Hooking old Rotary Phone up to old GE tape recorder

Discussion in 'The Projects Forum' started by PartyLine4, Jan 31, 2013.

  1. PartyLine4

    Thread Starter New Member

    Jan 12, 2013
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    I have attempted to connect my old rotary phone up to my old GE tape recorder.

    I tried taking a 3.5 MM plug and hooking up the red and green to the phones microphone. I was able to understand what I was saying, but there was a lot of static.

    I am curious to if I could possibly damage the microphone or the recorder in any way by experimenting. I want to keep them, as they are the only ones I have as of now.

    The old phones are almost indestructible so I am mainly worried about the recorder.

    Do the old tape recorders send current out through the microphone and wait for interruptions like the rotary phone does?

    I know for a fact that the phone's microphone uses a bit more than the recorder would.

    Here is the recorder I am using:

    http://i4.ytimg.com/vi/SExPbI6WBjE/mqdefault.jpg

    For the sake of time the phones back then are not that different from the phones of today. The only difference is that you can actually mess with the old phones.
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    You would be better off to get a microphone that attaches with a suction cup to the handset of the phone at the earpiece. That way, there is no electrical interference or impedence mismatch and no chance of damaging your recorder. As an added bonus, you record both sides of the converstation.

    Here's one: http://compare.ebay.com/like/110698248294?var=lv&ltyp=AllFixedPriceItemTypes&var=sbar
    Radio Shack may still have them as well.
     
  3. PatM

    Active Member

    Dec 31, 2010
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    You could also try to Google
    Acoustic Coupler for a Telephone
     
  4. BReeves

    Member

    Nov 24, 2012
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    I'm reading he just wants to use the phones mic but could be wrong.

    That phone has a carbon mic and needs to have a voltage passing through it. Google carbon microphone.. You will need a low voltage source (1.5 volt battery) and a cap to isolate the DC from the recorder input.
     
  5. PartyLine4

    Thread Starter New Member

    Jan 12, 2013
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    Ok so I am more of a guy who likes to do it himself. I always know there is something I could buy, but what the heck would that teach me?

    According to the post above me, I need to add a CAP to the phones mic?
    One thing that drives me crazy is how people figure out what values to make everything. SO i I ask you, how do you know what size cap to put on the phone.

    I remember an instructive thing that let you turn your phone into a computer mic, he used a resistor and a capacitor I THINK.

    OK let me explain what I did with a Diagram what I have accomplished so far:

    You see the phones microphone is hooked in parallel with the recorder. This seemed to record both sides of the conversation, but when played back on the recorder the volume was SUPER loud! I had to turn the volume to level 1 to hear it normally. However the conversation is not perfectly clear, such not as music being played on the recorder.

    There has to be a simple way to do this efficiently and effectively.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    You forgot to tell us if the phone is connected to a phone line so it is powered.
    Connected to a phone line its microphone will have a huge output signal that will overload the mic input of the recorder.
     
  7. PartyLine4

    Thread Starter New Member

    Jan 12, 2013
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    Yes sir, I know this by doing it the hard way. Luckily I had minimal volume selected when I tested the playback.

    So is there anywhere on the phone I could safely hook the recorder to?

    The phone is powered by a phone line supplying 49 volts to it. The recorder only needs 7.5 to operate.

    The recorder has a monitor feature that I suppose automatically starts recording when it detects sound. Does this sound right to you?

    Amazing.
     
  8. PartyLine4

    Thread Starter New Member

    Jan 12, 2013
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    Hooked up just the mic to a 1.5v battery in series with a 3.5mm plug.

    Worked very well except for the massive amplitude from the mic.

    So a capacitor takes out the noise from DC right?

    And a resistor is probably needed to get the desired amplitude effects.
     
  9. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Yes, a 1uF cap in series with the center wire going to the jack on the recorder would remove the DC bias.

    A resistor between the center wire and other wire going to the recorder could be used to reduce amplitude, however, you are also effecting the loading of the circuit you are tapping into, so start with a high value and work lower until it sounds right and still works.
     
