Crown D-75A Scratching my head on Bridged Mono Mode.

Crown D-75A Scratching my head on Bridged Mono Mode.

I purchased two Crown D- 75A Amplifiers to use in Bridged mono mode. 1st I tested them in Dual mode and all Good. Then I tested them in Mono Bridged mode. If I hook the speaker up as per the manual figure 2-7 ( speaker connected to both the positive terminals) I get a low output and the amp really heats up. NO GOOD ! Forgot to mention , inside there is a jumper that must be placed in Mono or Dual depending how you are hooking up.
If I hook up on just on the speaker channel 1 terminals + and - , I measure the output at 51 watts.
This sounds ok but I am suppose to have 110 watts as per the specs. I have been ripping my hair out trying to figure this out and I don't have much. Anyone with some Ideas? It says Clearly in the manual not to use the negative terminals in bridged mono mode.
I'm sure im missing something, Just have no idea what.
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Joined 2003
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If I hook up on just on the speaker channel 1 terminals + and - , I measure the output at 51 watts.
This sounds ok but I am suppose to have 110 watts as per the specs.
If I understand correctly from the above you expect to have 110W with therminals + & -. You will only get the 110W in Bridge mode.
Speakers should be 8ohm, not less.
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Here's an experiment that might lend insight:

With amp set for bridged operation, try speaker connected + to - for left channel only and note volume. Then repeat with speaker connected + to - for right channel, only. If the amp is working properly and is phased correctly for bridged operation, the volume should be equal in both experiments and channel dissipation comparable.

If things still don't make sense, a service manual may help.

Good luck!
Hi BSST, thanks for jumping in. I just finished some more tests. Here is the results.
Note :the owners manual shows one speaker in Bridged mode connected to the two positive terminals. also it states do not use the negative terminals.
Speakers 8 ohms.
Test 1 : input on channel 1 only, Bridged mode , two speakers connected + - terminals . Result 46 watts on each channel
Test 2 : input on Channel 1 & 2, Dual mode , Two speakers connected + - terminals . Results 37 watts on each channel
Test 3 : Input channel 1 only, Bridged Mode, one speaker connected to + + on each channel ,as per the owners manual . Result no good.

Conclusion : 110 watts not achieved ,46watts x 2 test 1..
something stinks.
I am waiting for the second D-75A to arrive, ill compare the two. should tell me what's going on.

i wonder if using an unbalanced input makes any difference?


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Joined 2011
Are you certain this was properly done?

"To activate the bridge-mono mode, both the top and bottom covers must be removed.
After the covers are removed use the following instructions.
1. Remove the jumper from the pins marked with the words "DUAL" and "MONO"
2. Reinstall the jumper so the two pins closest to the word "MONO" on the circuit board are shorted.
3. Reassemble the unit. To convert back to the Dual mode, simply move the jumper back to the "DUAL" position."
Be sure that you don't connect the scope ground, or other instrument ground to the speaker negative since that is not ground in bridge mode.
Be sure the mono jumper is connected between pins 2 and 3.
Test the AC voltage between the two + terminals with a signal generator driving Ch1. Do not drive Ch2 at the same time. Driving Ch2 will cancel the bridge input from Ch1 but having to sink the current from Ch1 will make it run warm. The bridge jumper does not deactivate the Ch2 input, and they assume you leave it unconnected.
I'm not convinced the bridge mode is being set up correctly. I think rayma is correct in post #15.

In bridge operation, both speaker terminals are driven and are "hot" with resect to ground. In bridge mode, the amp is internally arranged to drive channel 2 with an inverted version of channel 1 input; see rayma sketch showing the two opposite phased signals. These are the two signals appearing at the hot binding posts, chanel 1 and channel 2. When a speaker is connected between the two hot terminals, it sees twice the voltage and four times the power that it would in normal hot-to-ground operation.

The cautionary warnings about not connecting instruments to bridge outputs are warning you to not connect a scope ground to a hot amplifier binding post. The two center binding posts in Crown's pictorial are still both at ground potential and it's safe to connected instrument grounds there; that's what rayma is suggesting you do in post #15.

The experiment I was suggesting in post #7 is essentially what's described in the last paragraph at the bottom of the Crown page in the original post. If you don't find sound in channel 2 as Crown describes, the internal connection to channel 1 input must be absent; if there is audio present but then very little audio when speaker is bridged, that suggests that the signal inversion hasn't been implemented.
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I should add why the input needs to be balanced in case you use bridge mono mode.
If you use a TS jack, you only feed one amplifier's channel.
The other one is therefore connected to ground.
With a TRS or XLR you apply the non-inverted part of the signal (tip or pin 2) to channel 1 and the inverted part (ring or pin 3) to the other channel of the amp.
This will give you the full power you expect.