Measuring power output of a speaker amp to the speaker?

[NoxMorbis]

It looks like Hatto asked this question about a headphone amp, but what about a speaker amp?

Could I use a DC bench power supply to power the amp, and then just let the power supply show the watts?

I'd be using this power supply, and was also wondering if it would be safe enough to use or not:

Second, what about a DMM on the speaker wires?

 

[staticV3]

Could I use a DC bench power supply to power the amp, and then just let the power supply show the watts?

Sure, but it would display the input watts. Not the output watts.

 

[Doodski]

Voltage RMS = Vpeak x 0.707
Where R load = The speaker impedance.
Watts RMS = (Vpeak * 0.707)^2 / R load

You will require a oscilloscope to view the 1kHz sine wave peak voltage and then you can use the formula I provided for you.

 

[NoxMorbis]

Sure, but it would display the input watts. Not the output watts.

I figured somethikng like that. Is there anyway to ballpark the input vs output?

 

[NoxMorbis]

Voltage RMS = Vpeak x 0.707
Where R load = The speaker impedance.
Watts RMS = (Vpeak * 0.707)^2 / R load

You will require a oscilloscope to view the 1kHz sine wave peak voltage and then you can use the formula I provided for you.

Not really worried about the 1kHz. I'm not being that specific about it. But, no scope.

 

[Doodski]

Not really worried about the 1kHz. I'm not being that specific about it. But, no scope.

1 kHz is a/the standard for measuring power. If you want to compare your specs with other amps you will use a 1kHZ sine wave.

 

[Doodski]

I figured somethikng like that. Is there anyway to ballpark the input vs output?

Not really. Any sort of estimation will be a guess.

 

[NoxMorbis]

Not really. Any sort of estimation will be a guess.

What about an inline DMM measuring amps on the speaker wires?

 

[Doodski]

What about an inline DMM measuring amps on the speaker wires?

Sure. Use this easy to operate online calculator to make it easier and faster for you.

...and... Use the 1kHz sine wave.

 

[Blumlein 88]

What about an inline DMM measuring amps on the speaker wires?

You could measure amps, then repeat and measure voltage. However with a speaker it isn't just a pure resistance, but a reactance. You will get volt-amps which is not quite the same thing as watts. Volt-amps will be more than watts. It is apparent power and not real power. Power factor is the real power divided by apparent power. That will not be 1:1 when there is a phase difference from inductance or capacitance involved. The reactance (either inductance or capacitance) the lower the ratio.

For instance if you measure an 8 ohm resistor with 1 picofarad cap in parallel with it your power factor is nearly 1 at audio frequencies. If instead you have 1 farad in parallel with the resistor it will be very much less than 1. Also if you have capacitance or inductance in series with a resistor it will not be 1 to 1.

 

[NoxMorbis]

1 kHz is a/the standard for measuring power. If you want to compare your specs with other amps you will use a 1kHZ sine wave.

k got that. I just wanted to see what 25 watts x 2 sounded like.

 

[NoxMorbis]

You could measure amps, then repeat and measure voltage. However with a speaker it isn't just a pure resistance, but a reactance. You will get volt-amps which is not quite the same thing as watts. Volt-amps will be more than watts. It is apparent power and not real power. Power factor is the real power divided by apparent power. That will not be 1:1 when there is a phase difference from inductance or capacitance involved. The reactance (either inductance or capacitance) the lower the ratio.

For instance if you measure an 8 ohm resistor with 1 picofarad cap in parallel with it your power factor is nearly 1 at audio frequencies. If instead you have 1 farad in parallel with the resistor it will be very much less than 1. Also if you have capacitance or inductance in series with a resistor it will not be 1 to 1.

Translation: No.

 

[Doodski]

I need to EDIT the parameters for calculating the RMS power of the amp. Use a 8 Ohm load resistor and not the speaker as the load. The speaker impedance is varying a lot so it is not suitable for a measurement. Sorry it slipped my mind before when I stated, "Where R load = The speaker impedance." >>> R load should be a 8 Ohm resistor.

 

[NoxMorbis]

I need to EDIT the parameters for calculating the RMS power of the amp. Use a 8 Ohm load resistor and not the speaker as the load. The speaker impedance is varying a lot so it is not suitable for a measurement. Sorry it slipped my mind before when I stated, "Where R load = The speaker impedance." >>> R load should be a 8 Ohm resistor.

