Measuring output Wattage, power amp only VS preamp + power amp

[danny davis]

When measuring output wattage of a power amp only ( not with the preamp, just the power amp section) , Does it give you a HIGHER wattage than measuring the output wattage with the preamp + power amp?

The preamp+power amp says on the outside label 200watts total

I measured the power amp section ONLY AC voltage output, the O-scope measured 44 VAC Peak to Peak

44 peak to peak VAC by the power of 2 divided by the speaker ohms 8 ohms is = 242 watts

It's over 42 watts? How and Why?

The preamp + Power amp output wattage was 190 watt total, so it's under 200 watts

The Label on the product says 200 watts , what's going on ?

When Measuring the AC voltage on the output of a power amp or preamp+power amp, do you measure the Peak to Peak or just Peak AC voltage?


For a Bleeder Discharge Resistor , Do you know anything else I can buy that is like this?
**broken link removed**

 

[Audioguru]

The 200W total is probably the power supply power to the amplifier that causes the output plus heat.

44V peak-to-peak is 15.6V RMS. Then the output power is 15.6V squared/8= 30.4 Watts.

Amplifier power is measured with a sinewave. If a square-wave is used then the output power is doubled (60.8W for this amplifier) because an additional 30.4W of power is in the distortion harmonics. When the amplifier is playing a sinewave then it operates much hotter than if it is playing a square-wave.

 

[danny davis]

So what AC voltage should I inject into the power amp section? the label says 200 watts total

Power amp input, I put in 100mV or 1 volt ? sinewaveform at 1Khz?

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I'm trying to measure just the power amps wattage

But what power amp input voltage should I put in? that will give 200 watt total output?

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44V peak-to-peak is 15.6V RMS. Then the output power is 15.6V squared/8= 30.4 Watts.

is the power amp 30 watts only? or does it need the preamp to make it 200 watts?

 

[Audioguru]

We do not know what is "200 Watts". I think it is the amount of AC electricity that is used. Then 30.4W is the output power into an 8 ohm speaker and the difference is heat (200 - 30.4=) 169.6W of heat.

To measure the output power of an amplifier you feed it a 1kHz sinewave signal and adjust its level so that the output of the amplifier is barely clipping. Then measure the power with a power meter or calculate the power using simple arithmetic like I did.

The input signal to a power amplifier is from about 0.2V to about 2V. A preamp is used to amplify the 0.005V output of a microphone to 0.2V to 2V then the preamp feeds a power amp or recorder.

You measured 44V peak-to-peak output on the amplifier when it had an 8 ohm load. But maybe you did not adjust the input level high enough to cause the output to be barely clipping. If the output can go higher than 44Vp-p then the output power is higher.

 

[danny davis]

A preamp is used to amplify the 0.005V output of a microphone to 0.2V to 2V then the preamp feeds a power amp or recorder.

Yes they say guitar pickups are 50mV Peak to peak , other guitar pickups are 100mV peak to peak

So I set my function generator for that when testing the preamp section board

What millivolts do you set your function generator at when testing the preamp amp board?

But maybe you did not adjust the input level high enough to cause the output to be barely clipping.

True, Because I really don't know what voltage I should set on my function generator

The input signal to a power amplifier is from about 0.2V to about 2V.

So you do put a higher input voltage from the function generator in the input of the power amp?

What is the output voltage on most preamp section boards going into a power amp?

If the output can go higher than 44Vp-p then the output power is higher.

If the output transformer is damaged, What would the output waveform look like?

Can the output transformer cut the output voltage in half or lower the output AC voltage or power ?

 

[BradtheRad]

It's wise to design your amp (and preamp) to accept a wide range of input voltage. That way it will work with all your source devices.

You build your power amp to provide a certain power level. Therefore you must feed it an amplitude of signal that will drive it to its maximum power.

1V is a typical level of signal that will come from a preamp. Commonly called 'line level'. If your source device produces that much, then it probably does not need a preamp.

Likewise you want to build enough gain into your preamp, so that it will amplify your softest source device, to drive the power amp.

