DDA D-Series (16:8:2) Mixer
Saw this fixer-upper come up on the usual place, and after some negotiating with
the seller I got it for a very reasonable price. Untested, without power supply,
ex-theatre company, looking a bit sad. Perfect!!!
It's not your usual mixing desk. As well as the obvious PPM meters in place of
the more common VU meters, it turns out that nine out of the sixteen input channels
have Neutrik mic transformers fitted. It also has custom multi-way
connectors on the rear, so it will need a complete new set of rear channel plates
making and fitting, and it did not come with a power supply, so that needs sorting
out as well.
And So It Begins...
First task was to remove all the channels, cabling, meters, and metal work down
to the bare parts. Then clean everything and start rebuilding. So here is a
photo that explains why this mixing desk is so heavy!
Yep, that's right, a welded steel tubular chassis that must weigh close
to 20kg on it own! This 30-slot frame is 1m (40 inches) wide, to give you some
idea of scale.
The first stage is now completed and the metalwork is all back together again.
Main changes were to replace all the old screws with new blackened ones, clean out
some of the threaded inserts, treat the rust patches on the base plate with rust
neutraliser, and give everything a thorough clean with disinfectant wipes and
sticky label remover. There was plenty of dust, fluff, and a dead spider.
Once the main metalwork was complete it was time to reupholster the armrest
and refit the reconditioned wooden side panels. The meter panel has also been
stripped down and cleaned (loads of masking tape goo on the rear!):
Next task is to start on the wiring harness and rear panels.
Then start on the modules (master,
inputs, outputs, and auxes). Power supply will likely be last as I can
test the system on a bench PSU for now.
First foray into the electrical side of things is to test and refit the meters.
This desk was fitted with the BBC PPM (Peak Programme Meter) option (white needle against
a black background). I originally thought to replace these with Sifam VU meters, which
would be a drop-in replacement. But after some digging around and costing it up I
decided to stick with the present PPM meters. A gentle introduction to programme meters
is found in
BBC Training Supplement No. 6, together with technical details of the classic
ME12/9 and ME12/10 PPM meters. You can find more information about BBC PPM meters at the excellent
BBC Eng.info website in the "ME" section.
Most PPM meters require a driver board to correctly drive the needle with the
correct response and ballistics. DDA were no exception, and every PPM meter
has a driver board bolted on the back of the meter:
Before refitting them I
decided to test them, using a bench supply and a signal generator courtesy of my
Unfortunately two issues were found. Firstly, whoever re-wired the mixer with
the weird Bendix connectors messed up the meter power wiring: instead of connecting
them to +18V and 0V they wired them directly across the +18V to -18V rails, putting 36V
onto the meters. While not a problem for the TL071 and TL072 opamps
(although rather close to their abs.max limit), the mid-rail
circuit was no longer mid-rail, and the op-amps got rather toasty. And secondly, the
meters for channel 5 and Right were faulty, so needed to be fixed.
Fixing requires a schematic, and unfortunately the excellent
DDA Consoles website
did not have it. So some time spent with a photocopier, coloured pens, and
TinyCAD resulted in
(click to view).
Note that the -4 trim corresponds to "3" on the meter scale, and +8 corresponds to "6",
with "4" being the 0dBu level in accordance with the standards.
The meter input is fully differential and capacitively coupled simplifying the signal connection
(no need to worry about which pin is ground or signal).
The 9V1 zener diode generates the mid-rail. After that it's a fairly standard diff.amp into full wave
rectifier into meter driver with piecewise logarithmic function generator feeding the
The only issue to note is that the polarity of the input capacitors requires the local
mid-rail reference to be positive with respect to the mixer's 0V. This means I cannot run
some of the meters off -18V and 0V as that would put a rather unhealthy negative DC bias across the input
capacitors which would probably let the magic smoke out. An option to be explored is to generate a local
-6V/+12V supply for the meters, a couple of beefy zener diodes would be good enough as the load is
With the meters all working electrically, attention turns to the way they are mounted.
This is done with a black plastic bezel and two little spring things on either side of the meter.
Unfortunately the combined weight of the meter and driver board looks like it is a wee bit
too much for the two springs, so an application of Dow 744 RTV silicone is enough to hold them
in place without causing damage to the meter or metalwork:
Finally, with new wiring harnesses assembled and fitted the meter bridge can now be
refitted to the frame:
The Curious Case of the Cue Switches
The first module for bringing up is the monitor master module, for two reasons: it brings the
power into the mixer and onto the mix bus, and it has the headphone amplifier and test tone generator.
It also has two switches for operating the cue lights:
Tracing out the four combinations of buttons shows that while the operation of the
LEDs is as expected, the behaviour of the external connections does not exactly follow the panel
|Up||Down||--||2 - 3|
|Down||Up||Green||1 - 2|
|Down||Down||Red||2 - 3|
The issue is the connection to the red/grn switch. Swapping two of the PCB traces round will remove the
odd state and make the cue and light controls work as expected (cue lights on only when the CUE switch is down,
and the colours match the front panel indicators). A couple of small bodge wires tacked down to the board should
fix this little oddity.