Not another discrete I/V stage

[georgehifi]

Can I get some comments on this I/V stage if it is fed from 4 x PCM56's, the output buffer is also discrete but the owner wants to keep that to himself. VR1 he says is the dc offset adjuster and VR2 the bias.

Cheers George

[cetoole]

What kind of comments are you looking for? Its a pretty standard common base amplifier, followed by a folded cascode, passive I/V, and JFET buffer. Lower bias than I would use, in the range of 4-5mA, and I wouldnt use a 2sk170 for a CCS. I also think it will have some stability problems with the DC offset, as I have seen the same issue in my own I/V which uses a very similar topology, so I used a DC servo to handle that. The HDAM seems kinda pointless though, as the 2sk170 provides output buffering for you. I did something similar again in mine, but LSK389C output buffer, with one half biasing the other half and 75ohm resistors in each JFET source, and it does a very good job of keeping itself stable. The I/V resistor seems a bit low for the output current of a PCM56, and I would parallel it with a cap for a LPF.