A little too basic, maybe?

I know this might be very basic to some of you but I really would
appreciate an answer.

I have made an extension to my telephone handset to plug in my
headphones (speaker and mic) and use it as a hands-free phone {don't
ask, it's not important to know why}. The problem is that I get low
volume in the headphone speaker. The other party hears me fine so the
mic is ok. What I need is a way to enhance the volume in the speaker
line. I've tried so many speakers (some with volume controlers, some
without) and all gave the same result: low volume.

I don't claim to be an expert but I know a little about electronics;
little but enough for making connections, and for fixing stuff like
radios and other small things (haven't tried the TV yet). I'm
learning, or trying to, more about electroncis but I just finished
high school and before now school wasn't giving me enough time to
practice electronics or read more about it.

I know this is possible, and I have knowledge of basic electronic
components and I know how they work (these physics lessons in high
school weren't so bad after all) but I still donno how to make a
circuit to increase the voltage/volume. Please help me with this...

I'd appreciate all your replies.

TIA
rA-nD-oM-iZ-eR
-----------------------

 

hAVe a LOok aT iMPEdaNces iN ur BookS

 

You need an amplifier in the speaker line.
It can be a very simple amplifier, and it can be battery driven to
simplify the construction.

Try to find a readymade small amplifier, or build one from a
schematic, there are lots of such schematics on the web.

A single op-amp might be enough.

An amplifier raises the signal strength and makes the signal suitable
for the loudspeakers in your headset, that is what the other person
meant with his talk about impedances.
The signal needs more "muscles" to handle a loudspeaker.
It needs the ability to deliver more current.

You have a mono signal, so if you have a stereo headset you can
connect the two speakers in series, or in parallell if series is
impossible. Or maybe you are happy with sound in one ear only.

 

Hi.
If they're stereo headphones, try wiring the two speakers in series
instead of in parallel. That'll increase the impedence presented to the
phone and make it seem a bit more like what was there originally.
Thoughts anyone?

