Favorite quad op-amps for synth design?

Batz Goodfortune batzman at all-electric.com
Thu Mar 23 10:30:55 CET 2000


Y-ellow Grant 'n' y'all.

At 01:20 AM 03/23/00 -0600, Grant Richter wrote:

>One note of caution regarding the 833/837 is the somewhat high
>input bias current of (gasp) almost a micro-ampere.
>
>These will not work so well in sample and hold applications that
>rely on the JFET input of the op-amp for microscopic bias currents.

Well this would go without saying. They're designed with audio in mind. As
previously stated. The critira for audio is somewhat different to the usual
run-o-the-mill op-amp. In that things like offset, input bias and even
rail-to-rail aren't important, noise is. Another factor is the ability to
virtual earth. Something that may also be lending to problems with the 324
in audio...

Traditionally, op-amp have been used in audio because they were cheap. I
mean you wouldn't use a 741 for a pre-amp in your right mind these days.
But the only alternatives 20 or more years ago would have been to use one
of a dozen or so equally crappy op-amps or build a discrete design. Which
of course companies like sound craft and others use to do.

Most other op-amps, when used in audio, were a compromise against cost and
complexity. Even the humble TLO8x/LF34x (which are the same thing in fact)
are not ideal for audio use. It's just that they got used because compared
to a 741 or an LM301, (or  Heaven forbid, a TCA220) they were a god send.

Enter the NE5534 circa 1981 and the LM833 circa 1984 and you suddenly saw
an explosion of cheap, high quality audio stuff. Consumer products which
traditionally had the expectation of not being very good quality compared
to the home stereo, -such as car audio- suddenly started to sound good.
Till today when the expectation is that if your car hi-fi isn't as good as
you're home hi-fi, then it must be broken.

I'm not putting this entirely down to the LM833/NE5534 but it was about
this time when manufacturers woke up to the fact that the market for
op-amps in audio was a market unto itself. And that if they were to start
producing op-amps which were specifically tailored to that end, they'd be
on a winner. Which of course they were.

It also marked the explosion of the special purpose op-amp as opposed to
the general purpose one. The 741 tried to be all things to all people. It's
trade offs were such that whilst it was the jack of all trades, it was the
master of none.

Let me digress a little here. You're still better off using something like
an LM324 in a DC or control type application than you are a 741. Even if it
cost more. Even if you only needed 2 of the 4 op-amps in the package. Even
though the  741  will work as a voltage buffer it's less than ideal. You
wouldn't use it for audio either  even though you don't need rail to rail.
Because it's noise level is atrocious.

But that's what we're talking about here. Op-amps used for audio. So the
best rule of thumb I could think of is, when dealing with audio, use an
op-amp designed for audio. Now  that could be an 833/837 by what ever
criteria you need package wise. Or it could be an 5534 or an OP-07.
Whatever gives you the most bang for the buck. The fact that these things
now have a bandwith of 20meg in some  cases and that you have to tame them
as a result is another issue altogether. It just shows what sort of quality
you can expect these days.

However you  wouldn't use them in some kind of other more specialist
circuit. Like a sample hold. Some of these things have way too much offset
anyway. And there's no provision to trim the offset because it's not
important in audio.

If  you were going to do a sample hold, you'd want to use a 3140 or
something. With a MOSFET input stage. As you've already suggested thus..

>Weren't there some op-amps that had MOSFET inputs?
>Does the CA3140 or 3130?

But these  guys are  traditionally more specialized and more expensive.
Mandatory in some cases. You can't even use a TLO81 or an LF351  in these
kinds of apps even though they are a JFET input stage. You might get  away
with them as an integrator but not as a sample hold.

For the record, the 3130 is a different beast though I can't remember why.
I've never used one. 3140s also cost the earth. Well they do down here
anyway. Worse  luck!

But getting back to the original question. You have a range of audio
op-amps. Whether they be 833s or 5534s or Op-07s etc etc. There isn't a
vasst lot of difference between them. The op-07s are probably better than
the previous 2 but I wouldn't go out of my way to get one. Or a bucket
load. Unless they were cheaper and easier to get. The only way you're going
to get a better mic preamp out of an IC solution is to use something like
an SSM2017. Then again you wouldn't use these as a general purpose pre-amp
in a mixer either.

BUT! Use one of these for the  input stage and use your choice of audio
op-amp for the rest. If you need a sample/hold use a 3140. If you need to
do CV control then use an LM324.

And this leads me to the last hint on mixer design. If it can at all
possibly be  helped, never put a gain stage in the middle of the mixer.
Amplify the small signals at the top of the chain and let it trickle down.
If you amplify in the middle of the mixer you not only amplify the input
noise of that stage, but also, all the noise  from all the preceding
stages. And the practical upshot of this is that even if you're using a
less than noiseless op-amp for these sages, because  you are not requiring
it to amplify anything greatly, you're not amplifying the noise as well.

Hope this helps.

Be absolutely icebox.

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