LD Systems PM 12-2

Photo: Mark Ewing

This traditionally-designed mixer/amp packs quite a punch, with 1400W of power under the hood, and also boasts very sturdy construction and built-in digital effects.

The idea of putting power amps and a mixer into the same box together isn't a new one, but today's powered desks are offering a lot more power and a lot more features than ever before. Requiring only the addition of loudspeakers and signal sources, the LD Systems PM 12-2 mixer we're about to look at is designed as a complete all-in-one small PA system, and contains a built-in graphic equaliser section and onboard digital effects unit, partnered with a high-power Class-H 2 x 700W amplifier section and a few flexible configuration options.

The first striking physical feature of this mixer is the substantial carrying handle, which is in the form of an aluminium bar extending right across the front panel. It's a very sturdy assembly, which is just as well, because this mixer is a fairly heavy item, weighing in at 23kg. (In an official 'place of employment', that weight would attract official guidance on how best to lift it!)

Channel Hopping

The tour of facilities starts with eight mono input channels, which are equipped with everything you'd expect in a live sound mixer. At the top of each mono channel strip are the input connectors — XLRs for mics and jacks for line inputs. The line inputs are unbalanced, although the jacks are stereo and double as insert send/return points. Also present on each mono channel is a 48V phantom-power switch and accompanying LED indicator. While it's not unusual to have phantom power on a mixer at this market level, one usually finds it's globally switchable, and I like the provision of per-channel phantom on the PM 12-2.

The mono and stereo channels of the PM 12-2.On to the control section, and the first knob in the signal path, which is the channel gain control. This is used to match the input signal to the mixer's nominal 0dBu operating input level. Just below this is a low-cut filter switch, which slopes off signals below 100Hz and is useful for reducing unwanted LF signals such as stage-floor noise, spill from bass instruments into vocal mics, and so on. The switch is a push-button type that has a nice positive action, and quite a long 'throw', which makes it easier to tell which position it's in.

Next up is the equaliser section: the HF and LF controls are fixed shelving circuits focused on 12kHz and 60Hz respectively, providing up to 15dB of cut or boost. The mid-frequency circuit is a semi-parametric design and allows the same +/-15dB of control, but with a variable centre frequency of between 250Hz and 7kHz.

The next group of four controls, labelled Mon 1, Mon 2, Aux and FX, is for the auxiliary sends. The Mon 1 and 2 sends are always pre-fader and are primarily designed for providing two independent monitor mixes to the stage; the 'Aux' send is switchable between pre- and post-fader, so it can be used as a third pre-fade monitor feed, or as a post-fade effects/recording send, or to feed an auxiliary PA, such as a theatre foyer system or outside relay. The FX send can be used to send an external signal (for example, in case four foldback mixes were required), but this control taps off the channel signal post-fader and cannot be switched. Its normal function is to feed the input of the internal effects processor, and therefore requires no external patching.

The final section of the mono channel strip contains the pan control, for placing the channel signal within the main left and right stereo image; a channel all-mute switch; a channel pre-fade listen switch; and, finally, the 100mm channel fader. The mute and PFL switches have red warning LEDs built in, which is useful, although if they had been different in colour it would be a bit easier to see at a glance which mutes and which PFLs are selected. The mute buttons affect all outputs, both pre- and post-fade, and effectively kill the channel in question.

The PFL function is essential for achieving optimum channel-gain settings, and for listening to individual channel signals in the headphones: the gain of a channel can be adjusted while you use the PFL switch, even if the channel is muted. One further feature of the PFL button is that its associated red LED functions as a channel peak-signal indicator, which will light up when the (pre-fader) channel signal reaches about 5dB before the onset of clipping. The channel faders move smoothly over their full travel and are 'calibrated' to a nominal operating position (labelled '0'), but a further 10dB of gain is available above this position.

Channels 9/10 and 11/12 are arranged as two stereo channel strips, with independent left and right line-level jack inputs on each. They don't feature microphone preamps. If a source is connected using only the right (lower) jack socket, it will be treated as a mono signal and fed equally to both left and right sides of the strip. Stereo channels have a simpler, two-band equaliser section; all other features for the stereo channels are the same as for the mono channels, other than the Pan knob, which becomes the 'balance' control, and the use of grey fader caps instead of white ones.

Master Section

The clearly laid-out master section incorporates all controls for the main and auxiliary outputs, returns and effects. At the top of the mixer is the main patchbay section, which includes all inputs and outputs except for loudspeaker connections and mains power.

