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Drawmer Three-Sum

Band-splitting Processor By Paul White
Published January 2006

Drawmer Three-SumPhoto: Mark Ewing

This intriguing new unit from Drawmer lets you split a stereo signal into three bands and then process each band with a different piece of outboard.

Low-cost plug-ins and multi-band hardware boxes, such as the TC Electronic Finalizer and Drawmer DC2476, have made more people aware of the possibilities of multi-band signal processing, especially in mastering applications. However, many of the big-name mastering engineers still prefer to work with analogue equipment, albeit very high-end, expensive analogue equipment. What's more, they like to be able to pick which equaliser or compressor to use depending on the material they are working on, and it isn't unusual for mastering engineers to have their own custom switching and mixing systems built to enable them to do this easily. For those on a lower budget who still prefer to work in the analogue domain and who demand a very high technical specification, Drawmer have come up with an ingenious piece of equipment that enables those old analogue processors you locked away in the cupboard to be given a new lease of life.

Analogue Hardware For Multi-band Processing

What Drawmer have done here is build a processor that can split a stereo audio signal into three frequency bands, providing individual connection points for each band so that the user can insert their own stereo processing devices into the signal path. This enables the user to patch in different external devices for processing each frequency band. Although compression is the most obvious choice here, there are some other intriguing possibilities opened up by such a device, not least that of using controllable distortion to add energy and punch to specific parts of the mix spectrum.

This last point was really brought home to me when I bought a Drawmer DC2476 Masterflow three-band digital processor, which has, amongst more familiar processes, adjustable tube saturation emulation for each of its three frequency bands. The difference this makes in enabling me to warm up the bottom end of mixes or to add breath to the top end is far greater than I ever expected, and it's a feature I now employ to some degree on most of the pop and rock mastering jobs I get involved with. They really ought to make that one process available as a plug-in!

Drawmer Three-SumPhoto: Mark EwingWhile the theory of multi-band processing is too wide to go into here in any great detail, it is worth pointing out the main benefits, specifically as they apply to compression. Conventional compressors turn down the level of the whole signal, no matter which part of the frequency spectrum is responsible for the level peak that triggered the compressor. In a pop mix, this often results in the kick drum triggering the compressor and causing high-frequency sounds such as hi-hats to be dropped in level whether they need it or not. By processing each band separately, peaks at the bass end can be controlled without disrupting what's going on higher up the spectrum. Overall this allows more compression to be applied without unduly affecting the subjective transparency of the mix. Additionally, there are benefits to being able to use different amounts of compression in the different bands — for example, you can increase the density of the low end by using a higher compression ratio or a different compressor threshold, without altering more subtle settings for the mid- and high-frequency bands.

Split & Mix

Conceptually, the Three-Sum is pretty straightforward. It's a 1U box that uses precision crossover-style circuitry to split the incoming audio into three bands. The three sets of stereo signals are then routed to rear-panel balanced XLR connectors, and the returns from the externally connected stereo devices are brought back into the Three-Sum and added back together to give a full-range stereo signal. A precision two-stage brick-wall limiter is included in the summing section to prevent overloads caused by combining the three sets of processed signals, and there's a pair of meters that can be switched to monitor the input or output levels. Apparently, the limiter deals with high frequencies separately, so as to retain transparency during limiting.

The limiter has been designed to operate transparently, but will also give a classic analogue pumping effect if driven hard.The limiter has been designed to operate transparently, but will also give a classic analogue pumping effect if driven hard.Photo: Mark EwingThe first thing the input signal encounters when it enters the unit via the rear-panel XLRs is a simple Level Trim control with ±10dB of gain range. From there, the signal is split into three bands, where the lower split frequency is continually variable between 18Hz and 1.6kHz, and the higher split frequency between 530Hz and 42kHz. A fairly gentle filter slope is used to help avoid artefacts at the crossover points, specifically level 'humping' when compression is being used. Each band has a Normal/Mute switch, which is useful if you need to scrutinise just one or two bands, and each band can also have its external connection points bypassed to remove the effects of the externally connected processor.

A further Gain control is placed after the point where the returned signals are summed, and this feeds directly into the limiter, which can be set to operate at levels from 0dBu to +16dBu. A four-LED meter shows the gain reduction caused by the limiter's action, and two moving-coil meters with VU characteristics can be switched between input or output levels. Because many digital recording systems require a very high input level to reach digital full scale (typically around +16dBu), the meter scale can be switched from reading 0VU full scale to +10VU. In fact the only obvious omission is a level control for each band, so if you're patching in your own processors, they'll need to have their own level controls to enable you to balance the contributions from each band. Compressors invariably have a make-up gain control of some kind, so this shouldn't be a problem. However, it would have been better from an ergonomic point of view to be able to fine-tune the band levels from the front panel.

A 115V/230V voltage selector switch is only provided inside the casing so that it cannot be accidentally switched to the wrong setting during use.A 115V/230V voltage selector switch is only provided inside the casing so that it cannot be accidentally switched to the wrong setting during use.Photo: Mark EwingOn the rear panel are the IEC power inlet, balanced XLRs for the stereo inputs and outputs, and balanced XLRs for the stereo sends and returns from each band. There are no jack alternatives, which would have made life easier for me, but in a serious mastering situation XLRs would probably be the connectors of choice. That's a total of 16 XLRs, so no wonder there's no room for jacks! A mains voltage selector is available inside the case, which means you won't reset it accidentally.

