Caps on 741 supply voltages

You want to prevent parasitic oscillations in the chips. I always drop a 100nF cap from each supply to ground at each chip, and I usually try to make sure there's a 10uF electrolytic somewhere on the board.

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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Excellent, thanks! What's the 10uF electrolytic for though? I've always been taught to stay away from electrolytics, though I guess that's for RF purposes only.

jesse

The little caps decouple each chip at high frequencies, the big caps decouple the whole board at lower frequencies.

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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

I've got a nasty suspicion that the big electrolytics have enough equivaent series resistance to damp any high-Q resonances you might get between the wiring inductances and the 100nF ceramic disks.

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Bill Sloman, Nijmegen

The 100nF caps handle 200Hz and up pretty well, the 10uF is for lower frequencies.

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JosephKK

When I was growing up, [ ;-) ], these were called bypass capacitors. I'm anal about them - in this case, I'd put probably about a 1~10 uF aluminum from each V to ground - you _do_ have a good low-resistance ground plane, right?

There may be people who disagree with me, but for audio and maybe even video/low band HF, and always, always, with digital, you can't overcapacitate.

(something in me wants to put a smiley there, but the answer isn't supposed to be flip - just its form. ;-) )

Cheers! Rich

Might, indeed! Your answer is the correct one, Bill, because a sea of 0.1 caps certainly makes a nice resonant circuit, somewhere in the 5 to 20MHz region. A single crummy electrolytic is just what the doctor ordered to kill that resonance. Better be crummy tho, no low-esr tantalums need apply. Ahem, crummy, but not too crummy!

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 Thanks,
    - Win

We ARE talking about 741s here. Adequate for mid-fi audio but not high performance by any stretch.

Yes you can !

Lots of bulk capacitance to ground simply provides a path for any ripple. You end up with a dirty ground.

Graham

What do you mean by 'handle 200Hz and up'. Is the power supply no good for that on its own ?

I think you have a fundamental misunderstanding of their purpose.

Graham

The original 741 suffered from "pop-corn" noise, so-called because that was what it sounded like in audio circuits. Back in 1970, one of my then colleagues was using LM308s in a high-quality audio frequency front end purely because the processing necessary to make the super-beta transistors eliminated the pop-corn noise.

Modern 741's are supposed to be free from this vice, but the last time I replace a 741 in a circuit, back in 1988, we did seem to get rid of a lot of pop-corn noise, and the effect was quite noticeable - it was controlling the induction heaters on a GaAs crsytal puller, and they went from bang-bang operation to the sort of proportional control they'd been designed to provide. The operaters were grateful and the GaAs crystals seemed to have fewer defects. This was the machine that made 95% of the single-crystal GaAs produced in the West (as it was then).

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Bill Sloman, Nijmegen

Hey, even 741s can suffer from ringing on their supply bus lines. They pass this right on to the next stage, where common input-stage BJT nonlinear RF rectification turns this into distortion.

Bill, what did you replace the 741s with in 1988?

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 Thanks,
    - Win

OP-01. I subsequently did a fair bit to the front end electronics - we weighed the crystal as it grew with an LVDT on a spring, and the LVDT monitoring circuit got somewhat fancier under my care - but that was the sinble most effective improvement.

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Bill Sloman, Nijmegen

That's an interesting case.

1/f noise may actually have been more problematic than 'poporn' in your instance though.

Graham

The 741 hasn't got any spec for either, and neither does the OP-01. IIRR the OP-27 and OP-37 were amongst the first that did specify !/f noise - the LT1001 certainly does (0.3uV ptp typical 0.6uV max for

0.1Hz to 10Hz) and I'd have gone for something with a specification if 1/f noise had been a real problem.

The LVDT was excited at a few kHz, and I amplified the output as much as I could before demodulation, so the OP-01 was more of a buffer than an amplifier by the time I'd finished.

I'd have loved to have used a proper AC-bridge on the LVDT output, but there wasn't enough space for much electronics directly above the LVDT, and there was a great deal of expensive mechanical engineering in the way of getting any more space.

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Bill Sloman, Nijmegen

You *do* have a fundamental misunderstanding.

Graham

No, it is not, that many trace inches away. That is why they are adjacent to the parts and not clustered elsewhere.

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JosephKK