15 year-old integrated - Op-amp upgrade

[Lenin]

Hi guys,

Just giving an old Musical Fidelity a 'once over' for a good pal (nothing drastic - electrolytic replacement, replaced about 10 signal path resistors).

Could some of you fine folks come up with some suggestions for upgrading the op-amp?

Thanks in advance...

[mlloyd1]

What's in it now? TL071, TL072, NE5534 or NE5532? Something else?
What particular part of the circuit : Line driver stage, RIAA stage, Headphones driver, etc.

mlloyd1

[Lenin]

Help! The JPEG details appear in the 'attachments' box, but not in my post. Arrrrrrghhhhh!


It's an LM318 line driver by the way. I believe some later models used the marginally superior LM118.

[Lenin]

I hpe this works...

http://www.geocities.com/ResearchTri...elity_a370.jpg

[Lenin]

C'mon, help me out here guys...

[mlloyd1]

Oh, I see. You were talking about a power amp. The LM318/LM118 was never a favorite of mine for much of anything. How much experience do you have? As drawn here, the LM318 has some frequency compensation applied. Playing with it will alter stability related characteristics (frequency response, phase margin, etc.) of the amp. If you're not careful (and even if you are) you could cause the amp to oscillate (especially with the MOSFETs) which could create some "difficulties" for you. Do you at least have access to a o'scope?

Now that we have the caveats out of the way, just for grins, I'd drop in a NE5534 first, then an OPA604 as experiments that won't break the bank if you damage them. You should probably remove the 47pF & 22pF caps and the 3K3 resistor. And I'd recommend tweaking the 120pF while watching the amp's response to a LOW level square wave. And Oh Yeah, DON'T do square waves at high power unless you know what you're doing.

That should be enough to get you in really deep trouble !



mlloyd1

[peranders]

Replacing the opamp can be tricky because it's inside a feedback loop. LM318 is rather fast (very fast in those days). The basic idea is rather conventional so if you try to find an opamp with similar speed and remove 22p, 47p and 3.3k but you must be prepared to insert some compensation.

I suggest that you take a look at Linear's application notes about composite amplifers. Use "composite" as one keyword.

This type of arrangement don't need a very fast opamp, an OPA134, NE5534, OPA627(!!!), many AD-types or similar will do. I would say any good audio opamp will do.

[Lenin]

Thanks guys,

Looks like I'll be busy. I'll post my results.

Lenin