Well that makes good sense, though, so thanks. Still, no way to really measure it without a scope or machine, like a power meter:

 

[Doodski]

Well that makes good sense, though, so thanks. Still, no way to really measure it without a scope or machine, like a power meter:
View attachment 276106

You can measure the current going through a 8 Ohm load resistor. Then use that web page calculator that I linked you to. You choose whether it is Peak or RMS measurements in the calculator pull downs and require 2 parameters and then click on, "Calculate."

For example. Fill in the Current text box with 7.8 Amps peak. Then fill in the Impedance text box with a value of 4 Ohms, select a RMS value for the Power text box and click Calculate. You will get a result of 121.7 Watts RMS into 4 Ohms.

Very easy to use once you get the hang of it.

Alternatively you can use a voltage value and select 8 Ohms for the Impedance and play around with that.

Ohm's Law | AE Techron

www.aetechron.com

 

[NTK]

Below is the schematic for measuring the speaker impedance from ANSI/CTA-2034. It can also be used to measure the power into the speaker. You use a 2 channel DAC ADC (or oscilloscope or data acquisition system) to measure voltages V1 and V2. V1 is the voltage across the speaker, and V2 along with R_sense, gives the current. You can ignore Z_w (speaker wire resistance) and S2 (back EMF voltage from the reactive speaker load). R_sense, the current sense resistor, is typically 0.1 ohm. For Figure 9:

Voltage across dummy load speaker = V1​

Current across dummy load speaker = V2 / R_sense​

Below is an example (voltage and current into a power supply) of the type of data you can expect for a complex waveform into a complex load.

Apparent power is calculated as the RMS voltage times the RMS current (over a certain period of time). Real (active) power is the time averaged product of instantaneous voltage and current. Power factor is the ratio of the real power to apparent power, which is also the cosine of the phase angle between voltage and current.

 

[NoxMorbis]

You can measure the current going through a 8 Ohm load resistor. Then use that web page calculator that I linked you to. You choose whether it is Peak or RMS measurements in the calculator pull downs and require 2 parameters and then click on, "Calculate."

For example. Fill in the Current text box with 7.8 Amps peak. Then fill in the Impedance text box with a value of 4 Ohms, select a RMS value for the Power text box and click Calculate. You will get a result of 121.7 Watts RMS into 4 Ohms.

Very easy to use once you get the hang of it.

Alternatively you can use a voltage value and select 8 Ohms for the Impedance and play around with that.

Ohm's Law | AE Techron

www.aetechron.com

So just set it up like any amp measurement using a DMM?

 

[Doodski]

So just set it up like any amp measurement using a DMM?

Yes, that should work. You will need to get/calculate the peak current and not the swing from max negative to positive current as in peak-to-peak current. You need the peak current for that web page calculator that I linked you to. You can also use peak RMS current as a option if your DMM is a True RMS meter.

 

[Doodski]

@NoxMorbis ...and... When metering the current with the DMM you will need to estimate when the amp goes into clipping. When you start increasing the power output of the amp the current value should increase fairly linear/steadily and then when you hit hard clipping it should jump up suddenly I would imagine. Reduce the power output to the point where it does not increase radically and you should have the onset of clipping. Without a oscilloscope to see clipping you are flying kind of blind but the peak current should be measurable with the this measurement method I explained here.

 

[NoxMorbis]

@NoxMorbis ...and... When metering the current with the DMM you will need to estimate when the amp goes into clipping. When you start increasing the power output of the amp the current value should increase fairly linear/steadily and then when you hit hard clipping it should jump up suddenly I would imagine. Reduce the power output to the point where it does not increase radically and you should have the onset of clipping. Without a oscilloscope to see clipping you are flying kind of blind but the peak current should be measurable with the this measurement method I explained here.

I'm not trying to see what it puts out maximum, though. I just want to see what relatively 25 watts-ish a channel sounds like going through my speakers playing a song. I'm not comparing it to anything else, just my ears. I'd have to somehow calculate how many amps = (app) 25 watts to the speakers so like yuo state about estimating amps, but not before clipping, but around 25 watts? 25 watts-ish is my target.