Friday, 10 February 2017

Design made pretty

The new amp is not going to live with me... it's being made for a friend. 

Topology-wise, it will be a push-pull running fixed bias, ultralinear, just like the previous one. Except it'll be running EL84s on the output instead of KT88s. And the plate voltage will be 300V instead of 560.

So we're looking at realistically 12-15 watts, which for the intended usage will be ideal.

Owing to this one having a phono stage which includes a cathode follower, we have some fairly rigourous requirements for the power supply:

  • B+ of 300V for the output stage with some hefty reservoir capacitance
  • 2 X separate 280V rails for the phase splitter (DC-coupled, hence the slight voltage drop needed)
  • A separate 300V rail for the phono stage
  • 2 X 250V rails, heavily decoupled to avoid signal feedback through the B+

That's just the high voltage side. On the low voltage side this amp needs

  • 6.3V for the EL84s
  • 12.6V for the input and phase splitter and cathode follower. This rail needs to be elevated ... the 12.6 vac needs to be standing on around 60 VDC to avoid exceeding the Vhk on the follower. (I figured the other valves on that supply wouldn't mind an elevated heater supply too much either)
  • 25 VDC for the heaters on the two phono input valves (12AX7s with heaters in series to absolutely minimise the mA)
  • And last but not least a minus supply of about 20V for the fixed bias

As with my previous amplifier, there's a delay start circuit with a 555 timer and a relay which turns on the HT after about 30 sec from power-up, to allow all the cathodes to be at operating temperature before B+ is applied.

Since I'm challenging myself to build this amp on a small chassis, I decided that the power supply needed to go on the smallest board I could get it on. The design I sketched out in the last entry was the inspiration for the final layout for the board, see below:

Board design... single-layer and NO topside links

Once the design was done, it was time to employ my primitive stone-age PCB manufacturing technique: Print design onto plastic sheet, finely sand copper-coated board, apply plastic sheet face-down to board, then iron it! (Seriously... with a clothes iron).

When all the image is transferred to the board, the board is rinsed then placed into a tray of etchant solution to dissolve away the uncovered copper.

Leaving us with a nice board ready for drilling. The transfer is usually not 100% with this technique, but can be easily touched up with the soldering iron

Out of the etchant, ready for drilling

After the board is drilled it's a case of assembling the components and soldering them on, with a board of this nature I like to start with the smallest/lowest components first (resistors) and work up in size.

The finished product

Count the rectifiers on this board - 4 of them

8 power capacitors on a board not much bigger than a smart phone

Top-down view. At bottom right is the delay circuit

Sunglasses just for a scale prop

Top-down view, rotated

More build images as it progresses...

Friday, 3 February 2017

Design process - the ugly end

So when designing amplifiers, I like to put the power supply on a circuit board.

While I think point-to-point wiring has its place in the amplifier section, the use of a PCB gives a more compact and tidy layout for power supplies.

So in the next amp I'm building the power supply is going to be a little more challenging than the previous, as this one has a phono stage, which adds complexity.

This power supply needs to provide:

  • B+ 300V for the output stage
  • 280V (X2) for the preamp
  • 300V separate regulated supply for phono stage
  • 250V (X2) off the phono stage rail
  • -25V regulated DC for the heaters on the phono stage (and doubling as bias supply for output valves)
  • Elevation voltage of around 60V for the heater supply for the Cathode Follower
  • 555 IC circuit for 30sec B+ delay on power-up

So a number of resistors, capacitors and rails needed. And it all has to fit as compactly as possible onto the PCB since this amp is going into a compact chassis.

So, before starting to design the finished PCB, some thought needs to be given to the layout... in the old days this might have been a pencil-and-paper exercise, but that method isn't particularly flexible when it comes to making corrections or revisions.

Enter a tablet computer with a pen. This is how I make my rough sketches for PCB layouts... from this file I shall design the final PC board

It looks truly awful, but it makes sense to me.

Next step: designing the board from this sketch, when done I'll post the layout here