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cowboyro
Premium
join:2000-10-11
Shelton, CT

Phone line amplifier?

Anyone has seen one? Not an amplifier that goes between the base and the handset, but one that would actually go on the phone line.
I have a magicjack device, the volume is way too low. All the hacks known have only changed it from very, very low to very low.
I don't care about the FCC compliance as it doesn't go on an actual phone line.
Alternatively a schematic so I can build one myself without reinventing the wheel?



aurgathor

join:2002-12-01
Lynnwood, WA
kudos:1

I remember seeing at least 2 maybe 3: one in the defunct Radio-Electronics, one in an old electronics book 20+ years ago, and one somewhere else. There's a small chance that I can find that book and scan it in. I think they all used some strange transformer (hybrid?).

Actually, there are some circuits there: »en.wikipedia.org/wiki/Hybrid_coil

BTW, can't you just hack your phone and put in a small amplifier for the speaker, or increase the amplification?



cowboyro
Premium
join:2000-10-11
Shelton, CT

I want to use multiple phones in the house. While hacking one is acceptable, when I'm considering 6 phones it's no longer OK.
To an extent the system would be 2 phones mike to speaker and a power system for "my phone" side along with passing the ringer in the on-hook state.



tschmidt
Premium,MVM
join:2000-11-12
Milford, NH
kudos:9
Reviews:
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2 edits
reply to cowboyro

Have you thought about hacking the Magicjack rather then the phone line? If you put an amp in the phone line will need some sort of hybrid transformer to split direction. If you hack the device itself may be able to add a gain stage.

That being said are you sure this is not just a bad device or perhaps a problem with phone? I'm sure you have already done so but if you are using inside wiring to connect POTS phone make sure it is disconnected from the NID. If you are trying to drive more then one phone see what happens if you only use a single phone.

I did a search for "magic jack volume low" and got lots of hits.

»www.magicjacksupport.com/low-vol···192.html

»www.magicjacksupport.com/low-vol···997.html

Do all 6 phone have to ring? If so will need a pretty hefty ring generator depending on the REN of each phone.

/tom
fixed typos



aurgathor

join:2002-12-01
Lynnwood, WA
kudos:1

1 recommendation

reply to cowboyro

said by cowboyro:

I want to use multiple phones in the house. While hacking one is acceptable, when I'm considering 6 phones it's no longer OK.

I see. For that much of a load, you probably need an amplifier.

I was hoping lutful will post a transformer-less DSP solution -- but he seems to be slacking....
Apparently, there are some patents on them, so they do exist, at least in patents.

I think you can scavenge hybrids from phones or from modems -- just go to some thrift shop.
--
Wacky Races 2012!

lutful
... of ideas
Premium
join:2005-06-16
Ottawa, ON
kudos:1

1 recommendation

Click for full size
said by aurgathor:

said by cowboyro:

I want to use multiple phones in the house. While hacking one is acceptable, when I'm considering 6 phones it's no longer OK

I see. For that much of a load, you probably need an amplifier.

I was hoping lutful will post a transformer-less DSP solution -- but he seems to be slacking....

I was waiting for someone to actually post a convoluted analog POTS interface circuit that manipulates just the audio without impacting ring waveform and off-hook detection. Good quality telephony devices designed since 2005 actually use transformer-less POTS interface and include DSP.

Above block diagram shows how you can make a digital FXS-FXO repeater using off-the-shelf SLIC/DAA ICs and a DSP or FPGA that could do any kind of manipulation of the audio and DTMF. But AFAIK such a device does not exist but VoIP gateways that also have a proper FXO interface could be hacked to do this job.

cowboyro See Profile ... If the Magicjack is connecting to all the phones in the house, the "ring equivalent number" or REN seen by their line driver IC could be too high.


Jason
Stowage Class Traveler
Premium,Mod
join:2001-01-24
38.2967 Lat
kudos:3

But aren't most modern phones (with a wall-wart power supply) typically ridiculously low REN?

I just looked at my base, and it lists 0.1 REN
--
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.



cowboyro
Premium
join:2000-10-11
Shelton, CT
reply to lutful

said by lutful:

cowboyro See Profile ... If the Magicjack is connecting to all the phones in the house, the "ring equivalent number" or REN seen by their line driver IC could be too high.

