Products
Since all my tube consoles are custom built there are no standard products as such, so this section simply lists all the products to date starting with the latest and ending with the first prototype. Information on both completed products and work in progress is given.
Transition to Mark 3
Many lessons were learned from building the EZTubeMixer described below, the main ones being that the mechanical integrity of the modules needed to be improved, secondly that the modules should be about half the width so more channels could be fitted and thirdly that front panel wiring needed to be minimised or eliminated. The Mark 3 design intends to address all these issues and the current state of that design is that two of them can be incorporated into current EZTubeMixer builds as illustrated in the mixer shown below:
Apart from the wooden enclosure, at first sight this mixer looks very much the same as the EZTubeMixer below, but under the hood things are very different. First the modules are quite different.
As the picture above illustrates, there is a single 6U PCB housing both the mic pre and EQ. This PCB slides into PCB guides so plugging modules into the backplane is simplified. There is also room on this PCB for the output transformer so this no longer has to be mounted externally. On the right hand side of the module, the PCB is mirrored by a galvanised steel screen that creates a rigid module and also screens the module.
Inside the module there are further changes.
The picture above shows how the front panel electronics is arranged. Instead of hard wiring controls, they are all now fitted to and wired together by a single PCB. Connections from the front panel to the main board are made by leads terminated in Molex connectors which considerably simplifies assembly and improves reliability. Completed modules look much tidier.
The example shown above has four tube mic pres each capable of up to 70dB of gain. Each input has buttons for phantom power, 20dB pad and polarity reversal. Gain is selectable in 1.5dB steps from 26dB to 60dB with a 10dB trimmer. Each mic pre is housed in tis own completely screened plug in module to ensure minimum noise and interference.
EZTubeMixer
My early mixer designs were based on the mu follower circuit used in a gain structure based on early tube mixer designs by RCA from the 1940s. Although these give very good performance and allow the selective addition of tube tone, they do have a couple of disadvantages. The first is that their output drive capability is limited. They cannot drive both a mix bus and an output transformer at the same time so mixers with a direct out per channel are not possible. The second disadvantage is that their gain is fixed so mic pres with stepped gain switches are not possible. It was to solve these problems that a new preamp called the Eurochannel was developed. This has two identical preamps on a single Eurocard sized PCB that plugs into a backplane using a 32 way DIN connector. One amplifier has some additional components that turn it into a versatile mic pre i.e a microphone transformer, 48V, phase, pad and mic/line switches plus a stepped gain control covering a gain range from 26 to 60dB and a relay switched HI-Z instrument input. The output from this preamp provides a pre-fade bus feed and can also be fed to a fader and/or passive EQ circuit and back to the second preamp which provides gain make up and has its gain set by a pre-set pot. The second preamp provides post fade bus feeds and is also capable of driving a transformer to provide a balanced direct output (if a pre-fade direct output is required this can simply be taken from the output of the first preamp).
This PCB, along with new versions of the Helios 69 and Pultec EQP1A EQ are used to build channel modules that are 6U high and 2.8 inches wide. These modules plug into the backplane of a standard Eurocard sub-rack. Building relatively complex tube mixers now became so simple it was christened the EZTubeMixer and became a DIY project at groupdiy.com. The mic preamp has provision for fitting input transformers from Sowter, Jensen and Cinemag.
Three standard forms of passive EQ are currently available. One is based on the Helios Type 69 which is popular with musicians . The second is based on the Pultec EQP1A with an added mid boost/cut section based on the MEQ5. (above). The third is based on the bass and treble EQ found in the REDD 47 desks with an added mid boost/cut section based on the RS124 'brilliance' module.
Mic preamps and passive EQ are combined into channel modules which are 6U high and 2.8 inches wide. Six modules will plug into a standard 19 inch sub-rack.
To facilitate wiring between channel modules, a backplane PCB is used. This supplies HT, heater, phantom and relay power to all the modules, connects the mix buses across channels and provides fader, mic/line input and line output connection for each module.
The backplane is in turn fitted into a standard sub-rack:
And the sub-rack is fitted into a frame:
And here is the finished mixer
6 Into 2 Mixer
My first custom all tube mixer was for a client who makes on location classical recordings. For maximum fidelity a minimum signal path design was used with a simple passive switched high pass filter per channel. Each channel also has switchable phantom power, phase reverse and 20dB pad. Rather than pan pots, each channel has Left and Right assignment switches which allow each channel to be routed Left, Right, Centre or Off.
First Prototype
This was built in 2010 as a test unit so I client could check out the sonic signature of the design.
The intention was that the metal chassis running along the centre and the tag board holding components above it would be replaced by a single PCB. Here is a picture of the bare PCB.
