musical fidelity p170 power amp

[whats gan on]

i have modified my p 170 using AD979 op amps this makes a big improvement to the sound but when i unplug the interconnects the amp goes in to motor boating with repeated low frequency popping from me martin logans does anyone know how to stop this also i want to replace the two main audio transformers does anyone know the va rating of these transformers thanks

 

[indianajo]

You don't say what op amp it had before or provide a link to the schematics. A lot of times when you put in a faster op amp than a 741 or a 4558, oscillation occurs. This happened on my disco mixer at 1 Mhz. It didn't cause my CS800S amp to motorboat, but it sure ran the cooling fan really hard. You might try putting xx pf (picofarads) of capacitance around the feedback resistor on the op amp. This cuts the gain of the circuit at high frequencies. I used disks marked 22 from a R**** S****** grab bag, probably about 22 pf, and probably mismarked, knowing R**** S******. No more oscillation.
The feedback resistor is the one that runs from the point of the op amp (output) to the minus input.
You might also put in a shorting jack on the input of the amp if it is 1/4 phone or mini phone jack. This shorts the input when the source plug is not attached and prevents picking up RF from from the PC or TV in your room, etc.

 

[jaycee]

you also want power supply decoupling capacitors as close to the opamps as possible. 100nF from each rail to ground, and 100uF between rails.

Honestly, in a circuit like this, you're probably not getting much over using NE5532's

 

[whats gan on]

p170

thank you for the advice i will try that i bought a single version of the lm4562 and it sounds to my ears as good as the ad 797 and no problems