Circuit Design
Guidelines
Here are a few basic guidelines to follow when designing
audio amplifiers. I do not want to elaborate on circuit
topologies, there are hundreds of topologies to choose from, each
with its pros and cons. I have tried to keep the guidelines
general and applicable to all designs.
Try to make each amplifying stage as linear as possible, use local linearization by e.g. emitter degeneration (bipolars). Do not use large amounts of feedback as means to reduce distortion, it will measure well but fall short of high-end performance. Especially the dynamics of the amplifier will suffer from excess use of negative feedback.
Try to minimize the number of active devices in the signal path, remember each active device adds distortion far above what any passive component could do. My 20W design has only 10 transistors. A line stage should not need more than 6 to 8 transistors.
Be very careful with
topologies where the distortion is dominated by odd
harmonics. The use of differential amplifiers
unfortunately elevate the odd harmonics above the even
ones. Try to add a distortion mechanism than injects
"pleasant" even harmonics above the level of
the "unpleasant" odd harmonics. This will
increase the THD but sound better than if you did not add
the even harmonics. How to do this ? I use two methods,
one involving circuit design and another involving PCB
design.
1. If a push-pull driver stage is resistively loaded on
the collector side the even harmonics will increase (and
the open loop gain will decrease). Use this with care as
excess loading will give audible THD and softening.
2. A PCB-trick I use only works on Class B or Class AB
power amps. By creating an inductive coupling between the
output transistor power supply loop and the low-level
input stage one can induce even harmonics. The effect has
been described in AES where it was presented how to avoid
this distortion. I use the effect to inject distortion
instead....
All electrolytic caps shall have a film type cap in parallel. At high frequencies the electrolytic caps become inductive and serves no use.
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