As I already pointed in my post #6, we already have this nice thread "ASR dummy load configuration" where @Langston Holland's post #97 and post #98 are really thorough in-depth and worthwhile.
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Parts Express Dummy Load Performance Measurements
- Thread starter MAB
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MAB
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- #22
Yes, I already saw your post #6, now see you repeated that post again. I'm not sure why you are confusing this thread with the ASR Dummy Load thread.
This thread, which has nothing to do with ASR dummy load, is about a Parts Express dummy load that they sell, which doesn't perform well, and is causing people on various forums (including myself here) to produce distortion plots that are incorrect. If I was simply repeating a test of ARCOL loads I wouldn't bother posting the results, or I would post in the other thread.
Regarding your post #6 in this thread, it was off topic of this thread. This thread is about distortion testing of amplifiers with a Parts Express Dummy Load. I'm fine with people posting off topic, but I can't figure out why you repeatedly tell me "we already have a nice thread on ASR dummy load." And I'm not sure what the lost of equipment and links to random pages have to do with these Parts Express Dummy Loads.
Regarding post #6
You are burning in your amps
, why would you do something like this?
Also, why would you use expensive ARCOL resistors to do such a mundane and pointless activity (and likely counterproductive). If you want to take some life out of your gear by burning them in, just use an electric water heater element as a load. Or maybe these Parts Express Dummies that I test in this thread, they are really only good for soak testing.
Hello @MAB,
Thank you for your kind comments on my post #6.
Now I fully understand your points, and I am sorry for your frustration caused by my post.
My intention of the post #6 was just reminding all the people onboard (or will onboard) this thread that another thread of intensive measurements and reports on dummy load, especially on safe robust precision ARCOL resistors,
The main reason for my utilization of ARCOL resistors for burn-in (and/or initial short-time high-load tests) of my amplifiers is just their robustness and safety.You are burning in your amps, why would you do something like this? Also, why would you use expensive ARCOL resistors to do such a mundane and pointless activity (and likely counterproductive). If you want to take some life out of your gear by burning them in, just use an electric water heater element as a load. Or maybe these Parts Express Dummies that I test in this thread, they are really only good for soak testing.
In the rare case of accidental disconnection (open circuit) within the dummy resistors, it gives no damage to the amplifiers.
In the rare case of accidental "short circuit" within the dummy resistors, however, it would highly possibly damage and/or destroy amplifiers...
Based on the same perspective, I use ARCOL 8 Ohm 100W resistor(s) for Fq-SPL measurement of amplifiers' SP high-level outputs "in silence" using flat white noise under actual working-load (ref. here) ...
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Dosnt make sense to me. Do you say the speaker is distorting the voltage at its terminals? Where are those measurements? A 2 terminal device (resistor) only has one voltage across it, and thats the driving voltage, thats what your measuring, and thats the voltage the amp is "making" not the load. Resistor voltage coefficient (temp and voltage) should have no effect on distortion. Does an amp change output level when you change the load from 8 to 10 ohms (different freq. driving a speaker)? If it does do you blame the distortion on the load?Actually resistors do. Resistors have noise too. All of this can be measured, and is.
If you have a time varying signal from a perfect voltage source, and that signal is fed through a resistor with a varying resistance, the output will show distortion unless the time-varying resistance is perfectly in phase with the signal and the magnitude of the variation is linear in the amplifier's output - In this special case only the gain is reduced. As soon as either of these conditions is violated, odd and even order distortion is measured depending on the nature of the non-linearity in the resistor, even if the amp is a perfectly invariant voltage source. Voltage Coefficient of Resistance is primarily responsible for this in most cases, and usually resistors don't have significant VCR until operated above 50 Volts. In most cases, this would be a simple reduction in resistance as voltage is increased, and would show up as second order harmonics, even on a perfect straightwire + gain voltage amp. If the resistor is inside of the amp, I guess semantically you can call it 'amp distortion'. In the case of the PE Dummy load the normal VCR mechanisms normally discussed in literature (activation of resistive paths in the conducting films at high bias) are swamped by a much different mechanism (the resistor dissipates electrical energy acoustically). And the effect is quite massive, which is why I can hear the resistor whining under stress.
Also, I drove a much more reactive load than the Parts Express Dummy, tiny elevation in distortion, perhaps you missed that post. Which also throws cold water on the idea the amp can't handle the reactive load.
edit: typo
If your talking about the current, that will be distorted by these effects but the current is not the signal.
And I said nonlinear load not reactive, as in a load that changes impedance (resistance, inductance and capacitance, which is what I would expect from a vibrating coil) with instantaneous voltage, or has hystereses.
