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#1
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Ground loop between phone/computer
I'm trying to record telephone interviews using a Mac iBook. It doesn't
have an audio input so I bought a Griffin iMic, which runs off the USB port. My problem: I can't seem to get rid of ground loop hum. I bought a Radio Shack ground loop isolator to put between the phone and the iMic but that doesn't seem to help much. Any bright ideas would be appreciated. My set up.... * Radio Shack phone jack from the headset cord, to 1/8" inch mini plug. * 1/8" female to RCA mail adapter * RCA female to female adapter * Radio Shack ground loop isolator (RCA male in, RCA male out) * RCA female to 1/8" mini male adapter * Griffin iMic, using either mic or line settings (hum is worse on mic setting). * G3 iBook 700 * Audio In recording software for OS X |
#2
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In article ,
"Sushi" wrote: I'm trying to record telephone interviews using a Mac iBook. It doesn't have an audio input so I bought a Griffin iMic, which runs off the USB port. My problem: I can't seem to get rid of ground loop hum. I bought a Radio Shack ground loop isolator to put between the phone and the iMic but that doesn't seem to help much. Any bright ideas would be appreciated. My set up.... * Radio Shack phone jack from the headset cord, to 1/8" inch mini plug. * 1/8" female to RCA mail adapter * RCA female to female adapter * Radio Shack ground loop isolator (RCA male in, RCA male out) * RCA female to 1/8" mini male adapter * Griffin iMic, using either mic or line settings (hum is worse on mic setting). * G3 iBook 700 * Audio In recording software for OS X The ground loop isolator doesn't isolate well enough for telephone use. It is meant to isolate against about 2V, not the 25V or more that's on telephone wires. You need a real telephone audio transformer. You can add it to the Rat Shack transformer for maximum isolation. Put a non-polarized capacitor of a few µF and at least 50V between the telephone transformer and the phone if you have problems with low voltage. - Phone Laptop - ------ -------------- ------ Line in ) || ( ) || ( ) || ( ) || ( ) || ( ) || ( ) || ( ) || ( ------ -------+------- ------ Line in |------------------- Laptop Ground 600 Ohm Rat Shack Telephone Ground Loop Transformer Transformer |
#3
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Normal telephone conversations work around 500Hz.. your hum will be much
lower than this. I could be wrong here (and would like to know if i am) but. Perhaps a simple High Pass Filter could cut out this hum. Rob "Kevin McMurtrie" wrote in message ... In article , "Sushi" wrote: I'm trying to record telephone interviews using a Mac iBook. It doesn't have an audio input so I bought a Griffin iMic, which runs off the USB port. My problem: I can't seem to get rid of ground loop hum. I bought a Radio Shack ground loop isolator to put between the phone and the iMic but that doesn't seem to help much. Any bright ideas would be appreciated. My set up.... * Radio Shack phone jack from the headset cord, to 1/8" inch mini plug. * 1/8" female to RCA mail adapter * RCA female to female adapter * Radio Shack ground loop isolator (RCA male in, RCA male out) * RCA female to 1/8" mini male adapter * Griffin iMic, using either mic or line settings (hum is worse on mic setting). * G3 iBook 700 * Audio In recording software for OS X The ground loop isolator doesn't isolate well enough for telephone use. It is meant to isolate against about 2V, not the 25V or more that's on telephone wires. You need a real telephone audio transformer. You can add it to the Rat Shack transformer for maximum isolation. Put a non-polarized capacitor of a few µF and at least 50V between the telephone transformer and the phone if you have problems with low voltage. - Phone Laptop - ------ -------------- ------ Line in ) || ( ) || ( ) || ( ) || ( ) || ( ) || ( ) || ( ) || ( ------ -------+------- ------ Line in |------------------- Laptop Ground 600 Ohm Rat Shack Telephone Ground Loop Transformer Transformer |
#4
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"Sushi" wrote in message ... I'm trying to record telephone interviews using a Mac iBook. It doesn't have an audio input so I bought a Griffin iMic, which runs off the USB port. My problem: I can't seem to get rid of ground loop hum. I bought a Radio Shack ground loop isolator to put between the phone and the iMic but that doesn't seem to help much. Any bright ideas would be appreciated. I have a telephone that will record both sides of a phone conversation--but only for 15 minutes. Is this long enough? I only paid $9 for it! Norm Strong |
#5
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In article ,
"Rob Beech" wrote: Normal telephone conversations work around 500Hz.. your hum will be much lower than this. I could be wrong here (and would like to know if i am) but. Perhaps a simple High Pass Filter could cut out this hum. Rob The signals on phone lines are very badly distorted with respect to ground. A high pass filter will let through all the high frequency distortion induced by the base 60Hz signal. You still need to start with a telephone transformer so that the input is well balanced, causing the noise present on both wires to cancel out. A HiFi ground loop filter has too much capacitive coupling between windings to do that well for telephone line voltages. |
#6
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Thanks for the replies.
This would explain why the Rat Shack ground loop isolator didn't do much. What/where is a telephone transformer? Can i easily make one? I can solder a connection but I'm a novice at just about anything beyond that. "Kevin McMurtrie" wrote in message ... In article , "Rob Beech" wrote: Normal telephone conversations work around 500Hz.. your hum will be much lower than this. I could be wrong here (and would like to know if i am) but. Perhaps a simple High Pass Filter could cut out this hum. Rob The signals on phone lines are very badly distorted with respect to ground. A high pass filter will let through all the high frequency distortion induced by the base 60Hz signal. You still need to start with a telephone transformer so that the input is well balanced, causing the noise present on both wires to cancel out. A HiFi ground loop filter has too much capacitive coupling between windings to do that well for telephone line voltages. |
#7
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"Rob Beech" writes:
Normal telephone conversations work around 500Hz.. Ttypical telephone specifications list the analogue telephone system (PSTN) frequency range to be 300 Hz - 3.4 kHz. your hum will be much lower than this. Perhaps a simple High Pass Filter could cut out this hum. There can be considerable mount of mains frequency (50 Hz or 60 Hz dependign on country, plus harmonics of them) noise on the telephone line signal. In typical telephone line there is both common mode and differential mode noise. Normal telephones are built in such way that they do not play back this noise (high pass filter on the circuitry, acoustic design of handset speaker etc..) -- Tomi Engdahl (http://www.iki.fi/then/) Take a look at my electronics web links and documents at http://www.epanorama.net/ |
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