Behind DiGiGrid’s Top-Grade Microphone Preamplifiers
A microphone preamplifier is not just about amplifying small signals. If that were the case, designing preamps would be easy! Preamps have to perform in the harshest environments. A microphone signal often starts off at very low levels—sometimes a millivolt or so. It then has to travel down very long cables, possibly through patch bays, before arriving at the amplifier input. Along the way it is subjected to many problems. One of them is electromagnetic interference: a long mic cable makes a perfect antenna for picking up radio interference, and stages today are full of such devices, from radio mics and walkie-talkies to wireless Ethernet, to mention just a few examples. Other factors, such as static, can also cause interference. The preamp also has to withstand abuse such as shorting phantom power to ground. This can result in large -48V transients being applied to the delicate front end.
So, a good microphone preamp has two tasks:
1. It has to faithfully amplify and reproduce the small signals from the microphone
2. It has to prevent the pickup of any erroneous noise or abuse.
Designing a very low-noise preamplifier that can work in harsh conditions is not a simple task. This is why DiGiGrid audio interfaces feature top-of-the-line mic preamps by DiGiCo, who after more than 30 years of mic preamp evolution have arrived at their best design yet.
Let’s start at the front end:
• DiGiCo preamps have state-of-the-art transient suppressors that can clamp spikes within 5 ns and absorb up to 600-watt peaks.
• Following this, a LC Delta/PI network prevents any RF from getting through.
• Finally, there are two clamp diode arrays just in case anything made it through the first two barriers.
We then get to the mic preamp itself. Its input stage provides an impedance to the microphone which is constant over the audio spectrum. This is important for preventing the introduction of artifacts into the microphone signal.
The front end of the preamp is based on the best available bipolar amplifiers today. They provide a very consistent ultra-low-noise input, so each amplifier sounds exactly the same. These inputs are also capable of handling very high-signal levels (at low gains) of over 26 dB without clipping.
The preamp gain is 100% digitally controlled, providing a smooth padless gain range from -20 dB to +60 dB. The gain steps are carefully controlled and utilize zero voltage crossing detection to ensure that any gain change is done silently. On top of this, the preamp is instructed to mute when phantom power is applied in order to prevent loud transients.
By applying digital control to the front end gain, we can ensure that the dynamic range and the signal-to-noise ratio are always optimized.
A microphone preamplifier in a digital mixer can only be as good as the A-to-D converter. Having a low-noise front end can often be compromised by the selection of the converter. At DiGiCo there is no compromise. After looking around for the market leader in performance and ignoring the price, DiGiCo found the ideal partner in Burr Brown. Their multibit delta sigma converter provides over 123 dB of input dynamic range, ensuring that the converter would never compromise the performance of the preamplifier.
So there we have it: an ultra-robust front end mic preamp that can faithfully amplify and reproduce delicate microphone signals, even in the harshest environments.