Finally got back to work on my 4bsst klone - got interrupted by a nasty bout of genuine influenza. I finished the input board and everything tested fine. There's a small gotcha if you test the input board by itself - the signal ground and the stabilised power supply ground are not connected on the pcb. So if you, like me, use a floating power supply as input to the pcb stabilised power supply then there is no common ground and the input board does not work. A temporary wire bridge fixes this. The overall gain of the input board from the phono input is x2 not x3 a I previously stated.
The discrete component opamp has a gain of x3 but there is x0.667 attenuation at the input so overall the gain is x2. The real Brystons have their phono input sockets attached to the pcb and not the metalwork, so there is a point of weakness. The klone phono sockets are soldered to the pcb but also have mounting nuts to fix them to the back panel, so no weakness there. You can see this in the attached photograph. I ran a quick fft spectrum analysis with the input board producing a 1v rms output at 1kHz and any distortion was lost in the noise. I probably could have dug distortion out of the noise if I averaged for long enough but with a figure of well under -100dB I wasn't inclined to bother.
As well as documenting my build of the Bryston 4bsst klone I intend to redesign both the input board and the power amp board. The power supply and protection board will remain unchanged. Why bother to do this when I'm sure the klone will measure and sound fine? Well the input board and the input/vas of the power amp board use a common discrete component symmetrical opamp design which performs fine, but can definitely be improved. The opamp has npn and pnp input long tailed pairs and the collector currents of neither pair perfectly balanced - close, but far enough apart to give rise to small extra distortions.
The choice of push pull vas transistors in the power amp is sub optimal and suitable replacements will yield further small reductions in distortion, particularly at hf. The thermal tracking of the Vbe multiplier needs some investigation ( it may be ok ) and the circuitry around the quiescent current setting potentiometer needs to be put back to something resembling the Bryston original. Otherwise its far too easy to blow up the output devices unless the pot is set correctly before switch on. I'll redraw the original discrete component opamp circuit and aslo sketch out some alternatives. For the moment I intend to leave the output stage of the power amp as it is.
