MAB
Major Contributor
I have some Parts Express Dummy Loads for testing amplifiers. They have low inductance, high-power rating, are cheap, and get good reviews.
Unfortunately I was surprised to see they distort badly at high power, both the 8 Ohm and 4 Ohm versions. Here are some measurements and discussion.
I don't do much high power amplifier testing, but when I recently tested a vintage Yamaha CR-1020 receiver, I noticed the THD tracked a few other amps identically using the Parts Express Dummy Loads. I wired a different load together for comparison using some spare Parts Express wirewound low-inductance resistors, marketed as 'crossover' resistors. The wirewound load I put together is limited to below ~150 Watts, and gets hot even with a short 100 Watt sweep, so are only good for momentary use at high power.
Despite that large thermal issue, the comparison of the old Yamaha's measured THD indicates the Parts Express Dummy Load is distorting dramatically, and the quick and dirty load I made of the Parts Express wirewound crossover resistors is working great:
Wow, the Parts Express Dummy Load has significant distortion above just 1 Watt. I tried a few more cobbled-together loads from some additional chassis-mount Dale resistors and got the same good performance as I got from the PE wirewound resistors.
Regarding the source of the distortion in the PE Dummy Load: If this is solely caused by Voltage Coefficient of Resistance (VCR), or Temperature Coefficient of Resistance (TCR), these would be easily measurable at high power due to the large amount of distortion. I have an LCR meter with a DC bias adapter and a high power DC supply. I measured the Parts Express Dummy Load from 0 V all the way up to 28V bias, the VCR is smaller than I can measure even with a fairly competent Hewlett Packard LCR. I am able to measure R and Z to 100kHz, across DC bias, I see small change due to inductance, but an order of magnitude less than needed to explain the observed rising distortion with power of the PE Dummy Load, >30dB at 100 Watts.
5 Watt Dashboard Comparison:
The poor performance of the Parts Express Dummy Load affects the dashboard measurements, here is the Dale chassis-mount on top vs. the Parts Express Dummy Load on the bottom:
The Parts Express Dummy is has 15dB degraded THD at 5 Watts. Note the odd HD with the PE Dummy Load.
I did a search and see our own @RickS reported the same issue on the QuantAsylum forum some time ago:
I also notice; all of these dummy loads make audible noise when high power is applied.
The Parts Express Dummy Load makes a particularly odd sound compared to the noises emanating from the other resistors. I want to check this out more. With a microphone.
I also don't have a good load resistor that can withstand high power. So I constructed a (hopefully) good load, that can take high power. And I made measurements of the performance, including the sound.
I will also avoid doing any more high power testing of the 47 year old Yamaha
, so instead measurements will be made with a Bryston 3B-ST, which I purchased new in the late '90s.
High power load resistor:
I built the 8-Ohm load resistor out of eight 1-Ohm Caddock MP930 30 Watt thick film resistors. I have some MP9100 100W parts somewhere but can't find them. In any case, the MP930 give me a maximum power of 240 Watts, so long as they are sufficiently cooled.
If cooling isn't adequate these tend smoke-out rapidly. So I built a water cooler out of HVAC tubing and copper plate:
The copper tubing is brazed onto the backside of the thermal transfer plate. I can use water cooling on the resistors in case of long term power dissipation. It can be used for single measurements with short duration even without water.
I got the surface smooth to mount the TO-220 package resistors:
This contraption connected to my sink with just a trickle though the hose is really overbuilt
, limited by case to copper conductivity, not by the cooling capacity. So I should get the full rated 240 Watt power handling out of this load if I need to do a long duration measurement.
Comparing THD vs. Power of a few load resistors:
The below graph compares the Bryston's measured THD vs. Power with the Parts Express Dummy, the wirewound made from several Parts Express crossover resistors, and the Caddock Thick Film load.
A few things stand out. The cobbled together wirewound load performs the same as the thick film load, even if it is under-speced for long term power handling. It's thermal change in resistance is small even though it got uncomfortably hot to touch right after this measurement. Larger FFT sizes probably would get the load into trouble. The PE Wirewond and Caddock MP930 are identical for short term measurements. The PE Dummy Load isn't appropriate for distortion measurements above 1 Watt for measurements below 0.1% or so. The Bryston amp is almost 30 years old, never been touched, and still has great performance. Here is it's dashboard with the Caddock MP930:
The fact that the Caddock MP30 don't exceed the performance of the various wirewound I used is somewhat relieving. I wasn't interested in going into the sound of a resistor here. But, let's talk about the sound of resistors anyway in just a bit.
