[Scott Dorsey]

Andre Majorel wrote:
On 2008-06-10, Scott Dorsey wrote:
Andre Majorel wrote:
Can anyone point me to photos of the inside of a mixing desk ?
The kind that is made of strips. I'm curious what the wiring is
like.

There are many different kinds.

Some employ a backplane board that all of the individual
modules fit into. This seems like a good idea since you just
drop a module in, and it's possible to build connectors so
that the channel strips can be hot-swapped. However, it's not
very rugged at all and they invariably fail if they are taken
out on the road. This is most popular with the broadcast
guys, who need the ability to hot-swap and don't need
roadability.

By "fail", do you mean the cards need to be re-seated into the
backplane connectors or something more serious, like cracked
traces ?

Cracked traces, cold solder joints, connectors broken completely off,
boards with actual cracks through the material.

Drop a Radio Systems console six feet, it won't ever work again. Drop
a Midas console six feet, it'll probably be fine.

Some employ ribbon cable connectors. This is a lot more
rugged because the cable itself is flexible and will take a
lot of shaking around. Both pin and edge card connectors are
used. Sonically this can be a problem if the signals are not
laid out on the connector properly, but if big conductors are
available for very low-Z busses, and the signal lines are
separated by grounds, it can be pretty good. You'll find most
PA consoles are built like this. Most of them have seperate
power busses, though, not on the ribbon.

There is twisted-pair flat cable. Not shielded but perhaps
better than straight flat cable. That would double the pin count
on the IDC connectors, though ! Have you seen that used ?

No, the pin count won't increase since the busses are already balanced.
But that stuff doesn't really buy you as much as you'd think... ribbon
cable itself is surprisingly good, even if it's not completely shielded.

Some employ hand-wired cable harnesses going between boards.
This gives you the best of both worlds; it's possible to make
them hot-swappable, and use good shielded twisted-pair lines
for all of the busses. It can be very rugged, and if the
metalwork is built right some of them are set up so you can
just drop a module in. This is, however, the most expensive
of the three options.

I see, thanks. How are the signal busses done ? I understand
it's better to use current-mode summing. Is it just one wire for
each summing bus, running from strip to strip, each strip
injecting a little current into it, the sum being converted to a
voltage by whatever is at the end of the bus ?

Good consoles today have balanced busses, each side of which is driven by
a low-Z amplifier through a small-valued (often 600 ohm) summing resistor.

A lot of cheap consoles have unbalanced busses, or they use summing
amplifier stages that are chained together into a pyramid. This brings
the noise floor down without using lots of current drive, but it means
there are a bunch of additional gain stages in the signal path.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."