That's never the case with modern phones. And it's usually one base with multiple handsets...
I put the scope on the line and the incoming sound from the echo test number is ridiculously low compared to the outgoing level, so that rules out any issue with the phone.

lutful
... of ideas
Premium
join:2005-06-16
Ottawa, ON
kudos:1

1 recommendation

Typical VoIP ATAs can't easily drive the convoluted wiring that go with having too many phones. By contrast, Telco port has a very robust line driver ... but your audio spectrum is still being attenuated.

Anyway, you will need to detect off-hook status to insert and remove the audio equalizer/amplifier device in the phone line.


public

join:2002-01-19
Santa Clara, CA
reply to aurgathor

said by aurgathor:

a transformer-less DSP solution

I think you can scavenge hybrids from phones or from modems -- just go to some thrift shop.

A hybrid can be realized with two op amps. No dsp needed.

lutful
... of ideas
Premium
join:2005-06-16
Ottawa, ON
kudos:1

Click for full size
said by public:

said by aurgathor:

a transformer-less DSP solution

I think you can scavenge hybrids from phones or from modems -- just go to some thrift shop.

A hybrid can be realized with two op amps. No dsp needed.

Just a bit more than that. See above complete schematic from Clare app note. FYI the dsp core comes as a baggage in modern telephony chipsets which incorporate the hybrid. See earlier block diagram from Silabs sales material.

*** Question to all the "hybrid" proponents ... are you suggesting putting two hybrids back to back and amplifying the audio using the hybrid's TX/RX op-amps? Or are you talking about the "hybrid" boxes used by radio stations to put callers on the air? There is a huge difference.


aurgathor

join:2002-12-01
Lynnwood, WA
kudos:1

said by lutful:

*** Question to all the "hybrid" proponents ... are you suggesting putting two hybrids back to back and amplifying the audio using the hybrid's TX/RX op-amps? Or are you talking about the "hybrid" boxes used by radio stations to put callers on the air? There is a huge difference.

Methinks the answer should be pretty obvious from the original question and the answer given.
--
Wacky Races 2012!

lutful
... of ideas
Premium
join:2005-06-16
Ottawa, ON
kudos:1
reply to aurgathor

It is not actually obvious what you guys were suggesting even after public See Profile mentioned that hybrids can be built with op-amps. That is why I asked a clear question about a "plausible" back-to-back hybrids solution which was not actually proposed by anyone.

said by aurgathor:

I think they all used some strange transformer (hybrid?).

Actually, there are some circuits there: »en.wikipedia.org/wiki/Hybrid_coil

That wikipedia page shows transformer-type hybrids and says:

Hybrids are used in telephones (see telephone hybrid) to reduce the sidetone, or volume of microphone output that was fed back to the earpiece.

Without this, the phone user's own voice would be louder in the earpiece than the other party's.

Such hybrids also had their windings so arranged as to act as an impedance matching transformer


Later there was talk of pulling hybrids from PCBs.

BTW I mentioned the "hybrid" boxes used by radio stations to put callers in the air because modern digital versions actually follow the block diagram I posted earlier in response to your "transformer-less dsp" quip.

They can equalize/amplify caller and host audio any way they want because of the DSP and not because of the hybrids. Obviously they can be used for this task, but they are quite expensive.

It is always good to have some resolution before the thread goes stale. Someone please draw a sktech or even a block diagram of the complete in-line audio amplifier circuit for POTS.


aurgathor

join:2002-12-01
Lynnwood, WA
kudos:1

While there are several uses of hybrids (for one, I didn't know it is, or can be used by radio stations) but to clarify my first post, this was one intended answer to the OP's question: »en.wikipedia.org/wiki/File:TwoWi···ater.jpg
While it doesn't have all the components drawn out, it shouldn't be too difficult to implement, once someone has the right hybrids.

Of course handling the ringer is left as an exercise for the implementer.

I'm sure there are other valid answers.
--
Wacky Races 2012!


lutful
... of ideas
Premium
join:2005-06-16
Ottawa, ON
kudos:1

1 recommendation

Click for full size
said by aurgathor:

this was one intended answer to the OP's question: »en.wikipedia.org/wiki/File:TwoWi···ater.jpg
While it doesn't have all the components drawn out, it shouldn't be too difficult to implement, once someone has the right hybrids.

