As it takes up rather a lot of room in my small workshop, it is currently being used as storage space while I work on other aspects of the Mark 3. However, I think you can see the how the 4U fader section works and at the rear there is a 9U section. The bottom 3U of this would house the rear mounted line amps and the 6U above would be used for connectors.
One of the other aspects I have been working on is the module electronics. I have now built the first prototype channel module PCB as shown below:
All the basic enhancements over the EZTubeMixer PCB are included:
- Provision for Molex KK connectors for internal wiring
- Ground reference the gain resistors so each amplifier can be used as a virtual earth mixer
- Moved the mic line switch to be before the phase switch so the latter works on both line and mic
- Amplifier gain preest pots moved to be accessible from the front panel
- Tubes moved so as not to foul the enclosure
- Output transformer (direct out|) included
- Provision for several EQ types
The next step is to test the board and then finish one of the EQ boards and try that out.
In the meantime I have been giving further thought to the first mixer I will build with the new modules. I said a few posts back that I would use the 500 frame as a test bed for the new modules and I still intend to do this. However, it occurred to me that it might just be possible to make a compact 8 track all tube mixer based on this frame so I have been investigating that possibility further. Also, the 500 frame is pretty simple; there is no room for lots of modules so it might even make a fairly feasible DIY project. Here is by first stab at a block diagram of the 8 tracker:
At the top left are the 8 identical channel modules. These have similar facilities to the modules I am building for the EZTubeMixer demo in that the mic pre, EQ and routing are all in the one channel module. The main changes are:
- A permanently connected PAN control instead of the smart pan
- Addition of pre/post switching for the two AUX sends
- Addition of pre-fade listen (PFL) switch to allow level setting and quality checking
- Addition of a channel mute switch. This mutes the signal feeding the PAN and AUX sends but leaves the direct out unaffected.
At the top right are the audio bus amps for the two main buses (L & R), the two AUX sends and the PFL. The AUX sends and PFL would normally have simple rotary controls but you would expect the main bus output to be on a slider fader. Unfortunately, as we only have 8 slots, and all 8 are used for channels, there is no sensible place to put a main bus slider fader. So I have decided to use a single large rotary knob in the meter bridge section for the master fader. It will be on the front panel of the Twin Line Amp used for summing the buses so it can easily be a couple of inches in diameter. As I am right handed, I think I will place it in the right most module in the meter bridge. It's a compromise but a small one I think.
At the bottom left is the talkback and direct inputs. The talkback uses half of a Twin Line Amplifier (the other half is used for the PFL bus amplifier). A simple pot sets the gain and I envisage a front panel mounted XLR into which the talkback mic can be plugged (much the same way as we used to do it at Neve). Two momentary action switches control the routing of the talkback. One switch routes it to the L & R buses for slating and the other routes it to AUX1 for artiste communications. When either switch is operated, a dc signal labeled 'DIM' is produced which is used to operate two relays in the monitor section to reduce the level fed to the control room monitors to prevent howl round.
The direct inputs are modelled on the AUX Returns of the EZTubeMixer demo and indeed they can be used for FX returns. Each direct input has level and pan controls and uses half of a Twin Line Amp. I think there is enough room in the meter bridge section of the 500 frame to house four direct inputs which means there will be 12 inputs available at mixdown.
Lastly, at the bottom right are the monitoring and metering sections. I envisage just a pair of meters placed centrally in the meter bridge of the 500 frame. At present I am thinking of using a couple of Sifam AL20SQ meters for a nice retro feel.
Each meter has its own 5 way switch that selects between four of the direct outs and one AUX send thus covering all eight channels and both AUXes between them. The output from these two switches feeds a second switch that connects the meters either to the 5 way switches or straight to the main bus outputs. This allows the meters to monitor the main output and easily be switched to read other outputs when desired. The signal fed to each meter is also fed to the monitor section.
The monitor section is quite straight forward. It consists basically of a three way monitor select switch and a level control. The monitor select switch selects between the the main L/R buses, a two track playback input or the meters. Normally you would listen to the main buses and occasionally switch to the two track playback to check sound quality. Selecting the meter position allows you to listen to whatever the meters are connected to. The output of the monitor select switch goes via a couple of relays to a 10K:600 transformer and thence to the monitor level pot. The 10K:600 transformer is there to ensure the monitor section applies a minimal load to the signals it monitors. It also means a simple passive 1Kohm level control can be used on the secondary for the monitor level control (this is pure Neve).
The relays affect what reaches the monitors and its level. RY1 and RY2 are the PFL relays. When a PFL button on a channel is operated, the PFL dc operates the relay and automatically routes the output of the PFL bus amp direct to the monitors so it can be heard. RY3 and RY4 are the DIM relays which apply a 20dB attenuation to the monitor level. These relays are operated by the talkback switches and, as mentioned above, are designed to prevent howl round when talkback is used.
As it stands, this design uses 8 channel modules and 5 Twin Line Amps for a total of 39 tubes.
The next step in the design is to sketch out some front panels for the modules and the components of the meter bridge.
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