SRS Labs Pro 220

The SRS Labs Pro 220 is a 2-channel processor that enhances the spatial aspects of stereo source material based on the way the human ear localizes natural
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The SRS Labs Pro 220 is a 2-channel processor that enhances the spatial aspects of stereo source material based on the way the human ear localizes natural sounds. At the heart of the Pro 220 is the proprietary Sound Retrieval System (SRS) technology, which analyzes stereo signals for sum and difference content. The sum is what is common to left and right channels (referred to as L+R); the results are in the center of the stereo field. The difference signals — left channel minus right channel and vice versa — consist of ambience, reverberation, and echo signals originating from effects processors or indirect sounds picked up by microphones.

The Pro 220 allows you to independently adjust the amount of sum and difference signals for stereo program material. That not only affects the width of the stereo image but also changes the apparent level of ambient effects: widening the image adds ambience and narrowing the image makes the mix seem drier.

The Pro 220 can also synthesize a stereo image from mono tracks. The 3D Mono feature uses patented filters to put six specific frequency bands 90 degrees apart: three bands create sum (L+R) content, and three create difference (L-R and R-L) content. That creates a pseudostereo signal that is fed into a specific SRS processor, designed for mono, with a predetermined mix that the user cannot change.


The front panel has three continuously variable knobs for processing stereo input — Space, Center, and SRS Level (see Fig. 1). The fourth continuously variable knob, 3D Mono Level, works only on mono sources.

The Space knob controls the amount of difference signal sent to each output. Turning this knob clockwise increases the amount of perceived echo, reverberation, and ambience while widening the stereo image. Turning the Space knob counterclockwise makes the perceived sound drier and narrows the stereo image. A fully counterclockwise setting removes the spatial effects without attenuating the input signal.

The Pro 220's Center knob controls the amount of sum signal in the output. A clockwise turn increases the level of center-panned tracks in a stereo mix, which are typically lead vocals, bass drum, and bass. A counterclockwise turn of the Center knob reduces the output level of center information. A fully counterclockwise setting preserves the original input signal so that none of the summed signal is added.

The SRS Level knob controls the combined output of the Space and Center controls. Turning the SRS Level fully counterclockwise completely attenuates the source signal and SRS processing. You can also bypass the SRS effect entirely with the SRS/Bypass switch. This front-panel switch has a status LED that tells you whether SRS processing is active or bypassed. Two other LEDs, one each for left and right SRS channels, turn green when the SRS output level reaches -20 dBu and red when the SRS output exceeds +13 dBu.

The 3D Mono Level knob controls the left and right output levels of the Pro 220's 3D Mono processing section. Turning the 3D Mono Level knob fully counterclockwise completely attenuates the mono source signal and processing. Engaging the front-panel 3D Mono/Bypass switch bypasses the 3D Mono effect so that only the dry source is passed. The status LED lets you know whether the 3D Mono process is active or bypassed. As in the SRS section, two additional LEDs, one each for the 3D Mono block's left and right channels, turn green when the 3D Mono output level reaches -20 dBu and red when the output exceeds +13 dBu.

You can adjust the SRS Level and 3D Mono Level controls so that the unit's processed-output levels are equal to those of the bypassed levels. That facilitates A/B comparisons of processed and unprocessed sounds. A power switch is the final element on the unit's front panel.


The Pro 220 has separate analog I/O for the SRS and 3D Mono processors (see Fig. 2). The unit offers RCA and unbalanced ¼-inch jacks: there are stereo pairs for the SRS inputs and outputs, a single RCA and ¼-inch input, and stereo pairs for the outputs of the 3D Mono processor.

You can use the ¼-inch and RCA outputs simultaneously, but only one type of input per channel can be used at a time. Unfortunately, the Pro 220 accepts just semipro, -10 dBV nominal line levels. A captive two-prong AC cord rounds out the unit's rear panel.

The Pro 220's slim owner's manual suggests that you process individual tracks by patching the unit to an unpowered recording console through inserts or by placing it in-line between the mixer's multitrack bus outputs and your recorder. It is often preferable to send tracks to the Pro 220 using aux sends, for reasons I'll discuss shortly. You may also choose to patch the line-level output of musical instruments or preamps through the Pro 220 or place the unit between your mixer's main outputs and your P.A.'s amp in a live setting.