Some additional thoughts on Temperature Coefficient of the Resistance:
I did sweeps with the resistors soaked under power and hot. The Dummy load is so thermally massive, yet it behaves the same with a fast sweep from cold as it does when cooking hot. I would think it would measure worse at high temperature. It has the same measured distortion even after soaking at >100 Watts until the case was too hot to touch.
The various wirewound and Caddock thick film resistors also have negligible changes across temperature too, and retain their good performance even when warmed up. I even ran hot tap water though the 'cooler' of the Caddock MP930 resistors to see if anything changed, no measurable change above the level of noise was observed. I think TCR would need to be very large to be the cause of the level of distortion observed in the PE Dummy resistor. Same for VCR, which should be easily measurable with the DC bias and a decent LCR meter. In fact, TCR and VCR don't appear to be an issue in the measurements I made. I am purposely avoiding running the power so high something runs away, which will affect the measurement.
Measurements of the audible sound from the Parts Express Dummy Load:
I used a microphone placed close to the surface to measure the what I was hearing coming from the PE Dummy Load:
It's not loud, but I can hear it clearly. It sounds bad, like a crappy reproduction of a 1 kHz sine wave. I subtracted out the background noise to get the lower trace. The dummy load acts like a really crappy speaker, ~15dB SPL at 125 Watts maybe slightly higher, frequency response 3.2 kHz to 20 kHz +- 5 dB.
I measured the acoustic distortion of the resistor cum speaker:
Wow, all of the stuff I heard with a 1 kHz sine tone is higher order harmonics. Lots of ringing of these various harmonics. For example, here is the response feeding 1.325 kHz sine tone at high power, ringing the dummy load at 5.3 kHz.
Most of the sound is the higher harmonic ringing. And the FR of the block of metal and resistor and encapsulation is preventing me from making any meaningful discoveries.
I wonder if the potting of the resistive materials is vibrating. It's not clear what the vibrations are doing. They may be modulating the geometry of the conducting films, altering the impedance with voltage and frequency. There may other sources of distortion, depending on the material of the conductor and insulator, and if the vibration creates friction or pressure. Or all of the above, and some things I can't think of. In any case the resulting distortion above 1 Watt makes these not useful for distortion measurments. All four of PE Dummy Load resistors I have show the same behavior, both 8- and 4-Ohm. Unfortunately, this makes them useful only as a soak resistor, not useful for performance measurements.
8 Ohm 200W Non-Inductive Dummy Load Resistor
8 Ohm 200W Non-Inductive Dummy Load ResistorNon-inductive dummy load resistors are a must for bench testing amplifiers. Also useful for testing loudspeakers and other audio circuits requiring an 8 ohm load. 5% tolerance. Dimensions: 6-1/2" L x 2-3/8" W x 1-3/16" H.
www.parts-express.com
I don't do much high power amplifier testing, but when I recently tested a vintage Yamaha CR-1020 receiver, I noticed the THD tracked a few other amps identically using the Parts Express Dummy Loads. I wired a different load together for comparison using some spare Parts Express wirewound low-inductance resistors, marketed as 'crossover' resistors. The wirewound load I put together is limited to below ~150 Watts, and gets hot even with a short 100 Watt sweep, so are only good for momentary use at high power.
Despite that large thermal issue, the comparison of the old Yamaha's measured THD indicates the Parts Express Dummy Load is distorting dramatically, and the quick and dirty load I made of the Parts Express wirewound crossover resistors is working great:
Wow, the Parts Express Dummy Load has significant distortion above just 1 Watt. I tried a few more cobbled-together loads from some additional chassis-mount Dale resistors and got the same good performance as I got from the PE wirewound resistors.
Regarding the source of the distortion in the PE Dummy Load: If this is solely caused by Voltage Coefficient of Resistance (VCR), or Temperature Coefficient of Resistance (TCR), these would be easily measurable at high power due to the large amount of distortion. I have an LCR meter with a DC bias adapter and a high power DC supply. I measured the Parts Express Dummy Load from 0 V all the way up to 28V bias, the VCR is smaller than I can measure even with a fairly competent Hewlett Packard LCR. I am able to measure R and Z to 100kHz, across DC bias, I see small change due to inductance, but an order of magnitude less than needed to explain the observed rising distortion with power of the PE Dummy Load, >30dB at 100 Watts.
5 Watt Dashboard Comparison:
The poor performance of the Parts Express Dummy Load affects the dashboard measurements, here is the Dale chassis-mount on top vs. the Parts Express Dummy Load on the bottom:
The Parts Express Dummy is has 15dB degraded THD at 5 Watts. Note the odd HD with the PE Dummy Load.
I did a search and see our own @RickS reported the same issue on the QuantAsylum forum some time ago:
I also notice; all of these dummy loads make audible noise when high power is applied.
