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Old 24th November 2015
I have restored an S series that I have used for about two years now. The power supply was rebuilt, and all electrolytics have been replaced in the PSU and all channels of the desk. Signal path caps were upgraded throughout to 220uf, and the main caps in the PSU were raised to 15000uf from 10000uf.
Recently I added .1uf WIMA polypro bypass caps to the signal caps in the Master section and this made the biggest improvement on the overall sound of the mixer. If I remember correctly, there were only six of them, so quite a bargain, IMHO.
The high end just shines now on the board, and mixing drum overheads is a delight. Instead of struggling to get definition, I am finding I have to be subtle with any EQ, and generally don't need it if the tracks are well recorded, other than HPF.
Jim Williams has said that changing the signal path caps to 220uf solves some phase issues on low end, and I have done this to a few TL072 equipped mixers with great results.
Changing opamps just seems like too much trouble to me, and the few times I've tried it, the sound results were not even close to the electro and bypass cap changes mentioned above.
Would consider the bypass caps on input channels, but almost afraid it will make the board too bright. guess I should try a few and see what I think. Same for the group or bus channels.
Haven't considered the transformer additions mentioned in this thread, because I'm not using the mic pres for recording, and from the schematics, it looks like the transformers are only on the mic inputs, not the line inputs. It also would get expensive real fast. Don't think this board has the output transformer option.
Just fine by me. I love this thing now. The only other thing I might attempt is going to WIMA polypros in the channel EQs.