[sdiy] Headphone amps using op-amps

[ChristianH]

> You really need an audio power amplifier 
> IC that has a sufficiently beefy output stage to maintain the output voltage 
> waveshape even into low impedance headphones.

That's my first thought on seeing the subject line.

There's LM386/380/389/1877, all dirt cheap, and made for that very
purpose.

Chris



On Wed, 12 Dec 2012 21:30:54 -0000 "Richie Burnett"
<rburnett at richieburnett.co.uk> wrote:

> The distortion is most likely due to the limited current sourcing/sinking 
> ability of the standard op-amps you mentioned if driving into a low 
> impedance set of headphones.  Some additional series resistance will help 
> limit the current (and prevent distortion) into low impedance cans, but you 
> will loose volume and efficiency.  You really need an audio power amplifier 
> IC that has a sufficiently beefy output stage to maintain the output voltage 
> waveshape even into low impedance headphones.  Most synths that i've looked 
> inside use a TDA something-or-other 8-pin IC that is a generic stereo 
> headphone amplifier.  I don't have the part number handy but whatever is the 
> current incarnation should be on the manufacturers website.  Phillips I 
> think.
> 
> You might also need a Zobel network at the output as some op-amps will play 
> up if presented with just the right combination of inductive and resistive 
> load.  I'd be tempted to go with a dedicated headphone amplifier IC myself, 
> rather than try to beef up a general purpose op-amp with a push-pull pair, 
> particularly if low THD is an important requirement.
> 
> -Richie,
> ----- Original Message ----- 
> From: "Oren Leavitt" <obl64 at ix.netcom.com>
> To: <synth-diy at dropmix.xs4all.nl>
> Sent: Wednesday, December 12, 2012 8:08 PM
> Subject: Re: [sdiy] Headphone amps using op-amps
> 
> 
> >
> >
> > On 12/12/2012 12:22 PM, John Speth wrote:
> >>> If I'm driving a pair of cheap headphones from a standard op-amp (I've
> >> got a
> >>> TL072, a MC1458 or a LF353 available - you don't get much more 'bog
> >> standard'
> >>> than that lot) is there anything special I need to do?
> >>
> >> Great question!  I suppose the answer lies in how do we design an
> >> inexpensive circuit that can reliably drive a low impedance inductive 
> >> load
> >> with reasonable sonic fidelity.
> >>
> >> JJS
> >
> > It's basically an impedance matching issue - I've gotten good listening 
> > volume just putting a small 1 kOhm / 8 Ohm audio output transformer 
> > between op-amp output and speaker.
> > A better way is to boost the drive current with an added complimentary 
> > pair of transistors at the output. There are many examples in op amp app 
> > notes and data sheets.
> >
> > - Oren
> >
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