Ben
~~~

 

I had a problem with a set of studio phones once where the amplifier
would distort before the phones got loud enough. I ran the signal
through a multitap audio transformer and it raised the voltage going
into the phones which effectively raised the wattage that the phones
drew from the amp. Radio shack sells tiny audio transformers that you
might try experimenting with, however I don't know if they are the
right impedence for the job. The transformer I used was big but then I
had the space for it.

 

---
If I remember correctly, your telephone handset receiver looks about
like 600 ohms, while your headphones probably look more like 8 ohms. If
that's true, then a passive solution would be to use a 600 ohm to 8 ohm
audio transformer. Failing that, a single audio power opamp like an
LM386 http://cache.national.com/ds/LM/LM386.pdf would prob'ly do it.

 

I got it parallel from the start, so that's supposed to lower the
resistance; am I right? I tried connecting the speaker line to my pc speaker
that uses AC power, and most probably has some complicated amplifier, and I
got good volume. If u'd be generous enough to point me to a diagram of a
circuit for this amplifier I'd appreciate your help.

Thank you for responding...
rA-nD-oM-iZ-eR

 

Yes, when you parallell two equal resistances (impedances) the result
is half the resistance.
When you put them in series the result is double the resistance.
If your signal source is having trouble with low resistance loads you
may get higher volume if you connect the headphones in series.

Other amplifiers are made for low resistance speakers, and then
parallell connection is good because it gives much more effect.

I don't have a link, but search the web for LM386 and you will
probably find a schematic of how it is to be connected.
It is a good little amplifier for speakers or headphones.

But that means you have to buy a LM386 chip and a few other components
and put it all together. Or two if you want stereo.

A simpler solution could be to use an amplifier you already have, or
can get easily.
Most stereo systems have an input and outputs suitable for this
purpose, so you can get and old stereo and put between your signal
source and your headphones.
Use the loudspeaker outputs from the amplifier if it doesn't have a
headphone output.

You only need a mono amp actually, if you can't find a stereo, so you
might be able to use a radio with input and output, or whatever kind
of amplifier you find.

If you want to use any op-amp you can get hold of you should put the
headphones in series as op-amps cannot handle low impedances well.
LM386 is an exception, it is made for loudspeakers or headphones.

 

A phone line impedance, at audio frequencies varies between about 1000
or more ohms at lowest frequencies (300Hz) to about 400 ohms at 4 KHz
although this will vary all over the place depending on how long your
phone line is, type of cable you phone company uses and on and on.

Telephone instruments use a nominal 600 ohm resistance across the line
as a cheap and cheerful method of more or less matching the line
impedance for maximum signal reception. This is really a lousy match
and causes all kinds of problems with echoes at the exchange, but is
what, for historical reasons, we're stuck with.

Your headset speakers in parallel, assuming they are 8 ohm speakers,
will present 4 ohms to the line. This means that almost all the voice
frequency power sent from your local exchange over your telephone line
will develop in the line or, if the line is short, in the 600 ohm
"matching" impedance at the exchange itself, leaving very little for
your speakers. I'm actually surprised you can hear anything!

Now, assuming that your headset is connected to the line, which is not
too clear from your description, you need to match your earphones to
the impedance of the line. A 600 ohm to 4 ohm transformer will do
this, but they tend to be bulky, expensive and hard to find. Also,
this will affect the operation of your telephone and might piss of the
phone company considerably. If you have your speakers connected after
the line connection, that is inside the phone instrument after the
transformer, then use a matching buffer amplifier as described below.

A buffer amplifier will take care of the problem. It need not have a
high gain, in fact a gain of 1 should be sufficient, but it should
have a near 600 ohm input impedance,if you are connected to the line,
and a low output impedance. This will ensure maximum power transfer
between line and speakers and least distortion in the speaker output.
If the input connection comes from the interior of the phone, then
it's probably safe to leave the 600 ohm matching resistor out of the
circuit. If you are interested, I can send you a circuit diagram of
just such an amplifier, although I would need to know exactly how your
connection was made. If you are still interested, and I can find the
time, I can send you construction details.

As an interesting aside, the reason for the choice of 600 ohms for the
telephone instrument, which matches the impedance of no cable
presently in use anywhere in the world, is that 600 ohms was the
impedance at voice frequencies of old style telephone wires on poles
separated by wooden cross members. This was, of course, back in the
days when only the rich had telephones and the need for multiple
connections was not foreseen. Sort of an earlier version of: "I don't
see why anyone would want more than 20 megabytes on a hard disk."

Regards

John

 

Guess what, It worked. I changed the connections to series and it gave
my nearly the same volume that I got from the normal handset. Although
the connections were a little funny, being only 3 wires not 4, but I
did manage to do it.

I don't know how to thank you enough, guys, This was really helpful.
I'm sry I didn't reply sooner but I intended to do so when I wrote
half way through the reply and the electricity went off. I got
interested more about amplifiers, though, and I have read and will be
reading more about them in the coming days.

Once again, I thank you from all my heart for helping me; all of
you... I really do appreciate it.

Thx;
rA-nD-oM-iZ-eR

 

If it has a 3-terminal plug, they will be out of phase in series.
You have to do major surgery on the wiring.

 

This has been done many times before. Don't re-invent the wheel.
http://www.google.com/search?num=10...-u2+-u3+-u4+-ic2+-ic3+-ic4&btnG=Google+Search

 

He should try putting them in parallel to get ~4X the power.
It is likely that it still won't be enough (4X power isn't 4X
louder because of the logarithic nature of the human ear) but
it's cheap and easy, and thus worth a try.

 

I don't know... I personally like having my mono headphones
be out of phase. It makes it seem like the voices are in my
head. (and yes, I know that I just provided the perfect straight
line for a joke!)