The Care and Feeding of Classic Gear, October 1999—Keeping Your Mics and Connections Up to Spec

May 18, 2004 12:00 PM

KEEPING YOUR MICS AND CONNECTIONS UP TO SPEED

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Sound recording is unusual among the technical arts in that today's artists and technicians often combine current, state-of-the-art technology with equipment and techniques that are positively anachronistic. Does any modern filmmaker use a hand-cranked camera? Would any graphic artist willingly choose wax and an Exacto knife over a desktop publishing program? Probably not. Yet, recording engineers frequently use microphones, reverbs or tape machines from a previous, pre-digital generation.

Without going into the reasons for the appeal of vintage microphones and tube gear, there clearly is a growing demand for those who can maintain and repair such devices. Many popular vintage products are more than 40 years old, parts are hard to find and knowledgeable service people are even more scarce. With that in mind, I'd like to offer some advice on evaluating classic mics and other vintage gear, and provide some suggestions for improving their performance.

I have restored, repaired or evaluated numerous vacuum tube or ribbon microphones and have yet to encounter one that was totally free of problems. Admittedly, many of the mics came to me for repairs, but more than a few were considered operational and had been used regularly for recording.

My suggestions here are meant to extract the highest performance from vintage equipment, while maintaining "stock" condition. Some people may decide to go further by adding modern, higher-performance parts, changing capsules/diaphragms, or embellishing the design with modern circuitry, such as adding regulation.

POLARITY, CAPSULES AND RIBBONS
Be sure to check the polarity of the mic. Without using any test equipment, you can establish polarity by comparing the mic you want to check with another mic whose polarity is known (preferably a mic similar in pattern and type to the unknown one). Stand them as close as possible to each other, a few feet in front of a loudspeaker fed with pink noise or between-station FM radio noise. At the console, send both microphone signals to the same output. Using a polarity switch (or "phase reverse," as it is more commonly and incorrectly called), find the polarity that nearly eliminates the output signal of the console. This is the inverted polarity state. Correct (or mark) your unknown mic as necessary.

If the wiring/polarity is okay and your condenser mic lacks bass, a new capsule may be needed. This problem affects some condensers when the capsule becomes so dirty that the polarizing charge leaks off. You might be able to get the mic capsule cleaned, but repair or replacement will be necessary. On the other hand, noise or crackling from a condenser mic does not necessarily indicate a bad capsule-aging parts at the front end of the preamp may be the cause. Fortunately, this is much easier to fix, even if you still need outside help, and even the lowest-level noise that is not simple hiss is entirely removable.

Older ribbon mics have their own special problems. These mics are extraordinarily delicate—the force of wind generated by a person speaking at close range can destroy a ribbon mic—and surprisingly low output levels are normal. RCA ribbon mics originally included a cloth bag that was meant to be kept on the mic until the moment it was to be used. The implication is that a ribbon can be damaged by simply moving it uncovered across the studio. Don't even think about using one in a kick drum. Also, checking the output connector with an ohm meter can destroy the ribbon.

When it comes to repairing or evaluating a ribbon mic, there is really no substitute for looking inside. But remember, these mics have powerful magnets-hold on to your screwdriver! Inside, the "ribbon" is a thin strip of lightly corrugated aluminum foil deep in the magnetic gap. A good ribbon will be centered, both front to back and left to right. A twisted, bent or broken ribbon will need to be replaced-a job for an expert. Also, ferric dust trapped in the gap between the ribbon and the magnet can cause distortion and altered response.

Some RCA ribbons suffer from what appears to be poor quality control (or a rash of clumsy repairs). On some of the mics I've seen, the windscreen cloth was attached with so much adhesive that it was almost airtight. The cloth is easy to replace. Just get a similar-weave cotton at the fabric store and cut it using the old cloth as a pattern. Put Duco cement on the metal part and not the fabric. The difference in the mics on which I've performed this "repair" has been astounding. I expected improved treble, but I also noticed reduced midrange resonance.

OUTPUT IMPEDANCE, PREAMPS
Another issue that affects microphone performance is the output impedance. Standards varied greatly in the past, and, for this reason, manufacturers often put multiple windings or tapped windings on the output transformer. Today's equipment prefers an impedance of about 150 to 250 ohms. If you don't have a tap for this, your preamp may not get the best out of your mic. Transformer-coupled preamps are most sensitive to this. If your mic can't get to that impedance, you could use a matching transformer, or start looking for an old preamp that is optimized for that mic.

This brings us to another sticky issue regarding older gear. We have learned that transducers (such as microphones) do not work well in a terminated circuit, so all modern preamps place a gentle load of a few kilohms to improve performance. This principle was not always observed in older preamps. Though all mics will drop in level when terminated, this will also cause dramatic frequency response changes in some microphones.

