Hi. I have to repair a philips 22Ah380 amp and its 22Ah280 pre amp. With all the flat controls, comparing the set to any built-in amp, it sounds bad, lacks bass, I think it should sound much better. I test it with 8 ohms resistive load and signal generator and the response is flat, even beyond the audible range. I did a full recap on the preamplifier (electrolytic only) without any appreciable change. The amplifier had a previous repair, originally it has 2 pairs of transistors per channel, now it only has one higher capacity pair, 2sc5200 / 2sa1943. Can this cause the bad sound?
If the new devices are much faster than the old ones, that could make the amp more unstable (or closer to the edge of stability) and make it sound different, possibly more edgy and harsher. But could still work fine. Some manufacturers design stuff like that tough...
Impossible to known without hearing an original unit.
Impossible to known without hearing an original unit.
One of the problems with changing the output devices from multiple ones to single ones is ---has the bias been adjusted for the new active devices ?
By themselves those new ones don't cause "bad sound " but "tube rolling " and BJT "rolling " aren't the same .
Tubes can actually still work to a degree but solid-state devices need accurate circuit adjustment .
IG BJT,s are not my favorite active devices and even JLH had caveats about them ,getting the "best " of both worlds didn't work out perfectly when it came down to it but adjust the bias first if that has not already been done .
IF the original circuit did NOT use those types at the output then what Ohdsp said will occur without changing the values of the compensation capacitors .
I do not have a diagram so cant be positive ,could you post one please ?
By themselves those new ones don't cause "bad sound " but "tube rolling " and BJT "rolling " aren't the same .
Tubes can actually still work to a degree but solid-state devices need accurate circuit adjustment .
IG BJT,s are not my favorite active devices and even JLH had caveats about them ,getting the "best " of both worlds didn't work out perfectly when it came down to it but adjust the bias first if that has not already been done .
IF the original circuit did NOT use those types at the output then what Ohdsp said will occur without changing the values of the compensation capacitors .
I do not have a diagram so cant be positive ,could you post one please ?
R646 - balance 0.
R647 - current output BJT 25 mA .
Turn in parallel 0.82 ohm in each arm (paar).
R647 - current output BJT 25 mA .
Turn in parallel 0.82 ohm in each arm (paar).
By sheer co-incidence ? both the old BJT,s have the same bias as the new ones BUT there is a big gain difference .
The old ones have a much higher gain for the same bias so its going to seem as if there is a lack of "power " /drive " musically.
The biggest drain of current is on the low frequencies so you feel a "lack of bass " therefore although it seems an upgrade as the circuit stands you "cant get blood out of a stone " .
I didn't realize the difference in gain till you posted the circuit diagram with the original BJT,s .
Options ? either change them back to original or install much higher gain output devices.
Its your amp but as I said I am not a fan of IGBJT,s many are so others will tell you otherwise , in any case they aren't the right devices for your circuit.
The old ones have a much higher gain for the same bias so its going to seem as if there is a lack of "power " /drive " musically.
The biggest drain of current is on the low frequencies so you feel a "lack of bass " therefore although it seems an upgrade as the circuit stands you "cant get blood out of a stone " .
I didn't realize the difference in gain till you posted the circuit diagram with the original BJT,s .
Options ? either change them back to original or install much higher gain output devices.
Its your amp but as I said I am not a fan of IGBJT,s many are so others will tell you otherwise , in any case they aren't the right devices for your circuit.