Interior of a mixing desk frame

Andre Majorel <[email protected]> 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.

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.

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.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

On 2008-06-10, Scott Dorsey <[email protected]> wrote:
> Andre Majorel <[email protected]> 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 ?

> 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 ?

> 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 ?

--
André Majorel <URL:http://www.teaser.fr/~amajorel/>
"Cette supposition rappelle assez celle de ce prédicateur qui, en
pleine chaire, faisait remarquer à ses fidèles la bonté de Dieu qui
avait placé les rivières auprès des villes." -- Alexandre Dumas

Andre Majorel <[email protected]> wrote:
>On 2008-06-10, Scott Dorsey <[email protected]> wrote:
>> Andre Majorel <[email protected]> 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."