Allen & Heath System 8 MkII (24:8:16:2) Mixer
It all started one day when I decided I needed a decent-sized mixer to build
my new electronic studio around. The hunt was on, and after some months I found exactly
what I was looking for - an unloved 2416.
The new mixer just after arrival. You can see a few knobs and
slider caps are missing, and some remnants of masking tape.
Condition on Arrival
I mentioned unloved above.
The power supply is missing, some knobs and slider caps are missing, and it looks like there have
been a couple of front-panel incidents, with one pot shaft definitely pushed in, and one
input channel strip looking a bit out of line (you can just about see it in the picture below)
which I'm guessing is due to a pot replacement job on that channel.
Work to do... which is exactly why I bought it!
What do all those numbers mean?
This mixer has 24 input channels.
These feed into 8 groups.
There are 16 tape returns.
Final mixdown goes to 2 stereo (Left/Right) channels.
This desk is of the Split format, with the tape returns positioned above the group faders,
which are to the right of the master section. This is in contrast to the more compact
Inline format which puts the tape returns into the input channel strips.
A surprise bonus feature I've found is that at some point in its life (either factory-fit
or a rather neat tech.mod) is direct outs for all 24 input channels. The 24 jacks are mounted
on the rear of the meter pod, and the installer has even gone to the trouble of nicely
labelling them. Not sure yet if they are pre- or post-fader.
Also, being of vintage design, it is rather more spread out than modern budget mixing desks.
It is 1.35m wide, compared to just under 1m for a modern 24-channel 8-bus mixer (e.g., Mackie 24:8).
That's a whopping 43% difference, and it is certainly noticable!
You can almost see the other end on a clear day
What is it with the colour?
I've read several disparaging comments about the brown colour of the Mk I and Mk II consoles (the later Mk III adopted
a battleship grey colour scheme). I like the brown scheme. It reminds me of those early EMI consoles from
Abbey Road -- I remember one that was in the reception of EMI's Central Research Labs sometime around
early 1990. It is a warm colour, in tune with the warm sound this console reportedly has.
Knob Colour Scheme
While sorting out what knobs and caps are missing I can summarise the colour coding as follows:
- Green = EQ
- Brown = Aux send
- Orange = Level/Pan/Gain
Time for some spec oggling. I have pulled together what I think are the main spec
figures from the various manuals. Note that the manuals are not entirely clear in some places
(it looks like it was written by a contract writer who didn't understand the finer points
of the technical details) so I have clarified some details where appropriate.
MODEL 2416 - 24 input channels, 8 outputs plus stereo mix, 16 track monitoring.
Connectors: XLR mic inputs and main outputs; ¼" jack all other circuits.
|GAIN (input to output):||Mic||Line|
|minimum (includes mic PAD) ||+4dB ||-15dB |
|normal range ||+20dB to +60dB||-15dB to +25dB|
|maximum (includes fader boost)||+70dB ||+35dB |
Normal operating level (outputs, line and tape inputs): +4dBu or -10dBV selectable
Internal headroom: +18dB
Maximum output level: +21dBu into 5 kOhms or more. +18dBu into 600 Ohms.
Peak indicator warning at 3dB before clipping overload.
FREQUENCY RESPONSE (mic or line input to output): 20Hz to 20kHz ± 1dB ref. 1kHz
|HF Shelving equaliser||±16dB at 12kHz or 8kHz|
|LF Shelving equaliser||±12dB at 120Hz or 60Hz (see note 2)|
|MID peak/dip ||±12dB, 400Hz to 6kHz, Q=1.5|
DISTORTION (Mic input to output at +4dBu, +45dB gain): <0.05% THD 20Hz to 20kHz
CROSSTALK: Left and Right output separation better than 60dB, 30Hz to 10kHz
Interchannel separation better than 70dB, 30Hz to 10kHz
NOISE (20kHz bandwidth RMS noise ref. 0dBu (0.775v RMS)):
|Stereo output (one line input routed unity gain):||-79dB|
|Stereo output (all inputs routed and faders closed):||-79dB|
|Group output (one line input routed unity gain):||-81dB|
|Group output (all inputs routed and faders closed):||-74dB
|Auxiliary output (unity gain and inputs closed):||-74dB|
|Microphone equivalent input noise:||-125dB all models 200 Ohm source.|
|Width ||1350 mm (53")|
|Height ||245 mm (9 ¾")|
|Depth ||672 mm (26 ½")|
|Weight ||64 kg (141 lb)|
- The noise figures come from the Mk III manual as the Mk II manual does not include
numbers for the 2416. Comparing the 1616 model in both versions I'd say the Mk III was
about 1dB lower noise. Not a huge difference.
