Measuring audio amp output power

[Ralph Mowery]

"Cursitor Doom" <curd@notformail.com> wrote in message
news:mp681u$8ul$2@dont-email.me...

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).

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.

 

[MJC]

In article <mp681u$8ul$2@dont-email.me>, curd@notformail.com says...

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).

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.

 

[Phil Allison]

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?

** Not in the slightest.

Liebermann is blowing it out his arse, as usual.


..... Phil

 

[Phil Allison]

Ralph Mowery wrote:

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.

** And this is a sufficient test for the purpose.


> When I switched to a square wave

** Which has harmonics into the medium and HF bands.

I noticed a large spike on the leading and trailing edges, especially as I
went higher up in frequency.

** All due to the harmonics way above audio range.

From this rough test, I would say that if using sine waves you could get a
close to true test, but if music

** Music signals stop at 20kHz.

or other odd ball waveforms

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



..... Phil

 

[Phil Allison]

N_Cook wrote:

Cursitor Doom wrote:

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).




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

** Huh ??

Wot a classic Kookism.



.... Phil

 

[N_Cook]

On 27/07/2015 22:32, Cursitor Doom wrote:

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.

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

 

[Phil Allison]

Ralph Mowery wrote:

"Phil Allison"


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.

** And this is a sufficient test for the purpose.


When I switched to a square wave

** Which has harmonics into the medium and HF bands.


I noticed a large spike on the leading and trailing edges, especially as
I
went higher up in frequency.

** All due to the harmonics way above audio range.


From this rough test, I would say that if using sine waves you could get
a
close to true test, but if music

** Music signals stop at 20kHz.


or other odd ball waveforms


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



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

** Huh ????

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

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.

** 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.

 

[Jeff Liebermann]

On Tue, 28 Jul 2015 08:19:32 +0100, N_Cook <diverse@tcp.co.uk> wrote:

On 27/07/2015 22:32, Cursitor Doom wrote:
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.

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

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:
<http://www.ebay.com/itm/171376872247>
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
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Ralph Mowery]

"Phil Allison" <pallison49@gmail.com> wrote in message
news:0c799d3c-9477-4076-b521-a6a8a6bd7410@googlegroups.com...

Ralph Mowery wrote:



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.

** And this is a sufficient test for the purpose.


When I switched to a square wave

** Which has harmonics into the medium and HF bands.


I noticed a large spike on the leading and trailing edges, especially as
I
went higher up in frequency.

** All due to the harmonics way above audio range.


From this rough test, I would say that if using sine waves you could get
a
close to true test, but if music

** Music signals stop at 20kHz.


or other odd ball waveforms


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

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.

 

[Ron D.]

Audio load: http://www.mcmelectronics.com/product/28-048 (No mention of tolerance)

Done.

Scopes have a lot of errors. A DVM may work, but pay attention to frequency response. If you stick to a sine wave you won't need a TRMS meter.

Next problem, you need to know the resistor value accurately.

I don't think you want or need want 0.001% accuracy.

 

[Phil Allison]

Ron D. wrote:

Audio load: http://www.mcmelectronics.com/product/28-048
(No mention of tolerance)

** I use the same resistors for all amplifier bench testing.

Submerged in water, they can handle 1000W each.

Using a pair and a simple switching scheme gives 4, 8 and 16 ohms.


> Scopes have a lot of errors.

** That's a a bit harsh.

Many analogue scopes manage 2% accuracy for time and amplitude & modern DSOs
have on-screen displays of the same parameters with better than 1% accuracy.

A DVM may work, but pay attention to frequency response.

** Most can be trusted to about 1kHz on sine waves.


..... Phil

 

[pfjw@aol.com]

On Monday, July 27, 2015 at 6:09:33 PM UTC-4, Ralph Mowery wrote:

"Cursitor Doom" <curd@notformail.com> wrote in message
news:mp681u$8ul$2@dont-email.me...
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).


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.

NO!!!!

Most metallic conductors are Zero-Resistance at cryogenic temperatures. So, a wire-wound resistor may drop off to near-zero in liquid nitrogen.

