Y-ellow Jim 'n' y'all.
At 03:53 PM 12/27/99 -0800, patchell wrote:
> Yes, I know it is foolish, how ever, the more I think about it, and
>look at what I can purchase, it is looking more attractive. It would
>seem that for about $1000 worth of components (slide pots, pan pots,
>push button switches and low noise transistor arrays), I can build a
>24x8 mixer that does what I want.
>
> I am still not sure if this is the smart thing to do (I sure it
>would be fun), or should I put on my suit of armour and go out and joust
>those windmills?
Take door number 3. I've never Jousted a windmill. What's it like? I'm sure
it's more fun than building mixers. And I should know. I've built dozens of
them.
The big problem, as I think Barry Klein once wrote, or was it in NSC's
audio handbook? Anyway that Mixers are relatively simple to design but
they are repetitive to build. And this is a bigger problem than you might
think.
There are 3 basic ways of building a mixing desk.
1) Build each channel on a separate strip like Soundcraft use to.
2) Build them all on one big huge PCB like they do these days.
3) build it how ever you like and hand wire all the pots and switches to
the front panel.
All methods will drive you nuts but option 3 comes complete with a
prescription for prozac. If you aren't already taking in prior to
contemplating such an act.
The actual circuits are nothing to design. The design rules are pretty
simple and so time-tested that you can rarely go wrong. Although many
designers often do. The two main design rules are.
1) Keep all your gain at the top and let it trickle down. If you placed a
gain stage at the output you'd not only amplify the signal but also all the
noise created in preceding stages.
2) Try and keep to a centrally earthed principal. Usually focused on the
output sockets or multi-pin I/O. This helps avoids hum-loops in complex
audio configurations.
You can whack almost any required feature between input and output and mix
and match as you please. You might need a couple of channels dedicated to
DJ work while other channels require complex routing and EQ. It really
doesn't matter as long as you follow the rules above. But the mechanical
arrangement could send you right over the edge.
I would suggest from the outset that you get your front panels
professionally made. Perhaps Ask Juergen about that frontpanel designer
guy in Germany. I haven't tried him out yet but what ever it costs to have
it made and shipped, it'll be worth it. Trust me on this. After drilling
the 10th hole and wishing it was easier to cut slots for faders, you'll
wish you got someone else to do it.
Serviceability is the next big issue. The problem with most modern mixers
is that when something goes wrong, you have to pull the whole thing apart
to fix it. And it might only be something simple. A 5 minute repair job can
take hours. Especially when you think you got it sused, put it all back
together, put it back in service, only to find it's still not right. Or
that you've neglected to attach that one doover and something is rattling
dangerously inside.
This is why I liked the Soundcraft-esk approach. Where each mixer channel
was on a separate strip. But this it self causes problem. You then have to
organize a back plane of some kind. Often this is nothing more than an IDC
cable with lots of connectors on it. Or it could be like I once did, had a
back-plain printed circuit board made with connectors on the strips which
plugged conveniently into it. Only it wasn't a total success in that
locating the connects together was often troublesome.
Making all your channels on one big board (or several big boards as the
case may be) is good in respect of mechanical stress. Sound craft desks
were mainly designed for studios where they weren't being trundled from gig
to gig. But even then they occasionally fell over due to mechanical stress.
There was a big one in Studio 202 here years ago. Really nice 48 channel
mother but you had to search for a channel that was actually working on the
day. This was mainly due to variations in temperature. Expansion and
contraction. It unseats and moves the connectors and pins. Especially if
there's a little dirt in there. Suddenly you have a channel going down. And
here this is a common problem with almost anything that uses connectors. As
the seasons change I have to re-seat all the cards in my computers for the
same reason. Unfortunately all the so called engineers at 202 didn't know
this and would simply put up with the desk. Which got worse from year to year.
But! The advantage is that it's highly serviceable. If you have a clue. And
the fact that you're building one probably indicates you have more of a
clue than your average Adelaide sound engineer. In fact my dog drops better
engineers in the garden that your average Adelaide sound engineer. Which
probably explains why most music recorded in Adelaide is universally awful.
But I digress.
If you must mount expensive ICs in sockets, use a little hot-melt or
silastic underneath the IC to glue it into the socket. You can still prise
it out of course but it won't fall out of it's own accord. Keep your wiring
to a minimum. Use PCB as much as possible. Wiring is not only a pain in the
ass to do but also vulnerable to mechanical stress. Once again, if you must
use hand wiring then go over it with hotmelt of silasic to secure it. Don't
rely on wiring harness because they can cause their own kind of problems.
Try and use sub-chassis for things like the faders. If not all the front
panel components. This allows you to put some felt strips between the fader
actuator and the front panel it self. Which in turn helps prevent dirt
getting into them. You'll double the life of your faders. At least double!
Or you can use duncan faders which have an integral dust seal in them. But
they cost more. A lot more. Mine were $10 bux each 15 years ago. And that
was discounted from $70 bux each because they couldn't get rid of them. In
hindsight, a sheet of felt is a lot cheaper.
And there is another alternative to all of this.
Find yourself a second hand mixer that does most of what you want, replace
the faders etc as required and modify it so that it does exactly what you
need. Add extra channels and whatever else you want. In the long run it
will get you off the ground quickly, relatively cheaply and end up doing
what you need. Our main studio consol of 6 years was like that. It wasn't
the best designed desk in the world but it was laid out almost how we
needed it at the time. So I set to and made some modifications. And it did
a lot of work. 7 CDs, a few sound tracks, jingles. Even a live-to-air for
the ABC once. But the point is that it was working from the time we bought
it. All I had to do was taylor it to our needs.
And unless your requirement is so off the wall that there just isn't
anything like it on the market, (as with this g'damn MIDI mixer I'm still
trying to de-bug) I'd leave building a mixer from scratch to someone else.
Hope this helps.
Be absolutely Icebox.
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