[sdiy] transformers, power amps, and op amps.

Neil Johnson neil.johnson71 at gmail.com
Wed Aug 19 12:36:25 CEST 2015


Hi,

> i'm currently (perpetually) trying to wrap my head around some basic
> electronics ideas and i was hoping to ask a few questions.

Execllent!

> at the moment, i'm trying to design a small amplifier. based on this one, to
> be precise:
> http://www.allaboutcircuits.com/textbook/experiments/chpt-6/class-b-audio-amplifier/

I would strongly recommend completely ignoring that circuit.  It has
far too many problems to list.  There are much simpler, better, and
understandable circuits around for driving a small speaker.  Just
delete that page from your screen, erase the circuit from your memory,
and have a quiet moment to ponder the nature of the internet....

> i have the following questions:
> 1. to drive a speaker, the current is scaled up considerably - is it safe to
> send it back into an op amp? i know that operational amplifiers
> theoretically reject current, but how do i determine their limits? and does
> high current impact offset voltage?

The ideal model of an op-amp is no current into the inputs.  So you're
safe in not pushing any current into the op-amp (unless you exceed the
safe voltage!).

> 2. if i use a transformer at the output, i could potentially be stepping up
> the voltage considerably - is that safe to send that back into an op amp?

Depends on what the output is (see question below about what you mean
by "line level").  If the voltage is too high then you can always use
a potential divider to reduce it.

> my main reason for asking is that i'm interested in using line-level devices
> within a +/-12v system, so i know they need to be stepped-up via transformer
> at some point.. i'm essentially wondering if it's possible to use one
> transformer for multiple functions.

What do you mean by "line level"..?  Do you mean the 100V line system
used in PA systems?

Cheers,
Neil
--
http://www.njohnson.co.uk


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