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NEWBIE: Designing a Simple Audio Amp


N. Thornton

Jan 1, 1970
Fwom:Ricky Romaya ([email protected])
Subject:NEWBIE: Designing a Simple Audio Amp
I want to design and build a very simple audio amplifier to drive my
8ohms speakers from (onboard) pc soundcard. I figure I'm using op-amps.
I'm a newbie and all I know about op-amps are basic, mostly from text

Basically, I'm a little confused. Usually I see that the type of
amplifiers to drive speakers are called 'power amplifiers'. From the
basic building blocks of op-amps, I could only find circuits for voltage
and/or current amplifiers. Do those 'power amps' are really 'voltage

no, theyre power amps, delivering large amounts of both v and current.

cos I figure the main drive factor for speakers are voltage
variations (voltage are converted into air preassure by the speakers, and
hence creating audible sound)?
Further more, I figure that the input stage of the amps I want to build
will simulate that of LINE-IN/AUX/CD-IN lines found on commercial sound
systems. Anybody knows whether there are standards to the input voltage
and input impendance of those lines (LINE-IN/AUX/CD-IN), and if there
are, what are the values?

200mv input, R anything from 10k to 50k - but there will be some
argument over those values.

And how about output impendance standards for
driving 8ohms speakers?

much less than 8 ohms, in most cases, but not all.

Also, anybody knows how the input parameters (voltage, current, power) to
the speaker relate to the SPL (Sound Preassure Level) generated by

look at the speakers specs to find out. Expect around 80-95 dB at 1m
for 1w in.

This indirectly means I'm asking the relationship of the amplifier gain
to the loudness of the sound generated by the speaker.

none, it is amp power output and speaker efficiency that determine

Last, but not least, what are the recommended op-amp series for audio
processing (I only know the multi-purpose u741 series)?

many to choose from. Whatevers readily available to you is the first
way to narrow the field down.

Well, that's a long list of questions, and there are more. Any help is

A power amp IC is an easy solution, but doesnt teach much. An opamp
and output tr pair is another way to get what you want, and gives you
much more to play with.

You can drive a pair of AC127/AC128 (IIRC) germanium output trs from a
LM324 for a nice 1w audio amp. Before anyone replies, people always
expect it to produce awful crossover, but it doesnt at all, its a bit
more complex than it first appears - but simple to build. It wont work
with silicon trs.

btw if you use a TBA820M and stick a nice heatsink on it, run it on
19v, you should get 6w a piece from those. Only 500% over ratings, no
worry :)