jeffbearer

I'm trying to modify a speaker phone so I I can record the speaker output. I've installed a phono jack that bypasses the speaker and then I've attempted building this speaker level to line level circuit. But my mixer does not receive a proper signal.



The speaker that I'm bypassing is 0.3W 32hm: speaker.

I'm assuming that I need to adjust some impedance somewhere but I don't know enough to know what to do. So any help would be appreciated.

Nigel Goodwin

Impedance won't be a problem, but lack of information is - you don't give any clue as to what isn't working correctly - "not receiving a proper signal" is completely meaningless. Nor have you mentioned what type of input you're trying to feed it to!.

Depending how the speaker is wired in the phone it may not be possible to do it this way (if it's fed from a bridged amplifier), you would need to use one side of the speaker, and gnd in the phone.

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ljcox

Are you trying to record telephone conversations?

If so, and if it is legitimate, I'll post a simple circuit I designed for my daughter who records conferences she is engaged in.

I suspect your problem is that the speaker phone speaker terminals are connected via the phone to the phone line.

Do you have a circuit of the speaker phone?

BTW I spent 40 years in the telephony business.

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Len

Screech

It's legit.
Post the circuit.
Thanks

jeffbearer

I host a podcast and this is for recording interviews. The reason I want to record off of the speaker and not from a tap style recorder is because the speakerphone circuitry mutes when I'm talking, allowing me to record only the person on the other end.

I currently mic the speaker and record it, but thought it would be a cool project to hardwire it to the mixer.

I'm trying to take the signal going to the speaker and convert that to line level so I can record it with a run of the mill mixing board. The linked website above it says that speaker level is around 3 volts, and line level is 0.3 volts.

If I measure the power on the speaker feeds it is indeed 3 volts. But before I realized the difference I hooked up the mixing board at 3 volts and all I got as a buzz. When hook some headphones up to the leads I hear the dialtone, pretty loud.

When I drop in the speaker level to line level circuit that I built I get nothing on the mixer, and when I plug in headphones, I hear the dialtone, I assume 10Db lower.

When I try measure the voltage coming out of the circuit I built pictured above, it measures 0 not 0.3 as I would have expected.

And this is where I'm lost, I don't know anything about troubleshooting/designing audio circuits.

I do not have the schematics of the phone, and thus thought that interfacing with the speaker would be the easiest method.

audioguru

Connect the headphones to the speaker wires then connect your resistive attenuator to see if the headphones get cut off because the ground wire is shorting the signal. Maybe try swapping the connections of the attenuator to the speaker.

Check the values of the resistors by measuring them. Maybe the 10k is 10M or it is open.

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ljcox

There is a safety/regulatory issue.

In Australia & I assume in other countries, the telephone line must be electrically isolated from any equipment that is powered by the mains. This is to avoid electrocuting linesmen who may be working on the line when a short in the device causes the active side of the mains to be applied to the phone line. So devices such as Answering Machines have a line transformer in them that has insulation resistance that complies with the appropriate Australian standard.

I modified an old answering machine.

I used the transformer, the RJ12 connector, the PCB and the case.

I inserted 2 RCA sockets and a toggle switch in the case and removed some of the components from around the tranformer so I could mount the new components in the vacated holes together with some wire jumpers as necessary since the tracks were not designed for this purpose.

I left most of the parts on the PCB but ensured they were isolated from the monitor circuit.

The toggle switch provides 2 options. Monitor and Hold.

In the Hold mode, a loop is applied to the line to hold the call so the phone can be hung up. In the monitor mode, the loop is not applied and the call is held by the phone.

I don't have the circuit, so I'll borrow the unit from my daughter and copy it and post it.

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Len

ljcox

Here is the circuit and photos.

The instructions can be read in one of the photos.

The TN12 termination matches Australian cables. A different impedance may be required oversaes.

T1 and C1 were in the Answering Machine, I added the other components into holes from which components were removed. I cut a few tracks and inserted some wire jumpers as necessary.

I don't know what the law requires these days, you may have to ask those on the call permission to record.

Attached Images

	Monitor 1.jpg (63.4 KB, 17 views)
	Monitor 2.jpg (89.2 KB, 13 views)
	Phone monitor.gif (22.4 KB, 29 views)

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Len

Hero999

There again most recording equipment is isolated from the mains vai a small isolation transformer.

If you're really worried about isolation, you could use a small mains transformer with the primary as the input and the secondary as the output; it will provide both attenuation and isolation.

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Nigel Goodwin

Tut, tut! - as you're in England you shouldn't even be thinking about such things!, you're not allowed to connect anything homemade to the phone lines here, nor anything that hasn't passed strict (and expensive!) laboratory testing!.

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Hero999

There agian I would design it properly so I wouldn't get caught!

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ljcox

Electrical isolation is not the only consideration.

Phone lines are balanced with respect to earth in order to minimise noise and crosstalk. So the line transformer is necessary to maintain that balance.

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Len

Hero999

Still, providing the secondary side of the power supply isn't earth bonded it shouldn't make any difference whether you use an isolation transformer or not.

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ljcox

It depends on whether there is any common mode noise. The transformer may need a shield between the windings. And if it is connected to a computer, there are several possible sources of common mode noise, eg. the computer, monitor, printer, etc.

Besides, I don't know if it would be legal. You may still need the isolation trans for legal reasons.

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Len

Charlie123

Hi,

I'm collecting information to start my project,
my first step will be to make my PC playing a wave file to the caller after answering the call after 6 rings. (what programmation language do you suggest for phone(and VoIP), .net, C+, perl, Ruby ???

I will need you suggestion on below comment,
I believe that the best thing will be to use my USB port to adapt to the telephone compagny, to do that, I will need an usb to rj-11 adapter similar or equivalent to :

http://hpna.com/HPNA-NetgearUSB10mbReview.html (but at lower cost and this one seems that netgear are not producing them anymore).

Waiting for some hints, suggestions from you guys....

Brgds-Charlie