- The spec quotes the LF EQ as being 120Hz and 80Hz, yet the schematics and the front
panel text both say 120Hz and 60Hz.
Allen & Heath are good enough to support their old products by providing many documents
online through their website.
System 8 page at Allen & Heath
Refurbishment - Upgrades - Modifications
(Note: at the time of writing these are mostly just ideas, I haven't done any detailed
thinking or design work just yet)
I will also be using the details from the Mk.III model to indicate where
the original A+H engineers thought improvements could be made.
The approach I plan on taking with this project is to adopt design and component
choices in keeping with the age of the mixer. So, for example, op-amps will be
TL072 and NE5532, not modern devices from the likes of ADI or TI/BB.
Update: After some further investigation the plan is slightly modified.
- Keep TL072 where there's no benefit in upgrading.
- Keep NE5534 in the main post-fade amps.
- Replace TL072 with either NE5532 or OPA2134 depending on the surrounding circuit.
Other component changes will be natural upgrades, for example replacing
carbon film resistors with metal film resistors giving lower excess noise
and tighter tolerances (better for CMRR on the mic pre-amp for example).
Design changes will be kept to simple changes either to improve the noise
performance, or due to other component changes. For example, reducing resistor
values where possible to reduce Johnson noise, or because of changes in op-amp
type (JFET input to BJT input).
Capacitors are an interesting one. Obviously any ceramics in the audio
path will be replaced by film types (polystyrene or polypropylene or
polyester, in that order of preference). Electrolytics will be refreshed
with brand new Panasonic high-temperature (105°C) low-ESR types, with
appropriate changes to value and/or voltage rating.
Generally pretty good. No sign of rust on the main panel (I've seen some desks on
eBay with varying amounts of rust - not good). The front arm-rest will need some attention,
but that's removable and should be quite easy to treat any rust and respray.
The original mixer has the same colour slider caps for all sliders, in this case black
with a greyish-white line. However there are actually four different slider functions:
To spruce up the desk I'm replacing them with a brand new set of Rean slider
caps bought from Farnell. The new colour scheme will be:
- Input with tape insert (ch.1-16)
- Input without tape insert (ch.17-24)
- Master Left and Right
- Group faders (1-8)
|Input with tape insert (x16) ||Black + white line|
|Input without tape insert (x8)||Grey + black line|
|Master Left/Right (x2) ||Red + white line|
|Group faders (x8) ||White + black line|
These new caps are shallower than the original ones, the result being they are quite
high up above the panel surface. So need to look into some sort of spacer to go
between the slider and the panel to reduce this down to, say, 1mm.
There are some missing knobs, and some missing knob caps. If exact replacements are not
available from AHB then the only option is to fit a completely new set, of a style
in keeping with the rest of the board.
Check, and replace if necessary. Might go for Switchcraft or Neutrik if they
By all accounts the original power supply was a bit under-rated. According
to the schematic it provided ±15V @ 1A and +48V at 100mA or so. Mine
was missing, so a new one is required.
I have sourced an Allen & Heath RPS11
linear power supply unit in need of fixing from the usual place.
It can provide up to 5A per rail according to the
spec sheet. A high-current connector is on the back, and after much hunting around
the net and examining the connector on my PSU I have found the matching
cable-mounted connector is a Hirose JR21PK-10P(71) 10-way plug (Mouser order code
current price £13.28 + VAT as of 15th July 2017). All I need to do is
assemble a suitably beefy power cable to go from the PSU to the mixer with the Hirose
connector on one end and a 5-pin XLR socket on the other.
After fixing it up (one of the main bridge rectifiers had gone bad) and replacing
all the large bulging electrolytics, I decided to fit chrome handles to the front:
It is a bit rusty, but considering what it cost me to buy plus parts I don't mind a
bit of surface rust! And it is just cosmetic--the internals are all fine.
