Tuesday, 9 August 2016

Peer-review burns

In the Academic world, when you submit your work for peer-review, you can expect some criticism. If your work contains errors or draws a conclusion based on a misconception, you can expect that critique to be robust, to use a euphemism.

In the case of my amplifier design, I decided to use the forums in DIYAudio as my council of peers. Since this is the first amplifier I've ever designed, as expected my initial design contained a few omissions, and I'd fallen for a few Gotchas. The community were initially dismissive in their critique, but pressing the goodwill I continued being friendly and thankful for the information, and then some useful suggestions began to turn up.

So - as a result of that helpful advice, below is the amended version of the initial stages of the amp.

Note that the topology is unchanged: 12AX7 initial gain stage using Grounded Cathode, 12AX7 Concertina phase splitter to follow, followed by a 12AU7 gain/driver stage. However there have been some changes:

Click the image for full-size view

First: The volume control has been reconfigured. I had read that using the volume control as a shunt to ground resulted in less noise as the pot wasn't in the signal path. This suggestion was robustly challenged, so I've adjusted the configuration accordingly.

Second: The Cathode bypass capacitors. I was using the values TubeCad had given me but it was suggested these were too low. Researching this further it seems that the exact capacitance needed is not easily defined but too little is much worse than too much. So I increased them

Third, the most grievous error I'd made... the second half of the 12AX7 – the concertina phase splitter – I'd completely omitted to bias the grid. Fatal error! So now there's a voltage divider network of resistors to get the grid about 2V below the cathode. These resistor values may need fine-tuning by doing measurements in the circuit.

Fourth - the 12AU7 drive stage - adjusted anode resistor value and tied the cathodes and gave them a common cathode resistor and capacitor

I'm going to leave this alone now but at the time I begin building the circuit, I expect that some modifications by trial-and-error will be necessary, using actual measurements and traces on the oscilloscope.

Next I shall move on to the finals and then the power supply.

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