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.