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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.

Brown....yummy!

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

Specifications


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):MicLine
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

EQUALISER CHARACTERISTIC:

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)):

 2416
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.

DIMENSIONS:

Model 2416
Width 1350 mm (53")
Height 245 mm (9 ¾")
Depth 672 mm (26 ½")
Weight 64 kg (141 lb)

Notes:

  1. 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.
  2. The spec quotes the LF EQ as being 120Hz and 80Hz, yet the schematics and the front panel text both say 120Hz and 60Hz.

Documents


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.

Philosophy

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.

There is also the idea of placing smaller-valued film capacitors in parallel with the electrolytics to deal with the high internal inductance of the electrolytics which will otherwise impart an HF rolloff. This is to be the subject of some experimentation.

Cosmetic Condition


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.

Slider Caps

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:
  • Input with tape insert (ch.1-16)
  • Input without tape insert (ch.17-24)
  • Master Left and Right
  • Group faders (1-8)
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:
Function Colour
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.

Knobs

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.

Connectors


Check, and replace if necessary. Might go for Switchcraft or Neutrik if they fit.

Power Supply


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.

Instead 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 798-JR21PK10P71, 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.

Channel Strips


  • Recap, recap, recap - Panasonic FM or FC electrolytics, Wima FKP or MKS for smaller values.
  • Change all existing TL072 op-amps for OPA2134.
  • Change all resistors to 1% 0.5W metal film.
  • 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:

  1. The NE5532 has a much higher 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....).
  2. 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.

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!

Master Left


  • Recap, recap, recap.
  • Fit NE5532 in place of IC5, IC6 and IC7.
  • Fit OPA2134 in place of IC1, IC2 and IC3.
  • Change resistors.
  • Reduce resistor values where I can to reduce Johnson noise.
  • Check pots and sliders.

Master Right


  • Recap, recap, recap.
  • Fit NE5532 in place of IC2, IC5 and IC8.
  • Change resistors.
  • Reduce resistor values where I can to reduce Johnson noise.
  • Check pots and sliders.

Bus Out & Monitor


  • Recap, recap, recap.
  • Fit NE5532 in place of TL072. Add capacitor on fader wiper to block bias current.
  • Change resistors.
  • Reduce resistor values where I can to reduce Johnson noise.
  • Check pots and sliders.

Meters


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.

So, plan is to make an LED replacement module. Should last a lot longer.

After discussing this idea with Tony Allgood of Oakley Sound 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. Nice.

The MkIII Service Handbook summarises the improvements as below:

  1. 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.
     
  2. Fader reverse switch added to each of the eight groups to exchange monitor level control with group fader.
     
  3. Balanced output option available for the stereo and group outputs. May be retro-fitted to the mixer.
     
  4. Microphone gain is increased by 6dB to allow a maximum overall gain of +74dB.
     
  5. Noise performance of the stereo, group and auxiliary circuits is improved.
     
  6. Switch type is changed providing smoother mechanical performance and improved serviceability.
     
  7. Revised Group PCB assembly design replaced the two Group assemblies used previously.
     
  8. General specifications and performance other than the details above are unaltered.
     
  9. 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.

Channel


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 thermal noise.

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
Channel Input123.3kHz
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 Output91.5kHz
Channel Input to Bus Out 79.2kHz
Channel Input to Auxiliary Out71.4kHz

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.

Components

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 contenders.

Potentiometers (Pots)

The pots used in the System 8 as per the Mk II service manual:

SectionFunctionValueAHB Part No.
Input Gain 10kC + bktAI0049
EQ HF 100kA AI0047
EQ LF 100kA AI0047
EQ Mid 100kA AI0047
EQ Mid Freq100kC dualAI0048
Aux 1 Send 47kB AI0050
Aux 2 Send 47kB AI0050
Aux 3 Send 47kB AI0050
Pan 10kA + bktAI0046
Output Level 10kB AI0043
Aux 1 Send 47kB AI0050
Aux 2/3 Send47kB AI0050
Pan 10kA + bktAI0046
Master Left Aux 1 Master 47kB + bktAI0045
Aux 2 Master 47kB AI0050
Aux 3 Master 47kB AI0050
Cue 10kB dual AI0044
ER1 Level 47kB AI0050
ER1 Aux 1 Send47kB AI0050
ER1 Aux 2 Send47kB AI0050
ER1 Pan 10kA + bktAI0046
Master Right Talkback Level 47kB + bktAI0045
Headphone Level10kB 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 + bktAI0046

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. Maplin and 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.ValueMaplin Order CodeAM Order Code
AI004310kB FW22Y
AI004410kB x2 FX09K
AI004547kB bkt
AI004610kA bkt
AI0047100kA FW05F
AI0048100kC x2 OM-01-072
AI004910kC bkt
AI005047kB FW24B

The pots used by AHB use the following notation:

ALinear
BLog
CReverse Log
bktWith support bracket
x2Dual

Note: This is the "British" notation, whereas Alps swap the meaning of "A" and "B" (which is actually easier to remember: "A for Audio").

Faders

Original Alps audio-taper K faders are available from Audio Maintenance

Switches

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.

Meters

The SQ-10 meter is still being made by Anders, and is stocked by both RS (code 220-368) and Farnell (code 7758219).

Connectors

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 (see above).


Breaking Mixers


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 productions.

If you have any additional information, or corrections to the above, please contact me.


Progress


Ok, so I now have a dedicated (well, half of it is mine) studio space so time to crack on with this beauty.

First item to address is how to mount it. While 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:


Copyright © 2001-2017 Neil Johnson