Drawmer DL231 Expander-Compressor-Limiter

pic of drawmer dl231

Last Update 06-11-2015

The Drawmer DL231 is a dual channel dynamic processor consisting of an expander, compressor, and limiter in that order in the audio chain.  It is designed to improve signal-to-noise ratio of particularly noisy signals such as live vocal mics or the neighboring mics on the toms of a drum kit.  It has been replaced by the DL241 which has some differences - good thing because the 3-way switches go bad on these old DL231s, and they are a custom switch (US$18 !!! each) made for Drawmer.  I found this DL231 cheap in a music store that I frequent.  Drawmer dynamic processors are very transparent high fidelity devices and are desireable pieces in the pro audio industry, so it's not every day that you find used Drawmer gear in mom-n-pop music stores.

The operation of the three dynamic tools are a little different from your average compressor.

The expander is first in the chain but it doesn't operate as the traditional expansion device.  Expanders normally boost the signal once a threshold is reached.  This one operates more as a gate in that it attenuates the signal as opposed to boosting it.  When the source material is above the threshold, zero attenuation takes place and the green status LED is lit.  But when the signal falls below the threshold, then the "gate" is active and is attentuating the signal.  Yellow status LED means it is attenuating 50% of the panel setting, red status LED means it is attenuating 100% of the panel setting (sort of a "soft knee" transition).  Attack time is automatic and program-dependent while release time is set by the control in the compressor (they share the same release control).  The DL241 replaces the red-yellow-green with a single red LED that indicates the expander is active, and has a dedicated release control in the form of a slow/fast button.

While this gating action can happen during the compression stage, what this does is drastically reduce hiss and noise from creeping into audible range  Without the expander/gate, a compressor would make the noise floor audible as the signal decays.  This is very obvious when using high compression ratios combined with low thresholds.  The expander/gate prevents this "noise creep" by "gating off" the signal as the compressor is increasing gain, effectively shutting off the noise floor.  It takes some tweaking to make this work reliably.

Expander controls are a bit crude with a variable threshold control and two 3-way switches offering ratios of 1:2/1:5/1:20 and attenuation of -10/-20/-40dB.  The DL241 is a simpler affair with the ratio/attenuation switches replaced by a circuit that automates these parameters.  The expander can be bypassed independently of the compressor/limiter.  A dedicated key input on the back panel via a 1/4" jack (unfortunately no longer available on the DL241?!?) is normalled to the audio input and is configured as a TRS with ring carrying send for the source material and tip carrying return signal.  You use this as an "insert" where you can EQ the sidechain audio to obtain frequency conscious gating, which is essential if you want to gate only on selected spectrum of a crowded signal.  You can monitor the sidechain for fine-tuning the processed threshold but this feature no longer exists in the newer DL241.

The compressor following the expander is an RMS detection type with control over threshold level, attack time, release time, ratio, and output recovery gain.  This detector is a log type which happens to be very effective on percussion.  No peak detector is available here.  One LED display is provided which will show either output level or gain reduction although the newer DL241 has separate displays for both.  Drawmer cleverly oriented the LED bar for the display - in output level mode, the "fill level" goes left to right while it is reversed (right to left) in gain reduction mode.  Output level mode is a VU meter with associated colored LEDs in optimal and clipping, and it just so happens that the reverse order of the LEDs is ideal for displaying gain reduction.  Unfortunately this does not provide any metering of the input signal.

A dedicated key input - separate from the expander key input - provides the option of processing the sidechain signal (unfortunately no longer available on the DL241?!?).  The TRS jack operation is exactly like the expander and the sidechain can be monitored for fine-tuning (but the DL241 omits the sidechain monitoring).  A bypass switch is global to the entire channel, however it is not a hardware bypass and no bypassed audio is passed from input to output when power is removed from the device.  This is not normally a problem because you can perform a manual hardware bypass simply by removing the input/output XLR plugs from the device and connect them together.  It is worth noting that the newer DL241 bypass is a hardware bypass.

The last dynamic tool in the chain is a 50:1 ratio limiter whose threshold is fixed at +6dB (or +4dBu) and can be bypassed independent of the other dynamic tools.  Attack time is fixed at a snappy 50us with automatic program dependent release time.  When the LED is lit, limiting is active.  The easiest way to gauge whether your signal is approaching limit threshold is to use output level mode on the compressor display.  Limiting can be used as a creative "retro" sound or can be used to protect digital audio inputs from clipping.  Drawmer wisely added a variable threshold control to the DL241 limiter.

The channels can be operated independently or as a linked stereo pair (limiters are always independent).  Inputs and outputs are electronically balanced +4dBu XLR jacks, no -10dBv 1/4" jacks here but the DL241 adds a rear panel switch to select +4dBu/-10dBv operation.

These boxes do have one achilles heel - the 3-way toggle switches will lose their contact integrity with age (translated: the audio cuts in and out, and wiggling the switch handle confirms the fault).  I had to replace them on my unit.  Unfortunately these are a custom switch available only from Drawmer.  They are a two pole switch, but not the on-off-on action - in the center position one pole is shorted to one throw, but the other pole is shorted to the opposite throw.  And these are PC board mounted switches so the replacement must be an exact fit.  They are basically emulating the action of 3-way slideswitches.  You're bloody unlikely (Drawmer is british, mind you) to find these switches on any low quantity electronic supply vendor because of their custom operation, and I can't find ANY OEM marking on the case of these things.  So Drawmer referred me to an authorized service center here in the states, which quoted me US$18 PER SWITCH.  This device has EIGHT TOTAL, four per side (three in the expander, one in the compressor) adding up to US$144 not including shipping and tax.  Disassembly to get to these switches is a bit of a PITA in that you have to disassemble the front panel (including all knobs and mounting nuts/washers), then you have to spread the sides of the case to get the switch assembly clear of the unit.  Not fun...

This is a pretty sophisicated dynamic control tool best applied to percussion, although it is not my favorite for program material.  Drawmer is world renowned for their transparent compressors and effective gates.  Drums can benefit from gating and hard limiting so Drawmer made an all-in-one product (well, with the omission of frequency selection of gating) that caters to that crowd.  I haven't yet had the chance to experiment with live vocals but this box may be a good solution as gating live vocals almost never works.  This box is out of production and wasn't made in huge numbers as they do not often appear for sale.  Some years later I bought a 2nd one at the same store - when I brought it home I discovered that the serial numbers were consecutive.  Both purchased from the same store years apart, what are the odds of that...?