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recording from a house phone

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Thunderchild

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So I want to use the non invasive method of a coil attatched as close as possible to the earpeice of a house phone so that I can record or put on speaker what is being said on the other end. what sort of amplification will I need and what type of coil for example roughly how many windings of what size wire ?

or if a direct connection to the earpeice is a better solution what should I do I don't fancy putting 40 volts into and op amp that will only take 20 for example. If I remember correctly there are dc 40 V when the line is inactive that go to 8 volts when it is active so would a blocking condenser be sufficient ?

hm better go route out some op amps
 
Use an isolation transformer to connect to the earphone. An audio 600 ohm to 600 ohm is usually used.
 
oops not much of a chance of laying hands on one of them, what if I an op amp ,the op amp has a high input impedence so shouldn't disturb the phone, what voltages are involved ?
 
Not much amplification is required, these usually go to a Mic input on a tape recorder, so if need be build a mic amp.
 
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I take it you're talking about pickup coil, I think this is by far the easiest and safest methon.
 
well there is a 150 ohm earpeice in the headset I am thinking of making a direct connection if I use a coil what do i do ?
 
You just stick the coil on the back of the earpiece and plug it into your tape recorder.
 
will this diaghram give good results in amplifiing ? it should have (virtual) infinit input imedence and be able to acheive very high amplification as necesary
 

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Your 1st opamp doesn't have a DC reference voltage at its input so it won't work.
The phone line is balanced to cancel interference and mains hum. Your circuit is unbalanced and if it worked then it would just amplify the interference and hum.
You need a transformer to balance and isolate the phone line.
 
That's exactly how a pickup coil works, the earphone voice coil is the primary and the pickup coil is the secondary, you shouldn't need any amplification for a tape recorder but if it's needed a then a normal pre-amp will probably do.
 
the off hook voltage is -48vdc (red to green phone wires)
and the ring voltage is +90vac
good to know if youre gonna connect anything more than a pickup coil
 
The onhook voltage is 48-52V
the off hook voltage is 6-12V
the ring voltage is 90-120VAC
The different exchanges and PBX systems can vary.
 
ok audioguru what do I have to do to that circuit assuming I will connect it to a coil, am thinking of an old earphone. how do I balance it I thought those were standard non inverting and inverting op amp configurations, how do I balance it or is it better to use a dual power supply
 
Thunderchild said:
ok audioguru what do I have to do to that circuit assuming I will connect it to a coil, am thinking of an old earphone. how do I balance it?
A coil is the secondary of a transformer where the telephone's earphone is the primary, so your amplifier doesn't need to be balanced. The coil provides perfect balancing.

I thought those were standard non inverting and inverting op amp configurations, how do I balance it or is it better to use a dual power supply
Your 1st opamp didn't have a DC reference voltage for its input.
Opamps don't need a dual power supply. They just need their input DC voltage about half-way between ground and the positive supply when using a single supply circuit. Look at these schematics:
 

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thats vvery interesting thanks,
but now I am confused I've always seen the non inverting config like I drew it up and you say that if a coil is the input that is ok why ?
would I need your version if it was getting it's input for example from another op amp ?
do I need a coupling capacitor between the stages ?
 
Thunderchild said:
I've always seen the non inverting config like I drew it up
No. An opamp needs an input DC reference voltage. The 1st opamp in your circuit doesn't have its input DC-connected to any DC voltage.

you say that if a coil is the input that is ok why?
A coil is completely separate from the balanced telephone because its coupling is a magnetic field so the coil doesn't unbalance the telephone which would cause hum.

do I need a coupling capacitor between the stages ?
Only if the two stages have different DC voltages.

I have corrected your circuit.
 

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