Channel One

The channel module is the heart of any mixer. The combination of mic pre and EQ that makes up a normal channel module is what defines the basic sound palette the mixer can provide.  So, having developed the basic 35mm wide building blocks as detailed in the previous post, it is time to put them together to make a full blown channel module.

One of the benefits of having a modular approach to the PCBs that make up a module is that you can reuse them in other modules. For the first channel amp we reuse the front panel board of the the classic mic pre module (see previous post) and the front panel PCB of the REDDPLUS EQ (see previous post). I made a couple of tweaks to both of them but they are otherwise largely unchanged. These two together fit onto a 35mm wide 6U high front panel to make up the channel module. One thing we do need that is new is a 6U main PCB for the module but even this is mostly made up of PCB layouts we already have. The main mic pre section is simply the 35mm TLA stuck on a 6U PCB with some added transformers for input and output. The bottom half of the board just uses the main REDDPLUS EQ board.

One the left you can see the TLA circuit and to its right are the input and output transformers. To the right is a footprint for an additional transformer and just below it is the REDDEQ circuit. The normal signal flow is input transformer, mic pre, passive EQ, gain make up amplifier and finally output transformer. The output transformer would usually be connected to either a direct out or insert point. One advantage of having the output transformer in the channel module is it makes if very easy to build a 6U 19 inch rack containing a bunch of tube channel amps.

As you can see there is plenty of space on the board for later additions.

The red rings around the three tubes are for testing an idea to make the module more rugged. The rings are just thick enough that they are gripped between the PCB and the steel screen and, in combination with the tube sockets pins, hold the tubes firmly in place and also buffer them from external forces such as you might find in a mobile situation. In addition, it is a real pain to have to remove all the tubes from all the modules of a mixer in order to ship it. It also requires the customer to fit all the tubes to the modules before fitting the modules into the mixer. My initial idea was to stuff the space between the tubes with bubble wrap to hold the tubes in place during shipping. All the customer would have to do is remove the bubble wrap before plugging in a module. With the red rings, the tubes are already gripped firmly so none of this is necessary. The big issue is the red rings have to withstand the bulb temperature of the tubes.

The front panel has been attached so you can see the front panel mic pre and EQ boards. If you look to the left, centre and right of the front panel you will see the low cost Ettinger blocks used to connect the front panel to the PCB. You can also see pillars attached to the two mounting holes of the 32 way connector and another pillar in the top right hand corner. These, together with the Ettinger blocks, are used to mount the module steel screening plate.

The other side of the module is screened by the 0V plane of the PCB.

 And from the front it looks like this:

Now we have a proper channel amp we can think seriously about configuring a real mixer.