Measuring audio amp output power

[Cursitor Doom]

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.

 

[Gareth Magennis]

"Cursitor Doom" wrote in message news:mp2lbl$9no$2@dont-email.me...

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.





An electric fire/heater element works well.



Gareth.

 

[N_Cook]

On 26/07/2015 13:55, Cursitor Doom wrote:

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.

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)

 

[MJC]

In article <ZV4tx.58349$Ch1.13405@fx40.am4>, sound.service@btconnect.com
says...

An electric fire/heater element works well.

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!).

 

[Ralph Mowery]

"Cursitor Doom" <curd@notformail.com> wrote in message
news:mp2lbl$9no$2@dont-email.me...

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.

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.

 

[Gareth Magennis]

"MJC" wrote in message news:MPG.301f0a76649358d3989683@news.plus.net...

In article <ZV4tx.58349$Ch1.13405@fx40.am4>, sound.service@btconnect.com
says...

An electric fire/heater element works well.

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!).






These are really cheap:
http://www.amazon.co.uk/2Kw-Electric-Heater-Overheat-Protection/dp/B004KDRCUO/ref=sr_1_2?ie=UTF8&qid=1437924981&sr=8-2&keywords=electric+fan+heater


The heater element in the one I bought was actually several windings, and
easy to hack and series/parallel/tap etc to get the right ohmage.

I had hoped to use the built in fan to cool the element, but it turned out
the fan motor was actually 110v, and took this from half the 240v heater
element. Doh!
So I bought another one which sits on top blowing cold air over the elements
when required.

(It should be noted that the resistance of the element is not constant, it
increases somewhat with temperature, but I don't need an accurate fixed
resistance load for my purposes)





Gareth.

 

[N_Cook]

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

 

[Jeff Liebermann]

On Sun, 26 Jul 2015 12:55:17 +0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

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?

Have you searched the web for an 8 ohm dummy load?
<https://www.google.com/search?q=audio+8+ohm+dummy+load>
<http://www.ebay.com/itm/like/201057901616>
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.
<http://www.homedepot.com/p/Rust-Oleum-Specialty-10-oz-Glow-in-the-Dark-Spray-267026/204209388>

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
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Gareth Magennis]

"Cursitor Doom" wrote in message news:mp2lbl$9no$2@dont-email.me...

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.




Here's another idea:
Get a reel of this -
http://uk.farnell.com/pro-power/ecw0-80/wire-0-8mm-copper-enamelled-125m/dp/1230984

By my calculations it will have a resistance of 4.25 ohms. (117m of the
125m length gets you 4 ohms)
Although it is only rated at 0.8A, I reckon you could extend that massively
by unravelling it all and laying it in a large container of water.
You could even have a flow of water through the container to keep the
temperature down.

It would be interesting to find out what power rating this could actually
tolerate.

Or is this a really stupid idea?! (For some reason this amuses me)



Gareth.

 

[Gareth Magennis]

"Gareth Magennis" wrote in message news:TP7tx.48715$m04.7988@fx43.am4...



"Cursitor Doom" wrote in message news:mp2lbl$9no$2@dont-email.me...

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.




Here's another idea:
Get a reel of this -
http://uk.farnell.com/pro-power/ecw0-80/wire-0-8mm-copper-enamelled-125m/dp/1230984

By my calculations it will have a resistance of 4.25 ohms. (117m of the
125m length gets you 4 ohms)
Although it is only rated at 0.8A, I reckon you could extend that massively
by unravelling it all and laying it in a large container of water.
You could even have a flow of water through the container to keep the
temperature down.

It would be interesting to find out what power rating this could actually
tolerate.

Or is this a really stupid idea?! (For some reason this amuses me)



Gareth.







Just to extend that idea a little further:

You could probably put a wood former into the chuck of an electric drill,
and quickly and easily make a long coil using this enamelled copper wire.
You could then partition the coil into several smaller ones without cutting
the wire, and suspend them vertically in a container of water.

Quick back of a fag packet calculations:
Suppose you made a long coil 10cm diameter. You would then suspend 0.5m
lengths vertically in the container, so the windings are spaced 5mm apart on
average.
This gives 100 turns at 31.4cm per turn giving 31.4 meters of wire per coil.

