The first area that could benefit from improvement is the overall construction of the channel module. Most EZTube Mixer channel modules consist of two PCBs attached to a front panel. One PCB contains the main mic pre and gain make up amplifiers and the second usually holds a passive EQ. Routing switches and pots and some EQ pots and switches are mounted directly onto the front panel and hand wired to the PCBs. The principal strength of this design lies in its flexibility. By simply by changing the front panel and/or the EQ PCB you can build a wide variety of channel modules. The weak points of this design are:
1. Mechanical integrity. Each PCB is attached to the front panel by a couple of screws and the mic pre PCB interfaces to the rest of the mixer via a 32 way connector that plugs into a backplane PCB. As there are no card guides, it can be difficult to locate the 32 way connector into the back plane and when you push home the module, all the strain is taken by the PCB.
2. The flexibility means each module leads to a lot of hand wiring. This is time consuming, is prone to errors and is not particularly neat
3. The PCBs generally have an earth plane which provides some degree of screening but the overall mixer design relies a lot on the the screening provided by the sub-rack and its add-on screens for ensuring the channel modules are adequately screened. This is fine as long as you stick to off the shelf sub-racks for which add-on screens are available but as soon as you want move away from standard widths, screening becomes a problem.
4. If you need a balanced output, for an insert or direct out for instance, the output transformer has to be fitted external to the module.
The solution to the mechanical integrity and screening weaknesses is to house the channel module in a screened box of some sort. I looked at standard sub-rack modules by several manufacturers a few years ago but abandoned the idea because of the expense of these modules. However, I was recently introduced to the range of sub-rack modules made by Fischer in Germany. Many of these use a pair of extrusions to form the sides of the box, which makes them very strong, and front and rear panels are attached by screws that fit into tapped holes in the extrusions. They are thus much simpler to assemble than many of the modules made by other manufacturers and turn out to be substantially cheaper. Both 3U and 6U versions are available with built in ventilation grills in the top and bottom which is ideal for tube circuits. Here is a picture of one of the 6U units made by Fischer:
This version has black anodised aluminium extruded sides and a plain aluminium front panel. The extrusion includes a lip intended to engage with a card guide which will ensure the unit slides in easily and mates properly with the backplane PCB. In the 3U version the PCB is supported along its entire length so there is no danger of it bending when it is plugged into the backplane. The complete screening this enclosure provides means you need little or no additional screening outside the module as almost all audio connections to and from the module are made by screened cables.
The next problem to attack is the large amount of hand wiring required inside a channel module. As this hand wiring provides a great deal of flexibility, any solution needs to be able to retain as much flexibility as possible without requiring lots of hand wiring. Part of the problem lies in the pads on the existing PCB that are intended to be used for interconnections within the board and between boards. For example, there are pads for the mic input, the line input, phantom power, mix buses and so on. To avoid having to wire directly to these pads, a possible improvement is to replace them all with connectors. Now, instead of soldering wires, we simply plug in some pre-made cables. This is convenient for rapid assembly or disassembly but it simply moves the problem to cable assembly. Soldering cables to connectors is no more fun than soldering them to PCBs. It would be nice not to have to solder these at all. One obvious alternative is to use IDC connectors and ribbon cable but many of the connections need to be screened so ribbon cable is not an option. A better alternative is to use crimp connectors in the same way they were used in the EZTube Mixer for screened cable connections from the back of the mother board to external connectors and other components of the mixer e.g. faders.. They also work well for small numbers of unscreened connections. So I have altered the PCB layout of the mic pre PCB to incorporate Molex KK connectors for for all these links. I have also added connectors for the outputs of the two amplifiers and for the input to the gain make up amplifier so that board to board connections can be made by plug in cables.
