[sdiy] Headphone amps using op-amps

[Richie Burnett]

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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