crackly static from bass????

[Gregory East]

Gig last night. Bass into Sansamp RBI rack preamp. It's a 110V so running off an overgrown wall wart transformer off our 240V. Running as an effect box into bass ampo doesn't make crackles.

The RBI goes via a balanced xlr to the desk.

The desk out is unbalanced to the EQ, balanced connections beteen everything else until it goes to the power amp.

The crackle goes away when fingers are on strings, so I'm thinking its an earth problem. The RBI is earthed by virtue of the rack. Paint maybe interferering? Did I escape potential death?

[Mikey]

Check your cable runs to make sure that they're not running along-side other cables, especially power cables. All cable intersections should be at 90 degrees (if possible), and preferably not touching power cables at all.

Check all of your cables.

[bgavin]

The desk out is unbalanced to the EQ, balanced connections beteen everything else until it goes to the power amp.

You have an unbalanced system.
Any unbalanced component in the chain renders the entire chain unbalanced.
The differential noise cancellation of the balanced system is gone.

However, (unbalanced) hum is different than crackle/static.
Crackle implies a mechanical problem somewhere in the chain.

Single coil hum from passive guitars and basses can be minimized with a good copper foil cavity treatment.
The addition of a star grounding system inside the control cavity will reduce ground loops and static.
Grounding to a star screw in the copper shielding is far superior to grounding at the jack.

There is usually a grounding wire exposed beneath the bridge.
This is intended to ground the bridge and attached strings to the instrument ground.
A DVM will reveal any high resistance or open grounds.

Exposure to getting killed mostly went away with the retirement of 60s era amps with a Death Cap isolation capacitor.
This lone cap is the only obstruction between the instrument (and artist) to the high voltage tube circuits. Scary.
I just cured an amp like this, using an isolation transformer. This is the only cure for these obsolete designs.
Modern amps don't have this problem.

Link to Guts Photo

Link to IsoXfmr Photo

[Gregory East]

It's a dual coil pickup active bass, Seymour Duncan musicman style. I had it plugged into another PA the other day and same dealio. It depended on which way the bass faces as to the intensity.

iirc there isn't any sheilding in the bass cavitity. But how come it behaves perfectly into a bass amp?

[bgavin]

(Most) active basses don't have the high impedance requirements of single coil pickups.
They play fine into a megohm input impedance, but don't require it.

I suppose an active, poorly shielded bass can suck just as much as a passive one.
There is an excellent photo-article on TalkBass for step-by-step installation of shielding.
I have a copy of it, just in case you can't find it.

[Fish]

It depended on which way the bass faces as to the intensity.

This is usually a prime indicator of an RF noise source near the instrument's pickup. Get away from any electronic (SCR- or triac-based) lighting dimmers; those are the most common offenders, though they usually cause more of a buzz than a crackle. I would expect proper shielding of the internal electronics to minimize the problem.

Perhaps a ground lead has worked loose inside the bass? "Cold" solder joints (it's a bad/incorrect name, but it's in common use) could be the cause.

In any event, from your description I would focus on the bass, the RBI, and the power to the RBI as the most likely culprits. As already mentioned, cables should be checked as well.

Hope this helps.

[Mikey]

Try turning your pots, all the way around and back. Is there a crackling noise as you turn it? If so, you have a bad pot(s). Don't bother cleaning it if it's bad. Replace it with a new, quality pot. They're cheap enough to replace.

[Scott Brochu]

didn't have time to read all post and maybe it was hit but my bass player went through the same thing.
It was his pick ups, they were too close to the strings which gave it a cracking sound but stopped when the string was against his finger.
Worth looking into.