 

[Trevor Wilson]

On 31/07/2015 2:32 PM, Ron D. wrote:

Audio load: http://www.mcmelectronics.com/product/28-048 (No mention
of tolerance)

Done.

Scopes have a lot of errors. A DVM may work, but pay attention to
frequency response. If you stick to a sine wave you won't need a
TRMS meter.

**Digital 'scopes often have true RMS display capability, with excellent
accuracy.

Next problem, you need to know the resistor value accurately.

I don't think you want or need want 0.001% accuracy.

**Far and away the biggest source of inaccuracy lies with line Voltage
regulation. Or lack of it. I would certainly not panic about resistor
accuracy nor 'scope accuracy, when faced with typical line Voltage
swings. Naturally, most professionals use a variable auto transformer
(aka: Variac™) and a line Voltage monitor.


--
Trevor Wilson
www.rageaudio.com.au

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

 

[Ralph Mowery]

<pfjw@aol.com> wrote in message
news:ab86ef38-7eac-4270-93f0-017fbea84a34@googlegroups.com...

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.

NO!!!!

Most metallic conductors are Zero-Resistance at cryogenic temperatures.
So, a wire-wound resistor may drop off to near-zero in liquid nitrogen.

The liquid nitrogen was mainly a joke to the extreme of cooling. However if
enough power is being used, the resistor its self would be heated to a more
normal temperture and would be way above the temperature of the nitrogen.
This woudl bring the resistance back up.

 

[MJC]

In article <ab86ef38-7eac-4270-93f0-017fbea84a34@googlegroups.com>,
pfjw@aol.com says...

Most metallic conductors are Zero-Resistance at cryogenic
temperatures.
So, a wire-wound resistor may drop off to near-zero in liquid
nitrogen.

If only superconductors for liquid N2 temperatures (77K, -196C) were so
common! Even at liquid He (4.22K, -270C) only a minority of metals make
the transition.

My Kaye and Laby says: "At low temperatures the effects of impurities
etc become increasingly important and these largely determine the value
of the residual resistance to which many metals decrease at low
temperature."

Mike.

 

[Ron D.]

Phil: > Scopes have a lot of errors.

** That's a a bit harsh.

Analog scope days and having error sources and effects drilled into my head.
I just really wanted to say, know what your error sources are. At one point in my life numbers like 2E18 and 7E18 were considered "essentially the same".

 

[Cursitor Doom]

Just one final point. Is a single sine wave sufficient, or will it have
to be two, harmonically-unrelated sine waves? ISTR with RF power
measurements you need to perform the slightly more complicated 'two-tone'
test; just wondering if the same applies at audio frequencies?

 

[Phil Hobbs]

On 07/31/2015 11:54 PM, Ron D. wrote:

Phil: > Scopes have a lot of errors.

** That's a a bit harsh.

Analog scope days and having error sources and effects drilled into
my head. I just really wanted to say, know what your error sources
are. At one point in my life numbers like 2E18 and 7E18 were
considered "essentially the same".

Former astronomer?

Cheers

Phil "also a former astronomer" Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

 

[Phil Allison]

Cursitor Doom wrote:

Just one final point. Is a single sine wave sufficient, or will it have
to be two, harmonically-unrelated sine waves?

** A single sine wave is needed for audio power testing.

Amplifier power ratings are based on the *rms* value of a sine wave - hence the misleading term "rms power".

ISTR with RF power
measurements you need to perform the slightly more complicated 'two-tone'
test; just wondering if the same applies at audio frequencies?

** Using two tones tests for intermodualtion, always present where there is non-linearity.


.... Phil

 

[Cursitor Doom]

On Sat, 01 Aug 2015 18:29:58 -0700, Phil Allison wrote:

** A single sine wave is needed for audio power testing.

Amplifier power ratings are based on the *rms* value of a sine wave -
hence the misleading term "rms power".

Well that would make sense with RMS being, IIRC, equivalent to the DC
heating effect in a resistive load.