The Master (bottom), effects (centre) and graphic EQ sections of the PM 12-2.Hundred-millimetre faders are used to control the output levels of the mixer section, and it's a nice touch that faders, rather than rotaries, have been used for the auxiliary outputs (FX, Aux, Mon 1 and Mon 2) as well as the main L/R signal. The red fader on the extreme right is the stereo master output fader, controlling the level sent to the power amps or to the main and mono line outputs. Although a single fader controls both channels, there are individual rotary controls at the power-amp inputs, so that the left/right balance can be adjusted if necessary. Directly above this fader are two buttons: one that mutes the main output (called 'Mute', yes); and one, labelled Standby, that mutes the entire mixer apart from the tape-return path. This is a very useful feature, making it possible for the front-of-house and monitor sound to be silenced while background music is played, without the need to change a single fader position. The Standby switch is also a good last resort if something goes seriously wrong with the system at a very bad time!

The FX, Aux and Mon 1 and 2 busses, as mentioned earlier, are similarly equipped with faders, each of which has a mute button for its output, plus an after-fade listen switch (pre-fade listen in the case of the FX send) so that the individual aux mixes can be monitored. A rotary level control is associated with each auxiliary master fader, and these controls are used to return the effects signal into monitor mixes (in case you want a different amount of some effect in the monitor mix than you have in the main mix), or — in the case of the FX buss — to allow external effects to be mixed in.

Additional rotary controls in the Master section include the 'Aux in' control, which feeds the external input signal through to the main mix and has a pre-fade listen switch for monitoring the signal, and to allow you to set the correct level. The 'Mono' control governs the level going from the mono output jack. This output can be configured as a mono subwoofer output if you engage the switch marked 'sub bass', which introduces a filter into the signal path, allowing only frequencies below 100Hz to pass through.

There are individual controls for the left and right power-amp inputs (called channels A and B), and each input channel has two LEDs to indicate signal presence and warn of clipping. This is a nice detail that gives a useful guide as to the incoming level of signal and could also be very useful for fault tracing, should the need arise. A further example of flexibility is the option — via a switch next to the channel-A control — of feeding the power amps either with the main L/R signal or the Monitor 1 and 2 signals. This is an excellent idea. If, for example, you needed more power in a large venue, you could use the PM 12-2's internal power amplifiers as monitor amps, sending the main front-of-house mix to a bigger or installed system, or to powered speakers, without having to use a different desk or external patch leads.

Patch Me In, Scotty

There are numerous access points to the signal paths, which allows a fair degree of flexibility. Balanced jack sockets are provided for main left and right outputs, Monitors 1 and 2, the Aux send and the FX send, while the inputs are unbalanced high-impedance TS jacks, and main stereo line outputs are all equipped with insert jacks, for connection to additional outboard processors. Two mono inputs are provided for external effects, and these can, of course, be used together as a stereo return. Direct power-amp inputs are also available, which break the normal signal path and therefore completely bypass the mixer sections.

Wrapping up the connections are a single stereo headphone jack, a remote effects-mute switch socket, and unbalanced RCA-type connections for recording and tape replay, plus a BNC lamp socket. This will supply any 12V gooseneck light and is invaluable for working in the dark!The power-amp section of the mixer

Graphic Description

The nine-band stereo graphic equaliser sits neatly below the patchbay and employs a single set of slider controls that affect either the main left/right mix or the Mon 1 and Mon 2 mixes. The entire EQ section can be hard bypassed by means of a switch located alongside — which is handy for an instant 'with and without' comparison of the overall mix.

The EQ's bands are spaced at one-octave intervals, with the lowest being centred on 63Hz and the highest on 16kHz, while the slider controls give generous amounts of cut and boost, in the region of 12dB each way. This is a facility to be used with care: although the ability to cut and boost agressively is provided, it is best used sparingly — especially at the bottom end!

Built-in Effects

As the PM 12-2 is designed to provide a simple, all-in-one solution to live sound, you don't want to have to connect loads of other equipment to achieve a good basic working system. This is why it's great to have built-in effects, Although the integral digital effects processor offers fairly basic facilities, it's convenient and saves loads of time when setting up and packing away.

The internal effects provided are reverb, delay and echo, presented in the form of a number of preset programs that can be adjusted to provide what's needed. The display and controls are simplicity itself, with only three push-buttons and a two-line LCD panel to worry about.

Of the 14 presets provided, the first five are Room reverb settings ranging from short (0.2 seconds) to long (five seconds). The next three are Hall settings and have longer reverb times, of up to 10 seconds, while the final preset is a Church reverb treatment. There are two dedicated echo presets: 'Echo A', a mono echo adjustable between 100 and 700ms; and 'Echo B', a left/right ping-pong effect. Three further presets provide echo combined with, respectively, Room, Hall and Church reverbs. The basic reverb programs are quite usable as they are, and the small amount of user control offered — reverberation time for Room, Hall and Church presets, and echo interval (100-700ms) for echo effects, including the combined effects — should provide enough variation to cover a range of situations, although it is important to optimise the signal level sent to the effects section, to achieve the best signal-to-noise ratio.

