eval telephone board produces lots of noise

G

Gena

Guest
Hi all,

I am experimenting with a board based on this design:
http://www.melabs.com/products/labxt.htm
http://www.melabs.com/downloads/LABXTSCH.PDF

The board is basically working but at the audio output the signal from
the line is buried in a lot of noise... Could someone please look at
the schematics and suggest what is wrong with it? Is it missing a band
pass filter? When it is driven from a soumd card instead of a
telephone line the sound is good...

Thanks,
/G
 
"Gena" <gen_petrov@yahoo.com> wrote in message
news:29e50f7c.0501091108.71cd7466@posting.google.com...
Hi all,

I am experimenting with a board based on this design:
http://www.melabs.com/products/labxt.htm
http://www.melabs.com/downloads/LABXTSCH.PDF

The board is basically working but at the audio output the signal from
the line is buried in a lot of noise... Could someone please look at
the schematics and suggest what is wrong with it? Is it missing a band
pass filter? When it is driven from a soumd card instead of a
telephone line the sound is good...
Click to expand...
Could it be the polarity of your phone line? The schematic looks to be
sensitive to that.
 
"Anthony Fremont" <spam@anywhere.com> wrote in message
news:VIhEd.19305$q4.5977@fe1.texas.rr.com...
Could it be the polarity of your phone line? The schematic looks to be
sensitive to that.
Click to expand...
Which part of it is sensitive to the polarity? The speech circuit is
isolated with a transformer and I can't see how it can be sensitive...

....MM
 
"MM" <mbmsv@yahoo.com> wrote in message
news:34fmvfF4bch44U1@individual.net...
"Anthony Fremont" <spam@anywhere.com> wrote in message
news:VIhEd.19305$q4.5977@fe1.texas.rr.com...

Could it be the polarity of your phone line? The schematic looks to
be
sensitive to that.

Which part of it is sensitive to the polarity? The speech circuit is
isolated with a transformer and I can't see how it can be sensitive...
Click to expand...
Telephone lines are polarized, the interface shown in the schematic
contains diodes that would be sensitive to this. Many commercial phones
have bridge rectifiers so that it doesn't matter which way the wires are
connected, but the melabs device doesn't appear to do it that way.
Therefore, you could try switching the two incoming wires, but a better
method would be to check the polarity with a DMM and insure that the
diodes are forward biased.
 
MM wrote:
"Anthony Fremont" <spam@anywhere.com> wrote in message
news:VIhEd.19305$q4.5977@fe1.texas.rr.com...

Could it be the polarity of your phone line? The schematic looks to be
sensitive to that.
Click to expand...
Oh he just needs a charcoal toroid... one of those, and problem
solved! :)
 
"Anthony Fremont" <spam@anywhere.com> wrote in message
news:zPxEd.22952$q4.1872@fe1.texas.rr.com...
Telephone lines are polarized, the interface shown in the schematic
contains diodes that would be sensitive to this. Many commercial phones
have bridge rectifiers so that it doesn't matter which way the wires are
connected, but the melabs device doesn't appear to do it that way.
Click to expand...
Look closer at the circuits involving diodes, I don't think they are
sensitive to the polarity. The ring detector has a DC blocking capacitor and
the loop current detector has two diodes in opposite direction...

/MM
 
"MM" <mbmsv@yahoo.com> wrote in message
news:34g0qlF4a2jeiU1@individual.net...
"Anthony Fremont" <spam@anywhere.com> wrote in message
news:zPxEd.22952$q4.1872@fe1.texas.rr.com...

Telephone lines are polarized, the interface shown in the schematic
contains diodes that would be sensitive to this. Many commercial
phones
have bridge rectifiers so that it doesn't matter which way the wires
are
connected, but the melabs device doesn't appear to do it that way.

Look closer at the circuits involving diodes, I don't think they are
sensitive to the polarity. The ring detector has a DC blocking
capacitor and
the loop current detector has two diodes in opposite direction...
Click to expand...
You're right, I'm stupid. :-( I didn't look hard enough before and
completely missed the back to back loop current detector optos.

