Monday, 28 January 2013

Making Front Panels

I have now finalised the position of all the controls on both the Pultec and Helios type channels. The next step is to drill all the holes in the blank panels. To do this I print off copies of the front panels from Front Panel Designer with the 'Print Reference Points' option selected. This prints the panel with the centres of all the holes marked by a small red cross. I then carefully cut out the prints of the panels and glue them to the blank aluminium panels using diluted PVA. When that has been done I centre punch every hole ready for drilling. It does not matter if the paper legend gets damaged during drilling as it will be removed after drilling and replaced with a vinyl one. The picture below shows the four channel panels with their stuck on paper legends after centre punching.

Friday, 25 January 2013

Channel Front Panels

I have been trying to finalise the layouts of the front panels of the two different types of channel. I use Front Panel Designer for this. When the layout looks good I print it out and place the knobs on it to get a better feel for the overall appearance. Here is a picture of one I finished this evening. It is the Helios 69 style channel.

For prototyping purposes I print a version of the front panel layout with the centres of each hole marked. I then glue this to a blank front panel and use it as a template for drilling the holes. After removing the paper layout I print out another one in colour on sticky backed vinyl and carefully apply it to the metalwork.

The legend has gone through a number of revisions but I think I have now settled on a standard for character sizes and font. Section headings, like HIGH in the above picture, are 3.5mm high whilst elements within a section like the frequencies on an EQ switch or the labels for individual pots are 2.5mm high. The font used in all cases is DIN17, 1-stroke as defined by Front Panel Designer. Here is a picture of the Pultec channel. As you can see I have not yet finalised knobs. The larger knobs for the frequency selection and mic gain I have selected because they push on to the D shaped shaft of the quarter inch Grayhill switches used. You don't have to worry about getting the pointer lined up correctly with the legend because these knobs have removable caps so you can rotate the cap to the right position and glue it in place.

All the pots have splined shafts and take push on knobs. Again the exact orientation of the pot does not matter as you can simply set the pot fully anti-clockwise and push on the knob at the appropriate orientation. I think so far my favourite small knobs are the bottom right hand three in the above picure.

Sunday, 13 January 2013

EZTubeMixer Demo Mixer

The purpose of this blog is to document the building of a 4 channel all vacuum tube demonstration mixer intended to showcase all the key elements of my EZTubeMixer design that can be found at:

The design caters for three alternative microphone transformers by Sowter, Jensen and Cinemag, The mixer will be built with two channels using Sowter transformers and one each using the Cinemag and Jensen types. The design also supports a 3 band Pultec style EQ or one modelled on the Helios Type 69. Two channels with have the Pultec EQ and the other two will have the Helios EQ.

Each channel will have:

  • Switches for phantom power, phase, 20dB pad and mic/line selection
  • 12 position stepped gain control covering a gain range from 60dB downwards in 3dB steps
  • Smart Pan (pan control only selected when both left and right buses are selected)
  • AUX send switchable pre or post fader
  • Pultec or Helios 3 band EQ with EQ in/out switch
  • Rotary channel fader
  • Direct out

Channel PCBs fit into a 6U high 14HP wide standard Eurocard slot.

Six channels will fit across a standard 19 inch sub-rack. In this mixer four channels are used for inputs and the other two for the master section. The sub-rack is fitted with a backplane into which the channel PCBs are plugged using a 32 way 0.2inch pitch connector.

The master section will contain:

  • Master rotary fader
  • Master AUX send fader
  • AUX return with Smart Pan
  • Monitor selection and level control
  • VU meters

The mixer will be housed in an off the shelf enclosure made by Rackz

They make a very nice sloping 19 inch rack enclosure with an add-on meter bridge.

You can see where the two VU meters have been mounted in the bridge. A standard 6U high sub-rack which which house the channels and master section is fitted into the sloping front panel of the Rackz enclosure. The sloping area is 8U high which leaves a 2U space below the channels sub-rack for rotary faders and a scribble strip.

The picture above does not really show the sloping front so here is one that does!

I just ordered the sub-rack that will house the channel modules. I ordered this from SRS in the UK because, although sub-racks are available at similar prices from distributors, you cannot buy all the little bits and pieces needed to customise the sub-rack. With SRS you just phone up, tell them what you want and they email you a quote. In this case I have four channel modules each 6U high and 28HP wide which together use up two thirds of the width of the sub-rack. In the remaining third I want to have the mix amps, AUX send masters, AUX returns and monitor selection and level controls. The mix amps and AUX sends only need to be 3U high so I want to put them at the top of the remaining third so they plug into the backplane. Below these I want to have a panel containing the monitor section. So I need a little bit of mechanical wizardry that divides the 6U high space into two 3U high spaces and that's where SRS really comes into its own - their guys know just what parts you need to do this. I also want to fully screen the sub rack so I need top and bottom  vented screens and again SRS know what I need. Their MOQ is only 100GBP which a custom sub-rack like this easily exceeds so you get just what you need with no penalties for being a small order. Check them out