Is POTS telephone too old for a new project?

Ivan wrote:Hi Harry,
I do not seek for a "Schrodinger cat" in this phenomenon. IMHO there is simply a bug in the forum application.

BR from Ivan

Yes, Ivan,
It appears that there is a bug in the app. But I have now started to Ctrl+A and Ctrl+C to copy the text to my clipboard before I send. I always do that with G-mail since that also has a few "funnies" with the web application.
I tried to use a little levity to hide my frustration. Sorry!
Have a nice evening, Harry

Hi Ruud,
For the 2-wire to 4-wire you can trim the sidetone to quite a low level with the OpAmp method, but we deliberately crippled it to get the sidetone. I am not sure how low it can be, but since the cancelation was done in a resistive circuit it should be possible to get 40dB or more with very little effort.
I will see if i can dig out the circuit and let you have it. It was done around 1991, so I will have to search for it (Jeez!! is it that long ago?).
Best regards from Harry

(Time flies like an arrow, but fruitflies like a banana)

Yes, I know the phenomenon of 'acoustic feedback' to the caller, but in this case the 2-wire/4-wire adapter is for use in public radio. So you want as little 'leak' from your own signal as possible. Even 20 dB will cause some coloration of the audio.

Hi Harry,
I do not seek for a "Schrodinger cat" in this phenomenon. IMHO there is simply a bug in the forum application.

BR from Ivan

Hi Ruud and Ivan,
Interesting thing just happened, Ivan. You and I were writing at the same time, but you clicked SEND before me, so I lost my message. Dunno where it dissapeared to! Probably a quantum phenomenon.


@Ruud,
The problem we had in Riyadh was an accomodation building for 40 engineers, each having an extension from an internal exchange, plus a few extension in communal areas. An external caller caused ringing ding-dongs in the ceiling on all floors, so someone had to take the external caller and connect them to the correct extension. This was a pain in the arse at 3AM!!

With my circuit added, it connected an incoming line to a spare internal extension. It seized the incoming line and coupled the audio forward to the internal exchange. The caller dialled in, got a new dial tone and could dial the required extension directly.
The external call was released a few seconds after the call ended due to the 500mS DC-break.

This was all 2-wire to 2-wire stuff.


I did two extra boxes:
1 - 2-wire to 4-wire to feed a message recorder.
2 - 2-wire to 4-wire to feed a simplex VHF radio TX and RX so we could use the internal exchange from our vehicle 2-way radios.

For these I used an OpAmp to isolate the TX and RX audio paths, and achieved about 20db to 25dB isolation. As Ivan pointed out, any more than this would make the line "feel" dead: you need a bit of sidetone for comfort.

I will publish the exchange circuit, and I will search for my lab-book from that era to see if I can find the other circuits.

Very best regards from Harry - SM0VPO

Hi Ruud,
the common solution in POTS phones from the pre-semiconductor era employs a transformer. It is a part of a bridge indeed. The bridge is intentionnaly unbalanced a bit, so that the speaking person can hear himself a bit. The AF feedback causing noise must be avoided under any circumstances. If the speaking person get no acoustic feddback at all, he feels like the phone does not work.

BR from Ivan

Please DO publish it! It sounds interesting.
At the moment I am busy developing a 4-wire to 2-wire adapter for a telephone line.
It seems a telephone line is a very complex impedance!
The idea is that a 4-wire to 2-wire adapter acts like a 'Wheatstone bridge for audio'.
The goal is to get as little of the signal you are sending to the subscriber at the other end of the line back in the received signal; the received signal should only contain the audio 'from the other end'. But it takes a complex network to balance the bridge for all audio frequencies.

Hi Ivan,
Thank you for the comment. If I have little to do, on a plane/train or something I may well prepare the article.
Thanks for the response, then I will make it a low priority project.

BR Harry

Hi Harry,
the answer probably depends on where the device should be used. POTS seems to be absolutely obsolete   in Europe. (Btw, to be honest,
I still have a POTS + ADSL line at home.) Even ISDN is leaving the stage. VoIP attacks everywhere.

VBR Ivan OK1SIP

Hello again every one,

When I worked in Saudi-Arabia I built an "auto-answer" for an ordinary POTS telephone system using a BC547 and BC548 latch in series with an AF transformer.

When the incoming ring signal occurred the latch triggered, took the line, and coupled the audio to the internal exchange, so using DTMF tones the caller could dial in internal number. When the caller hung up the the voltage reverses, but after 30 seconds there is a 1-second break in the 50V standing DC. This break releases the latch to free the line.

Could this be an interesting project? Or is the POTS telephone system a bit too old? Any feedback would be appreciated before I spend any time on it.

Very best regards from Harry