The Ins and the Outs

One of the perennial problems facing a mixer designer is how many pins to use on a module connector and what to assign to them. You could use a really large connector with many pins but this will add unnecessary cost to many smaller projects. Alternatively, you could use one standard connector and an additional one only for more complex modules. The only downside of this method is you may need two motherboards for the complex modules.

To get a better handle on the problem, let's look at the existing Eurochannel 32 way connector pin assignment.

This is the original hand drawn schematic from 2012 that defined the 32 way connector pinout. The first four pins are assigned to the mic input and the second four to the line input. Notice how the screen of each input uses two pins. The reason for this is that, at the time, I was not using a backplane PCB for the modules to plug into. Instead I was using regular connectors and wiring the backplane by hand and I found it very awkward to wire a mic cable screen to a single pin on the connector. It was much easier if I used two adjacent pins.

Next are the 48V supply and the chassis connection (which is also the 0V of the 48V). Both these use two pins each simply because they are buses and it makes hand wiring a backplane easier.

Next we have the unbalanced OUT1 and its associated 0V closely followed by the FG and FS pins. OUT1 is the output of the first amplifier in the Eurochannel design. It is often fed to an external fader and returned to the module; this is the purpose of the FS (fader slider) and FG (fader ground) pins. Not all modules or applications use this feature but it is included for those that do.

Next is a pair of pins for relay power. Most modules will need some form of auxiliary power for LEDS or relays so these two pins are often used.

Then we have four pins assigned to buses. In many cases this is too few. It allows for a stereo bus and a couple of AUX sends but this means there are none left for a solo or PFL  audio bus and its associated dc bus. We could really do with twice as many bus pins.

This if followed by the main unbalanced output OUT2 and its 0V pin.

Lastly are the power pins each of which uses two pins. The HT needs tow pins so we can ensure there is always a suitable gap between these two. The heater supply needs two pins so we can easily bus the relatively high heater current along the backplane.

As you can see, this particular pin out is determined by a variety of factors, some relevant to the module and some relevant to the process of building a mixer. It has some limitations, particularly in the number of buses and there is no provision for balanced outputs simply because in the Eurochannel design they were external.  There are also some non-optimum assignments from the point of view of PCB layout. For example, all the power supplies are at one end of the connector. From the point of view of PCB layout it would be better if they were near the middle of the connector. Despite its peculiarities, this bus standard has served well for over five years.

The new 6U modules being designed for the Mark 3 include output transformers so they need a way for these to be connected to the outside world. The 6U size board allows a second 32 way connector to be used. This can provide balanced outputs and additional buses but a lot of its pins are not really needed and this solution only works for 6U modules. It is no good for 3U modules.

Holger Classen has also been working on new versions of Eurochannel modules. His approach has been to retain the 3U module size but to make the module deeper. This allows output transformers to be fitted onto the PCB. In this way he can fit two complete mic pres into a single 3U module. He cannot add another 32 way connector so he proposed a new assignment of the existing 32 pins to allow for two balanced outputs. We have discussed this at length and made a few modifications until we were happy the new assignment would work for us both. Here is the new assignment of pins:

Notice first that the way each pin is referenced has changed. This reflects the actual row and column in which the pin is located. It also makes it clear that a three row connector is used but pins are only fitted in rows a and c. This implies a 0.2inch spacing between rows. Also, only even numbered pins are fitted which again indicates a 0.2inch pin spacing.

At the top is the first balanced input and its associated balanced output. Note that only the input has a shield. The balanced output output does not really need a shield pin (its cable screen can be connected at the XLR end) This pinout is duplicated at the bottom of the connector for the second balanced input and output. A chassis pin is located at both ends of the connector.

All the power pins are located about the centre of the connector. This time the 48V and its ground have only a single pin each, just like the utility power (which replaces the old relay power). Heaters and HT supply all have two pins each for the same reasons they did in the original. HT 0V is renamed AGND to more accurately reflect its purpose.

We now have a total of eight unassigned pins, and they are genuinely unassigned. Their purpose will be determined by the design of the backplane PCB. This provides a great deal of flexibility for both Holger and myself (Holger already makes his own backplane PCBs) but all backplane PCBs will have this core pinout. So the supplies and the chassis connections will be bused on all backplane PCBs. In 1 and In 2 will both be brought out to 3 pin Molex connectors in the same way as the mic and line inputs are at present. Out1 and Out 2 will both be brought out to 2 way Molex connectors just as Out1 and Out 2 are at present.

