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Old 12th May 2016, 10:39 AM   #1
godzich is offline godzich  Finland
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Question Concept: Bridged amplifier with dc bias on the output

Hi,

Just want to probe you fellow audio diyers on the following:

I'm working on a new bridged amplifier idea that would float both speaker terminal outputs at about 50V with no signal. The differential offset (at no signal) would naturally be very close to zero.

As most (non grounded) bridge amps also this design has two speaker protection relays. Over current protection also covers shorts against ground from either output.

My question is: would such non symmetric balancing cause any problems outside the amplifier? Any contraindications?

Cheers,

Christian
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Old 12th May 2016, 05:19 PM   #2
Struth is offline Struth  Canada
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Hi Guys

There is no "non-symmetric balancing" here as you describe it, or as it must be for the circuit to work.

Voltage is relative, so whether the two amplifier outputs sit at "zero" or at +50V" or "-50V" is immaterial. The speaker only sees the voltage difference between the amplifier outputs and as long as there is never a DC difference the speaker will be happy.

As you've already noted, you have to protect each output from a short to ground AND provide DC protection for the speaker AND protection if the load itself is too heavy.

The appeal of being able to use a single rail is one that was more common in the early days of solid-state than it is now. However, for a given power rating, a bridged single-rail amp can use the same size PT as a split-rail amp where the single-rail non-bridged amp would need a PT twice the size.

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Old 13th May 2016, 09:45 AM   #3
godzich is offline godzich  Finland
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Hi STruth,

Thanks for your comments. There are other more important benefits than a common supply rail that I'm after. A good amp must be protected against any abuse, and that is already covered in my design.

My worries lies in the constant DC potential at the outputs. It's a fact that any high power amplifier produces even lethal voltages at the speaker terminals, but only during high volume passages. Otherwise the terminals are virtually at GND level. In my design there would be a constant voltage (let's say +40...+50V) even at idle.

Mostly I'm worried about user safety, an possible corrosion with a constant DC potential at the output. Safety can be handled at the amplifier end by using Speakon contacts, but the speaker end is often just bare wires or open plugs.

Anything DC abov 42V is not considered safe. Normally you can touch any amp outputs when muted, but this design would still carry the DC potential if turned on. Well, normally most people connect speaker wires with the amp turned off to avoid unnecessary shorts. In this case you must do the sa,e to avoid touching the 40..50 DC potential....

If there is enough humidity and if there is somewhere at the speaker end a path to GND even corrosion could be a problem...

These are the things that keeps me worried, and would therefore like to hear your comment and thought if this topology is acceptable?!

Cheers

Christian
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Old 13th May 2016, 11:13 AM   #4
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A standard bridged power amplifier, (Crown, Shure, too many to name), have potentially lethal voltages across the wires whilst being driven with signal. That is why loudspeaker wiring MUST conform to Class 2 wiring regulations. Because the centre voltage hovers around earth potential, there is no real issue.
What difference is yours? Are you allowing the output to sit at a positive DC potential? Will the positive DC potential be at earth potential?
I don't see an advantage to have the speaker terminals at DC potential as there is no need.
Seems like reinventing the wheel without a few pointers.
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Old 13th May 2016, 03:52 PM   #5
Struth is offline Struth  Canada
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Hi Guys

Assuming this amp is for your own personal use, there is no real issue with a standing DC voltage of 40-50V on both wires. Moisture is a concern with ALL wiring and there is nothing worse about your situation. We have extension cords and power cords lying all over the floor in our living rooms and every other room, and these have actual lethal voltages in them.

The DC is present at the output even at idle and when signal is present. I think the only pitfall really is that most people would not expect there to be voltage on the speaker cable except when there is a signal. You yourself might forget one day and unplug the speaker with the amp idling - forgetting it is 'on' if the noise level is low enough from the amp.

There are advocates of single-rail power amps but the benefits of split-rails generally outweigh everything else.

What is the appeal of single-rail to you?

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Old 13th May 2016, 04:04 PM   #6
sreten is offline sreten  United Kingdom
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Hi,

It has been done before. Bridging is a way to avoid the coupling
capacitor for single ended supplies. Very common in car audio,
due to the no capacitor, and extra output power available.

rgds, sreten.
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Old 13th May 2016, 04:52 PM   #7
CBS240 is offline CBS240  United States
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Use a DC servo circuit for each output phase set up to keep each output at Vcc/2, and to keep any drift common mode. No need for bi-polar power supply, no need for output coupling caps.

Circuit protection could be simpler as there is only one supply rail to shut down. Solid state relay for this and for speaker.
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Old 16th May 2016, 07:10 PM   #8
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