Recording - December 2015 - 34

B Y PA U L V N U K J R .

Great River Electronics is a Minnesotabased audio company run by gear
designer Dan Kennedy. Dan got his start
making industrial measurement and control systems, and he has carried that same
attention to detail and penchant for highend components into his audio designs,
most of which are a blend of analog
sound with the precision of digital control.
The Great River ME-1V mic preamp is
legendary; over the years the company's
product line has grown to include a range
of varied ME-1V configurations, equalizers, and the state-of-the-art Mixmaster
20-a full-featured summing mixer, monitor controller, DAW front end and more,
which we reviewed in June 2015.
Dan has been involved in the 500
Series arms race for several years now.
We took a look at his MP500MV (a 500
Series version of the ME-1V) and the thennew Harrison 32EQ 500 Series EQ module, back in our October 2011 issue.
Over the years, the one piece conspicuously missing from the Great River lineup
has been a compressor. I was pretty excited to see what Dan has cooked up with the
new PWM-501 500 Series compressor.
Pulse Width Modulation
The PWM part of its name refers to its
gain reduction circuit. Readers may be familiar with FET, Optocell, Variable-Mu and VCA

over the usual compressor suspects? A big
reason is that it can get really, really fast, both
in its attack and release times.
The PWM-501 up close
The PWM-501 is a single-slot 500
Series module dressed in a textured black
finish with red, while and blue knobs. It
perfectly matches the style and look of the
company's Harrison 32EQ.
It features a rock solid build inside and
out, with great clean circuit board layout,
smooth pots with great feel, and nice bright
LED metering. Internally it uses a transformerless balanced input and a transformer-coupled output stage. The input
stage is a THAT Corporation 1206 chip.
Feature by feature
The unit starts at the top with a red
knob labeled Output level. Even though it
appears first in the chain, this is the makeup gain control, not input; it has zero
bearing on how much signal goes in.
Unity gain is roughly at a 3 o'clock setting and its maximum throw is +15 dB.
The next knob down is the Threshold
control. It is set to operate differently at different settings. According to Great River, at
settings below 50 it looks at the whole signal and acts as a compressor, but as you
move above 50 it starts to only lock onto
the peaks and becomes more like a limiter.

Great River Electronics
PWM-501 500 Series Compressor
A novel approach to dynamics control in an elegant module format
compressor designs, but PWM (Pulse Width
Modulation) is something found more often
in synthesizers than in gain reduction gear.
In a synthesizer, PWM is essentially an
adjustment to the duty cycle of a pulse
wave, where the time between the rise
and fall (on and off) of the waveform is
highly variable. It's similar in the PWM501where the rise and falling of the pulse
controls the unit's reaction to gain.
This is an entirely analog process that
works in the time domain and the pulse peaks
and valleys turn the signal on and off very
quickly. Why use pulse width technology
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The Ratio control on the PWM-501 is
fully variable from 1:1 all the way beyond
20:1. According to Great River, these
labels are somewhat approximate, as
they are highly dependent on where the
other controls are set.
Next are the Attack and Release controls; Great River does not list their minimum or maximum times. One thing to note
is that the input circuit is an absolute value
peak detector and, as mentioned, can grab
on to a signal very fast... so fast that at its
quickest settings it may cause distortion on
some signals.

The next control, labeled FF/FB, gives
the unit the ability to switch between feedforward or feedback modes, or to variably blend between the two. This gives
the compressor extreme versatility.
In a feedforward design, the signal sent
to the compressing sidechain circuit
comes from the input audio. It is fast, modern and punchy. This is how VCA compressors work. The most common examples would be the dbx 160 and SSL's legendary bus compressor.
In feedback mode, the signal comes from
the output of the audio and it is comparatively

Table of Contents for the Digital Edition of Recording - December 2015