By Allen H. Burdick
Who Should Care?
You Should Care!
High-Frequency Intermodulation Distortion is a Critical Mic-Pre Parameter!
The mic-pre function is one of the most difficult challenges facing the audio engineer. A mic-pre is often the limiting factor in the audio chain. When selecting a mic-pre, everyone looks for great specifications, along with some magic, that will set their recordings apart from the crowd. Overall, the technology of low noise amplifiers has progressed superbly during the past 30 years from warm, but noisy, tube amplifiers, through the harsh sound of the early discrete transistor amplifiers, to the relatively clean op-amp designs of today.
One electronic element that has historically been a part of almost every mic-pre design is the input transformer. We have listened to preamplifiers with transformers so long and so often that we've been educated to expect the sound of iron without even realizing it! When we listen to a truly clean transformerless mic-preamp we often say that something is missing in the low end. And, of course, we're right! What's missing is the distortion generated by the non-linear core of the transformer. The engineers at WGBH-FM in Boston are among those with time to do extensive listening, and who possess the best in both types of mic-preamp systems. They have been able to make the long term comparisons everyone wishes they could. Now that their ears are re-educated, they consistently pick the clean amplifiers for their recordings.
But "clean" does not only come from the absence of iron and nickel. High frequency intermodulation (IM) distortion can also ruin otherwise good performance in a mic-pre. Essentially, all distortion is caused when signals are passed through a non-linear element. Harmonic distortion and intermodulation distortion are both created by the same mechanism. This may be a narrowband amplifier, an amplifier that is slew rate limited, or an intrinsically flawed design element in the amplifier, such as the output stage. At low frequencies, the large amount of feedback in today's audio amplifier elements all but eliminates distortion products. However, at high frequencies the intrinsic gain of an amplifier element is significantly reduced and therefore the percentage of gain available for use in feedback becomes severely limited. Excellent high frequency performance requires a careful use of wide bandwidth, intrinsically clean circuit elements, and the proper amount of feedback.
High frequency IM distortion can result from two significant sources in a mic-pre. The first is intermodulation between genuine high frequency audio signals that are present from the source, such as the sounds from a triangle, rich in harmonics. The late Deane Jensen measured significant energy from cymbals out past 30 kHz. If your mic-pre can't properly amplify those signals, the intermodulation products that reflect back into the normal audio band will be most unpleasant.
The second source, and perhaps the most pernicious, is that of RF induced IM distortion. This is a result of 1) amplifier stages that have not been protected from strong external RF signals, and 2) from a lack of proper feedback compensation which allows the amplifier to intersect its open loop gain curve. When this occurs in the presence of RF, the amplifier becomes non-linear and intermittent IM distortion is the result. And intermittent it often is. A product may measure well on the bench, but when placed into a system or when taken into the field, users may find that its performance is far less than stellar. Here at Benchmark, we are convinced that most of the "bite", "edge", and otherwise undesirable characteristics of many amplifier designs are a result of poor RF immunity. RF causes non-linearity to create new, unexpected, and unwanted audio signals from the incoming audio. And whether you want it or not, this new extra audio comes free with most mic-preamps! To achieve truly clean audio at 30 kHz, the 3 dB bandwidth should extend past at least 200 kHz and still be RF stable. This is no trivial task!
Who should care? At Benchmark, we care! We have created "clean" with a very careful transformerless design. The Benchmark MPS-420 has wideband - 500 kHz for outstanding performance at 30+ kHz; flawless square wave response - a powerful measure of RF stability; and a common mode filter that removes RF from the microphone input line. RF protection has been accomplished without limiting the bandwidth, without degrading the 1 dB noise figure, (see the "Noise Primer" in "A Clean Audio Installation Guide™") and, most notably, without compromising the distortion performance. See the 100 kHz DIM, and the CCIF twin tone IM sweeps.
Still want "warm" (2nd harmonic distortion)? Fine. Keep a good tube mike, an Aphex® Aural Processor, or a tube compressor in your bag of tricks. That way, YOU are in control. But don't settle for high frequency IMD that comes free. The cost is too high: it eliminates the magic!