From: kehoe@netcom.com (Thomas David Kehoe)
Newsgroups: sci.electronics
Subject: Re: Telephone Headsets
Date: 19 Dec 92 17:32:10 GMT
Organization: Netcom - Online Communication Services (408 241-9760 guest)

>I've been looking into telephone headsets (the things that telephone
>operators usually wear) and the prices are much higher than I expected:
>$160 - $200 and UP.  Basically, the thing consists of half a headphone,
>a microphone, and a small amplifier.  I would have thought that the price
>would be $60 to $80.  Does anyone know why they are so expensive; or,
>does anyone know a source where the price is more reasonable??  Thanks.

Radio Shack sells a headset & universal amplifier for $49.95.
The frequency response of the headset is seriously non-flat
(too much high end).  The $230 ACS headset I tried sounded
similar.  I haven't tried Plantronics or Hello Direct, the
two other major headset manufacturers (I can send you their
phone #'s if you want).

Instead, I bought a Yaesu YH-1 ham radio headset for $31
($25 wholesale).  Excellent sound, attractive styling,
folds up for carrying, and a standard 1/8" stereo plug.
The microphone connects directly to my phone, which has an
elecret microphone.  The microphone seems a tad less
sensitive than my handset microphone, but very few people
complain.

The earpiece is much quieter than the earpiece in my 
handset (and far better sound).  Turning up the volume
on my phone works, but with lots of distortion.  So
I put in a little LM386 op-amp.  Sounds great.

Total cost is $<40.  Which leaves the question why
does the same thing from Plantronics cost $185?

-- FCC certification costs $5K+.  That seems ridiculous,
for someone to see that my microphone connects straight
through, and my earpiece has a little op-amp.

-- My headset only works with one type of phone (AT&T
700 series).  I had to add a 0.1uF capacitor from the
earpiece (red) wire to ground (black and green) wires.
Who knows what other things you have to add to other
phones.  The Radio Shack headset interface has three
settings: I think they are insensitive electret,
sensitive electret, and carbon microphone.  So it has
some type of amplifier for the microphone.  Remember
that carbon (round) microphones vary resistance, and
electret (square) microphones vary capacitance.  Also,
I called AT&T with a question, and was told by six 
people that technical support is never given because
modifying a telephone is a federal crime (using an
uncertified headset is such a modification).

- The better amplifiers have compression and other
features to limit the headset to 85dB (an OSHA rule).
This is so that if someone else in the office sends
a fax on line you're talking on, you don't get 
130dB of high-pitched sound in your ear.

Now, I have a question.  I'm using a bucket brigade
delay between my microphone and earpiece, as a 
treatment for stuttering.  The delay circuit can
either be powered by battery or plug-in power supply.
The latter goes throgh a 7805 voltage regulator,
which eliminates hum.  I can plug this into the handset
jack of my phone and it sounds fine.  But when I
plug it into the Radio Shack universal telephone
amp (which plugs into my phone), I hear a 60 Hz hum.
Specifically, connecting the microphone (yellow) wire
causes hum.  The other three wires don't.  Disconnecting
my phone from the phone line eliminates the hum, but
unplugging my phone's power doesn't (it just turns off
the answering machine).

I assume this hum has something to do with the phone line's
ground being 3 volts off of the power line's ground.
The hum is eliminated when I power my delay from the
battery.  I tried a capacitor between my microphone
(with its own driver) and the yellow wire.  This eliminated
the hum, but people said they couldn't hear me.  That
makes no sense, as the capacitor should pass AC signals.
Anything else I should try?  Maybe a giant (4700 uF)
capacitor between +5v and ground on my delay circuit?
I tried connecting the delay circuit ground to earth
ground, but this caused a loud, nasty sound.
-- 
"Why my thoughts are my own, when they are in, but when they are out
they are another's." - Susannah Martin, hanged for witchcraft, 1692.
Thomas David Kehoe          kehoe@netcom.com         (408) 354-5926
Newsgroups: sci.electronics