Strange preamplifier output stage

This is the an output stage of my Technics SU-C1010 six channel preamp. I want to change the opamp for a LM4562, but I have no idea how this circuit works.


Would it be wise to give it an own symmectrical power supply? Whatfor are the 330P caps across the inputs? Whatfor is the voltage from Q511 to inputs? Or why does it have no symm. power? :confused:

Your help is strongly appreciated
The FET is used to bias the opamp inputs at half supply. As there is only a single supply, this is necessary to get max pos and neg output swing from the opamp.
If you want to change this to a double supply system you should also take the other parts of the amp in consideration. My guess would be that it isn't worth it, but check it out.

Jan Diudden


Paid Member
2007-07-17 2:35 am
Central Berlin, Germany

it's not so strange as it seems...

Q511 provides a virtual GND, about half the supply voltage (look at the voltages stated, 4.6V at the source).

The amps run in standard inverting config, with high gain (-3300/47=-70 which is a little too high IMHO, there probably is another source impedance in series not shown here).

Those caps do the high-frequncy stuff, the 330p rolls off the gain to zero for high frequencies and the 100p shunt HF to the virtual GND node.

The outputs must be capacitively coupled, not shown here, because they swing around the virtual GND voltage, not zero.

One would need a little more info about the enviroment of the circuit to make proposals for replacement.

- Klaus (arrgghh, lost the race to Jan, I see:D)