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.