The Parts Express Dummy Load makes a particularly odd sound compared to the noises emanating from the other resistors. I want to check this out more. With a microphone.I also don't have a good load resistor that can withstand high power. So I constructed a (hopefully) good load, that can take high power. And I made measurements of the performance, including the sound.
I will also avoid doing any more high power testing of the 47 year old Yamaha
, so instead measurements will be made with a Bryston 3B-ST, which I purchased new in the late '90s.High power load resistor:
I built the 8-Ohm load resistor out of eight 1-Ohm Caddock MP930 30 Watt thick film resistors. I have some MP9100 100W parts somewhere but can't find them. In any case, the MP930 give me a maximum power of 240 Watts, so long as they are sufficiently cooled.
If cooling isn't adequate these tend smoke-out rapidly. So I built a water cooler out of HVAC tubing and copper plate:
The copper tubing is brazed onto the backside of the thermal transfer plate. I can use water cooling on the resistors in case of long term power dissipation. It can be used for single measurements with short duration even without water.
I got the surface smooth to mount the TO-220 package resistors:
This contraption connected to my sink with just a trickle though the hose is really overbuilt
, limited by case to copper conductivity, not by the cooling capacity. So I should get the full rated 240 Watt power handling out of this load if I need to do a long duration measurement.Comparing THD vs. Power of a few load resistors:
The below graph compares the Bryston's measured THD vs. Power with the Parts Express Dummy, the wirewound made from several Parts Express crossover resistors, and the Caddock Thick Film load.
A few things stand out. The cobbled together wirewound load performs the same as the thick film load, even if it is under-speced for long term power handling. It's thermal change in resistance is small even though it got uncomfortably hot to touch right after this measurement. Larger FFT sizes probably would get the load into trouble. The PE Wirewond and Caddock MP930 are identical for short term measurements. The PE Dummy Load isn't appropriate for distortion measurements above 1 Watt for measurements below 0.1% or so. The Bryston amp is almost 30 years old, never been touched, and still has great performance. Here is it's dashboard with the Caddock MP930:
The fact that the Caddock MP30 don't exceed the performance of the various wirewound I used is somewhat relieving. I wasn't interested in going into the sound of a resistor here. But, let's talk about the sound of resistors anyway in just a bit.
Some additional thoughts on Temperature Coefficient of the Resistance:
I did sweeps with the resistors soaked under power and hot. The Dummy load is so thermally massive, yet it behaves the same with a fast sweep from cold as it does when cooking hot. I would think it would measure worse at high temperature. It has the same measured distortion even after soaking at >100 Watts until the case was too hot to touch.
The various wirewound and Caddock thick film resistors also have negligible changes across temperature too, and retain their good performance even when warmed up. I even ran hot tap water though the 'cooler' of the Caddock MP930 resistors to see if anything changed, no measurable change above the level of noise was observed. I think TCR would need to be very large to be the cause of the level of distortion observed in the PE Dummy resistor. Same for VCR, which should be easily measurable with the DC bias and a decent LCR meter. In fact, TCR and VCR don't appear to be an issue in the measurements I made. I am purposely avoiding running the power so high something runs away, which will affect the measurement.
Measurements of the audible sound from the Parts Express Dummy Load:
I used a microphone placed close to the surface to measure the what I was hearing coming from the PE Dummy Load:
It's not loud, but I can hear it clearly. It sounds bad, like a crappy reproduction of a 1 kHz sine wave. I subtracted out the background noise to get the lower trace. The dummy load acts like a really crappy speaker, ~15dB SPL at 125 Watts maybe slightly higher, frequency response 3.2 kHz to 20 kHz +- 5 dB.
I measured the acoustic distortion of the resistor cum speaker:
Wow, all of the stuff I heard with a 1 kHz sine tone is higher order harmonics. Lots of ringing of these various harmonics. For example, here is the response feeding 1.325 kHz sine tone at high power, ringing the dummy load at 5.3 kHz.
Most of the sound is the higher harmonic ringing. And the FR of the block of metal and resistor and encapsulation is preventing me from making any meaningful discoveries.
I wonder if the potting of the resistive materials is vibrating. It's not clear what the vibrations are doing. They may be modulating the geometry of the conducting films, altering the impedance with voltage and frequency. There may other sources of distortion, depending on the material of the conductor and insulator, and if the vibration creates friction or pressure. Or all of the above, and some things I can't think of. In any case the resulting distortion above 1 Watt makes these not useful for distortion measurments. All four of PE Dummy Load resistors I have show the same behavior, both 8- and 4-Ohm. Unfortunately, this makes them useful only as a soak resistor, not useful for performance measurements.