Now, after looking at it some more, I wonder which "audio output" the OP
is talking about. The one marked "line audio" is the one that appears to
be the received audio. The one marked "voice output" on the schematic
("audio out" on the chip) is the playback pin of pre-recorded messages.

That's a pretty neat board.
 
"Anthony Fremont" <spam@anywhere.com> wrote in message
news:TzAEd.23806$q4.11078@fe1.texas.rr.com...
Now, after looking at it some more, I wonder which "audio output" the OP
is talking about. The one marked "line audio" is the one that appears to
be the received audio. The one marked "voice output" on the schematic
("audio out" on the chip) is the playback pin of pre-recorded messages.
Click to expand...
Since I happen to know the OP I will answer. The signal in question is the
"Line Audio" taken from pin 4 of the J6.

/MM
 
"James Meyer" <jmeyer@nowhere.net> wrote in message
news:v6d6u09j80om58ibk66q3me8l87uob5nfd@4ax.com...
If you expect to hear the audio going *out* on the line, it will be very
weak and distorted when compared to the audio comming *in*.

Could that be your problem?

If not, then the component values around U9D should be checked again to
see if they are proper. R33 in particular is supposed to be the same
value as
the phone line impedance. If you're not connected to an ordinary phone
line,
the impedance of whatever you are connected to could be higher or lower
than R33
and that could be a problem.
Click to expand...
The problem we are trying to solve is with the signal coming from the
central office to the line out. The value of R33 is a bit strange, it is 1
kOm as you can see. We tried decreasing it to 680 Ohm. It improved things a
little, but not dramatically. As the OP mentioned we also tried using a PC
soundcard instead of a regular line. There is no noise when the card is
used...

/MM
 
"Rich Grise" <richgrise@example.net> wrote in message
news:pan.2005.01.11.21.24.08.965616@example.net...
Is the line noisy when you plug in an ordinary, preferably old-style,
telephone into it? Is there anything else on the line? And I can't help
but think somebody's DSL might be crosstalking - try borrowing a spare DSL
filter if there's DSL in the building.
Click to expand...


Rich,

It was the DSL!!! We should have thought of it ourselves! Thanks a lot!


/MM
 
On Wed, 12 Jan 2005 13:40:04 -0500, MM wrote:

"Rich Grise" <richgrise@example.net> wrote in message
news:pan.2005.01.11.21.24.08.965616@example.net...

Is the line noisy when you plug in an ordinary, preferably old-style,
telephone into it? Is there anything else on the line? And I can't help
but think somebody's DSL might be crosstalking - try borrowing a spare DSL
filter if there's DSL in the building.

Rich,

It was the DSL!!! We should have thought of it ourselves! Thanks a lot!

Kewl! I'll try not to dislocate my shoulder! ;-)
Click to expand...

Cheers!
Rich
 
On Mon, 10 Jan 2005 23:16:22 -0500, MM wrote:

"James Meyer" <jmeyer@nowhere.net> wrote in message
news:v6d6u09j80om58ibk66q3me8l87uob5nfd@4ax.com...

If you expect to hear the audio going *out* on the line, it will be very
weak and distorted when compared to the audio comming *in*.

Could that be your problem?

If not, then the component values around U9D should be checked again to
see if they are proper. R33 in particular is supposed to be the same
value as
the phone line impedance. If you're not connected to an ordinary phone
line,
the impedance of whatever you are connected to could be higher or lower
than R33
and that could be a problem.

The problem we are trying to solve is with the signal coming from the
central office to the line out. The value of R33 is a bit strange, it is 1
kOm as you can see. We tried decreasing it to 680 Ohm. It improved things a
little, but not dramatically. As the OP mentioned we also tried using a PC
soundcard instead of a regular line. There is no noise when the card is
used...
Click to expand...
Is the line noisy when you plug in an ordinary, preferably old-style,
telephone into it? Is there anything else on the line? And I can't help
but think somebody's DSL might be crosstalking - try borrowing a spare DSL
filter if there's DSL in the building.

Good Luck!
Rich
 
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