Microphone Jack connection.

[lord loh.]

I am trying to use winscope to view waveforms on my PC. I connected a stereo jack as a mono jack by shorting the outer most terminal and the middle terminal and using it as one terminal of my probe . The second terminal of my probe is connected to the inner most terminal of the jack.

Is this okay? could this create short circuit in my system?

Also I twisted the two wires to form a twisted pair, as I believe that balanced signals shall flow through it. Will this cause attenuation? (Theoretically?)

 

[akg]

i think that is the proper way to do it. , and sound card will have dc decoupling at i/p

 

[Nigel Goodwin]

i think that is the proper way to do it. , and sound card will have dc decoupling at i/p

The input to a sound card (as in pretty well any audio system) is AC coupled, NOT DC coupled - this is what prevents you making DC measurements with Winscope.

 

[Nigel Goodwin]

I am trying to use winscope to view waveforms on my PC. I connected a stereo jack as a mono jack by shorting the outer most terminal and the middle terminal and using it as one terminal of my probe . The second terminal of my probe is connected to the inner most terminal of the jack.

Is this okay? could this create short circuit in my system?

Also I twisted the two wires to form a twisted pair, as I believe that balanced signals shall flow through it. Will this cause attenuation? (Theoretically?)

There's nothing balanced there at all, twisting the wires wouldn't be a good idea, but scope performance is so low it probably doesn't matter much. Twisted balanced conductors, along with an outer screen, are used for low impedance balanced signals - neither of which applies here.

 

[akg]

i think that is the proper way to do it. , and sound card will have dc decoupling at i/p

The input to a sound card (as in pretty well any audio system) is AC coupled, NOT DC coupled - this is what prevents you making DC measurements with Winscope.

yup that's y "....and sound card will have dc decoupling at i/p ."

 

[lord loh.]

There's nothing balanced there at all, twisting the wires wouldn't be a good idea, but scope performance is so low it probably doesn't matter much. Twisted balanced conductors, along with an outer screen, are used for low impedance balanced signals - neither of which applies here.

Thanks.... I knew I was doing something foolish.... I'll untwist it.... :wink:

I still am not able to understand why it is not a balanced siganl...

 

[Nigel Goodwin]

There's nothing balanced there at all, twisting the wires wouldn't be a good idea, but scope performance is so low it probably doesn't matter much. Twisted balanced conductors, along with an outer screen, are used for low impedance balanced signals - neither of which applies here.

Thanks.... I knew I was doing something foolish.... I'll untwist it.... :wink:

I still am not able to understand why it is not a balanced siganl...

A balanced signal needs a negative signal, a positive signal (these two are 180 degrees out of phase) and a ground reference - although mike inserts usually don't have a ground reference at their end. The classic way is to use a center tapped transformer at either end of the balanced link.

Certainly the soundcard end isn't balanced, I've no idea what you are feeding into it?, but if both ends aren't balanced, nothing is balanced.

 

[audioguru]

You need shielded (screened) audio cable to avoid hum pickup from low-level, high impedance or both sources.

 

[audioguru]

Oh, I just saw the title saying microphone input.
The microphone input on sound cards has a DC voltage on it for electret microphones. The voltage might mess-up your source if a coupling capacitor isn't used.

 

[lord loh.]

okay...

I am actually trying to watch a 555 output...

Also I tried to watch the output of my 12-0-12 transformer and saw square waves... :shock:

180 degree phase difference would mean hat the signals are equal in magnitude but travelling in the opposite direction. And if I am thinking on the correct lines, it is only the current that ought to be of interest as current creates a magnetic field that cancels out on a balanced line thus giving noise immunity and attenuation...

Most of the 220v power cords are twisted... The current coming in is the current going out. Does this not cause any minor attenuation?

So to be called a balanced signal, one wire must carry +x and the other -x with respect to an arbitrary ground? Does this mean that a third wire of the common ground must run between the devices as well? (eg USB :Vcc, GND & a twisted pair)

 

[audioguru]

Your microphone input is designed for an input from a low level microphone of about 10mV. It is severely overloaded with the signal from a 555 and from your transformer, producing square waves. You need an attenuator to see signals with an amplitude of volts.

A telephone line is a balanced twisted pair, and doesn't have a 3rd wire as ground. It is at a high enough level that it doesn't need a shield like a low level microphone needs.

 

[lord loh.]

A telephone line is a balanced twisted pair, and doesn't have a 3rd wire as ground. It is at a high enough level that it doesn't need a shield like a low level microphone needs.

So what do I consider the basic criteria to have a balanced signal?

1. +v in one wire and -v in the other.
2. A ground needed? The telephone does not have one... :? :? :?

I am totally at sea.... :? :? :?

 

[audioguru]

Quote from Nigel: "A balanced signal needs a negative signal, a positive signal (these two are 180 degrees out of phase) and a ground reference".

The two wires of a balanced line have opposing signals, not DC voltages. The signal on one wire swings in a positive direction when the signal on the other wire swings in a negative direction, then the swings continue in the opposite directions. A balanced line doesn't transmit much interference and doesn't pickup much interference. The receiving end amplifies the difference in signal between the wires. Interference will occur on both wires in the same direction so won't make a difference signal.

Your mic wires have a signal on one wire with the other wire grounded to the signal, so isn't balanced. The wire with the signal is an antenna to interference pickup so shielded cable is needed.