noise when cable is unplugged

[GloBug]

No , why would you ground a ground.

"I'm using a plain cheap cable, no shielding. It's about 6 ft. long."
"If I use a 6" cable, there's no noise."

Why not put a RF cap across the pins and be done with it.

Leave it plugged in, seriously why do you feel the need to run the amp with no input?

When you learn more you might make a one that doesn't hum with no input, but why???

 

[epicyclic]

Think i might have an answer for you now you tell us the cable your using is unscreened ,though why !!!!!!! .

Your using a balanced line receiver but unless your using the David Burt modification to said it actually presents different impedances on the hot and cold ( not normally a problem ) . In your circuit 18K hot , something less than 6K cold . This inbalance dosnt give you the Common Mode Rejection you require to connect 6ft of unscreened cable . Thats why when you use a transformer your curing the noise because the transformer TRULY balances the line .

 

[simon7000]

I'm using a plain cheap cable, no shielding. It's about 6 ft. long.

If I use a 6" cable, there's no noise.

I don't have a oscilloscope, unfortunately.

Is this a transmission line problem? If vp is 2e8 m/s, and my cable is 2m, then that gives 100mhz. Is connecting a resistor across the input jack the same thing as connecting 2 resistors (it's a mono jack) from each pin to the circuit's ground, from a termination point of view?

Yes 10K should do it.

 

[epicyclic]

Ref .......Post22

Im correcting myself the impedance mismatch refers to the differential signal not the common mode signal and so my statement is incorrect .......apologies.

The transformer does truly balance the line tho........but you probably new that already.

 

[godfrey]

(it's a mono jack)

That sounds ominous. For a balanced connection there should be 3 wires; two for signal plus one for ground.

 

[Speedskater]

That sounds ominous. For a balanced connection there should be 3 wires; two for signal plus one for ground.

In a balanced interconnect, there are only two signal conductors. The third connection is for the shield which is connected to the chassis at the send end. At the receive end the shield may or may-not be connected to the chassis, or it may be connected with a capacitor (hybrid). The shield is not part of the audio ground (although the audio ground is probably connected to the chassis).