DanTayloe: Thanks for the information about the TS924. I looked at
some information on the TS924 on the web including a suggested circuit
for the TS924 as a headphone driver. Why do I need the LM4808? Why
not use three of the four amplifiers in the quad as voltage amplifiers
and the fourth as the headphone driver? I would then need each
amplifier to have a voltage gain of 16 rather than 8, and the noise
situation might be somewhat worse. I see that the TS924 is capable of
driving 16 Ohm phones which should be what I need. Thanks. Bill
Young WD5HOH














Dan Tayloe wrote in message ...
There is not much available in the 3v range that is really low noise.
Lowest that you can get at any voltage is 0.9 nV/squarerootHz (LT1115,
etc.). There is a 5v TI device that good to 3 nV/SqrtHz (OPA2228), but
about the best you can do at 3v to 4.5v range is 8-9 nV/sqrtHz, at least
in the audio range. Some low voltage op-amps specify lower noise, but
usually upon further examination of the noise curves, that low noise is
only at a couple hundred KHz.

I would recommend the TS924, which is a quad device. These 3v devices
do not have much gain, 40 db maximum, compared to 80-120 db from other
higher voltage devices, so you will need to string together several of
them to get the proper gain and still keep distortion low.

I recommend the quad package so that you can use each of the four
devices at a 8x gain each. This will total about 72 db for the four.
Maybe use 1K input and 8.2K feedback. Keeping the input resistor to a
smaller value will help reduce resistor induced noise.

Mouser carries the TS924 in both the DIP and the SOP package for a bit
under $2. I like the SOP surface mount for deadbug construction use
because it take ups a lot less space and because that size works really
well with 1/8w resistors. I like to use #30 or #32 gauge enamel wire
for hook up wire, burning the enamel off the end of the wire with a blob
of solder. Stick the end of the wire in the blob so that the copper
gets exposed directly to the heat.

To keep hiss at bay, I would recommend placing the volume control about
40 db from the speaker, and using high frequency gain roll off
capacitors across the 8.2K feedback resistors, perhaps in the 0.0033 uF
range.

Good luck and God bless!

- Dan, N7VE

Bill Young WD5HOH wrote:

Does anybody have a suggestion for a good audio IC preamp to go ahead
of an LM 4808 headphone driver? I have been using the LM 386 for home
brewed receivers up to now, but I would like to go to the LM 4808
because it can run on 4.5 volts (three D cells), and it will drive my
Sony stereo phones better than the LM 386. But it is a low gain
device, so I need about 70 dB of audio gain from an audio preamp. One
or two IC stages, that doesn't matter. I know that the LM 3808 is a
surface mount chip, but I think I can solder it OK. Thanks. Bill
Young WD5HOH