At first glance, the persistence of this problem might seem a bit strange, but remember that most mic manufacturers were in the mixer business, too. The products were optimized for each other. This resulted in difficulties when mics and consoles were mixed among manufacturers.

For example, RCA, manufacturer of low-output ribbon mics, built mixers with high-gain preamps. When American engineers tried out European condensers in their studios, the mics distorted in their consoles. In those days, gain trims were not used in preamps. The U.S. importer of Neumann recommended attenuators that were fitted in the power supplies. By the time the U67 was introduced, the attenuator was installed at the factory. Modern preamps are designed to accept a wider range of input levels, so those attenuators can be considered vestigial. Removing them will only serve to improve your dynamic range.

TUBES AND VOLTAGES
Concern about tube wear is not always justified. Power tubes and rectifier tubes (which also handle power) do have a short lifespan, but some tubes can last indefinitely when given the proper circuit to live in (I have a tube that is at least 60 years old, and it works just fine).

The simplest test of a tube is visual-examine the tube while it is operating in the circuit for which it is designed. A burned-out filament or shorts between the electrodes indicates failure. A tube tester will confirm this condition, and will also identify a "gassy" tube while in its early stages. Since air leaks only get worse, the filament will eventually give out because it burns in the air. A white powdery appearance inside the bottle is evidence of leakage-throw the tube out!

Finding replacement tubes is not as hard as you might think. Of course, old used tubes can be worn out, and even old, unused NOS (new old stock) tubes can be poor in quality. However, the art of good tube manufacture was never lost in Russia or China, and modern tubes from those countries, including new versions of some of the old tube model numbers, are in every way superior to the old tubes.

A tube checker is useful, but has limitations. For example, a tube checker uses a single generic circuit for testing tubes that can be used in many different ways. So, although you can use a tube tester to track tube's gain over time to see if it is wearing out, some tubes will exhibit more gain than others in the tube-tester's circuit. What's best for you is what works best in your circuit.

Also note that a tube tester does not test for microphonics, the result of mechanical vibration in the tube shaking the internal electrodes to the point that there is an audible signal. Depending on the tube's position in the circuit, microphonics may be an extremely important parameter. Preamp tubes are the most affected by microphonics. The best test for microphonics is to listen to the unit with no input. Tap on each tube one at a time, and, if you have a batch of tubes with the same part number, swap them out and grade them for microphonics. Save the good ones for those applications where they are important. The others will be usable elsewhere.

Line voltage sensitivity is another problem in older tube circuits. Some circuit designs are more susceptible to voltage fluctuations than others, and I have heard of mics that will not operate if the AC voltage is 10% low. Modern tube designs often use solid-state regulation, but performance may change with line voltage fluctuations.

Many recording standards have changed over the years, and classic equipment may require a special interface to conform to the modern standards. The old standard used 600-ohm terminated connections for all line-level equipment. By 600 ohms terminated, I mean that the output of one stage had 600 ohms in its output (which makes its output level sensitive to loading) and the next item in the chain was designated to provide a 600-ohm load, effectively reducing the output voltage in half once the connection was made. Don't go looking for resistors to rip out. Often that impedance was developed as a virtual element formed by the circuit topology. These aspects are designed in, and even older solid-state gear may be looking for a terminated studio environment. Check your manual.

Today's equipment can be built with a vanishingly small output impedance, which allows it to drive long cables without much signal loss. We ask that the subsequent devices draw only tiny amounts of current, which is to say it has a high input impedance. Now we can "Y" an output with abandon. We call this method "bridging" as opposed to termination. Despite the appearance of incompatibility, we can use terminated gear in a bridging environment with all participants happy about their sources and loads. The best way to deal with this is to connect a 620-ohm resistor across the output terminals of the older device. Any subsequent load will only bring the net impedance closer into line. Failure to do this properly will definitely alter the gain of the unit and will in most cases affect high-frequency response, too. As far as the other end is concerned, a modern device will usually have enough power to drive any older-style load without difficulty, but probably not two of them at once.

Sometimes there is a real input load resistor that can be removed. Actually, the preferred method is to change it to a much higher value of, say, 10 to 50 kilohms. This helps keep the unit from picking up buzzes or radio stations when nothing is plugged into it.

Meanwhile, don't expect the older units to be comfortable driving a long output line no matter what standard is being used. A finite (non-zero) output impedance will lose high frequencies when faced with cabling capacitance. Also, it will be more susceptible to noise and hum pickup. Keep those runs under 25 to 30 meters and you should be all right. For all the same reasons, it is with this equipment where you will probably notice how much benefit a premium cable can give to the sound, especially in terms of interference and cross talk reduction. You probably spent some serious money on that unit. Cabling is no place to start cutting corners.






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