This solves part of the interconnect problem but not for the EQ. The three band Pultec is the worst. It has 6 pots and two switches external to the EQ PCB, a total of 27 wires to connect. Assembly would be simplified if all these additional controls could be mounted on a separate PCB and connected to the main EQ PCB.These only connect over a short distance so screened cables are not essential especially as the channel amp is now inside a completely screened enclosure. So a ribbon cable might do for this but it means relaying out all the EQ PCBs to incorporate a ribbon connector as well as laying out the PCBs for the extra controls for each EQ. Since I have three standard EQs at present, that means laying out a total of 6 PCBs. Not an impossible task but it does mean that for any new EQ it would be necessary to lay out two more PCBs. Again, flexibility involves a lot of work so I looked for ways of reducing it. I realised that all the current EQs are 3 band and they all use three Grayhill switches in more or less the same positions so it occured to me we could have a single three switch PCB wired to standard IDC headers and include the EQ itself on the PCB holding the additional controls. All current and future EQs would use the three switch PCB and an EQ specific one. This reduced the number of PCB layouts from six to four and meant that new EQs would only need one PCB to be designed rather than two. So far so good. It then occurred to me that, if this three switch PCB was going to be used in every channel amp then we might as well incorporate it with the mic pre PCB and have a single standard 6U PCB that fitted directly into the Fischer module. This would have the further advantage that the 6U PCB would be fully supported on two edges just like the 3U ones would be.
I then spent some time laying out a prototype 6U PCB with three Grayhill switches placed below the mic pre. It turns out that this leaves a good sized area behind the switches, probably big enough to house a VTB2291 output transformer. However, I was not happy with the way a VTB2291 would be attached to the PCB and I thought it likely the mass of the transformer could damage the PCB in when transported if secured only at two points. I then contacted Colin at Audio Mainteneance to see if there was any possibility of obtaining a VTB2291 in a PCB mounting format similar to the VTB1847. It turns out that this is no problem and Colin was able to send me a prototype to try out. Not only is it no problem, but because it is just a slight tweak to the process of manufacturing an existing transformer, there is no price increase either.
There is also room on the PCB for an extra input transformer, so to test out all these ideas I designed a prototype 6U PCB and had a few made. Here is a picture of the prototype 6U PCB:
As I mentioned at the start, PCBs in the EZTube Mixer were simply attached to the front panel by a couple of screws and the new EQ boards could be mounted in the same way but, in the interests of improving mechanical integrity, I decided not to rely on this alone. So I have added positions on the 6U PCB for four pillars to provide additional support for the EQ PCB. These are labelled P1, P2, P3 and P4. Here is a picture showing how the PCB plus pillars slides into the module:
As well as adding connectors for internal connection, I made several other improvements to the mic pre PCB layout ,some of which were suggested by existing users (thank you Holger and Pierre).
1. The gain setting coupling capacitor and resistor positions of each amplifier have been swapped so that the resistor is connected to 0V rather than the capacitor. The main reason for doing this is that the junction of the gain setting resistor and capacitor is a virtual earth. As the resistor is now ground referenced, you can connect a large number of other resistors to this junction and use the amplifier as a virtual earth mixer (rather than the passive mixing scheme that is used at present).
2. The current design has the phase change switch before the mic/line switch. Since it is useful to be able to change the phase on line inputs as well I have moved the phase change switch to be after the mic line switch.
3. There is a pre-set trimmer resistor on the PCB to set the gain of the gain make up stage. This needs to be set to give overall unity gain when the passive EQ is in circuit. The trimmer is not conveniently placed to do this so I have moved it to the front of the PCB so it can be accessed in a fully assembled module via a small hole in the front panel. The Twin Line Amp version of this PCB might be used as a pair of gain make up amplifiers so the trimmer for the first stage (not normally used when configured as a mic pre) has also been moved forward so that it could be accessed through the front panel if the stepped gain switch were not fitted.
3. The holes at the top of the current mic pre PCB, used to hold the mic and line input cables, make the cables interfere with card guides (which can be used in 3U versions of the EZTube Mixer).These holes have been deleted.
4. The tops of the two output tubes and their bases are close to the metal parts of the module extrusions. They have therefore been moved in towards the mid line of the PCB to keep them well away from the metal extrusions.
I have yet to build one of the 6U PCBs and I am currently designing a prototype matching Helios EQ PCB to try with it.
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