The effects can be switched in and out (actually, the effects path is muted) by means of an external footswitch. I also discovered that access to additional effects parameters can be gained by pressing and holding the Enter button. This brings up three additional screens, where pre-delay, noise gate and mix percentage (wet/dry ratio) settings can be adjusted. Note that the manual doesn't mention these. I also established that if you adjust them, they stay adjusted after re-powering the mixer, so perhaps it's best I don't mention this at all...

Heavy Metal

At last, we come to the very things that make this a powered mixer: the power amps. When the PM 12-2 first arrived, I was surprised by the weight of the thing — it's a heavy beast, but then it does contain all of the above goodies, plus some 1400 watts of audio power.

A view of the individual circuit boards used for each channel.Taking a quick look inside, it's easy to see the reason for the weight. This thing is pretty solidly built and has an old-fashioned quality about it that's quite reassuring. The two amplifiers are laid out as completely separate blocks, each with a cooling fan mounted well inside the case, at the end of an aluminium-finned heatsink. A substantial toroidal transformer sits between the amp boards (held in place by a massive bolt through the chassis), and the whole power section is shielded by a metal plate that's screwed directly into the heatsinks. The mixer section appears well constructed, too, and has individual circuit boards for every channel, which should make life much easier should a fault ever occur.

The power amplifier is a Class-H design, which means that it employs a variable rail voltage depending on the demand placed on the circuit at any given time. This approach offers increased efficiency over traditional class A/B designs, and results in less heat output and lighter component weight overall. The manual states that 800W is possible into a 4Ω load with one channel driven, which reduces to 700W per channel when operating in stereo. Bridging, apparently, is not allowed. The amps are protected against short-circuiting or overloading the output, DC components, clipping and over-temperature, and the two cooling fans are speed-controlled and designed to stay off — and therefore silent — when not needed (ie. in low-temperature situations).

Through Its Paces

To fire this beast up, all you do is plug it in, connect speakers to the rear-panel Speakon connectors, and away you go. I experimented with vocal mics, high-impedance instruments (direct into the unbalanced stereo channels) and lots of recorded music that I normally use for testing and setting up.

The PM 12-2 certainly has loads of power for use in a small venue, and it delivers plenty of low-end punch even when driven hard. I hooked it up to a full set of Turbosound Impact speakers (18-inch subs with '12-inch + HF' tops) that have built-in passive crossovers, and the results were very acceptable, much as you'd expect from the maker's figures. Driving a small set of DAS eight-inchers in a conference room at a much lower level produced good, controllable performance too, and the variable fan speed meant that mechanical noise was hardly noticeable — an important point if the mixer is used for speech and audio-visual presentations. The graphic equaliser does what it's supposed to, and the graphic in/out switch is useful when you want to check and compare your settings. The digital effects processor is simple, but it does the job and is very straightforward to operate. Although there's a limited number of presets, the user-defined settings remain in memory after the mixer is switched off, so it's worth experimenting and saving the settings that work well for particular applications. The user manual doesn't tell you how to restore the factory presets, but apparently more detailed technical information is available from the ever-helpful people at the UK distributors, Adam Hall, should you need it.

Rear panel, showing Speakon speaker connectors, mains power inlet and power switch, and coolong fan outlets.When using the internal effects, I did notice that there are guide marks on all the channel 'fx' send knobs which suggest a notional setting at the three o'clock position (about 75 percent up). These settings do, in fact, represent the kind of level you need to drive the effects processor hard enough to optimise signal/noise performance and do provide a good starting point when setting up.

I like the 100mm faders; they are consistent and smooth in operation and there's plenty of panel space between most of the controls for the average finger sizes. The main panel is easy to read, understand and operate, and it's organised logically enough — although I did wonder why the auxiliary masters are laid out in a different order to the channel Aux sends (the channel sends are arranged, top to bottom, as Mon 1, Mon 2, Aux and FX whereas the masters are arranged, left to right, as FX, Aux, Mon 1 and Mon 2).

The 12-2's 23kg weight means that careful handling is required, although the carry-bar is comfortable enough to allow you to transport it single-handed, provided you don't park the van miles away! General build quality is good, but you may want to keep an eye on the screws that secure the carrying handle, and also on the big bolt holding the transformer in place (easily visible underneath the mixer) just to check that everything stays nice and tight. A transformer of this size could do a lot of mischief if it came loose. On the review model, there was one carry-bar screw and a couple of side-panel screws missing, and the plastic window over the digital effects display was untidily glued into place.

Conclusion

Speed of setting up, flexibility and ease of use are my favourite things about the PM 12-2. It's a substantially built, one-box solution and packs a very powerful punch, much more than enough for most pub and club gigs.

On the down side, I'm not entirely convinced by the power-to-channels ratio of this product: it's got a monster power amplifier and it's a solidly built affair, but it only has eight mono channels, and I can't think of many live sound jobs where this combination would be enough for me. On the other hand, for a smallish band who want to own a rig capable of doing larger venues without having to add more equipment, or for smaller installation applications, it's definitely worth serious consideration.