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

Neil Johnson neil.johnson71 at gmail.com
Wed Aug 19 17:46:52 CEST 2015


Alfred,

I suggets you grab some books by Doug Self as he covers a lot of what
you're looking for, from microphone pre-amps to power amps.

You could use transformers for microphones - some like the sound - or
adopt a purely silicon solution with transistors and op-amps.  As for
driving different output devices, you can either keep it simple and
adopt a basic high-current voltage driver (which is what most "power"
amps really are) or do something that allows for different load
impedances as well.  But that's a bit more complicated.

Cheers,
Neil


On 19 August 2015 at 16:42,  <alfred.pear at gmail.com> wrote:
> 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
>
>



-- 
--
http://www.njohnson.co.uk



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