Live Sound International | Designer Notebook: Sussing out the Soundcraft MH4

Designer Notebook
It's a Dual Purpose World: Sussing out the Soundcraft MH4

By Soundcraft Product Manager Andy Brown & Technical Director Graham Blyth

(This Designer Notebook was submitted by Soundcraft. Live Sound makes every effort to eliminate any use of marketing inspired hyperbole.)

In the past Soundcraft produced dedicated consoles for the very different jobs of both house and stage sound in the firm belief that this was the best way to ensure optimum performance. The commercial realities of today's competitive rental market, however, now dictate the economic advantages of a single console that can live -- and earn its keep -- at both ends of the snake. And since efficiency in the concert industry is the trend, the move toward more flexible console designs is a part of this process.

With this in mind we at Soundcraft decided on a dual-purpose console. At a product planning meeting some 18 months ago, the target market and price level of the MH4 were defined. We had been also considering for some time the idea of developing a high-quality, modular, VCA-equipped console that would slot into the range above the non-VCA K3 but at a price-point below the Series FOUR and FIVE.

The mighty MH4.

This console would perhaps have ended up as a Series Three, but the decision to go dual-purpose meant that this was now going to be named the MH -- short for Monitor or House.

Development started by considering which features would be needed to make a console dual-purpose?

The answer of course is that the bussing system needs to be extremely flexible -- the FOH application needs a moderate number of FX Aux sends and some subgroup busses, plus the main mix busses, whereas for monitors the subgroups and main mix are not needed and instead, as large a number of Aux sends as possible are required, ideally some stereo, for the in-ear monitor (IEM). For FOH, only the subgroups and main mix need insert points, but for Monitors, all Aux outputs need to have inserts and good AFL soloing.

Which Way To Go?

One way to achieve this would be to simply provide all the busses that would ever be needed in either application -- so maybe 16 Aux sends plus another 8 busses for subgroups, plus the main mix. This might be fine at the top end of the market, but the cost of providing all that hardware, much of which remains redundant in one or other application, would be out of the question for a mid-price console.

Another method often successfully used on lower price consoles is to structure the busses like a conventional FOH console, that is, with separate groups and Auxes (with perhaps a couple more Auxes than usual), and to provide a "fader swap" facility on the output section to allow output faders with insert points to be used for the Auxes when doing monitor work. Those same faders can then be used for the group busses in FOH mode when the Aux (FX send) masters are relegated to rotary faders. This provides a very compact and therefore cost-effective output section, but for the same cost reasons it is not usually possible to provide more than about 12 Aux sends for monitors.

Rear master panel.

The shortcomings of these first two schemes leads to the inevitable conclusion that to get enough Aux sends for monitoring, but without the cost of adding extra busses dedicated to subgroups, there must be some busses that can perform either as Aux sends or subgroups, depending on a global selection.

This mode-switching of busses is the key to dual-purpose functionality, but it is also the area where there is the most potential for operational confusion or misunderstanding on the part of the console operator, because we are entering an area where signal flow is being switched "under the panel" from one route to another and the function of some controls becomes slightly less obvious. It is this tricky area that the various console manufacturers choose to implement in their own particular way.

The MH4 Solution

Our solution was to take a set of 16 busses and divide them in half, with one set of 8 remaining as Auxes, the other being switchable to either work as Stereo Aux sends for Monitors, or fixed-level routing switches for subgroups. This is not a unique idea in itself, but what we decided to do was that instead of making Aux Pre/Post or On/Off switches perform double-duty as subgroup routing switches in FOH mode, we would have dedicated, recognizable switches for the subgroup routing (1-2, 3-4, and so on) and make it very clear that they only worked in the FOH mode. Making the illumination inside the subgroup routing buttons only work when the global FOH mode is selected highlights this.

In fact there are eight subgroups -- four pairs of busses, and each pair can be switched between a subgroup and a stereo Aux send from the master section. In Monitor mode, the 4 stereo Aux sends would be ideal for stereo in-ear or wedge feeds.

Input circuit board.

As a second innovation, we decided that by placing those subgroup buttons physically along side the dual concentric stereo Aux send pots for the second bank of eight sends, the lower "Pan" section of the Aux send could be used as a dedicated subgroup pair pan when in FOH mode. This makes use of the otherwise redundant set of Aux send controls, but also means that each pair of subgroups has its own dedicated pan pot, rather than relying on the main channel pan pot, as on a normal FOH console. Effectively it also gives the equivalent functionality of eight individual subgroup routing switches, even though there are only actually four paired routing switches. So you have the choice between four stereo or eight mono groups.

A further feature allows the unused upper level control pot from the stereo Aux send to control an additional mono FX send when in FOH mode on that pair of busses. This gives four extra FX sends in FOH mode for a total of 12. The result of all this is a configuration of 12 mono FX sends plus eight subgroups in FOH mode, or eight mono plus four stereo sends (16 busses) in Monitor mode.

At the output section end, it's more important to bias the features and layout in favour of the monitor application, as this part of the console is far less critical in the FOH application. All outputs are therefore given fader control plus inserts, talkback routing, AFL solo, and bus inject via a level control.

