This one is an early version with each both of the protection circuits used for each channel and the three relays for the soft start circuit. Bit of a mess this one (with plenty of other people been in it! 🙁 ) all the output MOSFETs test fine! 32 of them and original...I'm shocked. Someone previously tried to fix this with a driver board from a SR202. They actually wired the drive and feedback wires the wrong way round. So the drive was connected to ground. Cue shorted driver transistors, burnt resistors and vaporized tracks and probably other shorted semis. I had a spare driver card for this from a SR707. Changed to the correct feedback and protection resistors for an SR606 , did the three 27K resistor mod, increased the feedback DC feedback electrolytic cap to 820uf from the 100uf, changed the DC block caps before the gain pots to 220uF bi polar.
powered up on dimbulb, clicked out of protection. DC offset settled at 30-40mV. Bias at 500mV.
Plenty of bass! Sounds very good. Very little square wave tilt at 20Hz. 10Khz square wave has a very slight soft edge. No ringing/oscillations.
Then when I put my hand on the case, DC protect triggers and my RCD even tripped! Took me a little while to work it out. The case sides had a different ground potential to the back. There should be a washer sandwiched between the bolts at the back to keep a good ground for the whole case. The back panel was also not tight. I put in new washers, tightened up the back bolts and no DC trigger. Mains earth wire was also secure to back panel. and mains cable entering amp in good condition.
The three soft start relays are getting weak or need cleaning, When amp is on its side the amplifier fails to start.
The signal grounding is a bit different in these amps I always thought pin 1 of XLR input should be connected to chassis but its connected to screen which goes all the way through the circuit ground. I believe the CMR can be improved too with a change of resistors around the opamp. If i remove the screen cable from XLR pin 1 the DC offset goes up to around 120mV.
Anyway, it plays and sounds good.
powered up on dimbulb, clicked out of protection. DC offset settled at 30-40mV. Bias at 500mV.
Plenty of bass! Sounds very good. Very little square wave tilt at 20Hz. 10Khz square wave has a very slight soft edge. No ringing/oscillations.
Then when I put my hand on the case, DC protect triggers and my RCD even tripped! Took me a little while to work it out. The case sides had a different ground potential to the back. There should be a washer sandwiched between the bolts at the back to keep a good ground for the whole case. The back panel was also not tight. I put in new washers, tightened up the back bolts and no DC trigger. Mains earth wire was also secure to back panel. and mains cable entering amp in good condition.
The three soft start relays are getting weak or need cleaning, When amp is on its side the amplifier fails to start.
The signal grounding is a bit different in these amps I always thought pin 1 of XLR input should be connected to chassis but its connected to screen which goes all the way through the circuit ground. I believe the CMR can be improved too with a change of resistors around the opamp. If i remove the screen cable from XLR pin 1 the DC offset goes up to around 120mV.
Anyway, it plays and sounds good.
