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human prototype 31st July 2001 02:27 AM

i've observed circuits (ccts) with various rail voltages
some 9V some 12V some more volts some less
are there any advantages/disadvantages to having a certain rail value versus another?
is it necessarily dependent on component selection?
are there ccts with rails of 11V or 19V or...?
how did these common values (9, 12, etc) come about? are they inherently better than others or is it a case of "beta vs vhs"?

GRollins 31st July 2001 03:42 AM

Historically, voltages (the lower ones at least) were sourced from batteries, even in tube equipment--it was quite common to have the plate voltage supplied from the AC line, but have the bias voltage come from a battery. (Note that there are people who claim that battery bias sounds better. I have no opinion.) Anyway, since the batteries were made up of collections of carbon-zinc cells--and due to the electrochemisty of a carbon-zinc cell, it delivers about 1.5V--batteries came in multiples of 1.5V. This led to a progression of common voltages: 1.5, 3, 4.5, 6, etc. Some, like 4.5V, never became very popular (excepting some 3 cell flashlights, for instance). Others, like 9 & 12V, became quite common.
No, there's no magic to these numbers. You are free to develop circuits that use 21V or 11V if it suits your needs. The trick is to be sure that you have plenty of headroom for any signal that you intend to amplify. Gain devices will respond just as well to 'non-standard' voltages as they will to standard ones. But be aware that for the reasons mentioned above, you'll find a bit of an inherent bias (no pun intended) towards certain voltages in parts values.


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