Wednesday, 17 May 2017

Last piece of the EL84 Amp design

So the circuit sketches and various measurements on assorted bits of paper have finally been consolidated into a coherent schematic... thanks in no small part to an incredibly patient and supportive wife who endured being an electronics widow for one final evening on this project!

The schematic is "as built" and there is one part I am less than happy with and if this amp is ever back on my workbench it'll get fixed: I am not happy with the resistance of R3 and the resulting voltage on the low side of it.

This leads to the voltage on the anodes of the RIAA stage for the second valve and the cathode follower. A little low, but notwithstanding, it passes the listening test with flying colours. 

But if it's ever back on my bench, R3 is getting swapped out for a 27K quick-smart.

Apart from that, you can see the implementation of the 555-based delayed HT switch-on circuit, the DC heaters for the first two valves of the RIAA stage, and the sneaky re-use of that voltage for the bias for the EL84s, and the 270K / 68K Voltage Divider for elevating the 12.6V heaters - the cathode follower needs it.

The Gain and phase splitter stages (V4 / V5) are based on the Fisher X100, but with higher quiescent current on the 12AU7.

This is the circuit. You'll need to click it to see full size and download / print / zoom / scroll, or whatever.




You'll probably need to download this to see it clearly

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