Thanks dear, for the info.
A dedicated low noise regulated supply is really worth it.
I swapped long ago the repro volume pot for a SMD resistor attenuator (only one shaft
) which sounds identical to the SRL out. The original degraded/colored the sound quite a bit.
I replaced for now VR103/203 (71/2ips) for your typical "bourns" (3296?) variable resistor soldered to the bottom. I do not know if it sounds better or not but certainly it sounds way cleaner now. What would be the lowest noise pot we can use there?
I decided to bypass VR106/206 since my SMD resistor attenuator sounds so transparent (it can manage the volume on its own) and it turned out to be a good move.
With these mods, and several capacitor replacement that you recommended, now the machine is clean sounding. Lots of clicks and weird noises have disappeared... more importantly, no apparent oscillations with the decoupling caps from power pins-to-nearest-ground, for OPA2604 as IC501.
About external repro preamplifiers: indeed I was searching for them when I stumbled upon your thread. The previously quoted tesis about "transresistance preamp" showed a Crown discrete preamp and their siblings can be found on hifiengine, with Studer B62 which is also feasible. The hard part would be the several speeds and the bias circuit and adjustement...
https://www.hifiengine.com/manual_library/crown/pro-800-series.shtml
*The head pressure brushes seem like a good idea/mod...
I will try to take good pictures of the transport mods.
Cheers everybody,
M.