Friday, 2 February 2018

EL84 Amp II: Progress Post 3

The design of the amplifier has continued since the last post although slower than hoped. This was die to a few random factors (including but not limited to a computer that died and needed replacing and setting up, and some non-electronics projects that came up)

Anyway, the progress this time is the completion of the headphone board, which is based on a White Cathode Follower using ECC99 tubes.

This completes all of the signal handling boards. Below they are shown in their location on the chassis. 


Headphone board front centre

This represents a significant achievement: The headphone stage was a late addition requested by the customer after the chassis had already been ordered. I was not at all confident that the entire amp would be possible to build in the small chassis. But I persevered, with the approach of taking a long time on the PCB designs to get them as compact as I could, including manually routing them (which gets more complex as each track and component is added)

The last remaining board needing to be designed was the power supply. This is going to sit inside the chassis under the transformers. This dictated its size, which – as with the rest of the amp – needed to be as small as possible.

The final board design measures 140 X 75mm.

The next step is the careful eyeball check before exposure, etching, drilling (141 holes) and stuffing.

This is the layout



On this board we have:

  1. Delay circuit for controlled power-up
  2. driver for the 2-colour power LED that starts red and goes green when the main power kicks in after the warmup delay
  3. Elevated 6.3vac power supply for heaters, with three sets of output terminals
  4. 6V DC power supply for heaters with three sets of output terminals
  5. 370V supply rail for output stage
  6. 265V rail for phase splitter stage
  7. 300V rail for headphone stage
  8. 300V isolated rail for phono stage (to avoid feedback through power rail)
  9. 280V rail with two outputs for tone control and preamp gain stage
  10. double-filtered and isolated negative bias with four outputs (one per output tube)


Regarding point 4: From a technical requirement, it's only strictly necessary to run the phono stage on DC heaters, but due to the complexity of this project it has a high tube count (13) the number of tubes exceeds the rating of the 6.3vac heater winding on the power transformer. This transformer also has a 5vac winding which will be passed through a diode and thereafter smoothed by a 47,000µF 10V capacitor (it's huge... has to lie down on the board!) and this rectified DC heater power will be used for other areas besides the phono stage, just to share the load.

Next steps: Make this board, then metalwork for the chassis...

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