On grounding bridged (-) outputs & Class D

[g1]

I know from experience that if you ground the neg. terminal of a bridged conventional amp, you will only measure half the power. Why nothing gets damaged escapes me at the moment, but again, from experience I know I've never damaged anything by doing so. I suppose the ground is lifted in bridged mode?

But I've come across some suggestions that it will cause damage to a Class D amp that has floating outputs. I seem to recall a Fender service manual for a Class D amp specifically cautioning against grounding the neg. output terminal?
Is it correct that a Class D out should not be connected to grounded jacks? If so, is it all Class D, or only some?

[J M Fahey]

Balanced output amps such as you mention are actually two standard amps, with the one labelled "- speaker output" getting out of phase drive signal.

If you ground it, you are shorting its output to ground.

Surviving fate depends on short protection quality and in any case is never a good thing.

Why would you ground it anyway?

If you are rich, you use a differential input scope.

If you are like most of us, you scope "+" output from terminal to ground and "know" full voltage and output is twice that; in a 2 channel scope you also scope the "-" output, just in case it misbehaves or to confirm itīs the mirror image of the first one, again from output to ground.

They are NOT galvaniclly isolated from each other, by any means, they still share grounds.

That the speaker which is a floating load does not use ground, is something else.

You can ground scope to "_" and use "+" as HOT, *if* you use a floating, batery powered scope.

[Dave H]

You can ground scope to "_" and use "+" as HOT, *if* you use a floating, batery powered scope.

...or you could invert one channel and use the scope in A+B mode i.e. with channel A connected to one output and channel B connected to the other.

[J M Fahey]

True, there you are turning the whole scope into a differential input probe.

[g1]

Why would you ground it anyway?

It came up with regard to someone wanting to plug the output into a DI box that was able to accommodate speaker level signals. They wanted the EQ'd and processed signal from the amp to go to a mixing board. The DI was an active unit so the problem was whether the DI in jack was grounded. If so, I guess it could damage the class D amp.

[Enzo]

Correct me if I am wrong, but the signal into the bridged amp is the same in the two channels other than the invert. You can't ground either side of the bridged output, but the two hot outputs are still referenced to ground in the amp, so for the DI, you can tap off one of the hots and ground. If absolute phase matters, you may have to pick one side or the other to suit.

[nickb]

g1, I think the confusion comes from comparing stereo PA's that can be switched to a mono bridged mode with mono amps that use a bridge topology of any class. In the stereo case you have four output terminals, one hot and one ground for each channel. In the mono bridged case both output terminals are hot.

Due to the low voltage and high currents typical of class D amps it is common for them to use a bridge topology to achieve the output power required.

[g1]

Due to the low voltage and high currents typical of class D amps it is common for them to use a bridge topology to achieve the output power required.

Yes, I think this is the issue. And often they are hardwired to the speaker so it does not become problematic, but this case was a head, with 1/4 inch outs.

Another weird example of where it becomes problematic:
Some of them have a ground wire to the speaker frames for RF/shielding (& possibly FCC?) concerns. Now in the extreme rare case when someone replaces the speaker with one that has the (-) terminal connected to the frame, the ground wire to the frame will cause a problem. I'd guess protect mode or boom!

[Enzo]

This was a caution on the Marshall Mode 4, it had bridged outputs and a jack on the rear. It was a Cliff jack, so no grounding issue there, but people could connect things that were grounded and boom...