R5 should be 10R if you will lift one pin and measure it. It should be ~9.45R on TP for 47//47R instead of trimmer. See about changing the Zeners, about surely having a Vout that allows at least 10V difference to Vin, and that also Q1 shows ~1-1.5V Vgs which you can also measure as Vds Q2 when you have the power on.
Hi Salas
Thanks for your thoughts on loading resistor values. I have been listening to the phono a lot over the last two days, I still percieve a little more gain in one channel than the other.
There was/is no evidence of oscillation on the scope with either of the SUT's. However the original setup with the Sowter SUT's inside the phono enclosure was very prone to RF interference and mains bourne spikes, so there could have been something.
I have the new 977's in an external die cast aluminium box, but of course now there is another set of cables and phono sockets inbetween the cart and the input grids. The sound is certainly top class, excellent bass resolution and very quiet with zero hum.
Now I know how it sounds I would like to rebuld the whole thing in a more compact enclosure. What would be great if someone on this forum could lay it out on a PCB and make some available as a group buy. Unfortunately I only have rudimentary PCB skills. I'm trying to learn Eagle, but I don't think I'm up to laying out a phono stage just yet!
Could you measure the difference in dB @ 1kHz with and without the SUTs? Use 50 Ohm generator when with the SUTs. Swap channels since the SUTs are external, If the gain difference swaps with channels its the SUTs difference. If you can not find enough difference by measuring, then try antiskating with a test LP in case its source related.
Check and note things one after the other as in #1034. Have you had that reg used before in another situation? Did it ever smoke?