anthony wrote:here is schematic. tried to replace ka3402 to opa4132. but overal volume became to low
spdif is not ttl
Holy smoke, you reverse-engineered and re-engineered the whole thing with signal names and all?
That is a very impressive task, just for everyone to know.
Yeah, replacing bipolar opamps directly with fet opamps might require some other changes as well, because in general, bipolars can accept and drive signals of higher amplitude with same supply voltage than fet opamps.
SPDIF coax output is acceptable. Using multiple buffers in parallel to boost output current is not the optimal solution, but it is cheap, it is a common chip and the output should work fine in home use nevertheless. As a bonus it could be isolated with a transformer, but most people don't need it.
By the way, what is the purpose of that 405x analog switch circuitry, some kind of output gain selection or output filter bandwidth selection?