For any mastering engineer to take a product like this seriously, it needs to have an impeccable technical specification, and Drawmer have been careful to keep the audio bandwidth wide enough to keep the tweakers happy, but without it being so wide as to behave as an impromptu radio receiver! The response is flat to within 1dB from 17Hz to 28kHz, while the 3dB down points are at 10Hz and 47kHz. THD + Noise is better than -85dB, with crosstalk lower than 63dB at 10kHz. Although the crosstalk figure may not look particularly impressive, it is rather better than that which many competing multi-band devices offer, and in any event some analogue buffs cite a degree of crosstalk as one of the factors responsible for the 'analogue' sound.

Studio Tests

The first thing to decide, after thinking about what processes you want to apply, is where to set the crossover points. Different pieces of music demand different solutions, but in most instances I start by keeping the mid-range fairly wide and open by setting the low crossover point to around 150-250Hz and the high crossover point at 2-5kHz. You can learn a lot by muting the various bands to see what part of the musical spectrum is being affected. You may also be able to manage without processing all three bands in all cases, though for mastering you may well want to. A possible approach is to leave the mid-band alone, compressing the bass end to pump up the energy a bit, and overdriving a tube processor slightly to brighten up the top end. In this respect, adding subtle distortion only to the high end works a bit like a harmonic enhancer.

My first test involved using a Drawmer DL441 quad compressor patched in to handle the low and high bands only. Each compressor pair was linked to ensure consistent tracking. I used a hard-knee low ratio on the bass end and a soft-knee low ratio at the high end, adjusting the thresholds to get just a few decibels of gain reduction in each band. The mid-band of the Three-Sum remained bypassed at this time, and I used the output gain controls on the compressors to balance the high and low ends against the middle. The result was an extremely crisp mix with bags of transient detail, loads of low punch, and a generally nicely produced sound. Using just these two units alone allows the user to polish mixes in a hugely effective and classy-sounding way.

Drawmer Three-SumPhoto: Mark Ewing

Adding a further compressor to work on the mid-band can enable the overall level to come up slightly, but where compression has been used on individual tracks at the mixing stage this may not be necessary. It's also worthy of note that the limiter works extremely well for catching transients, and that its effect is extremely benign when showing between one and three decibels of gain reduction. If you push harder into limiting by turning up the output gain, you get the classic analogue pumping effect — this can actually be used quite creatively on some rock mixes or drum submixes, but for conventional use it's best to set the limiter so that only one LED flashes on the meter, and that only infrequently. If you need to bully more loudness out of a mix without making it sound over-processed, there are more effective plug-ins that can do that.

Adding distorting devices to the high end can help create the impression of detail from a dull mix, but a lot depends on the device you're using. A dual-channel tube processor of some kind that can be driven into harmonic distortion is probably the best bet, and it will sound more subtle than using guitar distortion devices. However, for deliberately aggressive musical styles, more overt distortion mechanisms might just do it for you. Whatever you use, it is important that the left and right channels can be accurately matched, otherwise your stereo image will suffer.

Drawmer Three-SumPhoto: Mark Ewing


The Three-Sum has been designed so that it can generate the high-level signals demanded by many professional audio interfaces and A-D converters. However, when driving the unit this hard, the VU meter becomes pinned to the end stop, so Drawmer have provided a switch beside the meter which reduces the VU reading by 10dB when needed.The Three-Sum has been designed so that it can generate the high-level signals demanded by many professional audio interfaces and A-D converters. However, when driving the unit this hard, the VU meter becomes pinned to the end stop, so Drawmer have provided a switch beside the meter which reduces the VU reading by 10dB when needed.Photo: Mark EwingUsed with care, the Three-Sum can be very effective, but I also appreciate that it may not be a solution for everybody. In my view, it's more likely to appeal to mastering engineers and high-end project-studio owners who know their signal processors in some depth, rather than the casual user looking for the magic 'fairly dust' button. However, add in a Drawmer DL441 and you get pretty close to instant magic! With the benefit of the 20/20 vision of hindsight, Drawmer might have made this box more attractive by building variable analogue tube emulation into each band, because as it stands you can't do anything with the Three-Sum until you plumb in some external equipment. However, they obviously also had to meet a viable cost point, and it does provide a simple and effective means of combining other analogue equipment to create a bespoke multi-band processing setup at a UK project-studio price.

The quality of the results available with this system will be dictated mainly by the skill of the user and the quality of the connected equipment, but the whole can often be greater than the sum of its parts. My tests confirmed that analogue multi-band compression still has a lot to offer, and using the Three-Sum to achieve it is a lot less costly than buying a dedicated multi-band analogue mastering compressor. Of course there are some advanced features missing, but apparently Drawmer's engineers started by sketching out a unit that had everything, before pruning it down to the essentials to reduce cost and minimise the signal path. For example, some mastering engineers would have liked switchable filter slopes, per-band level controls, and an overall bypass switch, while I'm pining for my multi-band tube emulation, but in reality Drawmer have delivered the essentials without compromising on audio quality.

There are also applications beyond those of mastering, as this processing can also be used beneficially to polish up vocals, drums, bass guitars, and other sources, though I think you'd have to be pretty dedicated to patch up something like this just to work on a bass-guitar track! The Three-Sum may not be destined to be Drawmer's biggest-selling product, but it offers a genuinely useful facility for those users who do a lot of multi-band analogue processing, especially those project studio owners with analogue compressors who'd like to get more involved in doing their own mastering and who find digital plug-ins and processors too cold sounding. 


  • Conceptually simple.
  • Effective limiter built in.
  • Excellent audio performance.
  • Surprisingly affordable.


  • No per-band level controls.


A great idea for anyone who has a desire to combine more conventional analogue equipment for multi-band processing.


£546.38 including VAT.

Drawmer Distribution +44 (0)1924 378669.

+44 (0)1924 290460.