That diagram comes from classic telco two-wire repeaters. The type of hybrid is shown in more detail here ... won't be easy to find in any consumer telephony PCB.

The rectangles beside the hybrid transformers are impedance matching network which tries to convert whatever is on the other side to approx. 600 ohm. See the original notes regarding amplified echo from impedance mismatch.

To mimic that diagram today, you will need to design 20Hz-3500Hz amplifier which can handle 60V ring signal and still sensitive to the attenuated audio signal. Most probably that means using valves or high voltage op-amps.

Rebirth

join:2009-06-18
33333
reply to cowboyro

@ cowboyro

Maybe a telephone line bug is what you need. Something like this for eg ?

»www.instructables.com/id/Simple-···plifier/

Some better ideas in here - »www.epanorama.net/circuits/telei···ace.html


public

join:2002-01-19
Santa Clara, CA
reply to lutful

said by lutful:

To mimic that diagram today, you will need to design 20Hz-3500Hz amplifier which can handle 60V ring signal and still sensitive to the attenuated audio signal. Most probably that means using valves or high voltage op-amps.

Nothing of the sort is needed, since it is for intercom like use. Amp only needed when off hook.
Balance network resistors can be adjusted for best result.

lutful
... of ideas
Premium
join:2005-06-16
Ottawa, ON
kudos:1

said by public:

Amp only needed when off hook.

I agree. I wrote in my first reply Anyway, you will need to detect off-hook status to insert and remove the audio equalizer/amplifier device in the phone line thinking of good old analog relays.

Why don't you draw a sketch for us?


cowboyro
Premium
join:2000-10-11
Shelton, CT

said by lutful:

said by public:

Amp only needed when off hook.

I agree. I wrote in my first reply Anyway, you will need to detect off-hook status to insert and remove the audio equalizer/amplifier device in the phone line thinking of good old analog relays.

Why don't you draw a sketch for us?

It would be just a matter of detecting a drop in voltage below 15V or so for more than 0.1 sec (to avoid responding to the ~25Hz ring signal).


54067323

join:2012-09-25
Tuscaloosa, AL

1 edit
reply to cowboyro

said by cowboyro:

Anyone has seen one? Not an amplifier that goes between the base and the handset, but one that would actually go on the phone line.

The device you are looking for is called a loop extender and is commonly used to compensate for line loss in long POTS circuits.

There are two types out there one is span powered with 186 volts being sent out from the Telco CO, which is not what you need and there are local powered ones which are used on OPX's which will work for your application.

With the death of POTS I would think you might be able to get one surplus for a song, if not, knowing what the device is called might lead you to a diagram of one.

»www.proctorinc.com/46222.htm

»www.ebay.com/itm/Proctor-OPX-Lon···c7137ccd


54067323

join:2012-09-25
Tuscaloosa, AL
reply to aurgathor

said by aurgathor:

While there are several uses of hybrids (for one, I didn't know it is, or can be used by radio stations).

Nowadays in broadcasting the term hybrid is used generically to describe a piece of equipment that connects a dial-up line to a board, the hybrid allows caller audio to be sent to the board and pre-delay program audio to the dial-up line.

»telos-systems.com/one/default.htm

The term is left over from the days when such a device would utilize a hybrid similar to the ones used in the Western Electric 500 desk sets, the problem with coil based hybrids, such as the ones used in the early broadcast hybrids, is their transmit and receive amps needed to be set on the verge of feedback (ringing) to work as well as possible and if an incoming call was too hot, they would go into ringing, producing a hollow echoish sound, conversely of the incoming call was too long, they where incapable of properly compensating for the low audio level of the call.

lutful
... of ideas
Premium
join:2005-06-16
Ottawa, ON
kudos:1
reply to 54067323

said by 54067323:

With the death of POTS I would think you might be able to get one surplus for a song, if not, knowing what the device is called might lead you to a diagram of one.

»www.proctorinc.com/46222.htm

»www.ebay.com/itm/Proctor-OPX-Lon···c7137ccd

Thanks for pointing this out. You can't beat that $15 price since making one like that today will cost more than $100.