For the FOH mode, routing to Mix and Center busses for subgrouping is provided. All output faders are laid out in a linear fashion above the VCA Master faders. This type of layout gives quite a wide output section, and enough space to incorporate a "send" Matrix, where every output buss can be sent into any of the eight Matrix Outputs.

It might be assumed that a Matrix is redundant in the Monitor application, but in fact it provides an ideal way of creating a "patchbay" to send the same Monitor mix to two or more different locations on the stage, which can sometimes be required with the more mobile performers.

Output EQ and Buss Inject

In the area of the Output section controlling the dual-mode Subgroup/ Stereo Aux outputs, we found a very satisfactory way of using a four-band stereo sweep EQ. Four of these normally sit in the signal path of the four Stereo Group/Aux Outputs, and so are ideal for use in the Monitor mode as a sweetening EQ for stereo IEM mixes. This could eliminate the need for any external EQ for the IEMs in many cases. In this application, the stereo buss inject input with its level control is ideal for use as a reverb return for that mix.

Looking into the frame.

In FOH mode, the EQ and buss inject point are not redundant, as switches allow the EQ to be flipped into the buss inject path, and the buss inject itself to be routed to the stereo Mix rather than the Group/Aux busses -- all of which makes a fine stereo FX return into the Mix.

Bells and Whistles

Every console needs a few small features that may not seem essential at first, but which make the console much more enjoyable to work with. Here are four that deserve a mention; all of them are the result of ideas suggested to us by various engineers at one time or another:

The Auto-sensing Phantom Power Switch, with a rear mounted repeater LED is a simple idea to help when patching up the console. If a multicore line that already has 48V from the other end is plugged into the back of the console, an LED right next to the input socket will immediately light up. Not only this but on the top surface of the console, the LED inside the 48V switch will also illuminate in this condition, even if the switch is not pressed in. This means that if you see a 48V switch illuminated but not pressed, you know straight away that the phantom is coming from the other end of the line. This may or may not be how you want to run the phantom, but on a 48-channel desk it could save you quite a few intercom calls to the other console trying to verify who is powering which mic! Of course the 48V switch works in standard fashion as well, so it will illuminate (along with the rear LED) when pressed and there is no external phantom present -- in this case it is simply detecting its own voltage.

Inside the Soundcraft production facility.

The Littlite Flashing Call Detect system was an offering to the many engineers who have missed intercom calls because they haven't seen the belt-pack light up whilst concentrating on either the console or what's happening on the stage -- not everyone it seems has the luxury of an external strobe light. The intercom line can be routed though the console and any call signal will be detected and will flash the Littlites. The console itself will function like a belt-pack as far as audio is concerned as well, where pressing the "Talk to External" switch will allow two-way communication with the intercom line, using the console TB mic and headphones. The idea of this is obviously to try and avoid having to constantly swap between two pairs of headphones.

There's even a "sidetone null" control on the master module, allowing the local talk mic level to be reduced in the headphones for noisy environments such as load-in.

The rear panel Mic Split output is a simple passive parallel feed from the input XLR, which could either substitute for a complete stage splitter system if budgets were tight, or is an easy way of Y-splitting inputs to the console, such as on a Monitor console where two acts have to be accommodated on a single console and some lines are shared, or you need to split a signal to two channels to apply different EQ for different mixes. A ground lift switch works on the split output only.

The Output AFL remote control interface works with either dbx DriveRack or BSS Varicurve EQ systems, and is a great feature for monitor engineers. By means of a simple midi connection, pressing any output AFL switch on the console will automatically select the correct EQ channel on the dbx or BSS remote panel. This means that there is no chance of soloing an output channel and accidentally selecting the wrong output EQ for adjustment. The EQ is already on the correct channel by the time your hands get to it.

When used with the DriveRack, the system also knows when an input channel's been soloed, so that the DriveRack's "wedge" EQ can be automatically bypassed for the duration of the input solo.

Surface-Mounted

In addition to the adoption of dual-purpose architecture, the MH4 also marks another departure from previous practice, this time in its method of circuit board construction. Early on in the development phase it was decided to make substantial use of surface-mount technology for the printed circuit boards (pcbs).

For those unfamiliar with this, surface-mount (or SMD) construction employs the technique of soldering sub-miniature components to the top surface of a circuit board, using a solder paste, without the need for wire leadouts. It's been widely used in the computing industry and consumer electronics for some time and its main advantage is dramatically reduced size and weight. Moreover when compared to conventional board assemblies, a lot less board area is needed for the same amount of circuitry.

So we initially experimented with the technology on digital CPU pcbs. Large volume use on our small Folio range of mixers had prompted us to invest in the necessary specialized machinery to build this type of pcb. Initial concerns about the field serviceability of SMD proved to be unfounded as well: use in higher-end products, such as stereo modules for the FIVE Monitor, proved it is well suited to life on the road.

With this in mind MH4 was to be our first large-scale fully modular console designed to use surface-mount technology exclusively. The benefits have already been mentioned -- the console does without 30 sq. inches of glass-fiber pcb material on every input channel, amounting to a significant reduction in weight.

