Allen & Heath's latest range of 21 series mixers offers a new format for 'on the road' PA systems, club installations, stereo recording and community broadcasting. The three models in the range are called the 621, 1221 and 1821, each having six, twelve and eighteen inputs respectively, but the important feature for these mixers is the inclusion of a mono output derived from the main stereo mix. Sockets are also provided to link two or more 21 series mixer together.

Construction & Circuitry

The mixers are rigidly constructed, with four heavy metal panels at top, bottom and sides and polished wood end pieces. Dimensions (mm) of the mixers are 6:2:1 458 x 475 x 100, 12:2:1 674 x 475 x 100, 18:2:1 891 x 475 x 100. Weights are 11.30, 15.90 and 22.70 kilos respectively.

The main panel is finished in light brown with yellow and red legending. Red is used for channel numbers and is a little difficult to read. Push-fit pots are colour coded red, green and brown, with good sized black fader tops. Incidentally, production models are to have plastic inserts in faders to catch dust etc.. Construction is based on boards that have PCB mounted pots for direct fitting to the main front panel.

The 1221 instrument supplied for review has 4 stick-on rubber feet and uses an external power supply to keep hum at a minimum. The supplied unit has 2 metre mains and connecting cables for +15 and -15 volts DC output to the mixer that is derived from straightforward dual regulator circuitry. The PCB is mounted on four snap-fit plastic pillars with a strap securing it in place. The PSU is adequately rated for the mixer range and a phantom supply is also available in an alternative PSU.

Access for servicing is easily done by removing ten screws at the rear and removing the heavy back cover. The individual circuit boards are all hardwired without the use of connectors. All wiring is via 9 bus wires, LED and fader wires, or connecting wire neatly grouped together.

A&H have obviously taken a lot of trouble to find parts that reduce cost but maintain good quality. The advanced low noise circuitry with high slew rate of 13V/microsecond is in fact based on one type of low noise dual op-amp throughout the mixer, the TL072. Each input stage contains two TL072's and six BC214 transistors. The monitor board and 2 send/mix PCBs use ten TL072's, plus BC214 and BC109C transistors. A 12V relay operates the PFL system.

Input stages

Each of the 12 input channels receive signals via individual Neutrix 3-way XLR connectors. Mic or line signals are set with a mic/line switch to give electronically balanced transformerless mic or high impedance balanced line inputs. A Pad switch further reduces the signal by 20dB and a Gain control next adjusts levels from +25 to +60 dB. Maximum Gain boost is always best avoided here as background noise increases considerably and the microphone gain at full sensitivity is 60dB. This should be satisfactory for most mic inputs, e.g. a 1.23mV input (with the mixer set to 0VU = 4dBm) gives 1.23V at the output.

At this point, a stereo jack socket allows interruption of the signal for send/return of effects and other processors (or simply to insert a standard unbalanced jack line input, bypassing the initial mic/line stages).

The first of the two sends available is 'Cue' and is normally next in the signal chain, although an internal link change will put it after the EQ stage if desired. Either way it is a pre-fade send and a Cue control sets output send level.

A 3-band quasi parametric equaliser gives plenty of tonal adjustment, with high frequency control, mid-frequency sweep from 400Hz to 6kHz, and low frequency for adjustable boost or cut over +12 to -12dB.

One LED peak indicator for each input stage is inserted next and will turn on at 3dB prior to clipping. Its position in the chain ensures that your equalisation adjustments do not overload the mixer.

Just prior to the input fader is also a pre-fade listen (PFL) switch for mono checking of the signal.

Next the input fader sets the level of the signal on the stereo bus lines, via a Pan control for left and right placement, and a post-fade Echo send with its own level control completes the input stage.

PFL

The mixer offers comprehensive PFL monitoring at 19 key points - in each of the twelve input stages as mentioned, at Cue and Echo outputs, at main left and right pre-fader outputs and main mono pre-fader output, and also at echo return inputs 1 & 2. Each PFL switch activates a relay that turns on a red LED close to the meters.

Since the PFL system overrides any other monitoring that is taking place, the LED is an important reference to ensure that only the PFL points you require are in operation.

Final mix stages

Two identical boards inside the mixer conveniently accommodate send/return signals and main stereo outputs, whilst a third board incorporates monitoring and the main mono signal.