All other pins on the backplane PCB are assignable on a design by design basis. One new version of backplane PCB will bus pins 24a, 24c, 26a and 26c for use as mix buses. Pins 8a and 8c will be brought out to a 2 pin Molex as wil pins 10a and 10c. In many mixers these can be used for external unbalanced connections to faders or EQ. This version of back-lane PCB thus emulates the original Eurochannel backplane PCB. The designs of existing PCBs (Eurochannel, Twin Line Amp, Classic, EQ with gain make up etc) will be migrated to be compatible with this backplane PCB.

All new designs will adopt the new pinout and new backplane PCBs will be designed as and when the need arises.

Universal Vertical Tube Preamp (UniVert)

In the previous post I mentioned the first 1.4 inch wide (7HP) module I designed was based on the two tube Classic design. In fact it is a bit more than that. Rather than just repackage the Classic design I thought I would try to broaden its applications. The Classic is basically two identical gain stages. They are normally configured as mu followers for high gain and low distortion, but they can be configured as SRPPs for better drive capability. They are normally simply cascaded with a gain pot between them but there is no reason some overall negative feedback (NFB) cannot be used. This would make them similar to the Eurochannel and Twin Line Amp boards and they could be used in the same kinds of applications. The two stages of the Classic do not provide quite as much open loop gain as the Eurochannel circuit so either the maximum gain must be reduced or slightly higher levels of distortion must be tolerated.  Because this makes it  more universally aplicable I decided to call it the UniVert. Here is its schematic:

V2, the output stage, is configured as a mu follower using a 6922 tube. For better drive capability at the expense of higher distortion, it can be configured as an SRPP stage.

V1, the input stage, is also configured as a mu follower but with a few extras. The key additions are R6 and R18. R6 allows the cathode of V1B to be raised high enough that R18 can provide NFB right down to dc, just as in the Eurochannel, to ensure stability at all gain settings. The closed loop gain is set by R19.

I have not built this configured with NFB but I have simulated it and it does perform well. The first one I built was a straight pair of mu follower stages. Here is the schematic for this with component values added:

As you can see, the NFB network is not used. The 220 ohm cathode resistors bias the mu followers at about 12mA each which is necessary for them to be able to drive a 600 ohm load via a 2:1 step down transformer. The first stage could be biased at a lower current because it usually only has to drive a 10K gain pot. Here is a pic of the PCB:

The PCB has all the usual EZTubeMixer fixtures and fittings including the 32 way DIN connector, mic and line inputs, phantom power, external fader connection, buses and provision for fitting an input transformer. Notice the two tubes are at the top so their heat does not flow past any passive components. This is not always the case in the Eurochannel and Twin Line Amp PCBs. It is made easier of course by having only two tubes. I am still not certain that three tubes could be squeezed into this size PCB but I am pretty certain they would fit in a 6U version, but that is something for later.

 In the centre you can see  the tracking for the pair of 9 way right angled headers that connect the tube mounting PCB to this PCB. And that is the reason this is the V2 PCB. In the first version I managed to get this connector mirror imaged! At least the boards could be used for mechanical mock ups. Talking of mechanics, you will notice there is quite a bit of bare PCB on the left which will be behind the front panel. This is to leave sufficient space for the new front panel mounted controls PCBs which give maximum flexibility in positioning front panels controls. Also, the 100uF 250V capacitor used to couple the gain setting resistor is too tall to be mounted normally so it has to be laid on its side. I could find only one source of a 100uF 250V electrolytic capacitor so I decided to stick with radial leaded types.

In performance terms it seems to be virtually identical to the original 14HP wide version. However, I do need to operate this version ,without the bottom screen, side by side with the  14HP Fischer module version, to measure exactly how good the new mechanicl system screens the electronics.

Fat ones, thin ones, tall ones, short ones - Module Mechanics Revisited

At the start of this blog I mentioned the reasons for using a module enclosure, particularly the mechanical integrity and screening properties and also that the Fischer modules were reasonably priced. Except they are no longer so cheap. SInce I started using them the price has gone up by 40 percent. In addition, after a couple of years experience using them, their shortcomings are beginning to become noticeable.

Tube modules tend to be be quite large to make enough room for the tubes. In turn, this means tube mixers tend to be pretty big (Holgers 12 channel one is one metre wide). To put this in perspective, the modules are nearly twice as fat as 500 series modules. So I have been spending some time looking at ways to squeeze more functionality into a given space and also how to squeeze existing functionality into a smaller space. Put another way, I can get just six tube modules into a 19 inch rack space but you can get eleven 500 series modules in the same space.

