Re: Op amp distortion
- From: John Popelish <jpopelish@xxxxxxxx>
- Date: Sun, 02 Sep 2007 14:29:39 -0400
Mike Faithfull wrote:
(snip)
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 !!
Changing from single supply to dual supplies does not change the cross over distortion situation, if the output load is capacitor coupled. In either case, you decrease the distortion by adding a load resistor from the negative rail to the output pin, to maintain a small DC bias current through the output stage. The required resistor value depends on the AC load impedance and output swing. The combination needs to produce a unidirectional load on the output. But even with this band aid, the LM358 is not a very high performance opamp at the upper end of the audio spectrum. Okay for low gain, low swing applications, or low frequency (audio limited to 5 kHz, say) because of its low slew rate (only .3 volts per microsecond) and low gain bandwidth product (of only about 0.7 MHz). Normally used for audio only if very low cost is important.
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.
Are you located in the US? I have several old ones to sell for a little that I got from eBay and checked out.
.
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