Hello.I was given this amplifier one channel was blown so i desided to change both channels transistors and caps.
I used :
2SC1403A--->MJL21194 ( i already had those and used just for testing purposes)
2SA794--->2SA1837
2SC1567--->2SC4793
2SA921S--->KSA992F
2SD571L--->BC639-16
2SC1885--->KSC3503D (cant find E or F suffix on mouser)
2SA912--->KSA1381E
The problem is that when connecting the preamp out of the unit to main in amplifier,the amp is oscillating around 280khz. For testing purposes i have connected 50ohm high wattage series resistors in each of the supply rails.
When connecting my smartphone to main amp,amplifier plays fine with no oscillation.
When conecting main amps input to ground,amp starts oscillating again.
When connecting a wire to main amps input,the amp oscillates again...
There is no low pass filter at the input of the amp as you can see at the schematic.i could solder a 100p to 220p there.
Miller cap is 5pF.Maybe raise it to something like 47pF and leave lead compensation as is 7pF.
Any ideas?
I used :
2SC1403A--->MJL21194 ( i already had those and used just for testing purposes)
2SA794--->2SA1837
2SC1567--->2SC4793
2SA921S--->KSA992F
2SD571L--->BC639-16
2SC1885--->KSC3503D (cant find E or F suffix on mouser)
2SA912--->KSA1381E
The problem is that when connecting the preamp out of the unit to main in amplifier,the amp is oscillating around 280khz. For testing purposes i have connected 50ohm high wattage series resistors in each of the supply rails.
When connecting my smartphone to main amp,amplifier plays fine with no oscillation.
When conecting main amps input to ground,amp starts oscillating again.
When connecting a wire to main amps input,the amp oscillates again...
There is no low pass filter at the input of the amp as you can see at the schematic.i could solder a 100p to 220p there.
Miller cap is 5pF.Maybe raise it to something like 47pF and leave lead compensation as is 7pF.
Any ideas?
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If you now have insipid HF oscillation ( occurs under certain circumstances ) your circuit design has been compromised and will have to be restored to stability by component changes due to the change in BJT parameters .
What people don't realise is just "modernizing " by fitting up to date active devices without changing the bias setting components is asking for trouble due to changes in design gain settings .
Sticking an increased value Millar capacitor might be like putting a sticking plaster over an inch deep cut, it might work but for how long and then you have a change in frequency response .
The main compensation capacitor is C611 try upping that to 20pf (but no more ) but really you should be changing the bias resistors to suite the new BJT,s .
What people don't realise is just "modernizing " by fitting up to date active devices without changing the bias setting components is asking for trouble due to changes in design gain settings .
Sticking an increased value Millar capacitor might be like putting a sticking plaster over an inch deep cut, it might work but for how long and then you have a change in frequency response .
The main compensation capacitor is C611 try upping that to 20pf (but no more ) but really you should be changing the bias resistors to suite the new BJT,s .
I've had issues substituting fairchild KSA and KSC parts (small signal and medium power parts) into amps before, end up sounding harsh or just unstable.
Even when the spec's are close enough in all the key areas, there just seems to be something different about them. Had better results with more generic parts even if the output capacitance is much higher.
Picked up some cheap chinese small signal parts that are same spec as the KSA992/KSC1845 (which are the same spec as most other 120V small signal parts), but just seem to work...
Even when the spec's are close enough in all the key areas, there just seems to be something different about them. Had better results with more generic parts even if the output capacitance is much higher.
Picked up some cheap chinese small signal parts that are same spec as the KSA992/KSC1845 (which are the same spec as most other 120V small signal parts), but just seem to work...