Adjustability is a key factor. Normally you install a volume control in front of the preamp.

However if you send your source device directly into the power amp, then you might also want to install a volume control in front of the power amp.

You want to avoid any mishaps when you switch from one input device to another. You don't want to overload components, nor blast the speakers with sudden high volume.

 

[danny davis]

This is what I use at work for a load tester? do you guys know anything else like this , that has an oscilloscope output and an RMS output for a meter , take a look at this

**broken link removed**

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1V is a typical level of signal that will come from a preamp. Commonly called 'line level'. If your source device produces that much, then it probably does not need a preamp.

To Test the preamp section board, I put the oscilloscope probe on the Line level output jack , the output should be 1 volt peak to peak ?

I think the Line Level output jack is hook up to the power amp section not the preamp section

They do have a send and return jacks for the preamp section

But can I put the oscilloscope probe on the SEND jack to test the check the preamp circuit board? it should be around 80mV to 1 volt peak to peak right?






Some of the Effects loops are in series , others are in parallel

How do I use the SEND JACK to test and check the Preamp circuit board if the Effects loop is in Parallel? do I put a Dummy Plug in the RETURN JACK?

 

[Audioguru]

The rated power of an amplifier is produced when it has its rated load and the input sinewave level is increased until the output waveform shows a little clipping on an oscilloscope. Then you measure the peak-to-peak voltage, calculate the RMS voltage and calculate the output power.

 

[danny davis]

but if the output transformer is damaged can the output be cut in power , watts, volume?

hhow do u know if u got a damaged output tansformer?/

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Can I use the send jack? As a preamp output? Do i need to use a dummy plug in the return jack?i

 

[Audioguru]

I used a vacuum tube amplifier 50 years ago. The output power dropped and it got very distorted every 3 months. Replacing the worn out vacuum tubes made it like new but it dropped power and got distorted every time it needed its tubes replaced over and over. Vacuum tubes failed so often that they were sold in corner stores that even had tube testers. Many bands today replace vacuum tubes before each gig.

Lots of parts including the output transformer can fail and reduce output power. But a failure of an output transformer is not likely.

 

[danny davis]

I tested all 4 power tubes on a digital tube tester , they are 100% GOOD

is the wattage rating on the label mean the power amps wattage or preamp+power amp?

so how do u know if the output transformer is dropping the wattage or volume?

 

[Audioguru]

Why do you ask if the output wattage rating includes the preamp or not? How much output power does the preamp produce?

Hmm, did you do the simple arithmetic? Its output might be 1V and its load at the input of the power amp might be 100k ohms. Then its power output is only (1 squared)/100k= 0.00001W which is almost nothing.

You must trouble-shoot the amplifier to find out what reduces the wattage or volume.

 

[danny davis]

Ok thanks for clearing that up for me

So its 100% sure it's not the preamp section board that is reducing the wattage , because the preamps output wattage is in the micro watts or picowatts it looks like

A DVM meter is RMS , so I can put a DVM meter on the output load or on the output jack of the power amp to measure the AC voltage

guitar amplifiers have a SEND and RETURN jacks on them, they are called Effects Loop FX loop

They are either SERIES or PARALLEL

Can I use the SEND jack as a Preamp output?

Because if u look at how they connect the SEND and RETURN jacks they have a parallel wire that is connected from preamp to power amp even when you have a send and return jacks plugged in.

 

[audiomik]

Your 'Send' jack should output the signal from your Pre-Amplifier.
The signal should remain available to your Power Amplifier with this only connected so you can adjust your outboard FX units.
This should also 'double' as a Pre-Amplifier Output.
Depending on the specific Amplifier's design, sometimes the 'Send' jack breaks the signal to the Power Amplifier.......

The 'Return' jack should break the direct signal from your Pre-Amplifier to your Power-Amplifier and feed your Power Amplifier with the output signal from your FX units.
An input signal into your 'Return' jack should then only feed the Power Amplifier.
Switching is done using the contacts within the jack sockets to break the input from your Pre-Amplifier.
This is also known as 'Half Normal' connection when used for professional patchbays.

