Re: Op amp distortion


"Phil Allison" <philallison@xxxxxxxxxx> wrote in message
news:5jum6vF1b1u3U1@xxxxxxxxxxxxxxxxxxxxx

"Mike Faithfull"

As part of a "powered monitor" (not for the PC but wedge-shaped speaker
cab with built-in amp for use on stage) I assembled a commercial power
amp (already tried and tested for several years in another application)
and a 'front end' comprising two dual op-amps (LM358 or TL082) to provide
two balanced input preamps and a mixer.


** FORGET using LM358s for audio (other than maybe a guitar stomp box).

Stick with the tried and proven TL072 / 82 series, the NE5532, the RC4558,
4560 series and even the LM833.

All these can be trusted to be free of x-over problems and to have low
THD.

Note, the TLs and the LM833 can display a tendency to oscillate at
frequencies in the MHz range if wiring layout or supply decoupling is poor
OR if the output pin is not isolated from load capacitance by 100 ohms
or so in series.

Such oscillation will often result in significant audio distortion.



...... Phil

Thanks for all replies. Regrettably, my days of regularly getting to grips
with this technology have long gone, so no 'scope is available, only the
trusty old AVO 8 and a cheap digital test meter that doesn't actually say
"made in China" on it anywhere but we all know don't we .......

The LM358 was chosen originally because I got the impression from the
marketing stuff that it was "optimized for single supply operation" and I
wanted to use a +12v power source available from the power amp and avoid the
need for additional cost/components to make a +/- 15v supply. This meant
that I was having to make modifications to "textbook" circuits to run off a
single rail and that caused me some problems. Therefore, after a lot of
experimenting to no satisfactory conclusion, I bit the bullet and did the
right thing !!

The balanced line input stage circuit is almost a copy of the one shown in
Figure 2 found at http://sound.westhost.com/project51.htm . With the
benefit of hindsight I would probably have been better off "upgrading" to
Project 87 and building it on a pcb purchased from ESP, but with the
exception of the ICs I had (almost) everything I needed in my junk box to do
it the cheapskate way. The "almost a copy" in my version has the following
differences from the published schematic:

C1 not fitted ('cos the only things small like that I had to hand were
committed elsewhere (see below) )
R11 is 27K and R10 is 22K and a 10K pot in series (so I could "tune for max
common mode rejection" and because I had used the last of my 10K resistors
anyway!)
R12 is not fitted and the output on pin 7 connects through a 10uF capacitor
to the 'top' of a 10K pot in the mixer stage which is grounded at the other
end. The input to the next stage is then taken off the slider via another
10K resistor.
Pin 7 was temporarily connected through 3.9K to -15v to try to eliminate the
crossover distortion like in the mixer stage, but while it had the desired
effect in the latter, it didn't seem to make any difference here.

(Little light comes on in head ...) So perhaps crossover distortion is not
the problem here ... The cicuit is built on stripboard and although I did
connect some small value ceramic caps between the supply pins and ground at
each IC location, I haven't otherwise paid much attention to layout or the
possibiity that the circuit mught be hooting at some undetectable frequency
as Phil suggested.

Time to see if I can borrow a 'scope from somewhere I think.

I really am most grateful for all the help I'm getting with this, by the
way. Given the reason for the project, failure will inevitably bring egg on
face and comments about "thinking he's cleverer than he really is" from
other band members.

But surely life is partly about making mistakes and learning from them isn't
it? If you never make a mistake you're not trying very hard! :-)


.

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