The first option I looked at was stereo or two channel modules. These are the same width as the existing modules and fully compatible with the standard mechanics and motherboard interface, but just have two channels of electronics in them rather than one (these were mentioned earlier in the Mark 3 6U modules discussion). They are line level only modules so the active electronics is essentially the Twin Line Amp design. Many people now seen to be moving to line level only mixers and use external preamps for tracking either direct or via the line level mixer. This basically splits the functions of a classic mixer in two. From my point of view, the advantage is I can now get twelve channels in a 19 inch rack space instead of just six. The 8 tracker becomes a 16 tracker overnight.

However, the Fischer modules are beginning to become a limiting factor. The type T 6U module I use places the top surface of the PCB 14.2mm from the left hand edge of the front panel. This relatively large offset makes front panel design more difficult for a two channel module. The controls of the left half are 14.2mm plus their height from the left edge of the front panel. For the twin channel module to be symmetrical about the centre line, the right half controls must be the same distance from the right side of the front panel. So the controls are pushed 14,2 x 2 0 28.4mm towards the centre making the centre look cramped and the edges bare. Maybe a different mechanical scheme could reduce the 14.2mm offset sufficiently to fis these limitations.

These modules operate at line levels so screening is not so critical as it is for microphone preamplifiers. Maybe a simpler mechanical scheme can be used that will be more cost effective?

The second option I looked at was a making modules half the existing width, that is 1.4 inches wide instead of 2.8 inches. This means the tubes cannot be mounted directly on the PCB as they are at present. Instead they need to be mounted vertically which means they use up more PCB space. It turns out there is probably not enough space to house the three tubes used in the Eurochannel mic pre or the Twin Line Amp plus the input transformer and all the required passive components, but there is room for the two tubes used in the Classic mic pre design. So this was the first PCB I laid out after first designing an adaptor PCB to allow the tubes to be mounted vertically.

At this point it became clear that 1.4 inch wide Fischer modules, although available, were not going to have enough room. Most models of Fischer modules are quite wasteful of width. For example, the T types used for 6U modules mentioned earlier, where the top surface of the PCB 14.2mm from the left hand edge of the front panel. In a 1.4 inch wide module we only have just over 35mm to play with. The tubes are 22mm in diameter and if we lose another 14.2mm due to the module type we have already used more than 36mm. There was only one Fischer module that wasted much less width. This was Design I illustrated below:

This version only loses 6mm leaving an available space of 29.56mm above the PCB which is plenty for the tube. The module kit consists of:

• A plastic insulating panel that fits to the rear of the PCB (good idea)
• A standard blank front panel
• A piece of bent aluminium that forms the screen
• Four steel standoffs

Considering it includes a blank front panel I don't need (all my front panels are custom made), a piece of bent aluminium, four standoffs and a bit of plastic, it is not very cost effective. I did ask but Fischer will not sell any of the component parts separately.

In the meantime I decided to try a simple construction of my own. This starts with the original Eurocard method of fixing the front panel to the PCB which makes the top surface of the PCB just under 4mm from the left hand edge of the front panels (10mm better than current modules). Here is a drawing that shows how this works:

A neat little die casting is used. The PCB is fixed by a screw into a tapped hole in the die casting and the front panel is similarly fixed using another tapping. The result is the top surface of the PCB is just 3.97mm from the left hand edge of the front panel. Here is the fixing in detail:

It occurred to me that by using a longer screw we could fix a tapped standoff to the die casting and use this to attach a side plate at the right of the module. At the back of the PCB is the 32 way connector which is attached to the PCB in a similar manner. Again using a longer screw we could attach another tapped standoff to attach the rear of the other side plate. Here is a picture of the basic assembly:

This early version also includes a screen on the left side. This is held 3mm away from the bottom of the PCB by spacers. The screen is made from 1mm mild steel and is very rigid. A single long screw goes through the left screen, through the spacer and the PCB, through the threaded hole in the die casting and into the threaded standoff. At the back the stand off is fitted similarly using the 32 way connector fittings. On the right is the right side screen, also made from 1mm steel and held on by screws into the tapped stand off. The whole assembly is very rigid and strong, even though the front panel has not been fitted.

This then needed to be mated to the new PCB using vertical tubes, shown here:

You can see the two small boards on which the tubes are mounted. These are fixed to the main PCB using right angled headers. When the mechanics and PCB are mated they look like this:

The up side of this scheme is it is easy and cheap to build. It is also very strong so there is no likelihood of front panel controls being damaged by flexing of the PCB as in the EZTubeMixer design. The down side is there is only screening on three of the six sides of the module. However, the sub-racks I use have top and bottom screens fitted so the only side of the module not screened is the back where the 32 way connector is.