For anyone contemplating working on an RPS11 I would point out that from what I can
see the only difference between this and the RPS10 is the splitting of the output
transistors onto separate heatsinks on the back, and the change to a toroidal transformer.
You can find the RPS10 schematics on the
Allen and Heath website, but as the RPS11 is still an active product they haven't released
To bring the power into the mixing desk I have also made up a suitable cable. For those
playing along at home, I used a length of LAPP KABEL Olflex 150 18SWG 5-core, with a suitable
5-pin XLR at one end and the aforementioend HRS connector at the other. Note that for the HRS
connector I also used a Hellerman rubber strain relief boot to support the cable at the rear
of the plug. On the XLR end it is already a super-tight fit.
The connections between the two connectors are as follows:
|XLR Connector||HRS Connector|
|1 (-15V)||3 (-16V)|
|2 (0V)||5 (Audio 0V)|
|3 (0V)||4 (Chassis 0V)|
|4 (+15V)||1 (+16V)|
|5 (+48V)||10 (+48V)|
Note that the circuitry in the mixer (predominantly op-amps) are fine with 16V supplies,
giving a small (+0.5dB) increase in headroom.
There is much experimenting to be done here. I don't want to waste effort on changes that'll
have no beneficial effect on the audio (lower noise, lower distortion). Some changes will be
obvious age-related maintenance fixes (replacing electrolytic caps, replacing pots), while other
changes (I hate the word "upgrade") will be done with some experimentation and testing.
Some changes to definitely consider include:
- Recap with Panasonic FM or FC electrolytics, Wima FKP or MKS for smaller values.
- Reduce resistor values where I can to reduce Johnson noise.
- Check all pots and sliders, replacing if necessary.
- Replace pre-amp transistors with 2N5087.
Note: changing the fader amp from a TL072 to NE5532 needs to be done with care.
You can't just change the op-amp, for two reasons:
- The NE5532 has a much higher nput bias current than the TL072, which will impose a DC
voltage across the fader, which in turn will result in scratchy noises as the fader is
moved up and down. To fix this a series capacitor from the wiper is needed to block
the DC, and then a high (say, 47k-100k so it doesn't load the wiper too much) bias
resistor from the op-amp input to GND.
(Interesting to note that in the master boards they *did* put DC blocking caps in the
fader amps so it should be a lot easier to do the straight upgrade. Hmmmmm....).
- The other half of the op-amp is used to drive the peak detect circuit. Looking
at the schematic there are quite high resistor values used around it so these need
checking to ensure the peak detect continues to work as intended.
One option is to replace the TL072 with OPA2134, which has FET inputs for low input
bias current, but significantly lower noise and distortion than the vintage TL072.
However this needs some strong evidence to back this theory up considering the cost
of the OPA2134.
The ZTX214 PNP transistors in the pre-amp have a quoted noise figure (NF) of 10dB.
Not great, and you can see they tried to improve on that by paralleling two transistors
on each side, improving the SNR by 3dB. However, I am going to try the 2N5087, which
appears to be a drop-in replacement, and with a NF of only 2dB, giving an 8dB improvement!
See input channels.
See input channels.
Bus Out & Monitor
See input channels.
All those meters! 18 of them!! And each one has an incandescent bulb to
provide illumination. And they blow. And they're fiddly to replace.
After discussing this idea with Tony Allgood of
I'm going to experiment with keeping the incandescent lamps and add a
soft-start controller and also run them at about 70% of their rated current.
This should extend their operating life by several times, so that in theory
they won't need replacing so often, if at all. I might also make the brightness
manually adjustable (perhaps a small pot on the rear of the meter pod) so it
can be adjusted to suit the light levels in the studio.
What Has Changed Between MkII and MkIII?
It is reasonable to assume that the circuit changes made to the System 8 MkIII
were to improve the performance of the desk. I think we can ignore any effect on the
audio of changing the colour scheme to charcoal grey with grapefruit yellow and dawn grey.
The MkIII Service Handbook summarises the improvements as below:
- Cosmetic changes. Panel colour is now charcoal grey with grapefruit
yellow and dawn grey legend. Knob colour is grey with red, grey and blue knob
caps. The side cheeks are black.
- Fader reverse switch added to each of the eight groups to exchange monitor
level control with group fader.
- Balanced output option available for the stereo and group outputs. May be
retro-fitted to the mixer.
