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

[alfred.pear at gmail.com]

Wow - thank you everyone for the responses, it's nice to wake up in the
morning and read all of these perspectives on my questions..

Just to clarify a few things:

I do want to drive speakers. I'm essentially talking about a pre-amp and a
power amp in a single design. It sounds like I should worry about getting
one of them right before trying to get both of them right :)

Re: power amp -
I need to match a few impedances. I know that the circuit that I shared
outputs for 8-ohm matching, but I'm also going to need an output for
driving items with 1k impedance.  I'm attempting to drive some transducers
besides regular speakers - piezo elements, for example.

I would be interested in better, simple driver circuits, if people have
suggestions. I didn't realize the one I shared was so flawed. I had
experimented with the lm386 in the past and been very frustrated - I also
wanted to make sure I had something that worked on a bipolar scale, which I
had a bit of trouble finding.

Re: pre-amp -

I wanted to use a transformer to bring up the level of some pretty quiet
microphone elements. I messed with voltage gain and didn't seem to have a
lot of luck. There are probably a number of other alternatives to
investigate - it seems like I should worry more about setting proper
impedance with resistors- but just to clarify what seems to be the
consensus here: there is no reason to use a transformer at the input of a
preamp?


Thanks everyone..












On Wed, Aug 19, 2015 at 6:36 AM, Neil Johnson <neil.johnson71 at gmail.com>
wrote:

> 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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