mode signal levels. This is extremely important where there are high levels of RF or other interference.

TF: Are there any ways to make better choices about mics and preamps than "Read the specs, listen, if you like what you hear, buy it?"

GM: The people who ‘listen’ better do better work. There is no short-cut, nor is there a push-button answer here.

JH: There are some CDs available that demonstrate many mic preamps and mics, but since you were not there to hear the original performance, you do not know all of the details of the signal path, room effects, what the original sound source sounded like from the exact mic position, etc. You must try things under your own circumstances, listening from the mic position and eliminating as many variables as possible.

TF: There are now tube mics that are quieter than some FET mics. Other than a tube's absorptive capabilities as a result of plate saturation, have better components and circuit designs made the tube/solid state argument moot?

GM: Well, I don't know anything about tubes, but I can tell you that discrete components haven't evolved as far or as quickly as other semi technologies. I would really like to have faster, higher-gain, higher-voltage transistors to use, but it just hasn't happened. If anything parts are going away. We are constantly having to find replacements for parts that have been discontinued.

JH: There is certainly much confusion and misinformation regarding the supposed need for tube circuits to "warm-up" the often cold and harsh sound of digital circuitry. The cold and harsh quality is not the fault of solid-state circuitry. It IS the fault of CRAPPY solid-state circuitry that happens to sound cold and harsh because it is crappy. A well-designed solid-state mic preamp can do wonders to warm things up. Actually, a well-designed solid-state circuit is probably bringing things back to "room temperature", which I think is what most people really want. A tube circuit may be going beyond room temperature to a colored sound quality.

TF: Give us a glimpse over your professional horizons.

GM: I'd love to come up with a 'digital' microphone, and combine all that I know into one box right up near the capsule. It would be flexible, impossible to distort (i.e., it would auto-range), quiet and would certainly have transparency as a design goal. I don't think it would have a tube in it.

JH: I am always working on new products.

Hardy M-1 preamplifier (click on image for full size)

Any new mic preamps would probably have the same basic ingredients that I have used in the M-1, M-2, Jensen Twin

Servo 990 Mic Preamp, and various retrofit mic preamp cards for MCI and Sony consoles (MPC-500C, MPC-600 and MPC-3000 mic preamp cards).Those ingredients include Jensen's best mic input transformer (JT-16-B), the 990C class-A discrete op-amp, and the total absence of coupling capacitors from the signal path. Newer models would have different features and package options, but the heart of the circuit would be essentially the same. I'll save the details for later. Other items such as A/D and D/A converters, EQs, limiters, mixers, etcetera, are on the list. Suggestions are welcome!

Technique, Inc. Copyright 2001 Ty Ford may be contacted at http://www.jagunet.com/~tford