So you would need 4 of these series coils from the 117m of wire required to
get a 4 ohm load.


Now, how many Watts could this contraption handle?



Gareth.

 

[Phil Allison]

Cursitor Doom wrote:

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?

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


..... Phil

 

[Trevor Wilson]

On 26/07/2015 10:55 PM, Cursitor Doom wrote:

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.

**Lamps are a really bad idea. Don't do it. Incandescent lamps exhibit a
HIGHLY variable resistance, depending on how much current is flowing.

Just buy four of these of these:

http://www.newark.com/vishay-dale/rh0504r000fe02/wirewound-resistor-4-ohm-50w-1/dp/41K9167

Dump them in a bucket of water, or bolt them to a chunk of aluminium and
place a small fan on the aluminium.

--
Trevor Wilson
www.rageaudio.com.au

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"Just buy four of these of these:

http://www.newark.com/vishay-dale/rh0504r000fe02/wirewound-resistor-4-ohm-50w-1>/dp/41K9167 "

Why do those Vishays look so much like Dales ?

 

[Trevor Wilson]

On 27/07/2015 11:54 AM, jurb6006@gmail.com wrote:

"Just buy four of these of these:

http://www.newark.com/vishay-dale/rh0504r000fe02/wirewound-resistor-4-ohm-50w-1>/dp/41K9167 "

Why do those Vishays look so much like Dales ?

**Success breeds imitation. That's my take. You probably should ask
Vishay (who owns Dale). It would seem that Dale originated the aluminium
clad resistor (I may be wrong though) and others have copied the design.
Engineers don't have to worry about supply, since there are several
suppliers of essentially identical parts.

--
Trevor Wilson
www.rageaudio.com.au

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[Jeff Liebermann]

On Mon, 27 Jul 2015 06:56:53 +1000, Trevor Wilson
<trevor@SPAMBLOCKrageaudio.com.au> wrote:

Just buy four of these of these:
http://www.newark.com/vishay-dale/rh0504r000fe02/wirewound-resistor-4-ohm-50w-1/dp/41K9167

The RH variety is inductive. For non-inductive, you want the NH
series, which Newark and Digikey do not seem to carry. Mouser has it
at:
<http://www.mouser.com/Search/Refine.aspx?Keyword=NH0504R000FE02>
for twice the price.

Dump them in a bucket of water, or bolt them to a chunk of aluminium and
place a small fan on the aluminium.

An aluminum heat sink will keep my coffee warm. All the water does is
raise the humidity in the lab and create a huge mess when I tip over
the bucket. I'll take the aluminum heat sink, but without the fan,
which makes too much noise.

--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Cursitor Doom]

On Sun, 26 Jul 2015 20:18:28 -0700, Jeff Liebermann wrote:

The RH variety is inductive. For non-inductive, you want the NH series,
which Newark and Digikey do not seem to carry.

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

 

[Ralph Mowery]

"Cursitor Doom" <curd@notformail.com> wrote in message
news:mp59en$khn$3@dont-email.me...

On Sun, 26 Jul 2015 20:18:28 -0700, Jeff Liebermann wrote:

The RH variety is inductive. For non-inductive, you want the NH series,
which Newark and Digikey do not seem to carry.

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

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.

 

[Jeff Liebermann]

On Mon, 27 Jul 2015 12:50:31 +0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

On Sun, 26 Jul 2015 20:18:28 -0700, Jeff Liebermann wrote:

The RH variety is inductive. For non-inductive, you want the NH series,
which Newark and Digikey do not seem to carry.

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

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:
<https://www.google.com/search?tbm=isch&q=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:
<http://www.11junk.com/jeffl/pics/drivel/slides/dummy-load-inductance.html>
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
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Cursitor Doom]

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.

2. 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.

 

[Trevor Wilson]

On 27/07/2015 10:50 PM, Cursitor Doom wrote:

On Sun, 26 Jul 2015 20:18:28 -0700, Jeff Liebermann wrote:

The RH variety is inductive. For non-inductive, you want the NH series,
which Newark and Digikey do not seem to carry.

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

**No.

--
Trevor Wilson
www.rageaudio.com.au

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