The controls for these are located at the right hand part of the mixer and include monitor controls, VU meters, power on/off switch and its LED indicator. Controls for Cue Master and Echo Master sum the individual sends from input channels. There are two Echo returns which have Level controls and Pan controls for positioning on the stereo bus as well as Cue pots for an additional mix onto the Cue bus. The latter is a useful way of ensuring that treated echo, reverb and other processed sounds can be added to the Cue send output. Separate main faders for left and right output feed the XLR connectors at the rear, as well as a stereo bus output (via stereo jack socket) for connecting to another mixer or additional recorder.

Both stereo and mono outputs on the 21 series mixers are set for 0VU (4dBm), but internal links are provided to match outputs and tape monitor inputs to lower operating levels giving 12dB cut or boost respectively.

The big feature of this range is its main mono fader output in addition to the stereo faders (via XLR male socket). This will be ideal for use as a foldback mix on stage or as a feed to a mono PA system, without losing the facility of stereo recording during performance. It also offers a mono output for simultaneous video recording or for mono broadcast transmissions.

The use of stereo jack sockets for several of the rear connections, while being cost effective, does make linking up a little awkward.

Monitoring

Before actually mentioning these facilities, it is worth noting another aspect of monitoring - the control legends. Gain level, Cue and Echo controls are all numbered 0 to 12, EQ controls are divided into 8 parts, whilst input faders are smooth long throw types labelled +10 max, 0 nominal to -10 and off. I missed the graduated scales here and on the master faders (these have 0, -10, -20, and off line markings) although probably most pro users would simply chinagraph points straight on the mixer panel.

Monitoring is via two independently controlled stereo outputs to headphones and monitor amplifier. Thus each has a ganged stereo level control in its output stage to feed stereo jack sockets, at the rear for your stereo monitor amplifier/speaker system, and next to the VU meters/right hand end panel for your headphones (8-600 ohms). Output on headphones is entirely satisfactory with plenty of gain without extra noise.

It is worth noting again the basic order of the monitoring chain to understand the system's advantages. Main monitoring is first selected by 3 switches for Stereo, Mono and Tape In from a stereo recorder. This permits A/B monitoring during recording - a useful facility, although I would have liked gain control matching for the Tape-In to make instant switch comparisons without fiddling with levels. Nevertheless, if you are using the same 'master' recorder with the mixer, the internal links will enable suitable level matching.

The PFL switches then override any of the 3 main functions and the two VU meters monitor all chosen settings accordingly. The meters are illuminated and scaled -20 to +5 and 0 to 100%.

Rear Connections

The mixer basically consists of 12 input stages and input connections at the rear that feed 6 main bus lines for stereo left and right, mono, Echo mix, Cue mix and PFL. The latter also has a further bus line for DC control of the PFL relay and LED indication. There is also a 48 volt phantom power option, when using the MPS7P power supply unit instead of the normal MPS7 sold with the mixer. This operates in A-B mode on the two balanced line wires at pins 2 and 3 (1 is screen).

Apart from the input stages, the mixer offers standard jack inputs at the rear for Mono input, Aux input (stereo signal to Cue and Echo mix busses), Stereo input (on to L and R main busses), PFL input (with DC control signal) and Tape input (stereo signal to monitor system only). Echo Return 1 and 2 are via standard jack sockets and DC power from the supplied external PSU is via a Cannon XLR 5 pin connector.

Outputs available are as already mentioned: XLR L & R stereo, Mono outputs; standard jack Cue, Echo mono, (Cue/Echo) Aux and L/R stereo outputs, as well as Monitor and Headphone outputs.

Line inputs and outputs are set at a nominal level of 0VU (+4dBm), with a maximum level of +21dB (reference 0.775 V into 2K or more). With internal links for output pads and tape boost inserted, main output and tape input levels become 0VU = -8dBm (0.3V).

Conclusions

The 1221 mixer reviewed was well constructed and designed specifically to provide a low cost system for semi and professional use. Prices for the mixers (inc. VAT) are £443 for the 621, £569 for the 1221 and £742 for the 1821. Quality wise, it's likely to satisfy most users and although the circuitry is quite straightforward, the results are good with low background noise. Its robust frame as well as distinctive appearance will attract many electro-musicians who will benefit from this special system format and comprehensive in/out and PFL monitoring.

The Allen & Heath 21 Series Mixers are distributed in the UK by Allen & Heath Brenell Ltd., (Contact Details).