Audio vs Chassis Ground

[lowell]

I'm modding a Line 6 LD300 http://bmamps.com/Schematics/Line_6/...ice_Manual.pdf

I've added 2 speaker jacks. One for tweeter and one for woofer. I've isolated the Tweeter ground from the chassis because the negative terminal of the tweeter needs to go to the 51ohm resistor to audio ground. I've left the woofer negative lead physically mounted to the chassis ground. I didn't think this would be an issue, but wanted advice before I power on. So my main question is - What is the difference between audio ground and chassis ground in the schematic? I do not see one, but may be missing something.

[R.G.]

I'd have to do a whole lot more studying of the schemo to do a definitive answer, but after a short look and some minimal thought, I don't see anything that would make the stock answer wrong.

That being: Speaker return is a horribly "dirty" wire. It carries all of the considerable speaker return current, and if that wire is shared with any "ground" anywhere along the line, then the "ground" that attaches to the speaker return is jacked above and below the One True Ground by the speaker return current. So in general, unless the power amp is set up oddly, insulate the speaker return from chassis and any other grounds, and run a speaker return wire back to the star point at the center of the two first power caps.

I peeked at the power supply, and the chassis is connected to the power line safety ground, as it must be for safety. I suspect that chassis is connected to the audio ground only at the input jacks. It is connected there, but I didn't search the whole thing for any other connections. Connection only at input jacks or only at the star ground is theoretically correct. You could maybe upset this correct-ness by letting the speaker return currents flow through the chassis back to where the chassis connects to the audio ground at the input jacks, and from there back to the power supply through whatever network of audio ground exists to connect back to the power filter caps. That wobbles the *entire* audio ground network around. That might or might not have ugly consequences.

Insulating the speaker jacks from the chassis and running the return wire back to the power filters ensures it's not dragging the rest of the audio ground network around with currents it wasn't intended to handle.

It's possible you'd get away with it. Or maybe not.

[uneumann]

I'm modding a Line 6 LD300 http://bmamps.com/Schematics/Line_6/...ice_Manual.pdf

I've added 2 speaker jacks. One for tweeter and one for woofer. I've isolated the Tweeter ground from the chassis because the negative terminal of the tweeter needs to go to the 51ohm resistor to audio ground. I've left the woofer negative lead physically mounted to the chassis ground. I didn't think this would be an issue, but wanted advice before I power on. So my main question is - What is the difference between audio ground and chassis ground in the schematic? I do not see one, but may be missing something.

If you're working with a LD300 - unless I missed something, the schematic (page 8) shows it to be a bridged amp. There is no chassis or audio ground for the woofer. Both sides are driven. You'll need isolated connectors.

[J M Fahey]

What Uneumann says.
Woofer amp is bridged so speaker leads must be connected to matching power amps, 2 parallel ones each side, none is ground of any kind.

That said, I *hope* you are not trying to add more speakers to the ones already inside, the amp won´t tolerate that.
No big deal with the tweeter because it has a series attenuator resistor which even if halved to 25 ohms is still an easy load, but the woofer amp won´t tolerate another speaker-

Your customer should leave that amp as is and add a powered cabinet if he feels he needs more.

[lowell]

Yes you both are correct. I didn't realize the bridged arrangement, admittedly rookie mistake.

Customer simply wants the speakers accessible with an alternate amplifier. So this way, if this amp, which has been suffering from digital hiccups, is acting up, he can easily connect his backup head to the speakers.

So I added 2 jacks to the rear side of chassis, and 2 jacks mounted inside the cabinet. The tweeter and woofer have separate jacks and all 4 jacks are isolated. This way he disconnect the Line 6 from any load, and connect the other amp to the speakers. It isn't an option to accidentally connect an extension cab with just these jacks. And I'll make sure he's clear on that.

Thanks dudes!

[R.G.]

oops. I only looked down to the 150W version. Never mind.