Ancient Opamp replacement

Hi guys,

I'm bringing a Mcintosh MR80 tuner back to life and thinking about replacing the antique LM201 op amps in the signal path (used as buffers) with something more modern, perhaps OPA132's. If I decide to swap them out, should I;

1) remove the 47pF compensation (?) caps?
2) add bypass caps to the power pins?
3) anything else

Thanks!
 

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anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi McIntosh guy,
That is an easy question. The LM201 is the industrial version on the LM301, a decompensated op amp. You could upgrade him with an NE5534A, or try the OPA132. Given the new op amp is unity gain stable, you should remove the compensation caps. It never hurts to add capacitors to the power supply pins, but try it without first. You might be fine without them.

-Chris
 
IF they work, don't touch them, there's nothing to gain and *maybe* you could have stability problems.

Besides, they are called buffers but they are not, some gain is implied since those are inverting Op Amps, we see the NFB resistors R955/956 but not the input one, to the left of the screen.

Those excellent designs were carefully tweaked and matched, following rules we all ignore.
Who knows what were they trying to achieve, some 40 years ago?

"Newer" does not automatically mean "better" , and "different" may be the kiss of Death.
 
Seems like they employed feedforward compensation to squeeze the most out of the part. Especially in a tuner, I would then expect it to be one of your lesser worries. You might consider adding 3-5 mA of output Class A bias though.

BTW, these aren't strictly buffers, as the circuit is no doubt inverting. It'll have negative unity gain if source impedance is 33k.

EDIT: Looks like the first stage would have some gain, with the second stage being unity-gain buffers / active filters. Are there any series resistors at the output? Then again, I guess LF356s wouldn't necessarily need any, those run pretty generous output stage idle current and would be expected to be good capacitive load drivers.
 
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anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
In this circuit, I don't think that will be an issue. Just make sure you watch the outputs when you first power it up for unintended "features". I can't see any issues here with your plans. Try it and report back. Even better if you can measure the FM distortion before and after.

-Chris
 

chakija

Member
2007-09-27 11:43 pm
I am sorry for hijacking the thread...but I have same problem as McIntosh guy.
I am desperately trying to locate Harris HA-2630/2635. My vintage preamp is using these as output devices and one channel is toasted (oscillating).
If anyone has any information where I could get one, please give some info. :)
Thanks.
 
Hi McIntosh guy,
That is an easy question. The LM201 is the industrial version on the LM301, a decompensated op amp. You could upgrade him with an NE5534A, or try the OPA132. Given the new op amp is unity gain stable, you should remove the compensation caps. It never hurts to add capacitors to the power supply pins, but try it without first. You might be fine without them.

-Chris

So am I correct in thinking I should remove the 47pf caps if I try the OPA132?
 
Done. I installed SOIC OPA132's on Browndogs and pulled the comp caps. I then put a scope on the outputs and no oscillation.

From an audio standpoint, wow! Using the FM muting to silence between stations, the background noise & hiss is now nonexistent, even when volume is turned all the way up to 11 (that's a Spinaltap reference btw). Nice upgrade!

Thanks anatech (Chris)!

-Mike
 

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