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Class D Switching Power Amplifiers and Power D/A conversion

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Old 18th October 2019, 01:27 AM   #1431
xrk971 is offline xrk971  United States
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Quote:
Originally Posted by vacuphile View Post
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How could a bridged amp deliver more Volts PP to a loudspeaker than the potential between the two power rails?

Think about it.
That’s what bridged mode does. Both ends of the output are “live” - they are out of phase by 180 and output can theoretically be double the single rail.
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Old 18th October 2019, 10:02 PM   #1432
tonitonitoni is offline tonitonitoni  Croatia
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you get error lights beause you short one output to ground with your measuring device and for same reason you get half voltage because of same shorted output?
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Old 18th October 2019, 10:25 PM   #1433
voltwide is offline voltwide  Ireland
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I would suggest to measure both halfbridge outputs referred to GND with a 2-beam oscilloscope. This will immediately reveal whether both outputs deliver 50% level or one delivers 100% and the other 0%.
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Old 19th October 2019, 11:54 AM   #1434
tonitonitoni is offline tonitonitoni  Croatia
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also 48v pp output is max from said amplifier, cant be 100v.
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Old 19th October 2019, 05:09 PM   #1435
vacuphile is offline vacuphile  Netherlands
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Quote:
Originally Posted by xrk971 View Post
That’s what bridged mode does. Both ends of the output are “live” - they are out of phase by 180 and output can theoretically be double the single rail.
Yes, but only in a dual supply setup.

In a single supply setup, each output has the rail voltage as its maximum, and ground as its minimum, give or take some losses. So this is the maximum peak to peak voltage, the difference between the supply voltage and ground.

In a dual supply setup, it is basically the same: it is the difference between the two rail voltages, which defines the maximum peak to peak output voltage bridged. Since in unbridged mode the maximum pp is only half, that is, the difference between ground and one of the rails, bridging it doubles this distance and hence the peak to peak output voltage.
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Last edited by vacuphile; 19th October 2019 at 05:18 PM.
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Old 19th October 2019, 05:31 PM   #1436
FauxFrench is offline FauxFrench  France
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Quote:
Originally Posted by vacuphile View Post
Yes, but only in a dual supply setup.

In a single supply setup, each output has the rail voltage as its maximum, and ground as its minimum, give or take some losses. So this is the maximum peak to peak voltage, the difference between the supply voltage and ground.

In a dual supply setup, it is basically the same: it is the difference between the two rail voltages, which defines the maximum peak to peak output voltage bridged. Since in unbridged mode the maximum pp is only half, that is, the difference between ground and one of the rails, bridging it doubles this distance and hence the peak to peak output voltage.

It is not possible to run a classD amp on a single supply line in unbridged mode.

I believe the discussion on the total rail voltage being the limit for the peak-to-peak output voltage includes a small misunderstanding. Evidently, the instantaneous output voltage cannot exceed the total rail voltage. But, as the output changes polarity for the following half-wave, the top of one half-period to the top of the following half-period can be up to twice the total rail voltage with BTL coupling. Both views are right but based on different assumptions.

I have managed to run a TDA8932 from a single supply line, in unbridged mode (SE mode). Ouput from one line only and a coupling capacitor to the negative rail as used with the "old" class AB amplifiers running from a single rail.

Last edited by FauxFrench; 19th October 2019 at 05:37 PM.
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Old 19th October 2019, 06:20 PM   #1437
multisync is offline multisync  Canada
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Vacufile, I believe you are mistaken. It doesn't matter if the amplifier is single supply voltage or dual supply voltage. In bridged mode the output voltage is doubled. Suppose you have two amplifiers you wish to bridge amp A and amp B. Let's say they both have a single rail supply of 48VDC. The output of each amplifier will be around the halfway point or about 24VDC. The difference between output A and output B is 0VDC. As you apply an input signal out put A will go towards the positive 48 vollt rail and out put B will go towards the ground 0 volt side. Now you have output A 48 volts more positive than out put B. When the input signal swings negative. Output A goes toward the 0 volt ground side and output B goes towards the 48 volt rail. Output A is now 48 volts negative in respect to output B. We now have an amp that can out put + - 48 volts.
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Old 19th October 2019, 08:06 PM   #1438
Turbowatch2 is offline Turbowatch2  Germany
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A bridged amp is always made of two amps. Each single amp has a maximum output at given load.
If you look at what counts, the output in watt, it is exactly what you get if you leave the two amps separate, but half each amps load impedance. No need for double voltage!
The trick, why bridging amps is needed, you can not half a single speakers impedance without opening it up and modifying it´s voice coil!
If you had a speaker with a double voice coil´in series, you could open the series connection and put one amp to each single coil. No need to bridge the amps! Still same output as from a bridged amp at the single coil.

The misconception seems to be, some think only on the volts and not the ampere drawn of the amps. You can not bridge any amp, as it has to be able to deliver twice the ampere, too! So an amp you can bridge must always be able to run at 2 ohms, if you want to run it in a 4 ohm bridge.

Think of the amps as they where single battery cell´s. One got 1.5 volt, put in series, you get 3 volt.
Here, bridged, you only have one battery. So no way to get 3 volt out of a single cell!

If we double the available ampere from the single cell, we can connect 2 two times the halved load.
So to complete the picture, take two separate cells with half load each and connect them at positive and negative pole. Nothing will happen, the available power is 4 times the starting point. Open the + + and - - connection, again, still 4 times the power. Still 1.5 volt.

This remembers me of the discussion with some guy who could not see the benefit of a RCA line connection with two wires and a screen only connected at one side.

Last edited by Turbowatch2; 19th October 2019 at 08:11 PM.
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Old 20th October 2019, 07:54 AM   #1439
tonitonitoni is offline tonitonitoni  Croatia
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each half bridge output on this amplifier cant give more than 48v PP, it is impossible, you need two of those for 96v PP output swing wich is also impossible if he is shorting one half bridge output to ground with his oscilloscope.
no mistery and one side cant give 96v PP when other side is shorted to ground.
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Old 20th October 2019, 09:01 AM   #1440
FauxFrench is offline FauxFrench  France
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Quote:
Originally Posted by tonitonitoni View Post
each half bridge output on this amplifier cant give more than 48v PP, it is impossible, you need two of those for 96v PP output swing wich is also impossible if he is shorting one half bridge output to ground with his oscilloscope.
no mistery and one side cant give 96v PP when other side is shorted to ground.

You are right because you consider the instantaneous voltage.
If you, on the other hand, look at the load voltage the way we have done it for decades (X-Y diagram: voltage vertical, time horizontal) you will have +48V (peak) for a positive half-wave followed by -48V (peak) for the following negative half-wave. +48V to -48V is 96V peak-to-peak in our traditional terminology.
The trick of the BTL coupling is that it moves the reference dynamically such that the outputs stay within the supply voltage span.
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