SRM252mod

Hallo Mark!

Der umgebaute SRM-252 (siehe Anhang) funktioniert stabil.
Ich habe die vier 300kohm-Gegenkopplungswiderstände gegen 220kohm/2W/MOX getauscht,
dadurch reduziert sich die Verstärkung von 500x auf 345x und der maximale
Eingangspegel erhöht sich entsprechend von 0,59V auf 0,79Veff.
Ddadurch (fast) volle Dynamiknutzung der CD-Quelle und guten Regelbereich
des Lautstärkepotis (12 Uhr ist normal) sowie Erhöhung der
Leistungsbandbreite des Amps von 35kHz auf (zumindest) die 41kHz des SR202.

Weiterhin habe ich noch 2 (parallele) Elkos diekt vor der Hochspannungserzeugung ersetzt
und mit Bypass-Kondensatoren versehen (Murata HighCaps unter der Platine montiert):
33uF/50V/Elko -> 47uF/35V/Tantal/RM5 || 4.7uF/50V/X7R/G1206
1uF/50V/Elko -> 22uF/35V/Tantal/RM5 || 1uF/100V/X7R/G1206

Die somit mehr als verdoppelte Pufferkapazität
gewährleistet für die Hochspannungserzeugung
eine solide Baßleistung, und bei hohen Frequenzen
wirken die Tantalkondensatoren eh besser als Elkos
(durch die HighCaps zudem gut bis in den MHz-Bereich).

Der Baß ist zwar immer noch "flubby" und geht nicht sehr tief (50Hz?),
ansonsten klingt aber alles ok

Thanks AudioCats!

You didn't write which volume pot (including switch) will be better and fit.

You didn't write which HV rail caps will be better (lower ESR and/or higher capacitance) and fit.

You ditn't write which else low voltage caps you would repace and with what.

You didn't write why this amp hasn't enough potential to bother with

(isn't it the same circuit as the SRM323 except the power supply?)

But you advice me that this is not the DIY forum.

I also adjusted the bias offset by reducing the voltage difference of the output with TR2

and reducing the voltage between output and ground with TR1 to below 200mV (DC).

Finally I mounted rubber feets (17mm dia, 9mm height) with M3x10mm screws

instead of the taped foam feets below the case.

Seems the new version SRM252S uses some different transistors (instead of the SC1968)

and thereby might not work stable with reduced gain. I don't believe so, but it's possible.

I recommend using the tantals (+HighCaps) for stable operation anyway.

Seems there exist 22uF HV caps that will fit (height 20mm, diameter 10mm)

and have a higher voltage (350V instead  250V) than the original ones.

Maybe they have a lower ESR and/or will last longer.

By comparing the modded 252a with an unmodded one (serial number 1 apart),

the modded one has a tighter bass and sounds more precise, but

the soundstage comes nearer to the listener and the separation of the instruments

sounds less homogen, so the modded one sounds better with pop/electronics than with classics.

Maybe the diaphragm of (at least) the SR202 benefit from less control/feedback at higher frequencies.

This could be the reason for the commonly increased gain of the newer stax amp versions. 

The reduction of the gain from 500x to 345x increases the usuable poti range 

by not much more than 1 scale unit, so with a cdp source

I would try gain 250x or when using the SR003 even 200x.

Biasing the output of op's to drive them into class-a is common,

but an unsymmetric working balanced output stage is imho to avoid. 

.