Hiss problem with compressor -goes when you touch the case

[alextdel]

Happy New Year
I have built an orange squeezer clone. It is hissy when on, (quiet when bypassed) when put before a tubescreamer clone. When I touch the case of the compressor it looses the hiss and is fine. Any ideas where to start. Earthing is suspect but the continuity between the case and the signal return is good (0 Ohms).
[Sub question -  can an unused op amp channel with open circuit I/p and O/p introduce noise into the used channel.]

[Processaurus]

Hi, if noise goes away when you touch something, I would suspect a grounding issue.  Make sure all the pots, switches, and jacks are all grounded together, as well as the enclosure bottom.  Pretty much all the exterior metal stuff.  I had a problem recently with a pre painterd box that had paint on the inside and was insulating all the offboard components from getting grounded.   Actually if that stuff wasn't grounded you would get more of a hum than a hiss...  Maybe check nothing's contacting the case inside that isn't supposed to be.

The OS is a bit hissy, though, especially through a distortion.

I don't think the floating opamp inputs would cause noise on the other channel, but you could ground the non inverting (+) input if you're worried.  Better yet, use it as a gain recovery stage after the OS.  General Guitar Gadgets has a schematic of how to do that.  There is (or was, if its been fixed) an omission on the schematic, there needs to be a .1 uF or so cap between the level pot and the + input of the second opamp.

[alextdel]

Thanks, my OS is the Mark Hammer Mod. I couldn't work out a good way to cut in to the circuit board that I got from Generalguitargadgets so I put a second Op amp on a piece of veroboard along with the associated bits of extra circuit. Its a bit bodgy I know.

I really like the OS compression, but I hate the noise that is generated.
With the back off I get a pick up of noise when I touch the wire between the circuit board and the output jack. Is it normal to use shielded cable within a stomp box for the signal to and from the board?

[Processaurus]

Is it normal to use shielded cable within a stomp box for the signal to and from the board?

Shielded cable is a good idea for high impedance inputs to pedals if the input has a long run of wire (over a couple inches), or if it goes near a logic circuit putting out big voltage, or AC power.  Thats to prevent any noise from the circuit getting amplified by showing up on its high impedance input.  After that, when things are driven better, and have a lower impedance, (like the output) shielding would be less beneficial.  A lot of noise comes from the OS itself, though.  You can tell by turning your volume down all the way and listen for noise on the output.  Its a compromise with such a simple design.

Also the OS is picky about power it gets, it hums like crazy with anything but regulated supply, or a battery.

[alextdel]

thanks for the detailed info. I may try shortening some of my bodgy hook up, earthing the pot bodies and run it with a battery to test the power supply i'm using. (switched mode 9/12v)

[MR COFFEE]

Hi Alextdel,

You may be right to suspect the unconnected op amp, especially with the symptom of the noise going away when you touch the case.

I'd agree with Processaurus on the grounding problem if it was hum that went away, but since it's noise
going away, I'd suspect RF.

An op amp with unconnected inputs and no feedback from the output terminal to the inverting input acts like a very high gain amplifier for what ever signal it can pick up, usually capacitatively, including signal from it's own output terminal. Which means it acts like a sloppy oscillator. Since there is no negative feedback, the op amp puts out wideband noise with virtually a rail-to-rail swing which broadcasts itself into your audio circuit. Touching the case probably alters the capacitances involved just enough to damp your RF oscillation.

Ground the non-inverting input like Processaurus says, and connect the output to the inverting input. This is SOP for unused op amps, and I'll bet the problem goes away. Current drain will probably be a lot lower, too

[toneless]

Sounds like grounding problem...even your pedal or your guitar!

[alextdel]

Some comments after my investigation.
1. I had the power plug switching like a battery through the input jack (I don't use batteries) - this makes the case sit at 9V with the input jack out - ouch! I don't think this caused any noise but it was a bit tingley when the ground of a guitar cable touched my leg.
2. I followed the suggestion about grounding the non inverting input to the spare op amp channel. (Thanks Mr C!)
3. I also used single shielded leads within the case for the output lead from the board to the output jack.(Thanks Toneless and Proc')

I now have a very clean and clear pedal that sounds good. However after these alterations I did have a loud pop when turning the pedal on through the bypass switch. I took the back off and checked for bad input resistor but found nothing and since then the popping has dissappeared. I am assuming that I has a short to the top of a cap case or something. 

thanks to all for your help