You are here

Amek DMCL

Dual Mic Preamp & Compressor/Limiter By Hugh Robjohns
Published August 2002

AMEK DMCL.

Legendary designer Rupert Neve brings us the third in the Amek Pure Path range, a dual preamp and dynamics processor with unusually flexible filtering and side-chain facilities.

Although probably best known for their mixing consoles, Amek also produce outboard equipment, including their Pure Path range, which uses circuitry designed and developed by Mr Rupert Neve. Until very recently the range consisted of the CIB 'Channel In A Box' — a comprehensive, single-channel input stage with EQ and dynamics reviewed back in SOS September 2000 — and the DIB 'Driver In A Box' — a flexible multi-channel output driver. The latest addition to the range is the new DMCL: a dual-channel microphone amplifier and compressor-limiter which offers all of the sound quality and much of the flexibility of the Channel In A Box. Although it lacks the latter's parametric EQ and the facility to integrate a remote fader, it benefits from carrying a pair of channels with linkable dynamics, and also offers a digital output option.

The slim 1U rackmount box follows the established styling of the other Pure Path units, with illuminated badges on the rack ears, and a clear, uncluttered control layout. This is quite a deep unit, measuring almost 31cm from front to back, and it is heavy too, so some side or rear support in the rack would be advantageous, particularly in a mobile situation where physical shock is likely. The unit runs warm, but not excessively so, burning about 30W with the optional digital output module fitted. Unlike the other Pure Path units, though, the DMCL does not employ a fan in the power supply.

Interfacing With The DMCL

The rear-panel facilities are unsurprising, with XLR connectors for the mic and line inputs, a pair of quarter-inch TRS jack sockets for a compressor key insert facility, and another XLR for the analogue line output. There is also a high-impedance instrument input provided on the front panel which, although electronically balanced, is quite happy to accept normal unbalanced guitar inputs. Returning to the rear panel, the second channel's I/O is identical to the first, and is followed by another pair of TRS sockets which provide facilities to link the compressor side-chains with those of other units. If the optional A-D module is installed, there is also a comprehensive set of digital output facilities (see box).

The final rear-panel facilities are a fuse holder and the usual IEC mains inlet — the internal PSU is a multi-standard type, so there is no need for voltage selection facilities. Strangely, though, Amek have not provided a mains power switch on either the front or rear panel; the unit is 'live' from the moment it is plugged in.

AMEK DMCL rear panel.

Input Conditioning

The control panel of the DMCL is every bit as clear and logical as the rear panel, despite some quite complex internal signal routing possibilities. Again, the two channels are equipped identically, apart from an input/output metering button between the two channels, and the digital module's associated LED indicators on the far right of the second channel's control section. The fascia is divided for each channel into three discrete sections of signal processing; first the input selection and conditioning, followed by the high- and low-pass filters and then the compressor/limiter.

The input section starts off with an illuminated button that selects the front-panel instrument input in preference to the rear-panel microphone input. The instrument input is buffered with a bespoke 2.2MΩ high-impedance stage, and the selected instrument or microphone signal is then amplified by a classic Rupert Neve TLA (Transformer-Like Amplifier) stage. This is controlled from a rotary gain switch spanning 0 to +66dB in 6dB steps, and affords the mic input with an EIN figure of -128dBu at 66dB of gain (150Ω source), falling to just -127dBu at 36dB of gain. The microphone input impedance is relatively high at 5kΩ, with the benefit that long mic cables can be used with negligible high-frequency losses. Another button applies 48V phantom power to the mic input if required.

The line-level output from the TLA stage is routed through to another button that selects between the amplified mic or instrument input and the rear-panel line input. A polarity reversal switch affects whichever source is selected, and a second amplifier stage provides a continuously variable level trim of ±12dB. A specially designed input transformer immediately prior to the trim amplifier provides a fully floating, bridging line input which the handbook claims can maintain a flat frequency response even with signals over +20dBu in level.

An LED bar-graph meter at the top of the input section can be switched to show either the 'conditioned' input signal level, or the level immediately prior to the final output driver (where the optional A-D converter also derives its input). This I/O metering is switched simultaneously for both channels from a button at the centre of the panel.

Filtering & Dynamics

The filter section is equipped with two rotary controls and no fewer than five buttons. The high-pass filter is provided with a variable turnover frequency continuously adjustable between 20 and 300Hz but, more unusually, the slope of the filter can be switched from 18 to 24dB/octave. Similarly, the low-pass filter's turnover can be set anywhere between 2.5 and 28kHz, with either a 12 or 18dB/octave slope. Each filter can be switched in or out of circuit independently, and they can also be reallocated as a pair to the dynamics side-chain, allowing a degree of frequency-conscious compression — it is certainly possible to reduce pumping caused by excessive low-frequency energy, for example.

The compressor/limiter section is derived directly from that employed in the Channel In A Box unit, even retaining it's control layout and parameter ranges, including the distinctive '&MM' facility. This is a very traditional compressor design with familiar and logical controls. The five knobs adjust the Threshold (-40 to +22dBu), Ratio (1:1 to 40:1), Attack (0.3 to 300ms), Release (0.1 to 10 seconds), and Gain make-up (-10 to +24dB) — all working as expected. Five more illuminated buttons are provided in this section, the first switching the compressor in or out of circuit, and the third (Pre-flt) moving the compressor stage before the filters instead of after them. I have deliberately skipped the second button for a moment and will return to it shortly.

Two more buttons above the attack and release controls provide for stereo linking and the unique &MM feature. If you have not come across this before, the term stands for 'And Much More' and was introduced by Mr Rupert Neve to replicate the sound characteristics of his previous classic compressor/limiter designs. In effect, the button provides a soft-knee characteristic for a far more subtle, musical compression character. Other than where clinical, accurate limiting is required, I think the &MM button would remain permanently illuminated for most users, as the resulting dynamic characteristic is so much more musical.

