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In Reply to: RE: Wally Jung be damned :-) posted by 1973shovel on June 30, 2007 at 09:57:41
Definitely replace the 4558(s). I would not be able to sleep if I had one of those in the house even. A lot has changed in 35 years!
A few times the op amp swaps I made were audible and sometimes not. I'm very careful with bypassing/decoupling and always go with what the application notes for the op amp recommend and more.
Some people hear the words op amp and they freak out, waving a silver cross in front of themselves. They usually are not very knowledgable about anything circuit wise.
A lot has changed in 35 years.
Follow Ups:
Wow. An evil poltergeist, that 4558, eh?I have an OPA2604 sitting here, but it's slated to go into a Yamaha T-80 tuner I picked up, which also uses the 4558. That's a more critical application, for obvious reasons.
What I'd really like is a safe (non-oscillating) replacement for the 4558 which is known to have great bass. In this application, the sonics of anything above 100 Hz to 120 Hz will be less important, since it's a subwoofer amp.
I need to learn more about op-amps. I've done a lot of reading on this site and there's a lot of mixed information. At least it seems so to me. "Bypass this" "use ceramic caps", "NEVER use ceramic caps", etc.
Thanks for the input, nightdoggy. Just like your feedback on my JVC 1050 post, this is much appreciated!
WTF? I first wrote "unimportant", then decided "less important" would be the correct verbage, but forgot to delete my "un".
I love catching these things AFTER I post. Sheesh!
The rule is to use non-inductive caps on each supply rail and no sockets. This is so they will act as close to "ideal" capacitors at the very high frequencies, to provide stability and prevent oscillation. Remember that the op amps we want to use have open loop bandwiths in the MHz range. If you use a 1 uFd dipped tantalum paralleled with a 0.1 uFd ceramic disc or monolithic ceramic, you will not have any oscillation or supply noise issues. Remember to keep the cap leads short. The fact that both of these cap types are not appropriate for audio signal use is irrelevent. They are ideal in this dc application.
You can also add a .001 or .01 cap between V- and V+, (pins 4,8) but it is not essential.
There should also be a cap bypass across each feedback resistor. This is to reduce the op amp gain at very high frequencies. It will also address your slew rate concerns.
If you want to start rolling off the gain at about 100 kHz, use about a 33 pF cap between each output (pins 1,7) to each inverting input (pins 2,6). I generally use a 5 pF or 10 pF silver mica, depending upon the op amp and how much bandwidth I want. I like to minimize phase shift at the highest audible frequencies.
For subwoofer use I would go with an op amp with a bipolar input. Try a National LM4562 or 6172 or a Linear Technology LT1358CN8 or LT1361CN8 for example. Review the applications notes also.
I did find the 1050 service manual. It is pretty thick as service manuals go and it has excellent diagrams, board layouts, schematics, etc. etc. I'll never use it again at this point.
Happy tweaking!
Doggy
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