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Old 24th July 2002
  #12
Stan Cotey
Digidesign:
posted July 23, 2002 01:20 PM ___ __ _ _ _ _ __
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Regarding the clicks heard while changing the gain on PRE, I thought I'd take a moment to explain what's going on and why these clicks occur. First though, the clicks are a normal and necessary result of the high-quality gain control method used in PRE. The highest priority item for us with PRE was the sonic quality and we worked hard to obtain the performance with the initial design and keep it there through the remote control process.
As some of you know, PRE is a precision, transformer-less design, and uses a high-precision differential transistor pair (actually, a bunch on 'em in parallel to reduce noise) as a front end in place of the traditional input transformer.
The gain control method used in PRE utilizes relays to place fixed resistors in the feedback path. To vary the gain of the circuit, you cross-couple some feedback from one side of the input to the other. There are very slight DC offsets present across these resistors.
When you change gains, some of the DC offset from one side of the circuit is injected into the other side and we get what is called a "step change". Basically, this comes out as a small transient or 'click'. There are plenty of non-remotely controlled mic preamps that utilize a variation of this type of design and they use a potentiometer for gain control, in which case the step change never occurs. On some of these preamps, you can quickly rotate the gain control and hear a "whoosh" or a scratching sound (depending on the type of pot used,) which is really the same artifact in disguise.
As a manufacturer, the real problem comes in when you want to remotely vary the gain of one of these circuits. You can easily use a VCA (voltage controlled amplifier) or an MDAC (multiplying digital-to-analog converter,) which delivers the convenience and reduced artifacts of a pot but unfortunately impairs the fidelity and performance substantially. Remember, the audio performance of PRE in terms of noise and distortion is very, very good and due to the high-gain design and critical nature of the circuits, it's very easy to impair this excellent performance with a less-than-excellent gain control method.
When it comes right down to it, there are only two methods of remotely controlling a transformer-less mic preamp that I'm aware of.
The first might be using an actual servo motor to drive an actual pot. If you can get around the noise of the motor circuit leaking into the sensitive audio electronics, it's a workable solution. Unfortunately, servo motors cost serious money, as do motor drivers, as does the additional power supply required to run them, which would raise the price substantially, not to mention the fact that we would have to fit eight of them in the case and then deal with the extra heat, etc.
Then, you have reliability concerns since you now have an electro-mechanical assembly in a critical path. Topping that off, you now have to deal with the error of the pot, which means that unless you calibrate each pot to each mic preamp, you can't have reliable gain amounts, which in turn makes it hard to work in stereo. So the cost, size, complexity and heat all go up while the reliability and accuracy are reduced.
The other option is to use relays to switch fixed precision resistors in the circuit. The benefits are many; performance and sound quality are maintained, cost is kept reasonable, reliability is high and gain matching between channels is great. The downsides are simple but livable given the other options; you have fixed gain steps and there are small clicks when changing gain. Since most of the professional engineers I know rarely change the gain setting while recording, I believe it is a small price to pay for the increased sonic performance and benefit of being able to locate the preamp at the source.

Regards,

Stan Cotey
Product Manager