QSC GX3 - bridge mode?

[dincz]

Any reason why it can't be run in bridge mode - linking A and B inputs with TRS cable with balanced legs reversed at one end and Y output cable from A+ and B+?

[crez5150]

The reason it won't run in bridged mono is because they have not designed that feature into the GX3 series..... you need to move up to RMX for that. These are designed as amps at a price point.

[dincz]

I realise they haven't included that feature but it shouldn't be hard to achieve by reversing the polarity of one balanced input and using the appropriate output connections. Even on an amp with only unbalanced inputs it's a relatively simple mod with an opamp phase inverter.

As long as the outputs are ground referenced it should be a simple matter on this amp.

[crez5150]

Ah you mean that you want to bridge the inputs rather than the outputs?

[dincz]

To bridge the outputs you need to have the same signal feeding both channels but with opposite polarities. Otherwise both outputs track the signal and there's no voltage difference between them to drive the load.

[bremen]

I wonder why they don't offer bridge mode.

If you're lucky it's just to deter people from running it bridged into 4 ohms; there's also the possibility that it's already bridged, ie it's actually four power amps hard-wired in bridge mode. Their site offers schematics of just about everything except this amp, unfortunately.

[crez5150]

from their site -

Also, by removing bridge mono mode and 2 ohm operation - features that are never used by the majority of amplifier users, the price and weight can be lowered further while still allowing the amp to operate effortlessly into 8 and 4 ohm loads with ample headroom.

[dincz]

[quote name='bremen' timestamp='1342774478' post='1740640']
the possibility that it's already bridged, ie it's actually four power amps hard-wired in bridge mode.[/quote]

Aha, that's a real possibility! I've asked on the QSC forum but no reply yet.

[bremen]

[quote name='dincz' timestamp='1342774876' post='1740650']
Aha, that's a real possibility! I've asked on the QSC forum but no reply yet.
[/quote]

I don't know why they'd do it in a linear amp, though. Worth doing with Class D to avoid bus pumping.

Tee hee, I said " bus pumping"

[dincz]

Answer from QSC: "The GX Series amps should not be bridged in that way. The channels are not bridged, but the - side of each output is not connected directly to ground. Instead, the current monitoring circuitry is on that side, and so bridging the amp would bypass this protection."

It appears that they've designed out something that's normally easy to do - another amp crossed off the shopping list

Edited July 21, 2012 by dincz