User guide
other recording or mixing medium.
•Each channel features:
• Support for microphone (XLR), line level
inputs (1/4½) and direct instrument (1/4½)
• Selectable impedance settings for Mic input
• Input gain variable from 0 dB to +69 dB, in
approximately 3 dB steps, with an optional
–18 dB pad
• +48V power for condenser microphones
• Balanced Insert points (Send and Return),
for easy integration of outboard gear (such
as compressors or EQs)
• Phase invert
•85 Hz High-Pass Filter
• Up to nine PREs are supported through software
for up to 72 channels of remote-controlled
Mic Preamp channels.
• All features are available in Stand-alone mode,
making PRE ideal for any recording application.
PRE Index | All FAQ Index
[8273]
Why do I hear clicks when changing the gain on my PRE?
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.
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,