OC-2 ghost effect on guitar but not bass

[JimmyM]

Weird problem I discovered a couple days ago...

When I use my Taiwanese OC-2 with a bass, it works perfectly. When I use it with guitar, I can hear it faintly putting out a lower octave when I have it switched off. Quite disconcerting. I'm at a loss to figure this one out. Why would it only do it with guitar? Has anyone else ever had this problem with the OC-2? If so, what was causing it?

 

[Skrogh]

Maybe it's doing the same with the bass, but it's so low in frequencies, or your bass masks it so that you don't hear it?

Same amp, different amp?

I can try to reproduce it with my Oc-2 later today

 

[JimmyM]

The signal path was OC-2 > TAFM > Tech 21 British > REDDI > Fast Track > headphones for both. Not tried it in an amp yet. But I thought about it being too low to hear and tried the bass' high notes...doesn't do it on them, either.

 

[JonnyAngle]

I did the synth mod to a mit oc2 and found the ghost notes on bass as well. The solution was to move the wires on the inside so they weren't so close together. I don't know about a stock pedal though, but it might have given you an idea

 

[megafiddle]

Is there a large difference between the output impedances of the bass and guitar that
you were were using? A lower impedance instrument could be more immune to signal
bleed through from the sub-octave generator circuit (which is obviously always active).

A higher impedance is more susceptible to noise or extraneous signal pickup.

A passive P bass circuit would be most sensitive at a volume setting of -6db (5 on a
linear pot, about 8 or 9 on an audio taper). A passive J type circuit would be most
susceptible at low volume settings.

Same is true for guitar depending on how the output impedance changes with volume
setting.

If the instrument has a preamp, the output impedance would remain low regardless of
the volume setting, and would be more immune to any extraneous signal pickup.

.

 

[JimmyM]

Don't know impedance difference, but they were both passive.

Going to try the wire thing since it's easy My OC-2's at my gig, though, so it'll be tomorrow before I can report back. Thanks, folks!

 

[boomertech]

Jimmy, I think the problem that you are having is if you are daisy-chaining the Tech21 power with your other pedals. Here is some information from another thread:

http://www.talkbass.com/forum/f36/vt-bass-di-available-998085/index4.html#post14770113

Tech21NYC, are your pedals designed with a resistance between the power jack ground and the signal ground?

-Frank

Yes.

Thank you.

I asked because a while ago one of our customers had an issue with noise and pops when using one of your Sansamp pedals with one of my pedals when daisy-chained powered.

The FEA pedals bond the power and signal grounds together. When daisy-chained powered with the Sansamp pedal, the FEA pedal was providing the power ground path for the Sansamp pedal through the signal ground that was causing the noise issues. If I remember correctly the customer measured over 200 ohms between the power ground and signal ground on the Sansamp… resulting in nearly all of the Sansamp power flowing through the signal patch cable, through the FEA PCB grounds back to the power supply.

The solution I suggested to the customer was to use separate or isolated power supplies. He tried that and the problems were solved.

-Frank

I would have to run that info by our engineer. We in general don't recommend daisy chain digital switching power supplies with our products. They will work but we have received way too many complaints from end users and have experienced issues first hand. It would be cheaper for us to use those supplies but they aren't the best idea in audio circuits. I have one at home and sometimes just plugging the supply into the same power strip will cause my tube amps to get noisey and that is not even using the supply with an effect.

Thanks for the information.

The issue wasn’t with the type of power supply. It was with the shared power connection. When the Sansamp was connected with the daisy-chain power cable (regardless of the type of supply) to another pedal that bonds its signal and power grounds, all of the Sansamp current flows through the grounds of the adjacent pedal(s) due to the resistance in the Sansamp power ground.

I am not saying that the resistance between the power ground and signal ground is a bad design… I actually know why it is done. My point is that any pedal that has resistance between its power ground and signal ground should only be operated with isolated power. Otherwise when daisy-chained, the adjacent pedal(s) become a power supply path for it and it is a recipe for quirky noise issues.

-Frank

Try your Tech21 with isolated power or battery to see if the problem goes away. The massive ground loop that is created with sharing power with Tech21 pedals can cause other pedals to “look” bad when they are perfectly OK.

-Frank

 

[boomertech]

I just found an old Boss schematic for an OC-2 that shows a diode and 470 ohm resistor in the ground from the power jack. If the schematic is the same for your OC-2 then you have another pedal that will cause a ground loop when daisy-chained powered with other pedals.

The ground loops are even more complex because there are at least two pedals now sending their return power currents through the patch cables, through the other pedals.

Maybe you can try a battery with the OC-2 also.

-Frank

 

[JimmyM]

Ah, that would make sense. Was powering everything with a single 1-spot. Weird how it only did it with guitar, but maybe there's something weird about the guitar that needs addressed. Thanks, Frank!

 

[boomertech]

There is probably nothing weird about the guitar… ground loops just do crazy things. Sometimes, even if you don’t change anything at all, they can cause issues that seem to come and go.

Please let us know what you find.

-Frank