G'day Guys,
Commericially built equipment will normally have a stated power consumption.
For example:
If I was to do the same on a DIY power amp I am building.
Do I:
A) state the maximum possible power output of the power transformer, in my case 300w or 420VA (according to the pioneer service manual it was salvaged from)
B) the maximum power that is likely to be drawn from the power transformer. Eg: 100w into 4ohm
C) B + a derating factor for power loss in the transformer say 0.6. Eg: 100w/ 0.6 = 166W
Commericially built equipment will normally have a stated power consumption.
For example:
If I was to do the same on a DIY power amp I am building.
Do I:
A) state the maximum possible power output of the power transformer, in my case 300w or 420VA (according to the pioneer service manual it was salvaged from)
B) the maximum power that is likely to be drawn from the power transformer. Eg: 100w into 4ohm
C) B + a derating factor for power loss in the transformer say 0.6. Eg: 100w/ 0.6 = 166W
Thank you.
In terms of the likely power draw.
According to my scope I got to 51Vpp just before clipping into a 4R load.
Which equates to 18Vrms.
P=Vsquared/R
Therefore:
18V square/4R = 81Wrms
De-rating by 15% would be:
81W/0.85 = 95W
So call it 100W power consumption. Does that seem reasonable?
In terms of the likely power draw.
According to my scope I got to 51Vpp just before clipping into a 4R load.
Which equates to 18Vrms.
P=Vsquared/R
Therefore:
18V square/4R = 81Wrms
De-rating by 15% would be:
81W/0.85 = 95W
So call it 100W power consumption. Does that seem reasonable?
Set up a meter to measure the devices line current, and another to monitor the line voltage simultaneously.
Drive all channels into full clipping at the lowest allowable load impedance.
Note the line current and the line voltage at this condition. VI=W
Drive all channels into full clipping at the lowest allowable load impedance.
Note the line current and the line voltage at this condition. VI=W
If I understood the OP correctly, he's asking what should be specified or marked as the maximum consumption from the mains. What follows is an explanation of the principles involved:
If the amp is Class AB, it will consume 150 to 200% of the maximum output from the power supply. That's 150-200W for a 100W amp. The power supply itself will have losses, the biggest loss being in the transformer. This will be something like 10-15% for a medium sized unit, up to 25% for a small transformer like 9V 500mA.
So a 100W amp is likely to consume 170-230W from the mains when it's actually putting out 100W to the speaker(s).
Then there's the reduced power factor caused by the complex capacitive load faced by the power supply. The current drawn from the mains will be more than that indicated by the basic Power/Voltage formula. So, worst case, the amp will draw something like 0.9 to 1.5A from a 230V mains at full power.
That's 207 to 276 VA (not Watts) in NZ. The power company will bill you for 170-230W but the current (in amperes) is also important because it determines the minimum wire size that can safely carry the current. This is why a responsible manufacturer will also state the current, not just the power consumption in Watts.
If the amp is Class AB, it will consume 150 to 200% of the maximum output from the power supply. That's 150-200W for a 100W amp. The power supply itself will have losses, the biggest loss being in the transformer. This will be something like 10-15% for a medium sized unit, up to 25% for a small transformer like 9V 500mA.
So a 100W amp is likely to consume 170-230W from the mains when it's actually putting out 100W to the speaker(s).
Then there's the reduced power factor caused by the complex capacitive load faced by the power supply. The current drawn from the mains will be more than that indicated by the basic Power/Voltage formula. So, worst case, the amp will draw something like 0.9 to 1.5A from a 230V mains at full power.
That's 207 to 276 VA (not Watts) in NZ. The power company will bill you for 170-230W but the current (in amperes) is also important because it determines the minimum wire size that can safely carry the current. This is why a responsible manufacturer will also state the current, not just the power consumption in Watts.
Fantastic information. Thanks guys.
From all of that, it does seem like I am best to base it off the measured mains power consumption in future.
Setup dummy load, push power to just before clipping and see what the current being passed is on a clamp meter on the mains cable.
I may just do this experiment after I get my amps reassembled.
I have already gone ahead and stated my amps are 100W. It will be very interesting to see what is measured.
From all of that, it does seem like I am best to base it off the measured mains power consumption in future.
Setup dummy load, push power to just before clipping and see what the current being passed is on a clamp meter on the mains cable.
I may just do this experiment after I get my amps reassembled.
I have already gone ahead and stated my amps are 100W. It will be very interesting to see what is measured.
Are you familiar with this type of power meter? I got this one about 20 years ago for the then equivalent of about US$30. It's now available for less than $20.
I find it very useful at times. It can measure power, voltage, current, frequency, power factor, energy consumption up to a 10A load.
I find it very useful at times. It can measure power, voltage, current, frequency, power factor, energy consumption up to a 10A load.
Well,
I sacrificed a spare power cable to the gods of testing and non-compliance.
Chucked on a clamp meter.
Clipping into a 4ohm load. I got 0.7A on 230V mains.
So max power consumption is 160W
Clipping in my amp is at 80W.
Scale that up to 100W output
0.7A/0.8 = 0.875A or 200VA.
Very educational.
I always knew that one is best to de-rate (or oversize) a power transformer. The number I always had was 50%.
Turns out it was a pretty darn good rule of thumb.
I sacrificed a spare power cable to the gods of testing and non-compliance.
Chucked on a clamp meter.
Clipping into a 4ohm load. I got 0.7A on 230V mains.
So max power consumption is 160W
That was a pretty darn good guess. Or perhaps not a guess at all.
Clipping in my amp is at 80W.
Scale that up to 100W output
0.7A/0.8 = 0.875A or 200VA.
Very educational.
I always knew that one is best to de-rate (or oversize) a power transformer. The number I always had was 50%.
Turns out it was a pretty darn good rule of thumb.
Attachments
No, not a guess. Call it an estimate based on a combination of theory and practical design experience.
A Class B output stage has a theoretical efficiency of 78.5% at maximum output just before clipping. But that's with perfect (ideal) transistors with zero Vce saturation. Real-world transistors saturate and limit the output voltage swing below the power supply voltage. The driver and pre-driver stages have their own saturation voltages that further reduces the output swing as well as additional power consumption that does not contribute to the final output.
So a complete power amplifier stage is likely to have an efficiency below 70%, usually well below that. Add losses in the rectifiers, power transformer, regulators for lower stages, relays if any, etc., etc. ..... you get the idea.
A Class B output stage has a theoretical efficiency of 78.5% at maximum output just before clipping. But that's with perfect (ideal) transistors with zero Vce saturation. Real-world transistors saturate and limit the output voltage swing below the power supply voltage. The driver and pre-driver stages have their own saturation voltages that further reduces the output swing as well as additional power consumption that does not contribute to the final output.
So a complete power amplifier stage is likely to have an efficiency below 70%, usually well below that. Add losses in the rectifiers, power transformer, regulators for lower stages, relays if any, etc., etc. ..... you get the idea.
Ah, if you had the power meter I mentioned above, you wouldn't have had to mess up the power cable.
This type of meter started out as a gadget called 'Kill A Watt' purportedly for people who wanted to check their power consumption and possibly reduce their electricity bill, hence the name. I longed to have one for years but they were not available here and they were only for 110V systems anyway. Then the Chinese made their own versions for multiple voltage and plug standards. Mine is one of those.