I've done most of mentioned changes and I see great improvements to the sound. Very old (year 1979) SRM-1 (improved and modded to Pro) is still at least a little better, but I can enjoy the sound of SRM-252A right now, which was unacceptable before.
Both R1 and both C1 are removed. I have to remove both R2 also and place jumper here (it's stupid thing to buy an expensive cables just to listen to the unnecessary resistors). Hights are clearer.
C31 was replaced by 470uF/16V low ESR. C31 and C36 got also 3u3F/16 X5R (monolotic) SMD.
C4 and C5 are replaced by 2x Jamicon TH (low ESR) 47uF/250V 12.5x25mm (16x25mm is a safe max dimension - I'm looking for low ESR 100uF/250V of this size) supported by 2x SAMWHA X5R 1u/250 SMD 2225 (monolitic) and 2 x Miflex old Polish foil 68nF/250V caps (I'm looking for foil 220nF or 330nF/250V [R10 dimension]).
C37, C39 are replaced by another 2x 1u/250 SMD 2225 (monolitic X5R) SAMWHA.
R42 and R43 are replaced by endure Bourns 470uH/62mA/[15ohm] SMD 1812 coils/chokes.
Between both C4, C5 and C37, C39 both paths was cut and another 2x Bourns 470uH/62mA/[15ohm] SMD 1812 coils/chokes was placed.
This alone was very visible improvement to the "culture"/clearness of the sound.
By the way voltage rose from +209V and -207V to +233V and -230V what is also good.
Both channels D2, D3 was removed (jumpers was placed) and path was cut to separate both R19 and R20 from power line by 2Kohm Vishay/Beyschlag MMELF MMA0204 resistors (1x for each channel) - it's perfect value for keep the same currents and no tuning by use of potentiometers is really needed after.
This also has improved the sound (at least I've a feeling of the improvement). By the way, it well known that LEDs are not very linear with small currents ;) , so why Japanese "engineers" made another mistake here, in this amp and in higher amps models?!? Only for the faster visual inspection of the failure in the amp?!?
This modification do almost nothing with the open loop gain value, which I decide to keep as it was.
Both D4 ~1.54V RED LED was replaced to ~1.8V GREEN LED, just to rise a bit output current from 1.2mA to something about 1.5mA. This also has improved the sound especially when bass appears with higher volume.
Power consumption (measured before Stax XFF-1240 12V/400mA adapter) has risen from ~6.8W to ~7.5W.
R5, R6, R11, R12 have not been touched but I've a plan to replace it to safe for input JFETs values of 5Kohm and 2Mohm - it means GAIN of 400V/V (stock is 500V/V [200mV/100V]), but it may cause side effect, not an improvement.
I put the photos in few days.
When I finish, I call it SRM-252U (Ultimate) :D
I think I need second SRM-232A to compare following changes in one of them side-by-side... and I need fast switch box (with two cables ended with 5-pin Stax plugs and two 5-pin Stax inlets) - it will costs me... :D
Edited by borizm - 1/18/13 at 5:28pm