better amp configuration

[Steve Eddy]

Originally posted by AndrewT
Now let's look at the bridged amplifier.
Each amplifier in the pair is loaded by an effective impedance of half the actual load.
The supply rails supply half wave versions of the output signal.
This time the halfwaves are continuous rather than from alternate supply rails. (it looks like a full wave rectified version of the output signal).
The supply rails still supply the current to meet the demand from the effective load on each amplifier. But this time there is no rest period between half waves for the smoothing to recover. The transformer and rectifier work hard to deliver sufficent current during the small recovery period between adjacent halfwaves rather than alternate halfwaves and to compound this both rails must be charged simultaneously.
So I believe the smoothing capacitance needs to be doubled to take account of each amplifier feeding a half impedance effective load and doubled again because there are two amplifiers taking power from the supply. The effective load seen by the supply is one quarter of the actual load, due to two parallel amplifiers working in anti-phase but with half impedance loads on each.

Mmmm. If we compare apples to apples (and I think that was longthrow's intent), I'm not sure there's any problem here and it ultimately ends up being a wash.

Let's assume that both the single and bridged amps have the same voltage gain and the same output power. For example, the single amp has a voltage gain of 26dB and each of the two amplifiers being bridged have a voltage gain of 20dB for a total of 26dB.

To deliver the same power, the single amp's rail voltages would have to be twice that of the rail voltages feeding each of the two bridged amplifiers.

So while the single amp is alternately drawing current from one side of the power supply or the other for each half of the signal waveform, the half of the power supply that's being drawn from has to supply ALL of the energy being delivered to the load.

In the bridged amp, although it draws equally from both sides of the power supply on both halves of the signal waveform, each half of the power supply is supplying only HALF of the energy being delivered to the load.

And if you use two transformers, each with the same VA but one whose voltage is half that of the other, and each coupled to the same amount of reservoir capacitance, I don't see that one transformer's working any harder than the other.

se

[longthrow]

budget wise..

thanks to all!!!

[janneman]

If you really want a single high power amp, you need LOTS of output devices in parallel for reliability, because of Safe Operation limits (I'm talking BJT's here, with Mosfet's it's different) with supplies above 50V or so.

With a bridge, with same power, you can get by with a bit more than half the supply voltage. Each devices sees only half the max Vce and SOA is much greater. You can get by with less than half the number of devices in parallel. So the bridge only needs TOTAL the same number of devices, or less.
Which means no extra cost in transformer, heatsinks, output devices.

Jan Didden

[deleveld]

Yes janneman, for power it shouldnt make any major difference. But there are minor differences because the bridge output has voltage drops for two transistors and two emitter reststors instead of one for a single-ended output.

But consider the PSRR angle. A well designed bridge amp should have higher PSRR than a single ended. So any changing power supply lines should be less of a problem for a bridge than a single-ended output. So for a given amount of power supply signal bieng visible on the amp output, a bridge can have a smaller power supply capacity to reach the same quality.

Although, technically it has nothing to do with the bridge vs single-ended issue just PSRR which (should be) better with a well-designed bridge than with single ended. Of course, this is no guarantee that it actually will be...

Doug

[janneman]

Originally posted by deleveld
Yes janneman, for power it shouldnt make any major difference. But there are minor differences because the bridge output has voltage drops for two transistors and two emitter reststors instead of one for a single-ended output.

But consider the PSRR angle. A well designed bridge amp should have higher PSRR than a single ended. So any changing power supply lines should be less of a problem for a bridge than a single-ended output. So for a given amount of power supply signal bieng visible on the amp output, a bridge can have a smaller power supply capacity to reach the same quality.

Although, technically it has nothing to do with the bridge vs single-ended issue just PSRR which (should be) better with a well-designed bridge than with single ended. Of course, this is no guarantee that it actually will be...

Doug

Fully agree. Also you are right about the additional Vce-sat and Re drops, that's why I said in my post ...a bit more than half the supply voltage ...

Jan Didden

[m2003br]

Friends of Audio,

I don´t resist to give some opinions in this topic.
I have to agree with all points disclosed by Steve and Jan.
About the need of more capacitance in the supply, it´s not a problem, because the elcos may be only half of operating voltage, compared to single ended amps. One elco 10.000u 35V cost less than one of 4700u 70V
But, more important than all, the two biggest advantage of bridged amp is:
a) doubled slew rate for the same single ended circuitry, and,
b) a naturally balanced configuration, with identical impedance in both polarity inputs.

hope my coments are valuable.

Best regards,

Marcos