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MAB
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- #27
I will find, Amir discussed this in context of load resistors, and why you would never want to use a speaker as a load for amp testing, even if it seems to make intuitive sense. But we don't need that for the rest of this discussion.
You made some assumptions above, including the change in amp output. I already showed that even under large reactive load, much larger than the PE Dummy presents, the amp is invariant to within less than a dB.
The physics of a resistor with applied voltage:
The amp's output forms a voltage divider with the load resistance. You need to think of it in this way. What if the amp's output is completely linear and the the load resistor is varying?
Consider the following circuit.
R1 is the amp's output impedance
R2 is the dummy load's impedance
The amp has been replaced with a battery, constant DC voltage, with immeasurably low voltage variation
Can you tell me what happens to the output when you vary the R2 leaving every other parameter the same?
How would that output compare to the battery voltage as measured without R2?
Extra credit: Replace the battery with an Amp, the DC voltage with an AC voltage, and make the R2 a function of the applied voltage (this is called Voltage Coefficient of Resistivity), what happens?
VCR is one of the sources of even order HD in tube amps, even if the tube is operating perfectly linearly.
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MAB
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- #29
I fear we are talking past each other. Or you are making a semantic point, which I agree with if amp+load are considered together.
Resistors are not linear devices that we normally consider, like Voltage Coefficient of Resistance. In most applications it's OK to ignore these effects, and to ignore the fact that passive devices can be sources of distortion and noise when voltage is applied.
As I alluded, and @amirm commented in post #2, VCR is a studied phenomena, with conduction properties that are not Ohmic, and not described by a resistor's parasitic inductance or capacitance either, and whose nonlinearities lead to distortion in the passive itself. This is a good paper that describes the phenomena from about the time I was born.
The effects include tunneling between grain boundaries, defect scattering, and other phenomena more often associated with defective insulators and semiconductors. If this effect was due to a resistor in a tube amp causing a voltage-sensitive divider on the output, it would cause even order harmonics, and I agree it would be the amplifier distorting.
And if the resistor was an external one, that induced distortion in the amp (self-oscillation, input bias offset, etc.), I would also agree that this is the amp distorting.
But this is the Parts Express Dummy going non-linear and distorting all on it's own, as referenced in the paper. And it is an external load, so doesn't get included as amp distortion, unless it induced the amp to become unstable which I showed it did not. I am being careful to separate the amplifier being induced to distort, and the load distorting.
What's unusual about these Parts Express Dummies is they make a very audible noise when AC is applied, which I recorded and saw the sound aligned with the electrical harmonic distortions I measured. I am guessing the noise I hear are the bifilar wires rattling in the potting. I don't know if the nonlinearity is due to the sound energy being dissipated, or if there are piezoelectric or other effects, all of which are sources of some very odd distortion.
I would like to not do a semantic argument here, the paper I referenced above and many other examples in this field of research carefully uses the same language I am using, takes some time to distinguish between the passive device distorting vs. the intrinsic distortion of the applied signal. You can also read up on this thread. You can also use SPICE with a resistor model that includes VCR and simulate with an AC signal, especially interesting to apply a DC offset to the resistor!
edit: fixed the link to the Harris / Peterson paper
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What would you get with a perfect amp? No distortion. Would you then say the "load has no distortion"? Loads dont have distortion, they cause amplifier distortion. Some loads are harder to drive so you get more distortion. If you were to drive a speaker with a HP amp yould say its the speaker thats causing the distortion? How about a power amp driving a .1 ohm perfectly linear resistor, its the resistors distortion?
Any distortion on the output of an amp is amp distortion.
Mark185
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I had a load bank using the PE resistors until very recently with a Quant Asylum QA403. I had big problems with 60 Hz noise spikes and harmonics and I was using the differential inputs on the QA403 which I expected to cancel out any common mode noise. Long story short, when I replaced the PE resistors with Vishay/Dale 1% non-inductive 25 watt resistors, the AC power noise went away. I started out with 8 of these 25 watt resistors in a parallel/series arrangement to get 100 watt capability at 4 ohms. I ordered another 10 from Digikey now that I have seen good results and I will expand my load bank capability.
I don't understand how the PE resistors coupled 60 Hz into the analyzer but it appears that they acted like antennas. I was testing Class D automotive amplifiers so 60 Hz had no way to be coming out of them.
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MAB
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- #34
You are incorrect, unless you use an idealized resistor, rather than a resistor with significant VCR.
You also ignored the paper I linked.
This was studied in the last century. Perhaps read the links I posted. No resistor is ideal and Ohmic, some dramatically non-ideal. Like these PE Dummies, that make a loud noise under applied voltage, and no way a resistor can do that and still be considered Ohmic.
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