Paul White examines the role of the keyboard submixer in the MIDI/tape recording studio.
The way in which most musicians make music has been completely revolutionised by MIDI, yet aside from the introduction of the in‑line mixing console, basic mixer design has changed very little in the past two decades. It is true that prices have fallen dramatically and that automated mixing has become an affordable reality, but aside from some budget mixers popping up that incorporate a few token stereo channels, most are still designed for getting signals on and off tape.
If you have an in‑line console, then the monitor signal path can be used to handle MIDI instruments that are run in sync with the multitrack tape at mixdown, but you may still run out of channels due to the number of synths and samplers with multiple outputs. Furthermore, in‑line consoles invariably represent a compromise, as both the EQ and aux facilities are shared between the main and monitor signal paths. The reality is that to get enough channels to handle anything other than the smallest MIDI system means paying for a mixer that's bigger than you really need for multitracking alone.
With MIDI sequencing, it's no longer necessary to record your MIDI instruments on the multitrack tape, though there are still some occasions where it's useful to do so. And if you don't need to record your MIDI gear to tape, then you don't need mixer channels with full routing capabilities. From my own point of view, I don't think you need such a flexible EQ system with electronic instruments, because most sound pretty good straight out of the box. You may also find that you can make do with fewer aux sends, because most modern synths have effects already built in.
It amazes me that nobody has ever sat down and designed a mixer for the way we record and work today, because it seems patently obvious that while the traditional approach to mixer channel design works well enough for tracking, we need an entirely different channel design for the MIDI gear. The main criteria of such a design being plenty of stereo channels, a well designed input stage to prevent ground loops from multiple sources, and an EQ section specifically tailored to keyboard use. For example, I'd like to see a variable low‑pass filter in every channel, which would be useful both for warming up digital synths and for skimming off high frequency noise.
In the absence of any single mixer meeting all the needs of the tape/MIDI studio, the next best route is to use a submixer. In my own case, I've recently installed a Mackie 3204 purely because it offers the right basic facilities, it's clean and has lots of inputs. It also has four busses, which leaves some routing possibilities open. However, the concepts described in this article are applicable to any make of conventional submixer.
Making Connections
Most of you will be aware that a submixer can be used simply by patching it into a couple of spare inputs on your main mixer and then panning these hard left and right, as shown in Figure 1. In theory you can plug into any line‑level mixer input, be it a channel, aux return or whatever, but to retain routing flexibility you should ideally choose inputs that can be routed to the main mixer busses. This way you'll be able to record the outputs of your submixer onto any tape track, should the need arise.
If your submixer has balanced outputs and your main mixer has balanced inputs, then I strongly recommend you use balanced cables as this will reduce the risk of interference and hum. You should also ensure that both mixers are earthed and that both are plugged into adjacent mains sockets. If hum does show up, you may find that disconnecting one end of the cable screens in the two connecting leads cures it, but you can only do this if you're working balanced.
This arrangement works well enough insofar as it goes, but it doesn't do anything to integrate the aux send systems of the two mixers, so you could end up using one set of effects units on your main mixer and another on your submixer. This is fine if your studio is over‑populated with FX units, but less helpful if you only have one or two outboard units to play with.
There are ways available for linking the aux send systems of two mixers, which I shall explain shortly, but if you have one of the more modern effects processors with a stereo input, there's a much simpler solution. Simply connect the Aux 1 send from your main mixer into the left FX input, the Aux 1 send from your submixer to the right FX input, and set the FX output to all effect/no dry signal. Most effects combine the left and right inputs to mono before processing anyway, so as long as you're aware of any specific effects patches that provide different effects on the left and right channels, you can work as though both mixers were part of one large mixer. The FX unit outputs are patched into the aux returns on the main mixer in the usual way, as shown in Figure 2.
Unfortunately, this method of effects patching doesn't work if you have an effects unit with a mono input, or if you want to bring your aux sends out to a patchbay to retain full flexibility over which effect is patched where. In this instance, the only simple way to link the aux busses is to feed each aux send output from the submixer into its own channel on the main mixer. To make this work, you have to ensure the main channel is switched on but not routed to the left/right mix or to any of the groups (in other words, all routing buttons are up). Next, turn up the Aux 1 send control on the channel receiving Aux 1 from the submixer, Aux 2 on the channel receiving Aux 2 from the submixer, and so on. Figure 3 should make it clear how this works.
You should set the aux send controls on the main mixer channels to around three‑quarters up, then adjust the Aux Send masters on the submixer so that you obtain roughly the same amount of added effect for the same control settings on both the main and submixer. Note that under no circumstances should you try to combine the aux sends from two different mixers by simply using a Y lead. This is unkind to the output stages and invariably leads to a loss of signal quality, and occasionally to damage.
Using Patchbays
Most of the time, you'll be able to leave your submixer setup exactly as it is, but occasionally you may want to introduce a signal processor (such as a compressor, equaliser, or gate) between one of your MIDI instruments and the submixer input. Many line mixers have no insert points, which means you have the option of either unplugging your synth from the back of the mixer or installing a patchbay. A patchbay is obviously the most convenient option, as it saves disrupting your system every time you want to make a minor change. It also makes it easy to patch a particular instrument into your main mixer should you want to record it to tape, as many people do with their samplers to extend its polyphony.
Since most keyboard instruments have unbalanced outputs, an unbalanced, semi‑normalised patchbay is the best option here, because it will automatically route your instruments to your submixer unless a patch lead is plugged in. Plugging into the bottom row of the patchbay will disconnect the original source and allow you to feed a different instrument into that channel of the submixer; plugging into the top socket only will allow you to take a feed directly from a MIDI instrument without actually disconnecting the instrument from the submixer. Plugging into both the upper and lower sockets allows you to take the signal from the instrument, route it through an external processor and then return it to the mixer, just like a conventional insert point. Figure 4 shows how such a patchbay would be wired.
Summary
By using a submixer for your MIDI instruments, it is possible to get by with a smaller main mixing console yet still have a greater total number of inputs. The result is a saving in both space and cost, but without compromising your sound quality. Working this way may even allow you to budget for a main mixer with automation, and when it comes to your MIDI instruments, it's usually possible to automate their level and pan settings via MIDI from your sequencer, so it doesn't matter so much that your submixer has no automation facilities.
In an ideal world, all mixers would have accessible external inputs to their aux busses, enabling you to link recording mixers and submixers without the need to sacrifice channels. Though some manufacturers do indeed include such facilities, most do not. Similarly, some sort of standard connection that allowed you to link the Solo functions on both mixers would be useful, but as it is, they will work quite independently. To solo a channel on the submixer, you need to solo both the relevant submixer channel and the two main mixer inputs into which the submixer is routed.
One day we might get mixers that offer the best of both approaches, or at least an integrated linking system that allows submixers and mixers to be combined so that they can function as one large console. Until then, I can see a big future in submixers, especially as so many MIDI instruments have multiple outputs.