Active device substitutions needed for this schematic!


2002-02-15 1:42 am

Would someone be kind enough to take a look at this schematic and recommend some device replacements?

Before you take a look, be warned, as circuits go - it ain't pretty!
Its a big old Musical Fidelity, OK, OK, not the last word, but we've been together 15 years and the amps like a cosy old pair of slippers - you get the idea. Anyhow, I've decided it deserves a spruce up, but its with the active devices that I'm really and truly stuck.

I even tried some straight op-amp relacements (without changing any of the ancillary components - I just kinda said a little prayer and hoped for the best) but virtually everything LT1028, OPA134, OPA627 oscillate like crazy. OPA132's seem OK though...

I'm sure you guys have far better things to do but I'm really kinda stuck.

I hope the attached info is useful.

The output devices 2SK 226 and 2SJ 83 are enhancement mosfets, now discontinued.

Thanks in advance
parts replacement

Hi Lenin,
No wonder you got oscillations with other opamps.
If you place a fast opamp in front of a buffer/outputstage you get the effect of a dog chasing its own tail. You have to slow down the opamp by placing 50 or 100pF maybe more, from the opamps output to the inverting input. The opamp originally present had compensation too, but more in a custommade manner. This has to be removed for modern opamps.
See also the Application Note #47, High Speed Amplifier Techniques at Appendix C The Oscillation Problem-Frequency Compensation Without Tears.