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USSA-5 PCB GB

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#598
While I am procrastinating to start building the USSA5, I stumbled on two things regarding the USSA5 design:

(1) What is the purpose of R15 and R16 (referring to the schematic here: https://fabaudio.online/ussa-power-amp ). Are those resistors really necessary with the lateral MosFETs?

(2) Just as a thought experiment: let's say I want a balanced input, and I like tubes. Would it be a bad/crazy/wrong idea to replace the input stage with a long tailed pair of triodes? I know this would be a different amp, and I am not saying that I will do this. I am just enjoying the thought of this and would like to see what others think.

Ok, I need to finish the headphone amp that occupies my workbench so I can get started with the USSA5...
 
#599
1) Good point. R15/R16 purpose is for the specific output stage bias current measurement during the build. It also acts as very small local feedback. It is true that lateral mosfet do not require source resistors which also allows more square law effect. As a compromise I target both aspects (bias current measure and square law effect) by using a very low value (0.05 ohms) source resistor. You can also use an even lower value such as 0.02 or even 0.01 ohms ( however measurement will be less accurate at such low voltage with a standard multimeter). If you replace R15/R16 with jumpers (0 ohms) then you can also mesure PSU current and subtract input and driver stage bias currents. I find it more convenient to measure directly output current alone using resistors. In the FSSA amp which uses also lateral mosfet, the latest modified BOM indicates 0.01 ohms ( orignal was 0.05 ohms) for the same resistor purpose.

2) I suggest to first try the original version ( but you can use 0.01 ohms as source resistor) and then afterwards build another one with variations. Have in mind that USSA-3 amp uses the famous SK170/J74 jfet as input stage which gives excellent sonic...your pcb is also suitable for USSA-3 😉 with USSA-3 you can even adjust H2.
Long tail pair would be no more an Ultra Simple Symetrical Amplifier ..😉
Fab
 
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