Hello folks, I have had another stab at building a better RIAA preamp with tone controls. The devices used this time are 2 x NE5534, one for each channel of the RIAA preamp. This is followed by an OPA4134, 2 of the op-amps on it used for unity gain voltage followers (one for each channel) and the other 2 op-amps for Baxandall tone controls. Powered by a plus and minus 15V dc split rail supply. The circuit (page1), veroboard layout (page 2) and an image are attached. Well, it actually works well at low volume, and the original reason for posting here, i.e. poor treble response, has been solved - the treble and bass lift and cut work as they should and the outputs are balanced in volume. Unfortunately, there's the inevitable "but" coming.... If I stop playing music and turn up the volume, the left-hand channel has a very intrusive 50Hz hum. I know it is 50Hz (not 100Hz) because I have put the scope on it. With the turntable cartridge still plugged into the pre-amp, the hum outputs of the two NE5534 chips measured at the junction of R8 and R9 are 10mV pk-pk on the good channel but 140mV pk-pk on the bad channel. Swapping around the turntable leads has no effect. Swapping the NE5534 chips around has no effect - hum stays on channel not on chip. Cross-connecting the NE5534 outputs at the junction of R9 and C16 and turning the PA back on transfers the hum to the other speaker and therefore there is absolutely no doubt that the hum is being generated by the NE5534 shown in the pic at the bottom left; grounding the junction of R8 and R9 for that channel silences the hum. the top left (rear) device in the pic is working as it should, as are both channels of the OPA4134. I tried putting additional decoupling onto the dc supplies to the NE5534s using a 10R and 100uF electrolytic on the + and - rails to no effect. I even disconnected the front NE5534 circuit and replicated the layout of the rear circuit on a daughter board and connected it to the main board as a replacement for the front RIAA channel. The result was the same - the RIAA pre-amp on the daughter board now hums while the rear one still doesn't. Looking again at the image, the use of coax to one of the input sockets and an unshielded wire to other isn't the answer either - the unshielded wire provides the quiet channel!
Any thoughts and recommendations welcomed. Cheers, Jerry