- Microphone gain is increased by 6dB to allow a maximum overall gain of +74dB.
- Noise performance of the stereo, group and auxiliary circuits is improved.
- Switch type is changed providing smoother mechanical performance and
- Revised Group PCB assembly design replaced the two Group assemblies used
- General specifications and performance other than the details above are
- Owner Handbook Mk 3 and Service Handbook Mk 3 are now in use.
I have highlighted the changes that are of most interest. Fortunately there are only
four changes, so it should not be too much a task to reverse-engineer a set of change notes
for each block.
As already noted the only change that should be here is the increase in the mic gain.
The change box on the schematic confirms this: "UPDATE TO MK3 INCREASED MIC GAIN".
The specific differences between the two circuits are:
- R9 changed from 2k2 to 1k2
- R17 and R20 changed from 22k to 47k
This change is easy to understand: the pre-amp gain is increased by the claimed amount by
the changes to R17 and R20,
while R9 is adjusted to reduce the line input to the pre-amp to compensate for the increased gain.
Now while it may be a good idea for microphones, it is rather unfortunate to attenuate-then-amplify
the line-level inputs, since that will worsen its signal-to-noise ratio due to the additional noise
of the pre-amp.
Master Left & Right
On the LEFT board the following changes were found:
- C13 changed from 47u/25V to 100u/25V
- R26 (1k2) replaced with a link
On the RIGHT board:
- C1 changed from 47u/25V to 100u/25V
- R3 (1k2) replaced with a link
On the face of it these are reasonable changes: increasing the capacitors will improve
the low-frequency performance, and removing the resistors will remove one source of
Bus Out / Group Monitor
This is by far the largest change. On desks with sixteen tape inputs there are sixteen single-channel
input boards, one per channel. This is good as it means the smaller desks (128, 164, 168) only have the
four or eight PCBs they need. In the MkIII series they removed these smaller desks and focused on the larger
desks (1616 and 2416). Now that all desks have sixteen tape inputs they combined two boards
into one, which would have been cheaper to manufacture and assemble into the frame.
Ok, but what of the circuits? Have there been any changes? These are the component
changes I have found (excluding the reverse switch):
- C4 reduced from 47u/25V down to 10u/16V
- The 1k2 resistor on the +ve input to the tape input buffer (R20 in the MkII) removed
- LEVEL pot reduced from 47k to 10k.
Don't understand the first change -- it raises the lower -3dB point, cutting out some bass.
Could be a BOM optimisation?
The second change -- removing that resistor -- is a definite improvement (less thermal noise).
And the third change is understandable as a consequence
of adding the FADER REVERSE switch so that now both the LEVEL pot and the fader are the same impedance.
Note also that the level pot on the un-reversable channel is also now 10k to be consistent.
Analysis of Signal Path
A quick analysis in SPICE of the main function blocks shows some interesting
frequency response information.
|Block ||-3dB Frequency|
|Stereo Out ||158.6kHz|
|Bus Out ||113.1kHz|
|Auxiliary Out|| 94.6kHz|
Note: These figures assume that all 22p capacitors are fitted to the op-amp feedback resistors.
Combining the above blocks into the three main paths we get the following frequency responses:
|Path ||-3dB Frequency|
|Channel Input to Stereo Output||91.5kHz|
|Channel Input to Bus Out ||79.2kHz|
|Channel Input to Auxiliary Out||71.4kHz|
Where To Get Parts
One nice thing about old technology is that it is relatively easy to get replacement
parts, either brand new, or scrounged from other places. In this case pretty much
everything can be found new.
