Thought I'd better start off in this new section rather than the Lab.
My Edcor OPT has still not arrived but I managed to rig up a HiFi 8K:15ohms in order to get things moving. I'm still breadboarding this at the moment. I find that if I can make it work as a mess then it stands a better chance when its all tidy.
What I'm working on at the moment is separating all the frequency poles. Although there is no official feedback in the circuit, there is still the possibility of inducing feedback from the high ac voltages on the OPT back to the input stages. On the original 26W schematic you will notice that the wire taking the CV to the 12AU7 bridge tube is screened, this shows they understood the potential problem.
On the 26W, the high pass poles are 1.4Hz and 28Hz and the low pass is 450kHz or thereabouts. The low pass due to the OPT is not known but may be of the order of 60kHz.
My circuit replacing the interstage with caps and dif amps is obviously going to add more poles to contend with, so I have to make sure that no two are the same otherwise instead of nice safe 90 degree phase shift we will have 180 degrees and the possibility of ringing and oscillation.
best
DaveP
My Edcor OPT has still not arrived but I managed to rig up a HiFi 8K:15ohms in order to get things moving. I'm still breadboarding this at the moment. I find that if I can make it work as a mess then it stands a better chance when its all tidy.
What I'm working on at the moment is separating all the frequency poles. Although there is no official feedback in the circuit, there is still the possibility of inducing feedback from the high ac voltages on the OPT back to the input stages. On the original 26W schematic you will notice that the wire taking the CV to the 12AU7 bridge tube is screened, this shows they understood the potential problem.
On the 26W, the high pass poles are 1.4Hz and 28Hz and the low pass is 450kHz or thereabouts. The low pass due to the OPT is not known but may be of the order of 60kHz.
My circuit replacing the interstage with caps and dif amps is obviously going to add more poles to contend with, so I have to make sure that no two are the same otherwise instead of nice safe 90 degree phase shift we will have 180 degrees and the possibility of ringing and oscillation.
best
DaveP