LM358 not for audio

** The LM358 is a poor choice for most audio circuits - particularly if you want low distortion.

Why?

The LM358 and its quad version the LM324 are intended for single supply rail operation and to use very little current. To save current, the IC has an unusual output stage compared to most other op-amps. In audio circuits, there is a BIG problem is with this oddball output circuit - see schem on page 21 of this pdf:

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Qs 5, 6 and 13 form a complimentary, emitter follower, current booster supplying the output current - BUT the bases of Q5 and Q13 are linked so there is no forward bias on the stage.

Soooo - every time current flow into a load has to change between Q6 and Q13 - the collector of Q12 has to *jump* a nearly 2 volt gap created by the three B-E junctions involved. During the time this takes to happen, NO output current flows. This results in "crossover distortion" at the output which becomes worse and worse as the frequency rises.

On page 8 of the pdf, the makers allude to this fact:

" For AC applications, where the load is capacitively coupled to the output of the amplifier, a resistor should be used, from the output of the amplifier to ground to increase the class A bias current and prevent crossover distortion. Where the load is directly coupled, as in DC applications, there is no crossover distortion."

This is hardly an adequate explanation of the issue.

In the AC coupled case, one has to fit a resistor to ground of such a value that Q13 is never required to conduct current - which could mean a very low value resistor has to be used and is simply not practical.

In the DC case, the wording is quite misleading. Only if the load is connected directly to ground and the negative supply is also the ground is the comment true.

In the general case where the load is connected to a ground point that sits between the supply rails - then Q13 has to supply current and serious crossover distortion is the result.

BTW:

A low current, dual op-amp that IS suitable for audio is the TL062 and its quad version the TL064. Supply current is less than 200uA per amplifier and operation right down to +/- 2 volts is OK.

And definitely NO crossover distortion !!

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... Phil

Reply to
Phil Allison
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Interesting. Thanks for posting that. Looks like it's hard to overcome the 2 volt step needed between Q5 and Q13 to crossover.

-Bill

Reply to
Bill Bowden

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But there was a solution to some of this, I think it was a resistor at the= =20 output to ground, causing the output stage to act differently.

Of course, if the device is being used for the low current aspect, that=20 loading isn't likely to be good for current consumption.

Michael

Reply to
Michael Black

Did you only read the original post halfway?

Reply to
Bitrex

if you are using such an animal that allows 0 volts on the low side like a 358/324 etc, try placing a load R on the output, this will force the op-amp to attempt to maintain symmetrical output, at least that is what i've done.

Last time I looked at the internals of that, I seem to remember the output will go into a HI-Z state at the 0 volt level or some where near that, which gives it an advantage in single rail applications but causes issues else where.

Jamie

Reply to
Jamie

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