I've just been listening to the Pioneer with my GC amp, hmm. it certainly does sound bad
Still believe the mags?
I do happen to have a spare smd OPA2132 so I'll stick it in along with the 5v reg, remove those nasty muting transistors and lower the impedance of the output resistors.
You don't need to remove
the muting transistors.
After the output coupling cap you have: 22k to ground, and then 220R+muting transistor+220R, and then the RCA plug.
If you remove the two 220R series resistors the muting transistor doesn't connect to the signal anymore.
Then, put a 50R resistor under the circuit directly from the cap to the RCA.
Regarding the filter having radical changes
I've not seen op-amps implemented like whats used in the pioneer before
Nothing so different, it's an inverting op-amp with the voltage reference for NI input coming from the dac.
I don't have the service manual but I followed the circuit and after the dac you have:
- 100uf coupling cap (!!!!)
- ceramic cap to ground
- 10k feedback resistor
- ceramic cap from input to output
What I've done on the filter was:
- remove the SMD 5.6k and change it for a normal 4.7k film resistor (yes, there are holes to mount normal resistors on this position). This way I gave the output stage a little more gain (TI recommends 2x, even higher...).
- removed both
filter caps and installed a 27pf from input to output (feedback).
I didn't have anything else than ceramics at such a low value.
I tried 200pf styroflex but the sound was not so good, a much lower value was needed.
I've also noticed there is 100uf coupling caps before the op-amps.
Don't make me cry.
I changed those for 6.8uf bipolar electros (those pinky caps you see on the pic), bypassed with 100nf polyprop.
I wonder why the op-amps are run by single supplies, is it to keep costs down?
BTW, how to bypass a single supply SMD op-amp, my way:
- Bent the legs to size, cut them short, put the cap on top of the op-amp and solder directly at the V+ and V- pins.
I used a 33uf/40v Philips cap.