Writing about audio gear is an interesting exercise in trying to capture the emotional impact of an inanimate object. I think that it is less important how something works than how something makes you feel. When a piece of gear allows you to transfer a performance to a fixed format with emotion intact, it is a piece of gear worth owning. Plus, I love it when a metal box with knobs and wires pulls me out of complacency and makes me engage with the task at hand.

The new offering from Great River Electronics is a pulse-width modulation compressor in a 500-series form factor. As explained by Great River: "Pulse Width Modulation is a unique all analog circuit but operates in the time domain by turning the audio on and off very quickly to achieve gain reduction. This fast switching controls the signal power and allows the compressor to be very fast and transparent." Some PWM compressors you may know are the EMT 156, the Crane Song STC-8, and the legendary PYE compressor.

Out of the box, the Great River Electronics PWM-501 feels high in quality and carries substantial weight. In my API lunchbox, it looks very much the companion to the Great River 500-series Harrison 32EQ [Tape Op #89]. Its red, grey, and blue knobs feel precise, have a nice high-quality resistance, and are consistent from one to the next. Its faceplate is a classic matte black, and overall, the design and aesthetic are unlikely to go out of style.

The feature set is much like what you would expect to find on a well-appointed compressor: output, threshold, ratio, attack, and release knobs; switchable LED metering; and a hard-wire bypass. It also has some useful extras: an adjustable high-pass filter for the detector, and a knob for feed-forward and feedback detector modes. As is the case with many 500-series designs, there are a lot of features packed into a very small space. I have sausages for fingers, and although the knobs are relatively easy to access, the high-pass and stereo-link switches are in tight spots and a tad tough to get to.

The threshold, ratio, attack, and release controls operate similarly to traditional compressors but certainly have a unique twist. Every function on the PWM-501 seems to audibly affect the other, making this compressor capable of achieving an endless variety of options when it comes to dynamics control. More importantly, it forces true engagement and critical listening.

When my pair of PWM-501 modules arrived, I was in the middle of an unattended mix with no hard deadline, and I took advantage of this unsupervised time to creatively investigate these compressors. After becoming reasonably acquainted with the PWM-501 via the recommended starting points, I went down the rabbit hole, playing with its nuanced control set and ended up with some great new sounds and ideas as a result.

I started out with the recommended settings on an acoustic guitar track. This got me in the ballpark, but once I started fiddling with the controls, I began to see the power and versatility of the unit. Fine-tuning the controls got the guitar moving and breathing in a really nice way. It complemented the guitar nicely, helping it sit well in the mix, with a touch of extra warmth and nice sparkle around the edges. It maintained a nice shimmer without being overly bright. I know that sounds more like an EQ critique, but a compressor will certainly affect tone, and great compressors do it beautifully. I was also able to get the same sort of motion from the PWM-501 on a snare played with brushes. Instead of a hi-hat opening on the upbeat, we slid the brush across the head of the snare as a tonal shading and groove element. Again, a nice gentle compression was all that was needed to blend the snare nicely while still maintaining its slingshot propulsion.

I almost wish I had never put the PWM-501s on the drum overheads, because once they were there, it pained me to think of having to use them elsewhere. They added such a great life, warmth, and punch to the kit sound. Several times, I caught myself going too far with the amount of audible compression, because it sounded so good in that squishy-squashy way, and it was fun to see how far I could push the unit. I found the smallest of moves made the difference in determining the sweet spot, and in this way, the tonal possibilities of this unit remind me of a color wheel with fine gradations across the spectrum.

A nice feature on the PWM-501 is the high-pass filter on the detector circuit, and it is useful when you want to fine-tune how the compressor is influenced by low-frequency content. This, like all the other controls, can get very specific in terms of shaping the sound. I used it with great results on everything from parallel compression of the drums to acoustic guitar, and I found it played a part in helping to generate the desired amount of motion.

As a workaround to only having a pair of PWM-501s at my disposal, I went ahead and printed a stereo drum mix and then was able to use the units elsewhere. Their versatility lets them shine on both individual instruments and on stereo subgroups of guitars, vocals, loops, etc.

I used the PWM-501 following a Burl B1D preamp to record a Telecaster through an AC30. Before patching the compressor into the chain, I was fighting a small battle trying to determine the right amount of the amp's Brilliant channel shimmer while still having the guitar sit down in the track. With the PWM-501, I was able to shape and saturate the top end in a pleasing way without sacrificing its clarity or punch. It performed in a way I wish the amp's Cut control would have.

Using the PWM-501 as a peak limiter in a roughly 10:1 ratio while recording vocals was also useful - not hitting it too hard, but just knocking off the peaks. I quote the manufacturer: "The threshold control is unique in this design in that at low settings (0-50) it has the unit looking at the whole signal and acting more as a compressor, where as in higher settings (50-100) the detector looks for peaks and acts more as a limiter." It did just what I was looking for very transparently, leaving room for compression at mix-down.

The feed-forward / feedback feature is a variable control that lets you decide whether you want the input (feed-forward) or the output (feedback) as the source for the detector (sidechain) signal. Uniquely, the PWM-501 lets you choose one or the other, or blend the two if so desired in any amounts.

Feed-forward results in a more aggressive and in-your-face sound. This mode in conjunction with more extreme settings is capable of creating some not-so-nice sounds; so use your ears, and know that a little goes a long way in some applications. For example, feed-forward can bring grit and spit to a vocal. It is certainly nice to control dynamics with the option of not having it smooth out. I found it especially fun on drums. Going the other way to full feedback mode produces a noticeably rounder and subtly softer compression sound - useful when I wanted elements to be smoother or mellow out just a touch. I am not sure if the name influenced this feeling, but I did feel as though the difference between the two modes was like leaning forward or lying back in a chair. Finding some middle ground and then making smaller adjustments of a little more of one or the other provides yet another powerful tool for shading and influencing the feel of what's being compressed and where it sits among the other instruments in the mix.

The PWM-501 is a very versatile tool. In many ways, it reminds me of my much-loved, always-employed, Empirical Labs Distressor [Tape Op #32]. Although the two devices are capable of creating their own unique sonics, both are versatile and perform reliably at tasks ranging from very gentle and transparent compression to extreme audio destruction. Most importantly, they both do it musically. Everything I run through the PWM-501 feels as though it is being outlined in a pleasing way. Depending on the settings, the outline varies from a fine point to a substantial Sharpie.

In the hands of a knowledgeable engineer, the PWM-501 is going to deliver some impressive results. Conversely, a novice to compression may have a bit of a learning curve. If this is your first dance with a device of this depth and power, I would suggest starting with the settings recommended in the provided literature. Turn knobs, and try to understand the correlation between what you are hearing and what the meters are telling you. Use your ears, and start to understand what the attack and release settings are doing to your transients. Shape and position with feedback and feed-forward. Activate compression in the sweet spot with the high-pass filter. Slam it hard, or transparently control dynamics. You will not outgrow this device. In fact, it will help you grow as an engineer, and you may have to trade up for a bigger bag of tricks, because the PWM-501 is going to add significantly to your arsenal.

Tape Op is a bi-monthly magazine devoted to the art of record making.

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