Measuring audio amp output power

Hi all,

I have an Audiolab 8000 amplifier I picked up at a boot sale and would like to establish what it's capable of, since subjectively the power output seems a bit on the low side when driving recommended speaker loads. Anyway, ideally I'd like to use an 8 ohm 100W power resistor as a dummy load for each channel and then measure the p-p voltage output across it on a scope with the amp turned right up. Trouble is, I don't have such a resistor and was wondering if there's any suitable substitute? I recall someone somewhere using a car headlamp bulb but I doubt they come as 8 ohm units so some sort of elaborate series/parallel combo of lamps would be necessary to get that value. Has anyone a better idea? cheers, CD.

Reply to
Cursitor Doom
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I use a bank of 10 vitreous resistors spaced apart on tag board with a

60V sub 1W bulb across (for any DC problems etc) and a well droppered small low-wattage monitor speaker (for sound quality monitoring)
Reply to
N_Cook

I guess a 1kW unit for 110V would need ca 10A and hence be ca 10 ohm; not far off. But a 230V element would have higher resistance and several in parallel might be needed for each channel for a 8 ohm load...

Mike (retiring exhausted after all that mental arithmetic!).

Reply to
MJC

The headlight will not have enough wattage for that amp. A 100 watt light bulb may seem like a good subistute. Only problem is the resistance changes with 'brightness' of the bulb. When dark the resistance is very low and gets higher as the bulb lights up. Every time the power changes, the resistance of the bulb will change due to the heating of the filiment.

Reply to
Ralph Mowery

If you stick to 400Hz sine ,and a constant resistance, for general power monitoring purposes , a good quality DVM on the AC scale gives a good RMS reading. I only use a scope if there is quality of sound issues

Reply to
N_Cook

Have you searched the web for an 8 ohm dummy load? Note that the resistors should be bolted to the biggest aluminum heat sink you can find. Mine lives at a local auditorium and weighs about

20 Kg. It doesn't need to be that heavy to handle the heat, but heavy does tend to discourage those that would want to walk away with my dummy load. My collection of phosphorescent test cables lasted about a week.

Also, this load consists of 4ea 8 ohm resistors in series parallel. That was suppose to be useful to produce a 2 to 32 ohm load, for stereo or mono, none of which I've ever needed. It also had a pair of meters across the loads, but one of the stage gorillas stepped on one and I've never bothered to replace it for lack of a matched pair.

Hint: Leave room for one or two thermometers, which will help you determine when it's safe to handle.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

** 10 watt rated resistors will handle 100 watts if submerged in a container of water.

.... Phil

Reply to
Phil Allison

Is that inductiveness seriously likely to be a problem at audio frequencies?

Reply to
Cursitor Doom

I just did a quick check with a 10 ohm 60 watt wire wound resistor and a couple of other low value resistors. Used a 100 MHz scope, function generator and Fluke 87 meter.

From low audio up to 20,000 Hz and a sine wave with the 10 ohm WW there did not appear to be any noticable difference. When I switched to a square wave I noticed a large spike on the leading and trailing edges, especially as I went higher up in frequency. I did not see the spike with the resistor removed from the test leads.

From this rough test, I would say that if using sine waves you could get a close to true test, but if music or other odd ball waveforms then you would want the noninductive types.

Reply to
Ralph Mowery

Disclaimer: I am not an audiophile or expert on audio equipment beyond a few occasional repairs and some long past work at a recording studio and several radio stations.

Well, that depends on what you're trying to accomplish with the dummy load. If it's performance tests for a data sheet with calibrated equipment, then yes, every divergence from exactly 8+j0 is important. However, if you're using the load to simulate a real loudspeaker to test for ringing, oscillations, crossover distortion, resonant peaks, and such, then there's no way a purely resistive load is even close to a real world loudspeaker impedance:

Let's do some measuring and math. Digging through my junk box, I find a dummy load that I think was used to test power supplies: That's 2ea 2.5 ohm and 2ea 5.9 ohm inductive RH type resistors in series for a total of 16.8 ohms. Measured inductance of 16 uHy at

1KHz yields: Xl = 2*Pi*f*L = 2 * 3.14 * 1000 * 16*10^-6 = 0.1 ohms So, this load looks like: 16.8+j0.1 Good enough. However, if you were making measurements up to 100 KHz, where the load would look like: 16.8+j10 methinks a non-resistive load might be useful.

Considering that your original question was about testing a used amplifier to see "what it was capable of", y'er right. You can do that nicely without using overpriced non-inductive terminators. When you actually build your load, you might want to do the above measurement and calculations.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

Thank you, gentlemen.

So we're all agreed:

  1. there's no problem using wire-wounds for a load provided the power output measurement is carried out with sine waves from a suitable external generator.

  1. Even low rated power resistors can be used for the load provided their case temperatures are kept low by some effective means of external cooling (something more than mere heat sinks and fans, IOW).

Many thanks.

Reply to
Cursitor Doom

That is the way I see it.

You can probably go at least 5 and maybe 10 times the power on the load resistor if you put it in some water or oil. Many times that if you have liquid nitrogen.

Reply to
Ralph Mowery

their

To an extent. Seems to me that the winding temperature will rise because the insulation between case and wire will not have infinite heat conduction. At some power level the lifetime must start to shorten...

Mike.

Reply to
MJC

** Not in the slightest.

Liebermann is blowing it out his arse, as usual.

.... Phil

Reply to
Phil Allison

** And this is a sufficient test for the purpose.
** Which has harmonics into the medium and HF bands.
** All due to the harmonics way above audio range.

** Music signals stop at 20kHz.

** An audio amplifier reproducing a square wave will not pass harmonics much above 50kHz top the load - so you are wrong again.

.... Phil

Reply to
Phil Allison

2) only if you know the current carrying capacity of their winding wire
Reply to
N_Cook

** Huh ??

Wot a classic Kookism.

.... Phil

Reply to
Phil Allison

Nope. A fuse works by heating the wire high enough to where the wire oxidizes and turns into an metallic oxide which then crumbles. If you can extract the heat from the wire to maintain a temperature below this point, the fuse will last indefinitely. Dumping the load resistor into a bucket of water does this quite nicely. To do this, the wire is in intimate contact with a refractory ceramic which then transfers the heat to a metal case and finally to the water. No air gaps allowed.

You can demonstrate the principle with a common coffee cup heating coil: If you bypass the internal thermostat and run it in open air, it will blow up rather quickly. However, if you immerse it in water, the water will conduct most of the heat away, thus preventing the heater wire from fusing.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

Do try and keep up. This is about the output of the amplifier. It does not mater what the amp is going to pass.

From a quick test with a couple of wire wound resistors an audio frequency square wave was producing a spike on the leading and trailing edges of the ww resistor. It did not do that with a carbon resistor.

Reply to
Ralph Mowery

** Huh ????

Since amplifiers cannot pass the harmonic frequencies, they will NOT appear at the output.

** An "audio frequency square wave" must have its harmonics filtered out above the audio band.

However, what you have is a *WIDE BAND* square wave with its fundamental at an audio frequency.

I understand your test and a similar one to compare low value WW resistors for such inductance. Normally you see overshoot and ringing at frequencies in the low MHz range on the scope - using a 100kHz square wave.

... Phil

... Phil

It did not do that with a carbon resistor.

Reply to
Phil Allison

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