Here's a picture of a TAC Scorpion, a modular analogue desk.
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As you can see, each channel is on its own strip of metal, so you can pull them out individually and replace them. So (for example) you'd be limited to 32 channels by the chassis, but you can put whatever you want in those 32 slots.
Most desks (aside from really, really cheap ones) are designed in a very similar way to the scorpion, but you can't change anything because the holes in the top plate would be in the wrong place. Here's the inside of an old 8 channel Studiomaster. You can see the 8 single-channel PCBs, along with the different strips in the master section.
Each channel has it's own identical PCB, which makes servicing much easier, as you can repair/replace a single channel without affecting the others at all.
Here's a picture of an Allen and Heath GL3300 48-channel desk.
You can see that the channels come in groups of eight, with a master section in the middle that's the same width. This makes A&H's job very easy when manufacturing, as they can sell the 8,16,24,32 & 48 channel versions of their desks without having to manufacture any different parts other than the chassis.
Behold, the iLive
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My Church recently got the iLive system, and I have to say that it is
awesome. We got the IDR48 Mixrack (48 in, 24 out) and the T-112 surface (which has another 16 TRS ins and outs which we use for the various stereo ins/outs required across the Church).
(Off the top of my head), with the iLive Mixrack, you get something like 12 assignable matrices (which can be anything: main outs, recording matrices, groups etc), 24 mono auxes (or 12 stereo), 128 input channels (the 'brain' can support that many, even though there are only 64 physical inputs across the system), gate, comp, fully parametric EQ and De-Esser on each channel, a Lexicon effects rack with up to 8 different effects simultaneously (The most I've ever used in a service is 6, and that was pretty ridiculous), and 12 DCAs (Digital VCAs).
If you really had the need to expand from there, then you could daisy-chain in another Mixrack into the surface and add ins/outs/processing power. Having said all that, you do get limitations. Firstly, you have to use the same EQ for monitors and FOH. Also, if you want to use a sidechain with a compressor, your options are limited; A&H still use their 'blocks of 8' idea, and so your sidechain has to be in the same bank of eight as the channel you'd compressing/gating etc.
Wow, sorry for the waffle! It must be a slow morning.