This scheme is easily extended to wider or taller modules. Wider modules simply need taller standoffs. Taller modules simply need larger sheet steel screens. The only concern might be whether the larger steel screen flexes sufficiently in the centre of the screen to be a problem. If it is, another standoff could be used.

In practice we can probably eliminate the screen beneath the PCB. The left most PCB in a sub rack has its PCB right beside the aluminium side of the sub rack. Its right side is screened by its right  hand steel screen. But this also screens the bottom of the PCB of the module to its right and so on up to the other end of the sub rack. This may seem like a further compromise but it is a screening scheme I have seen recently in two professional broadcast mixers, one made by Glendale and one made by EELA, both for BBC local radio. If it is good enough for the BBC it is good enough for me.

The mechanical design could be refined further. For example, the the standoffs at the front could be eliminated if another pair of diecastings were used to mount the screen to the front panel on the right. This may or may not be an advantage. The scheme with stand offs does mean you can test a module without the front panel but it is not clear if this is an advantage or not. The general idea though is definitely worth exploring further.

25mm Separation

In the post before last I suggested using 25mm separation might work better than 20mm or 30mm both of which are not ideal. I finally found time to try this out. Here is a pic of the result:

The top switch now appears to be in a better position. I checked the distance from the top surface of the top PCB to the underside of the enclosure and it is pretty close to 25mm so the space above the top PCB is the same as the space above the bottom one. So far so good. I also checked the clearance above the inductors on the bottom PCB and and there is at least 4mm clearance (the top will be the same). So all round, in mechanical terms, it all seems to fit.

What about front panel layout?  To make this easier to explain let's rotate the module into is normal orientation:

The separation between the two sets of switches is now 25mm plus a board thickness, which is a total of 26.6mm. We already know the spindle of the left hand set of switches is 23.55mm from the left edge of the panel. The right hand set is 26.6mm from the left hand row so the right hand set are 23.55mm plus 26.6mm from the the left of the panel which is 50.15mm. The panel is 70.9mm wide so the right hand set is 70.9 less 50.15 from the right edge of the  panel, which is 20.75mm.

So the left hand switches are 2.8mm further away from their panel edge than the right hand ones. Possibly just enough to notice. I think we could spare 1mm less separation between the boards (24mm instead of 25mm) without compromising the headroom above components. This would reduce the difference to 1.8mm. I am still not sure how this would look so I will create some front panel layouts so I can judge them visually. An alternative would be to make a feature of the 2.8mm offset perhaps by running  vertical line down the left hand side of the front panel or including a long skinny graphic with some words of wisdom or marketing in them???

Fortunately both 24mm and 25mm standoffs are available.

General Standardisation or why can't I just use one set of pots, toggles, switches and pushbuttons

As I hinted a in the last post, a perennial problem is managing small variations on a theme, usually as a result of a customer requirement. At Neve back in the 70s ,we had this down pat - we had a separate department for it called the Module Group. The basic philosophy was no controls were mounted on the PCBs. They were mounted on a steel plate that sat behind the front panel. Pots and switches, toggles and push buttons were all mounted this way and then hard wired to the rest of the electronics. This is incredibly flexible but also incredibly expensive and time consuming but was typical of the 'instrumentation build' mentality of the day. I went some way towards this with the EZTubeMixer project by fixing some controls to the front panel and allowing others to be mounted anywhere but this still led to a lot of potentially error prone internal wiring and it did not look very neat either.

My problems are similar to those of Neve but different in important ways. Most customers are very cost conscious so, for example, they would change a Grayhill stepped gain switch for a REV LOG pot. Some like push buttons, others like toggles. Some want to improve on the Grayhill switches and fit ELMAs instead. This means that even supposedly standard modules are likely to require changes and I don't really want to lay out large 6U PCBs every time this happens.

Part of the problem is mounting components directly to the main PCB. Many mixer manufacturers went as far as to mount all the controls of a module on the PCB and interface this directly to the front panel. To allow for partial customisation they resorted to links on the PCB to allow AUXes for example to be set pre or post fader but that was the limit of customisation. The other problem with this approach is that the controls are now now right angle types and the height of their shafts varies a lot. This means they no longer line up and the front panel and looks a lot less neat. A possible solution to this problem is for the main board to be pretty generic with few if any built in controls (except perhaps for the EQ). Customisation is then done by further PCBs containing the required right angles components. In this way you can more or less get them in line, but you are still left with some differing shaft heights which is a real  pain when it comes to designing stereo modules.

