POTS telephone line question

BCAP

Well-Known Member
This is an unusual question -

For theatrical purposes, I would like to capture the microphone audio from an old telephone set (let's say a touch tone phone from late 80s) and get that as a line output. Don't need to capture the ring, the dial tone or the dialing, just the output of the microphone. In addition, it's not necessary for this rig to sound exactly like an old POTS line (further artifacts and processing will be added in).

I already own a Gentner Hybrid telephone line coupler box which could potentially make my job extremely easy.

While I've seen a number of POTS phone mods, I'd also rather not build anything or adapt the telephone in any way unless absolutely necessary.

Has anyone on this board achieved what I'm trying to do with a Hybrid coupler box?

I've done a little digging on POTS lines but I'm not 100% sure what voltage / current I would need to provide for the phone to operate. Assuming the input to the hybrid is an RJ11 jack and the phone to be connected can be connected to the set connection on the back of the Hybrid, it looks like I just need to provide between 3 and 9 volts DC between the tip (green, -) and ring (red, +) and 20 mA current.

Can anyone please confirm?

Thanks in advance.
 
Man, All I remember from my days of using Telephones with OLD wired headsets as Backstage communication... was 48vdc was 'standard' the voltage could jump to 85-100 when a ring signal came through. That's all I know. I bet some other old guy will know, though.
 
Man, All I remember from my days of using Telephones with OLD wired headsets as Backstage communication... was 48vdc was 'standard' the voltage could jump to 85-100 when a ring signal came through. That's all I know. I bet some other old guy will know, though.

That sounds about right. With the phone was on the hook I think the voltage was 48vdc, a ring was like 90v but when the phone's off the hook I think it's significantly lower voltage needed.
 
96V to ring, 48V for signalling... as measured at the Central Office.

Old handset, as in carbon transmitter?
 
That sounds about right. With the phone was on the hook I think the voltage was 48vdc, a ring was like 90v but when the phone's off the hook I think it's significantly lower voltage needed.
Ringing was 90 volts at 25 Hz if from your local telco's office; 90 volts at 30 Hz if from your business's internal switchboard. Decades ago 25 Hz was our common power line frequency. When our power companies shifted to 60 Hz our telco's installed MG sets, 60 Hz motors shaft coupled to 25 Hz generators. If you listened to the dial tone from your PBX then dialed up an outside line and listened close, you could hear the dial tone drop from 30 Hz to 25 Hz. In PBX's (Private Branch Exchanges) it was simpler to divide 60 Hz in half than to create 25 Hz within your PBX. You should be able to capture the sound of your phone's internal mic by bridging your phone line with an isolation transformer, add a cap in series to avoid DC saturation.
Toodleoo!
Ron Hebbard
That sounds about right. With the phone was on the hook I think the voltage was 48vdc, a ring was like 90v but when the phone's off the hook I think it's significantly lower voltage needed.
 
To build on the post above, 12 volt battery and an old telephone. Put the phone in series with a 600 ohm resistor. Decouple the DC by using a .1 mfd capacitor at the point where the resistor and phone come together. Your "line out" will be the output of the capacitor (tip) and the - of the battery (ring.) Will give you the actual telephone audio output.
 
Or look for al old western electric or Tellabs power supply that provides “talk battery” or TB as sometimes labeled

Alternatively hide a small DPA or similar lab mix and wireless transmitter in or near the phone - that way you don’t have to worry about keeping the gentner connection open
 
FMEng nails it: nearly any clean DC source will light up a POTS set; even as low as 6V (the voltage will probably affect the available level, but you can adjust for that).

If you want to *ring* it, you're gonna need 20Hz-ish (for a non-partyline ringer) at at least 30-40VAC for an electronic ringer, and probably 90VAC for a mechanical one if you want a good clean loud ring. There are ring-simulators around for this; I think Mike Sandman has one; might even have an isolated audio tap.
 
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FMEng nails it: nearly any clean DC source will light up a POTS set; even as low as 6V (the voltage will probably affect the available level, but you can adjust for that).

If you want to *ring* it, you're gonna need 20Hz-ish (for a non-partyline ringer) at at least 30-40VAC for an electronic ringer, and probably 90VAC for a mechanical one if you want a good clean loud ring. There are ring-simulators around for this; I think Mike Sandman has one; might even have an isolated audio tap.
I have one of these and it works for lots of phones. http://www.tele-q.com/

and of course +1 to the above.
 
I see that that unit does not have an interrupter - do you find this to be a feature or a bug?
Yeah I have noticed this. Never had an issue, and it may indeed serve to improve "flexibility" with older/newer phones and such. Honestly haven't done much probing into it. Just make sure never to hook up to an actual phone line!!!! That could be bad I am sure.

The variable frequency control is VERY useful, and in full power mode produces a very strong ring. Works GREAT for those times I don't want to use placed speakers and sound cues (or cant as easily).
 
Yeah; though power comes at the price of convenience: if the person running it knows what ring cadences are supposed to sound like, it's a feature. If not...

I'm likely going to grab one if they're not too pricey.
 

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