noise when cable is unplugged

[eccdbb]

I've been playing around with BUF634's, but something weird is happening. I am getting noise whenever:
1) 3.5mm cable is connected to the input, but not connected to a source
2) Headamp pico turned off, connected to the input

I am using a SSM2143 to receive the signal, in a 2x gain. It is connected to a LME49710 with BUF634's inside the loop. When I had 1 buffer installed, the noise was small, and after I put in the 2nd buffer, the noise became louder. Does anyone know why the noise happens?

 

[epicyclic]

Hi

can you post a schematic.

 

[eccdbb]

It's simple enough to describe:

SSM2143 in 2x gain -> unity gain LME49710 with 2 BUF634's inside the loop -> 2.5 ohm and 1nf zobel in parallel with the load. Each chip has either .1uf, .47uf, or 1uf capacitors from power supply rails to ground. There's also 100uf caps from rails to ground.

Power supply is +/-15v SMPS -> +/-12v linear regulators.

 

[epicyclic]

Does the noise go the instant the SMPS is switched off.

Excuse my ignorance why do you want 2 BUF634s in the loop.

 

[Minion]

Doesn"t having a cable not connected to a source just act like an antenna that picks up noise ?? I know with guitar amps not having your guitar plugged into the cord causes an evil noise , I believe that is why most guitar amps inputs have jacks that short the input to ground when a cable isn"t plugged in .......

 

[eccdbb]

The noise fades away.

I'm in the process of figuring out Iout vs. required loudness. Apparently 1 buffer is good for close range listening, but I want more.

 

[epicyclic]

Im inclined to agree with Minion....... however you have got a lot of gain available but your not using it , just 6dB, so on paper youd think it would be quiet . The thought that i do have is the very wide bandwith your setup has the potential for . The wider the bandwith the more noise you will have . The balanced i/p screen is earthed at the i/p connector ?

 

[theAnonymous1]

I assume there isn't a volume pot on the input? If not, try putting 100k resistors from the inputs to ground.

 

[GloBug]

Yes I agree with Minion, its normal.

 

[eccdbb]

There's no volume pot at the input.

I tried connecting the inputs to ground via 100k resistors, but that didn't help. What I did find out is that connecting the inputs together (by shorting or using a resistor) decreases the noise. Shorting and small resistors kill the noise; large resistors only decrease the noise a little. Also, using a transformer to couple the signal to the inputs gets rid of the noise completely. Any more suggestions?

 

[godfrey]

Any more suggestions?

Posting a schematic would help. What does the noise sound like?

 

[eccdbb]

The noise is is like a low frequency buzz.

I tried adding a low pass like: Balanced Interfaces Figure 10. It didn't help.

Then I made the circuit simpler- SSM2143 -> unity gain OPA2134, with power supply decoupling caps, and that's it. It still makes the noise, with G = 1/2 or 2.

Why does shorting or connecting a small resistor across the inputs make decrease the noise?

 

[epicyclic]

100R in series on both hot and cold inputs and 100pf to earth after on both . 2 RC networks .....low pass . Or 2 zorbell networks to earth on each hot and cold i/p ( 75R in series with 220pF ) .

Apologies i missed the above thread , you might try the zorbells tho.

 

[GloBug]

Don't connect the input without a source. It's normal.

 

[godfrey]

The balanced i/p screen is earthed at the i/p connector ?

Good question. Also, what type of cable are you using? shielded twisted pair?

Don't connect the input without a source. It's normal.

I don't think it's normal to pick up mains hum from a piece of shielded cable with nothing connected at the other end.

 

[GloBug]

Oh really? This must be new, OK I will hold your hand.

It's called RF, that's why guitar amps use a shorting jack.

The cable does not matter, or should I say antenna.

 

[godfrey]

Oh really? This must be new, OK I will hold your hand.

It's called RF, that's why guitar amps use a shorting jack.

The cable does not matter, or should I say antenna.

I'm open to correction here, but as I understand it, the "shorting jack" in a guitar amp shorts the amp's input when no cable is plugged into the amp, to kill the amp's self-noise.

It can't have anything to do with noise/RF/anything else coming in through a cable because it only shorts when there is no cable plugged in. As soon as you plug a cable into the amp, it's no longer shorted, whether there's a guitar on the other end of the cable or not.

I agree RF can be a problem but:
A) The OP has tried an RF filter and it didn't help.
B) The noise is "a low frequency buzz", which sounds like mains hum.

 

[Minion]

If you plug a patch chord into a guitar amp without a guitar you will pick up noise and hum and even radio , that goes away when you take the jack out of the amp because the input gets shorted to ground .......

In this situation he is connecting a cable into the input without any source connected replicating the same scenario as when plugging a Patch chord into a guitar amp without a guitar , the cable is acting as an antennae ......

There is no noise when there is a source connected , and there is no noise when the input is shorted to ground which through fairly convincing resoning points to the Cable picking up the noise .......

 

[simon7000]

Do you have an oscilloscope? Sounds to me like it is oscillating well above the audio band.

 

[eccdbb]

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?