Audio Video Receiver Power Limiting

[MichaelJHuman]

If you have ever seen the benchmark of an audio video receiver, you will see how the power drops off as more channels are driven at the same time.

For example, a benchmark done on my receiver, the Yamaha RX-V3900 showed 189 watts into one channel, and only 88 watts into seven channels.

Is anyone familiar with the factors which result in this power limiting?

One possibility is that the transformer used in the power supply limits the current which can be drawn. One theory I read, is that the internal resistance of the transformer starts increasing with current draw which causes a drop in in the output voltage. This drop affects the power supply rails causing clipping to happen at a lower voltage than the normal rail voltage.

Yet another possibility is that the power transformer has insufficient flux to transfer the needed current to the secondary. This implies that the transformer is saturating. I don't understand transformer saturation, so forgive me if this is not a reasonable reason for the limiting I am talking about.

Another possibility is that receivers have a limiting circuit.

I noticed in the service manual for my receiver that the early stages of each amplifier are connected to a different circuit than the final power transistors. This circuit takes the voltage rails as input, and outputs lines labeled +LB/+LB. Based on what little I know (and I know only basic electronics,) this could be limiting the rail voltage supplied to the early amp stages.

I am interested in understanding the main reason for the power drop off as more channels are driven at the same time. If anyone has any insights, I would appreciate it.

 

[f95toli]

It is due to the power supply as a whole, not any single component. Although the size of the capacitor bank will probably be the single most important factor.

 

[MichaelJHuman]

It is due to the power supply as a whole, not any single component. Although the size of the capacitor bank will probably be the single most important factor.

The size of the capacitor bank should only apply to peak power.

In continuous power, having more energy storage does not help as far as I know. You just need enough capacitance to handle their job as filter caps.

My receiver has two 18,000 uf caps, which you would think would not limit continuous power in any way, but would put a limit on peak power draw on transients over average level.

Correct me if I am wrong.

You say the overall power supply is the limiting factor. But that does not help me. We should be able to identify the specific factors in the power supply which are limiting the power output in the situation I described.

 

[vk6kro]

The output transistors used in the amplifier are not perfect and they have internal resistance.

If they have to operate into a lower resistance load, they have to deliver more current for the same power and this causes more voltage drop acoss the internal resistance of the transistors.

So, they deliver less power. Amplifiers are usually rated to give their best output into 8 ohm speakers and less into 4 ohm and 2 ohm speakers.

 

[MichaelJHuman]

The output transistors used in the amplifier are not perfect and they have internal resistance.

If they have to operate into a lower resistance load, they have to deliver more current for the same power and this causes more voltage drop acoss the internal resistance of the transistors.

So, they deliver less power. Amplifiers are usually rated to give their best output into 8 ohm speakers and less into 4 ohm and 2 ohm speakers.

Interesting, I have not heard anyone mention power transistors causing a higher voltage drop when the current is increased.

Do you think this is the main cause of power limiting?

 

[vk6kro]

Yes.
It still happens even if the power supply is regulated.

Another factor, though, is that the gain of most transistors drops off as the current increases, so you might get clipping in the drivers as they try harder to drive the main amplifiers.

This is all compromise when they design amplifiers.