All resistors, capacitors, diodes, transistors and ICs can be found new from all the
main component distributors. Here in the UK we have
Rapid (UK only),
RS (Allied in the US) and
Farnell (Element14) as the main
The pots used in the System 8 as per the Mk II service manual:
|Section||Function||Value||AHB Part No.|
||Gain ||10kC + bkt||AI0049|
|EQ HF ||100kA ||AI0047|
|EQ LF ||100kA ||AI0047|
|EQ Mid ||100kA ||AI0047|
|EQ Mid Freq||100kC dual||AI0048|
|Aux 1 Send ||47kB ||AI0050|
|Aux 2 Send ||47kB ||AI0050|
|Aux 3 Send ||47kB ||AI0050|
|Pan ||10kA + bkt||AI0046|
||Level ||10kB ||AI0043|
|Aux 1 Send ||47kB ||AI0050|
|Aux 2/3 Send||47kB ||AI0050|
|Pan ||10kA + bkt||AI0046|
||Aux 1 Master ||47kB + bkt||AI0045|
|Aux 2 Master ||47kB ||AI0050|
|Aux 3 Master ||47kB ||AI0050|
|Cue ||10kB dual ||AI0044|
|ER1 Level ||47kB ||AI0050|
|ER1 Aux 1 Send||47kB ||AI0050|
|ER1 Aux 2 Send||47kB ||AI0050|
|ER1 Pan ||10kA + bkt||AI0046|
||Talkback Level ||47kB + bkt||AI0045|
|Headphone Level||10kB dual ||AI0044|
|Monitor Level ||10kB dual ||AI0044|
|ER2 Level ||47kB ||AI0050|
|ER2 Aux 1 Send ||47kB ||AI0050|
|ER2 Aux 2 Send ||47kB ||AI0050|
|ER2 Pan ||10kA + bkt||AI0046|
There are eight different pot types used in this desk.
They can be a bit tricky to find as you really need to replace them
with the same make due to variations in mechanical dimensions.
Audio Maintenance stock most of the values, but without brackets.
The more esoteric pots with brackets are special orders direct from
OMEG who still manufacture them.
|AHB Part No.||Value||Maplin Order Code||AM Order Code|
|AI0043||10kB ||FW22Y |
|AI0044||10kB x2 ||FX09K |
|AI0045||47kB bkt|| |
|AI0046||10kA bkt|| |
|AI0047||100kA ||FW05F |
|AI0048||100kC x2|| ||OM-01-072|
|AI0049||10kC bkt|| |
|AI0050||47kB ||FW24B |
The pots used by AHB use the following notation:
|bkt||With support bracket|
Note: This is
the "British" notation, whereas Alps swap the meaning of "A" and "B"
(which is actually easier to remember: "A for Audio").
Original Alps audio-taper K faders are available from
The switches are all standard Alps SPUN series 2- and 4-pole push switches.
The tricky bit is that they are mounted in metal brackets to provide mechanical support.
So replacing them involves carefully prising apart the metal fingers, replacing
the switch, then pushing back the metal fingers to hold the switch in place.
So far the only vaguely tricky part to find is the PFL relay. The original
NF2-12 is no longer made, but a compatible replacement NFEB-12V is still available from
Farnell, code 1200946.
The SQ-10 meter is still being made by Anders, and is stocked by both
RS (code 220-368) and
Farnell (code 7758219).
Again, all standard parts so there should be no problems finding replacements.
Knobs, Buttons, Fader Caps
The lovely brown knobs are non-standard parts it seems these days. So your
best bet is either find someone who is parting out an old System8 (see below),
or get a similar vintage AHB mixer yourself as a parts donor. Same for the
button caps and switch caps.
The faders have 8mm tabs, and you can still find supplies of suitable fader caps
The following people are breaking System 8's for parts. If you need original parts for your System 8 then you could contact them.
NOTE: These people are listed here as an aide, I offer no guarantees of any sort. Any trade happens between you and them, nothing to do with me.
Famous System 8 Users
The System 8 mixers have been around for many years, and were there
at a critical time in British pop. I'm slowly gathering together snippets of
hints at artists that have or still use System 8's in their studios and
If you have any additional information, or corrections to the above,
please contact me.
This is a heavy mixer! While being a large desk it did not come
with a stand as standard, so for a while I was planning all sorts of design ideas
involving metal and/or wood. But time is precious so I had an idea to turn to the
Ikea catalogue. And lo and behold they have exactly the right size of
metal desk frame to make a perfect stand for this 2416. So without further ado
I give you BEKANT (Article number 302.553.25):
(Click on the picture to go the product page)
For £50 I couldn't get the raw materials for less, plus my time, so definitely
a great deal. Only assemble the metal frame
together (you don't need the plastic bits or the cable tray), and I recommend
putting some rubber antislip pads on the top of the
frame to stop the mixer sliding off in moments of enthusiasm!
Important: Adjust the frame height before loading the mixer - once it is
on the frame you won't be able to adjust the height as the mixer is much too heavy!
Once all done it
looks very professional: