Saturday, 28 September 2013

AUX/MON panels and Smart Pan PCBs

The panels for the AUX send and return modules and the monitor panel arrived the other day.

The AUX send is very simple; it just has the two AUX master send controls. The AUX return is a little more complex; as well as a level control for each of the two returns, there is a pan pot for each return. This allows two mono returns or a single stereo return to be used. You can compare the actual panel layouts with my original sketches posted at the beginning of August.

The monitor panel is shown at the bottom of the above picture. The monitor is entirely passive and balanced throughout. The lower hole in the centre is where a 4 pole 3 way switch is fitted that acts as the main monitor select switch, selecting between the stereo bus (centre), a 2 track play back device (right) or an individual signal source (left). The left position connects to an additional 2 pole 6 way switch that can be switched to any one of the four direct outputs or the AUX send outputs. As these are mono signals they are fed to both left and right channels of the monitor. Following the main selection switch is the monitor volume control, the output of which goes to a pair of XLRs at the rear of the mixer for connection to the studio monitor amplifier. In parallel with the monitor volume control is one for the headphones. This feeds the headphones amplifier housed in the meter bridge which in turn feeds a TRS socket at the rear of the mixer.

Smart Pan

I have also been given some consideration to the implementation of the Smart Pan controls. This is an idea I came up with when I was at Neve back in the 70s. The idea is that a pan pot has two switches associated with it. When both switches are up the channel is muted. If the left switch is down, the channel is routed directly to the left bus. The pan control is not connected so there is no crosstalk via the pan control. Similarly, if just the right switch is down, the channel is routed directly to the right channel. However, if both the left and right switches are down, the pan pot is engaged and works as you would expect. The advantage of this circuit is you can route channels directly to buses with no crosstalk or you can pan as normal and you get a mute for free. The problem is it means you have to wire up two double pole double throw switches and a stereo pot, that's a total of 18 connection points, and you have to get it right each time for it to work. Doing this by hand is going to be prone to errors as well as not looking very neat so I thought a PCB solution was called for. I could then include connectors for the cables to the buses and from the channel amp to make assembly easier too. However, the pins on the switches are at right angles to those on the pan pot so a single PCB was not going to be possible. However, the PCB area required was so small it seemed wasteful and expensive to use two of them. I then realised that I could lay out a single PCB as if all the pins were in the same plane, but leave a gap between the pan pot and its switches, and then cut the PCB into two pieces; one to fit on the pan pot and the other to the switches. And that's what I did.

The picture above shows the completed PCB on the left. It is 50mm long and 30mm wide. I got 10-off made at Iteadstudio for less than £10.  As well as all the interconnect between the switches and the pan pot, it also has space for the pan pot slugging resistors that determine the pan pot law and has provision for an input connector and two bus output connectors (Molex). The PCB in the middle is after I have crudely cut it in two with a hack saw and on the right you can see them fitted to the two switches and the pan pot. There are just four connections between the two PCBs for which I intend to use a short ribbon. Although the PCB attached to the switches extends upwards over where the AUX send pots are, it is high enough to clear those pots.

Friday, 6 September 2013

Rear Panel Connections

On the back of most mixers you will find a panel filled with an array of connectors, mostly XLR and TRS jack types along with an occasional D-type, power socket or IEC mains inlet. Many of these require quite large holes which are not easy for the DIY constructor to make simply due to their size. Often it is convenient to make the rear panels compatible with 19 inch racking. As 19 inch panels are only supported at the edges they are generally made of mild steel to stop them bending in the middle. For making large holes in aluminium I have used Q-Max punches. These work very well and make a nice clean hole. Unfortunately they do not work so well with mild steel and it is quite easy to break a Q-Max punch on a steel panel. Making lots of holes is anyway a royal PITA and something I therefore prefer to avoid if at all possible. What you really need is a simple way of building panels with a variety of sizes and types of holes.

Fortunately, manufacturers of rack equipment have also realised this and produced some nice modular systems. The one I prefer, because it is flexible, low cost and British, is made by a company called Monacor:

Monacor International

They do a number of pre-punched 19 inch rack mounting panels which are fine if you happen to want the number and type of holes they have available. However, if you want something more flexible they have a 2U frame onto which you can mount a number of different panels:


The basic frame has space for 10 segments which can be  filled with pre-punched panels one or two segments wide. Here's a frame with a selection of panels laid on it:

From right to left we have a 2 segment panel punched for Neutrik XLRs into which I have fitted four plugs, then we have the same panel unpopulated, next a 2 segment slotted ventilation panel (very handy for tube mixer designs), then a one segment blank, a one segment panel punched for two TRS jacks and lastly a 2 segment blank which I will use to mount the the IEC mains inlet connector. Monacor do have an IEC pre-punched panel but the mains inlet I am using on this mixer is rather bigger as it includes a fuse, an on/off switch and a mains filter. At least I only have one big hole to make myself. All the other connections needed by the mixer are taken care of by the Monacor pre-punched panels.

The only downside is that there is no means of easily labelling the connectors. Now you can get a different pre-punched panel system from Canford Audio that does include the ability to label the connectors but it is five times more expensive so for the moment I am happy to think of an alternative means of achieving this.

And before anybody asks I have no association with Monacor at all.