The only real solution to getting all the shafts in line is to use vertical mounting controls fitted to a PCB that runs parallel to the front panel. That way the control's shaft can be placed almost anywhere you like. This sounds like a great solution but in practice all it does is shift the problem. You now have to find a range of vertical mounting components that are all the same height. This had me stumped for a long time because there are plenty of vertical components available but finding ones of the right quality with the same height seemed an impossible task, and even if they were available, the sources were often obscure or involved purchasing lots of 1000 units. I managed to do it in a small way when the number of types of components was limited such as in the case of the 4toggles PCB. This little board is used with the Classic Solo design and holds four toggle switches which provide phantom, phase, 20dB pad and mic/line functions. The board is mounted directly to the front panel using the switches themselves, it has mic and line inputs and a single output all on Molex KK connectors.

The challenge is to extend the idea of the 4toggles board to more general interfaces like pan and AUX controls for example. Ideally we need the following controls available in a vertical PCB mounting format with identical heights:

• Rotary pot
• Toggle switch
• Push button
• Rotary switch

and the components all need to be of sufficient quality for them to be used in a pro audio mixer. It would also be nice if there was more than one source of each type.

An Answer

After a lot of research I finally came up with a set of readily available controls that appear to meet all the criteria:

Rotary Pot

I have chosen 9mm vertical pots made by ALPHA:

As you can see, the top surface of the pot is 10mm above the PCB surface. The threaded portion is 5mm deep so it will comfortably fit through a normal 2.5mm front panel leaving 2.5mm for the fixing nut (which is usually about 2.2mm thick). I chose these pots for several reasons:

• ALPHA have an excellent record of reliability in pro audio systems
• The range of values (including REV LOG) is ideal for audio use
• They are available from suppliers on both sides of the Atlantic
• They are available in round or D-shaped shaft styles which widens the choice of knobs.

Toggle Switch

The toggle switches chosen are the ones already in use in the 4toggles PCB:

The top of the toggle switch is only 8.64mm above the PCB but these switches are usually supplied with two nuts so one of them can be used under the front panel to bring the overall height to 10mm. Allowing for the lower nut and a 2.5mm panel thickness there will be just over 2mm of thread protruding above the front panel is is plenty for the small diameter nuts used with this switch. I chose this toggle switch because:

• They are widely used in pro audio
• They are available just about everywhere
• Several manufacturers make them
• The come in a good variety of options from SPST and DPDT types to oes with centre off or centre active positions

Push Button

The push button was perhaps the hardest component type to source. They are used in huge numbers in low cost mixers and are made by the truck load in China but their suitability for use in pro audio is suspect. The one I have chosen is the SPPH4 series by ALPS:

Once again, the top surface of the switch is only 8.5mm above the PCB surface but this does not matter because this part cannot be fixed to the front panel.Its only shortcoming is it has to be used with other components that attach the PCB to the front panel or else special fixings for the PCB have to be added for this purpose. I chose this push button because:

• ALPS has an excellent reputation
• the switches are rated for 10,000 operations

From the drawing it is clear that with the front panel 10mm above the PCB and a 2.5mm panel thickness, the button shaft protrudes 5.5mm above the panel when the button is not pressed. Most buttons are at least 6.5mm deep so these should recess nicely into the front panel.

Rotary Switch

For once we are spoilt for choice. There are four manufacturers of rotary switches that are mechanically compatible with this system. They are:

Knitter MRS18

As you can see from the above diagram, the top of the switch is 7.5mm above the surface of the PCB. A single nut and washer is enough to pad this out to 10mm. As the threaded shaft is 7mm tall, it will protrude 2mm above a 2.5mm thick panel which is just enough to attach a nut to secure it. It is available in 1 pole 9 way or 2 pole 4 way versions. It has 6mm diameter D type shaft. It does not appear to have an adjustable stop.

ALPHA SR17

This is identical to the Knitter MRS18 and is also available in 1 pole 9 way and 2 pole 4 way versions.

GRAYHILL 56 SERIES

As can be seen from the above diagram. the top of the switch is 9.58mm above the PCB surface. A single washer is enough to pad this out to 10mm. With a 2.5mm thick front panel, the threaded shaft protrudes 3.5mm above the front panel which is plenty to include another washer and a nut. It is available in 1 pole 12 way, 2 pole 6 way and 4 pole 3 way versions. It has adjustable stops and a 1/8th inch diameter shaft.