Above the &MM button is an LED bar-graph showing the amount of gain reduction being applied. This covers a massive 40dB range, but the scale is expanded at the top end so that very small amounts of dynamic control are clear too.

Stereo Linking

The stereo linking facilities seem entirely intuitive until close examination, when further complexity (and flexibility) is revealed. The first channel's link button is actually labelled 'Link A-B' and pressing this button does exactly what it says: the two compressor's control signals are combined so that the dynamics sections of both channels react identically to the strongest control signal. However, the control parameters of both channel's compressors must be set identically — there is no master/slave facility here.

The Link button on the second channel relates only to its associated rear-panel TRS socket. Pressing the button activates the socket to receive and share compression control signals with an external unit. Strangely, channel one's Link A socket is permanently active, regardless of the status of the Link A-B button.

Return To Sender

Back now to the skipped button, which is labelled simply 'Key'. This relates to the compressor key insert facility, using the pair of TRS sockets on the rear panel, and is probably the most complex aspect of the device — largely because of the degree of configurability Amek have bestowed on it. As supplied from the factory, this insert send-return facility is located in the signal path at the start of the compressor's side-chain (and prior to the filters if they are allocated to the compressor). Although the insert return is normally bypassed, the send output always carries the signal which is routed to the compressor's side-chain (ie. the main signal), and can therefore be used as an uncompressed direct output.

By pressing the Key button, the insert return signal is routed through to the compressor's side-chain, allowing a more elaborate equaliser to be inserted in the side-chain for de-essing purposes, perhaps. Alternatively, an external signal could be input to control the compressor, allowing voice-over ducking, for example. The send output is normalled across to the return, of course, so that if nothing is plugged in, the side-chain signal path is maintained.

After reading this, you may still be asking yourself how you insert an external signal processor into the main signal path — a pitch-correction unit perhaps? Amek have thought of this possibility, and have included internal switches (one for each channel) which reconfigure the key insert facility to operate as a conventional channel insert. Unfortunately, accessing this switch involves removing the top cover plate, and there is no external indication that the Key Insert function has been modified from its factory default. Nevertheless, some users will undoubtedly find this a useful feature.

When reconfigured, the insert loop is positioned in the signal path between the filters (unless they are allocated to the dynamics side-chain) and the compressor input. Thus the signal can be processed externally and returned to benefit from the DMCL's compressor section. As before, the insert point is bypassed until the compressor's Key switch is pressed — not ideal labelling or ergonomics, but useful nonetheless.

On The Session

The DMCL is priced to sit amidst some very worthy UK competition, but its specifications and excellent sound quality make it comfortable in such company, and it would be an excellent choice as the front-end to a high-end digital workstation. The mic amp is quiet, neutral and efficient, the instrument input is a thoughtful and effective addition, and the line input seemed completely bombproof.

Although this new unit doesn't incorporate the CIB's equalisation, the filters have very sensibly been retained, allowing precise control over the audio 'window'. The compressor is a very usable design indeed, whether providing precise control of peaks, or musically transparent compression of an overly dynamic source. The provision of the insert loop, and the possibility to reconfigure it, are nice features too, but I would have preferred the remote fader facility of the CIB to have been retained rather than the external dynamics linking ports, which I fear will find little use.

The digital module performed well in comparison with my own Apogee converters, although I would have preferred its controls to have been located more conveniently on the front panel. Overall, then, this is a very high-quality front-end unit with a well-focused feature set and the Rupert Neve design hallmark.

Digital Output Module

If the digital option is installed, the outputs from the two analogue channels are converted to digital audio at any of the four standard sample rates between 44.1 and 96kHz, and with 24-bit resolution. The digital output is made available on XLR (AES-EBU), phono and Toslink (S/PDIF) connectors simultaneously, and a pair of BNCs provide word-clock input and output. There is no facility to output in the ADAT format via the optical connector, which some potential users may find frustrating.

Four small buttons below the clock connectors determine whether the internal or external word-clock is used (at standard or Super-Clock rates) and whether the internal sampling rate is 44.1 or 48kHz, or double either of those rates. It is a shame these buttons are tucked away at the rear of the machine where they are hard to get at, but there are six LEDs on the front panel to indicate the sample rate, external sync source, and word-clock lock, so at least you know what is going on.

A workaround I found for this was to clock the DMCL's converter from an external reference (an Aardvark Aardsync) which acts as clock master for my entire digital setup. In this way, changing the master clock sample rate automatically reconfigured the DMCL's sample rate to suit any particular project. The only drawback was that the sample rate indicators on the DMCL remained resolutely at the internal setting, refusing to reflect the true rate.

I was also a little disappointed to find no facilities for dithering the digital outputs to a lower resolution than 24 bits. Consequently, connecting the DMCL to a CD-R or DAT recorder will result in truncation and quantising distortion. The handbook provides no warning or advice about this potentially serious problem. However, in reality, the prime market for a unit like the DMCL is as a front end to a high-end digital workstation, which will almost certainly be working with 24-bit input resolution.

Pros

  • Well-focused feature set.
  • Impeccable performance.
  • Mr Rupert Neve's revered analogue circuitry.
  • Optional A-D module.
  • No cooling fans.

Cons

  • Digital output fixed at 24-bit resolution.
  • Sample-rate controls hidden around the back.

Summary

A well-conceived dual-channel front end with versatile dynamics and an optional high-quality A-D module for easy integration with DAWs. With its Mr Rupert Neve circuitry, the DMCL's performance is superlative.

information

DMCL £1762.50. Digital output option £352.50. Prices include VAT.

Amek +44 (0)161 868 2400.

www.amek.com

Published August 2002