I tested the Pro 220 on individual tracks, effects returns, and complete mixes. When I used it on stereo drum overheads, the SRS processing widened the stereo field but thinned out the bass drum's low end and made the cymbals sound beady and cutting: it sounded as though I had boosted the EQ around 6 kHz.

Next, I patched the left and right outputs of my Dynacord DRP20 digital reverb into the Pro 220's SRS inputs. Surprisingly, the SRS processing made the reverb sound narrower and muddier. The Pro 220 also changed the signals' timbre too much, by boosting the reverb's bass and the lower range of the high frequencies. However, the SRS processing sounded excellent on synth pads, making them sound incredibly wide.

In all applications, you should hard-pan the left and right SRS outputs because the processing is not mono compatible. Collapsing the image to dead center almost completely cancels out the sound. That is important to consider if there's a chance your music will be played on mono AM radio or TV. For that reason and because SRS processing can significantly alter the timbre of the tracks, I routed tracks to the Pro 220 using aux sends. That let me retain my mono-compatible dry tracks in the mix and add a touch of SRS processing by way of effects returns to sweeten the mix.

When used on an entire mix, the Pro 220's SRS processing can widen the stereo image dramatically. Again, mono incompatibility is a consideration. But moderate use of the Pro 220's SRS processing won't skew the spectral balance of your mix nearly as much as some other spatial enhancers on the market.

It's too bad the Pro 220 can't accept professional levels. If it could, remastering applications would be one of its greatest strengths. Users could boost the level of center-panned tracks on mastered mixes and simultaneously cut the level of stereo effects and hard-panned instruments by simply cranking the Center knob and turning down the Space control.

Conversely, it would be possible to all but eliminate the lead vocal, kick drum, and bass by cranking the Space control and turning the Center control fully counterclockwise; all that would remain of the vocal would be the effects-return components. Intermediate Space and Center control settings can be used to fine-tune the balance of finished mixes as long as you're working with semipro levels and have a good stereo parametric equalizer to counter timbral changes introduced by the Pro 220's processing.


The Pro 220's 3D Mono effect is more mono compatible than its SRS processing, but it's not without fault. Collapsing the 3D Mono left and right outputs to center doesn't cause a noticeable drop in level, but it does significantly reduce high frequencies. When I used it on some kinds of source material, such as lead vocals, I could also hear a slight phasiness when the 3D Mono effect was collapsed to mono.

That said, the hard-panned 3D Mono effect sounded great on lead vocals when added to a mix using an aux send. Adding just a little bit of the effects return made the vocal spread out beautifully, contributing a wonderful sense of space without additional reverb or perceived delay. Collapsing the entire mix to mono caused little timbral change to the vocal, as the original dry signal contributed the most to the overall vocal sound.

The 3D Mono effect also lent a wide, natural-sounding stereo image to a mono acoustic-guitar track. I could make the sound even wider by daisy-chaining the 3D Mono outputs through the SRS stereo inputs and then routing the SRS outputs back to the mixer. However, the combined effect was decidedly incompatible with mono playback, and the acoustic-guitar track's timbre was changed dramatically so that the highs and low mids were accentuated significantly.

I also tried adding the 3D Mono effect to electric bass guitar. Although it was not artistically my finest moment, the results suggested that the effect holds great promise for processing synth bass.


The SRS Pro 220 offers good value. You won't want to apply it indiscriminately on every type of track, but used in moderation on vocals and select instrumental tracks, it can add that extra dimension you've been searching for on your recordings.

Pro 220 Specifications

Inputs(3) RCA; (3) unbalanced ¼"Outputs(3) RCA; (3) unbalanced ¼"Maximum I/O Levels+18 dBuNominal I/O Levels-10 dBVFrequency Response20 Hz-20 kHzTotal Harmonic Distortion<0.1% (active); <0.01% (bypass)Signal-to-Noise Ratio•90 dBAInput Impedance10 kžOutput Impedance200žDimensions19.00" (W) × 1.75" (H) × 5.75" (D)Weight7 lb.


SRS Labs
Pro 220
spatial enhancer


PROS: Widens stereo image. Synthesizes stereo signals from mono tracks. Independent bypasses and output level controls for SRS and 3D Mono processing. Low price.

CONS: Accepts only -10 dBV nominal input levels. Unbalanced I/O. Insufficient headroom for remastering applications. SRS process isn't mono compatible and can cause significant timbral shift. Mono playback of 3D Mono effect dulls the sound.


SRS Labs
tel. (800) 243-2733 or (949) 442-1070
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