So yes, you can use your 'Send' jack to test your Pre-Amplifier output and your 'Return' jack to apply a signal to your Power Amplifier.

On other points in this thread:
Output transformers can fail, especially if the Load Impedance is not correctly matched when the Transformer temperature will rise and insulation between turns of one of the windings can fail.

Output measurement is best performed by measurement of AC Voltage directly across the Amplifier Output terminals . Use of a True RMS AC Voltmeter is ideal as it will properly include distortion artefacts in it's reading, but you must make certain that the Meter being used has suitable frequency range for these measurements.
Power is then simply calculated from: V²/R = P (where R is a known Load Impedance)
In order to not cause damage to a Valve (Tube) Amplifier, always have a correct Impedance Load connected during testing.

Mains Input Power:
This only relates to Amplifier Output Power when you know the efficiency of the Amplifier.
Valve (Tube) Amplifiers are notoriously inefficient!

Hope this assists

Mik

 

[BradtheRad]

This is what I use at work for a load tester? do you guys know anything else like this , that has an oscilloscope output and an RMS output for a meter , take a look at this

**broken link removed**

That unit appears ideal for testing audio amplifiers. I have not used one like it. A heavy-duty resistive load. And real speaker voice coil assemblies (motors is the term they use), yet without the noise-making parts.

To Test the preamp section board, I put the oscilloscope probe on the Line level output jack , the output should be 1 volt peak to peak ?

1 V is simply a nominal figure. It's a convenient amplitude to work with, among low-power circuitry.

An audio signal might pulsate between zero and a few volts.

The waveform must not be allowed to get too high when powered by a 9V battery, or else it is prone to clipping.

 

[Audioguru]

So its 100% sure it's not the preamp section board that is reducing the wattage , because the preamps output wattage is in the micro watts or picowatts it looks like

No.
If the preamp has a fault that reduces its output level then the wattage output from the power amplifier will also be reduced.
Even if the volume control has a fault that reduces its output level then the wattage output from the power amplifier will also be reduced.

A DVM meter is RMS , so I can put a DVM meter on the output load or on the output jack of the power amp to measure the AC voltage

Maybe, check the spec's of your meter.
Most Digital voltmeters accurately measure the level of only 50Hz or 60Hz from mains electricity, not audio frequencies.

 

[danny davis]

yes you're right.

i think a fluke 87 can't measure 1Khz AC voltage , so it can't measure the rms ac voltage
it can only measure rms at 60hz ac voltage right?

 

[Audioguru]

I had a Fluke 87 meter about 31 years ago so I can't remember its specs.
A Fluke 87V EX has its Owners Manual in Google. Its AC voltage accuracy is 0.7% from 45Hz to 65Hz on all but the two highest voltage ranges and is 1% from 440Hz to 1kHz on all ranges So it is pretty good at the audio frequencies of 600Hz or 800Hz.

 

[FvM]

i think a fluke 87 can't measure 1Khz AC voltage , so it can't measure the rms ac voltage
it can only measure rms at 60hz ac voltage right?

Instead of "thinking", why don't you read the manual? It has specified accuracy up to 5 kHz.

 

[danny davis]

I use a BNC cable from my function generator to the input jack of the preamp amplifier

I put a BNC to 1/4 inch adapter plug into it the input jack

My problem is it's hard to find a ground to clip my oscilloscope ground to

I use to use the Plugs ground from the BNC

Is there a 1/4 to 1/4 couplier or something that I can plug into before the input jack so I can clip on my oscilloscope ground, so I can do troubleshooting?

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I can't clip my O-scope onto the jack because its PC mounted , its not one of those switchcraft jacks

There is no where on the preamp or power amp board to clip on to and the filter caps are hidden

I have seen these BNC couplers that is like a BNC splitter coupler , not sure what they are called tho

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BNC splitter connector
http://www.ebay.com/itm/like/330923866975?lpid=82

What what can I put for an adapter on the BNC connector so I can Clip my oscilloscope ground to , so it doesn't keep coming off?