Styling

Console styling can be an emotional subject, and opinions of users vary widely from those who see the console purely as an audio tool and don't care what it looks like, to others who may be heavily influenced by the look of the console when buying one. In reality the styling is a strong tool for us as manufacturers to communicate a part of our brand identity, and if done well can enhance the whole feeling of quality and solidity of the console, as well as its useability. We only get the chance to change styles once in a while; it makes sense to commit to a style and then use it on a range of products for a period of time.

With the MH4 we had such an opportunity to update a style that's been in use for probably five years. The idea was to update the cosmetics while still retaining a recognizable Soundcraft look. This was achieved by changing the panel color from our dark blue-gray to a more definite blue color using a metallic element. To lift the appearance further, visible frame extrusions that would previously have been painted were left with a natural anodised finish.

The biggest departure though was the meterpod design. A full-width meter bridge is great for flight casing but limits the shape of the console, and on large frame sizes each end usually ends up being filled with unattractive blank panels. Our centrally mounted pod design was an attempt to add some shape and style to the console. In reality, flight casing is not a problem as the pod does not project back beyond the rear load-bearing surfaces.

A final element of the new look was the side cheeks, and to tie in with the anodized extrusions on the frame we decided to use polished stainless steel inlays in the side. The design resembles the shape of a structural member inside an aircraft wing.

Graham Blyth waxes technical on the MH4 circuit design.

The MH4 mic amp is based on a circuit topology known as the Instrumentation Amplifier, and despite its title, this is actually a very suitable model for a live console input stage, due to its notably benign transient overload performance. The MH4 mic pre is also optimized to deliver particularly low noise in the most often-used mid gain area rather than only concentrating on the relatively easy to achieve -128 dBu EIN (Equivalent Input Noise) at maximum gain. The result of this is an EIN at 30dB gain of better than -122dBu, and a THD+Noise figure at this gain of 0.006% at +18dBu out of the pre-amp. The bottom line of course is how the circuit sounds, and it's gratifying to hear favorable comments already about the forgiving nature of this design, in terms of its high-level input and transient handling.

Equalizer

The equalizer in the MH4 is a totally new design. The swept HF shelving section has a much steeper slope than is usually seen, resulting from a novel element in the circuit design. The effect of this may require some getting used to, but it allows the user to focus the HF response exactly where it is wanted, with none of the usual undershoot and overshoot associated with such slopes. The Hi and Lo Mid sections are essentially the standard circuit that most designers have used for years but with a tweak in the side chain that removes the possibility of clipping at large amounts of cut and at wide bandwidth. The swept LF shelving section is unique in that it only uses half an opamp to achieve a well-focused 2nd order curve. This came about by analyzing what signal a certain key circuit node required and finding that it could easily be provided passively, thus deleting the more usual buffer amp. By rigorously applying this kind of thinking throughout the MH4 design, the total current consumption of the console's been kept remarkably low, resulting in a very cool running work surface, and greater console and power supply reliability.

LCR Pan

Most LCR Pan circuits use a special 3-gang potentiometer, followed by a buffer amplifier for each buss, but the MH4 circuit uses a new circuit design that, as well as using just a single gang pot, also delivers the required 3dB drop halfway between busses in both LR and LCR mode. (Many circuits have to accept a 6dB drop in LCR mode.)

MH4 in action at Chicago’s Cubby Bear Lounge.

It also achieves this without adding significant noise, which is very important as this point of the signal path is after the mute and fader sections and so adds to the final buss noise if the routing switches have been enabled.

Summing amps

The quietness of all the post fader electronics would be wasted if a conventional summing amp were used on the end of each buss, however high the quality of the opamp. Our semi-discrete summing amps deliver mic-amp levels of EIN and are very low noise. This enables an overall 48 channels routed mix bus noise of better than -85dBu.

MH4 Feature Check

Frame sizes:

24+4, 32+4, 40+4, 48+4, 56+4

Bussing:

Mic Preamp:

EQ: Fully parametric mids. Sweep HF/LF

High-Pass filter: Sweepable 30-400Hz

VCA Groups: 8

Mute Groups: 8

FX Returns:

Input Metering: 12 segment, plus separate peak LED in mic amp section.

Output Metering:

Matrix: 20x8

Solo System:

LCR Panning: From Inputs to LRM busses

Talkback:

Test Oscillator: Sweepable tone 63Hz-10kHz, or pink noise

Inputs and Outputs: All fully electronically balanced

Insert Points:

Buss injects: Balanced jack inputs to all busses

Automation:

Andy Brown has worked at Soundcraft in the UK for 18 years, the last eight producing live console concept design for the SM Series, K Series, Series FIVE and the new MH4.

Graham Blyth co-founded Soundcraft in 1973, and for the first 15 years was solely responsible for electronic design, from the 1S through to the S4, and S8000. More recently he has been responsible for the electronic design of the Spirit range, the Series TWO, and MH4.

For more information contact: www.soundcraft.com
Soundcraft USA +1 615-360-0471
Soundcraft UK +44 (0)1707 665000

January 2003 Live Sound International

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