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narsil said:If you don't have an oscilloscope, so how would you know if your amplifier has crossover distortion?
audioguru said:A lousy old LM324 or LM358 opamp has tonnes of crossover distortion that is very obvious. They designed them to be low-power so starved their output transistors of sufficient bias current. Their narrow bandwidth helps attenuate the distortion. :lol:
Correct, but only when the output is biased with a DC load resistor to operate in class-A, to eliminate the crossover distortion in the first place as recommended in the datasheets.Nigel Goodwin said:audioguru said:A lousy old LM324 or LM358 opamp has tonnes of crossover distortion that is very obvious. They designed them to be low-power so starved their output transistors of sufficient bias current. Their narrow bandwidth helps attenuate the distortion. :lol:
Not to mention that the feedback applied around the chip drastically improves the situation! - both devices have been commonly used in audio equipment with no ill effects, nor excessive distortion.
To reduce the power supply drain, the amplifiers have a
class A output stage for small signal levels which converts to
class B in a large signal mode.
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.
audioguru said:Correct, but only when the output is biased with a DC load resistor to operate in class-A, to eliminate the crossover distortion in the first place as recommended in the datasheets.Nigel Goodwin said:audioguru said:A lousy old LM324 or LM358 opamp has tonnes of crossover distortion that is very obvious. They designed them to be low-power so starved their output transistors of sufficient bias current. Their narrow bandwidth helps attenuate the distortion. :lol:
Not to mention that the feedback applied around the chip drastically improves the situation! - both devices have been commonly used in audio equipment with no ill effects, nor excessive distortion.
Single supply circuits usually have a DC load resistor.
I installed Goldstar (now called LG Electronics) made in Korea telephone systems in 1980. The telephone speakers were driven with a pair of complementary emitter-followers with their bases tied together, something like the output of an LM324 or LM358. They were driven by a pretty good opamp with overall negative feedback. The crossover distortion caused many complaints. :cry:
I added a resistor from the output of the opamp to the speaker so that the opamp drove the speaker directly at low levels and the transistors began working for higher levels. Luckily, the speakers were 63 ohms so the opamp could drive them to about 0.7V pretty well.
When users heard the big improvement I couldn't modify them all fast enough. :lol: