NEWBIE: Designing a Simple Audio Amp

Fwom:Ricky Romaya ( snipped-for-privacy@somewhere.com) snipped-for-privacy@daqarta.com (Bob Masta) wrote in news: snipped-for-privacy@news.itd.umich.edu:

>> Power amp gains are typically set so a "line level" of about 1 VRMS >> will drive them to their maximum output, which depends mostly upon >> the power supply rails... you size your devices to handle the

rails.

Care to elaborate on this? I've noticed that the power amp op-amps

have

their gain preset. Do you mean that to drive them to max output, you

need

1 VRMS input? Take TBA820M for example, it's gain is preset to 20 (if

not

mistaken), then a 1 VRMS input will give 20 VRMS output,

right, but the amp can only give that if its supplied with enough V to deliver that, which it wont be in practice. What the amp does is take the power supply V and limit it to whatevers needed for the speaker at any moment.

and the power on >the 8 ohm speaker is then (20^2)/8 = 50W? But it says that TBA820M is

2W power amp (on 8 ohm load and 12V rails).

Hence under those circumstances it would have a life maesured in seconds, maybe less.

Fwom:Ricky Romaya ( snipped-for-privacy@somewhere.com) Subject:NEWBIE: Designing a Simple Audio Amp

op-amps.

voltage

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

and

build

sound

voltage

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

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

power) to

it?

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

gain

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

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

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 :)