teemuk wrote:
Re: Different opamps in the OCD (08.02.2018 11:03, on
DIYStompboxes)
It may seem obsessive but the OpAmps are not all equal and do behave differently in certain conditions. Which can explain why they sometimes introduce different "tones".
Besides few details (of which most matter very little in "LoFi" -effect design) OpAmp circuits operate pretty much "uniformingly" as long as their feedback loop remains operational (no pun intented). This is pretty much the sole purpose of an operational amplifier to begin with. This all, of course, changes if the OpAmp is overdriven outside its linear range. This is the region where circuit architecture differences of the OpAmps begin to display themselves.
- Different OpAmps have different limits in how close to supply rail voltage(s) their output signal can swing. It is common for these limits to have some degree of "asymmetry". e.g. positive half wave swings within 1V and the negative swings only within 2V.
- Some OpAmps "latch to supply rail" when overdriven to clipping. The poor recovery characteristic introduces much "nastier" harmonc content than plain brickwall clipping,
- Some OpAmps reverse phase of conduction when their common mode voltage is exceeded. This generates sort of an "octave effect"
- Some OpAmps may introduce "ringing" oscillation bursts at sharpest waveform corners
- Low slew rate can prevent the gneration of most obtrusive very high order harmonics
- Noise ratio is usually an issue in "high-gain" -design
- Current limiting circuits may begin to operate if the opamp's load becomes too low in impedance.
- "Rail-to-rail" -type opamps are - for obvious reasons - designed to have decent overdrive characteristics (they pretty much just brickwall clip without any "side-effects") but their low limits of operating voltage can pose practical designing challenges.
Ok, where do we encounter overdriven opamps...
- Stages configured for sole OpAmp overdrive should be pretty self-explanatory.
- "Shunt clipping" circuits driven by an opamp gain stage. (Isn't OCD in this category?) These can often be deceiving. The opamp stage WILL overdrive with high-enough input signal or gain levels (though typically this happens well above clipping threshold of the shunt circuit). So, with "high gain" settings dialed in overdrive of the opamp stage is very much a possibity. The shunt clipping circuit will limit signal peaks exceeding its clipping threshold as usual, but it can not prevent any of the "ill-effects" it's driver stage introduces when overdriven. These may contribute to total distortion in certaim conditions.
- Feedback loop clipping in non-inverting configuration. If these circuits are overdriven excessively the opamp stage begins to clip to rails.
Could be some other examples too.
!
and thanks The G for the SPICE sims. I had initially thought 'common base amplifier', but couldn't see any way the NPN could be biased on without some extra resistor from base to a positive voltage (with respect to emitter). I've checked and checked again and I can't see any such extra resistor. I agree it should be a current source, to ensure the LED gets the correct current.
) on it here too
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