NKK MRK112

The NKK MRK112 is similar but not identical to the Grayhill 56 series switch.The top of the switch is 10.1mm above the PCB surface. The threaded shaft is 5.5mm tall so it protrudes 3mm above a 2.5mm thick panel which is ample for its fixing nut and a washer.It is available in 1 pole 12 way, 2 pole 6 way and 4 pole 3 way versions. It has a 3mm diameter d type shaft.

Conclusion

The above parts are sufficiently mechanically compatible to consider building small PCBs to hold them on a project by project basis. All are either currently employed in pro audio applications or have a life expectancy compatible with pro audio applications. I plan to emply them first in the 6U modules of the MKIII tube mixer.

6U Modules Part 3 and a hint of General Standardisation

It has been over a year since my last post here but a lot has happened. 6 months of work time was lost whilst I moved house and built and equipped a new workshop. Also,the separate Lunchbox project has really taken off and a version containing four tube mic pres and an integral power supply has proven especially popular. More on that later. The result is the Mark III has not received a lot of attention. Despite this, some progress has been made.

I finally got round to laying out one of the daughter board EQs, the REDD EQ. This turned out to be a lot easier than I expected. It essentially consisted of cutting and pasting the EQ from the main board to the new board.

It has four holes in the same positions as the main board so it can be attached using pillars. The only question now is how tall should the pillars be? At first I attached it to the main board using 20mm spacers as discussed in the original 6U modules post. Although there was a reasonable gap between the switches on the main PCB and the corresponding ones on the daughter board, the large EQ inductor was almost touching the underside of the daughter board. So I tried with 30mm spacers as shown below:

You can just see the large inductor behind the right hand switches. There is plenty of space between its top and the bottom of the daughter board. But the daughter board looks a little close to the module cover. WIll the switches and inductors fit on it?

As you can see, the switch fits comfortably but it is not clear if the inductor will fit. Also the switch looks a little closer to the to the top edge of the module than the bottom switch is to the bottom edge. We know that the bottom switches are:

 14.2 + 9.35 = 23.55mm from the left hand edge of the front panel.

Ideally, the switch on the daughter board should be the same distance from the right hand side of the front panel, and as the front panel is 70.9 mm wide, the daughter board switches need to be :

70.9 -23.55mm = 47.35mm from the left hand edge of the front panel.

Since the main board switches are 23.55mm from the left side of the front panel, the distance between them and the ones on the daughter board is just :

47.35 - 23.55mm = 23.8mm.

Since 1.6mm of this is the daughter board PCB itself, this means the pillars should be:

 23.8 -1.6mm = 22.2mm high.

So 30mm spacers are definitely too big but 20mm are definitely too small. Perhaps 25mm would be a good compromise.

Which brings us on to the other awkward mechanical problem we still have to solve. As you can see in the pictures above, the pan and AUX controls on the main board (red and blue knobs) are also soldered direct to the main PCB. But they are smaller than the Grayhill switches used in the EQ so their centres do not line up with the EQ switches. The original plan was for the EQ daughterboard to extend right across the module and also hold the second set of pan and AUX controls. There are a couple of problems with this. The smaller one is it would be much nicer if the pan/AUX controls lined up with the EQ switches. The bigger problem is that any time a customer wants a different pan/AUX combination we have to design a new main PCB and a new daughter PCB plus we still need to do seprate main board with EQ  for the mono mic channel versions of these modules.. I have already decided to go for a main board per EQ and a daughter board per EQ. Do I really want to redo these every time there is a minor change in routing requirements and do I really want to design another set of PCBs for the mic pre versions? The answer is very definitely NO which is why the daughter board only contains the EQ and does not extend into the pan/AUX area.

What we really need is some flexible means of adding any combination of mono/stereo pan/AUX mic/line controls in addition to the EQ. The obvious solution to this problem is to have a specific small PCB holding these controls that fits parallel to the front panel rather than at right angles to it as is the case for the main and daughter boards. This again provides the flexibility we had in the EZTubeMixer design but in a much neater fashion. The really big problem with this solution is finding a set of pots, push buttons, toggles and perhaps even rotary switches that can all be soldered directly to the PCB yet all interface neatly with the front panel. I have been working on this problem on and off for over a year but I think I now have a workable solution and this is what I meant when I included the phrase 'a hint of General Standardisation' in the title. This will be the topic of the next post.