Your problem is three fold.

(1) Load ...  Most of the new headphones are 32 ohm or less. If you are
diving a pair (ie left, right) with single power amp, the load is in
parallel and half the value.

(2)Power Supply.   If you are driving 16 ohm load with LM759, you need
atleast +15V and -15V.

(3) close loop gain at 1kHz is 60dB(with +15V, -15V and less with low
supply voltage). It is possible you are exceding this level.

National Semi has discontinued this part, If design is a new one try and
use LM380 or LM386 (low voltage device).

As a footnote........You only need 500mV to 800mV (rms) to your headhpone
to 80 to 100 dBr sound pressure.

Regards,

Jay




At 09:58 08/07/99 -0600, you wrote:
>I'd like to thank everyone who has taken the time to answer my question
>about the manufacturer of the CA810. My intention was to use this amplifier
>as a substitute of one that I am suing right now: the LM759. Let me tell you
>quickly about my situation.
>          I am performing an experiment that deals with sinusoidal signals
>from 1
>to 15 Khz. To start, I fed 1Khz to an LM759 power amplifier, and then the
>output of the amplifier is fed to a set of earphones. Here is the problem:
>When I monitor the signal without the earphones, I get a nice sinusoide of
>about 6Vpeak, but when I hook up the earphones, the signal goes down to
>about 0.9Vpeak. From here, if I try to raise the gain, the signal gets
>distorted. The rest of the experiment conists on having a microphone pick up
>the audible tone from the earphones and feed it to a digital signal analyzer
>to check that the signal is not distorted. What's my problem? It looks to me
>that the earphones are loading the amplifier big time, but why should that
>happen, since even the data sheets show examples with speakers connected
>directly to the power amplifier? I was thinking about using an audio
>transformer to isolate the earphones from the output of the amplifier. Any
>thoughts or pointers? Best wishes.
>
>
>