noisy power adaptor/transformer in telephone intercom

S

Simon Bartlett

Guest
Hello group,

I'm trying to create a phone intercom system, but am running into a
problem. The circuit
(http://www.epanorama.net/documents/telecom/telephone_intercom.html)
calls for a 12v power supply. I've got two power adapters/transformers
(the things you plug into the mains). One is 12v 1A, the other 12v
860mA. With the 12v 1A unit, the phone system works perfectly, but with
the 860mA unit, I get a loud buzzing on the line. If I use more
resistors to get rid of the buzzing, no voice is able to get through
either.

I've also tried a 20v unit I had lying around, and it too makes the
buzzing noise.

Now I can't use the 12v 1A unit, as it belongs to my computer router :-(

Does anyone have any idea? I blew up my multimeter, and am hoping to
get a new one for Christmas, so haven't replaced it yet, so can't use
for testing.

I'm very new to all this, so please don't assume a heap of knowledge on
my part in any response.

Many thanks,

Simon.
 
"Simon Bartlett" <sb@loa.moc> wrote in message
news:MPG.1a4fc635f4f86f4d989769@freenews.iinet.net.au...
Hello group,

I'm trying to create a phone intercom system, but am running into a
problem. The circuit
(http://www.epanorama.net/documents/telecom/telephone_intercom.html)
calls for a 12v power supply. I've got two power adapters/transformers
(the things you plug into the mains). One is 12v 1A, the other 12v
860mA. With the 12v 1A unit, the phone system works perfectly, but with
the 860mA unit, I get a loud buzzing on the line. If I use more
resistors to get rid of the buzzing, no voice is able to get through
either.

I've also tried a 20v unit I had lying around, and it too makes the
buzzing noise.

Now I can't use the 12v 1A unit, as it belongs to my computer router :-(

Does anyone have any idea? I blew up my multimeter, and am hoping to
get a new one for Christmas, so haven't replaced it yet, so can't use
for testing.

I'm very new to all this, so please don't assume a heap of knowledge on
my part in any response.

Many thanks,

Simon.
Click to expand...
Assuming you are using a DC wall wart it sounds like you need to apply some
filtering and smoothing to it's output. Like a choke or resistor with some
electrolytic capacitors of a few thou microfarads.

--
Regards ............... Rheilly Phoull
 
I'm guessing that your 860mA adaptor has insufficient filtering on it. When
the AC from the wall gets converted to DC, there is a considerable amount of
noise (known as ripple) that we hear as hum (maybe the buzzing you are
hearing). To get rid of the ripple, large value capacitors are used after
the rectifier stage in the adaptor. If the caps aren't big enough, there
will be ripple that you will hear as noise. The other possible problem is
that the adaptor is not regulated. If the DC voltage is regulated, it means
that the voltage will remain constant regardless of how much current is
drawn from it (within spec of course). Regulation also helps to reduce
ripple.

Maybe the 1A adaptor is regulated, maybe it just has better filtering.


"Simon Bartlett" <sb@loa.moc> wrote in message
news:MPG.1a4fc635f4f86f4d989769@freenews.iinet.net.au...
Hello group,

I'm trying to create a phone intercom system, but am running into a
problem. The circuit
(http://www.epanorama.net/documents/telecom/telephone_intercom.html)
calls for a 12v power supply. I've got two power adapters/transformers
(the things you plug into the mains). One is 12v 1A, the other 12v
860mA. With the 12v 1A unit, the phone system works perfectly, but with
the 860mA unit, I get a loud buzzing on the line. If I use more
resistors to get rid of the buzzing, no voice is able to get through
either.

I've also tried a 20v unit I had lying around, and it too makes the
buzzing noise.

Now I can't use the 12v 1A unit, as it belongs to my computer router :-(

Does anyone have any idea? I blew up my multimeter, and am hoping to
get a new one for Christmas, so haven't replaced it yet, so can't use
for testing.

I'm very new to all this, so please don't assume a heap of knowledge on
my part in any response.

Many thanks,

Simon.
Click to expand...
 
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