  10. PartyLine4

    Thread Starter New Member

    Jan 12, 2013
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    Here is a circuit I recently attempted to build.

    My Mic is a carbon mic at 5kΩ

    Instead of a 33k resistor I used a 51kΩ resistor.

    I also used a 47μf electrolytic CAP which, laying here I know realize, is in my circuit backwards :mad: . I also have in place of the 100kΩ POT, A 10KΩ POT.

    My circuit is different because all I have to mess with were these random components.

    If someone could please explain; how does the farads number effect this circuit? I pretty much know what the resistors are doing. The layout of this circuit is confusing to me, although i got it to semi-work :confused:

    The microphone seemed to work and was much lower in amplitude. But seeing as the current is in the micro amps, and the CAP is wired backwards, I guess the cap allows the AC to flow anyways?

    Someone please help before I destroy my tape recorder.....:confused:
     
    Last edited: Feb 1, 2013
  11. Audioguru

    New Member

    Dec 20, 2007
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    A carbon mic has a lot of distortion. Its level drops when the carbon granules pack together. If it is rapped on a table to shake up the granules then it will be louder.

    Here is the way the attenuator should be wired:
     
  12. PartyLine4

    Thread Starter New Member

    Jan 12, 2013
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    Could someone please elaborate on how this circuit works.

    I know hooking a battery to the mic with the mic jack in series works, but it is so loud.

    Now I have the circuit I posted a few posts back and it works but it is not loud enough!

    Someone please just elaborate on how you are getting these values for the resistors and such, and possibly what I could substitute for not having a 100k audio POT.

    I have taken basic ac/dc courses so I know all the formulas for everything.
     
  13. BReeves

    Member

    Nov 24, 2012
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    You need to tell us exactly what it is you are trying to do? Record phone conversations or just use the phone in place of a mic.
     
  14. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    I had a small brain fart last night and forgot to mention a resistor inline as well as across the line.

    The capacitor is blocking the DC component from the microphone. The inline resistor combined with the resistor across the line forms a voltage divider to allow a lower output by shunting part of the signal to ground.

    The pot doesn't need to be a logarithmic type (Audio Taper), a linear pot will do in this situation.

    You could start with 10k in series and 1k across the line after the cap and see how it sounds.

    The microphone may have an odd sound to it. Carbon microphones in telephones were built for durability rather than sound quality, since the phone lines were bandwidth limited to about 4kHz anyway.
     
  15. Audioguru

    New Member

    Dec 20, 2007
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    At work I measured the frequency response from one phone line to another phone line. 3kHz was way down at -12dB. Response was flat to about 1kHz. I complained to BELL and they said it was normal since their limit is -15dB round trip (-7.5dB to the central office plus another -7.5db to the destination).

    I built transmit equalizers for teleconferencing equipment that boosted 3kHz +10dB to make voices sound crisp and clear. Every time I demo'd one it was sold.
     
  16. PartyLine4

    Thread Starter New Member

    Jan 12, 2013
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    What is this " line " you speak of?

    How does the size of the CAP effect the circuit?
     
  17. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    I'm unsure what "CAP" means.

    Line is simply referring to the two wires from the carbon microphone that are going into the line in jack on the recorder.
     
  18. Audioguru

    New Member

    Dec 20, 2007
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    The value of the capacitor is calculated with the mic impedance in series with the recorder input impedance for the low cutoff frequency.
    A large capacitor value produces output down to very low frequencies. A small capacitor value cuts bass frequencies.
     
  19. PartyLine4

    Thread Starter New Member

    Jan 12, 2013
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    Does the mic have impedance? I would think of it more as a variable resistor.

    How could I calculate the size of capacitance I need in the circuit?

    The mic has a resistance of 4300 Ohms by the way.
     
  20. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Cutoff for a 2.2uF cap with 4.3k resistance is around 20Hz, any higher frequencies, up to the capabilities of the microphone will pass through.

    The capacitor is simply blocking the DC bias, allowing the AC signal to pass. If you look at Highpass filters on wikipedia, that may help your understanding.
     
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