Basic amplifier testing: dummy loads

[Ray Gianelli]

When testing amplifiers a dummy load is used. While a speaker can be used, a 1 Khz sine wave fed into an amp and ramped up to the amp's maximum output will be deafening. Hence the use of dummy loads.

When I first started doing this a friend and mentor gave me a pair of 50 watt 8Ω resistors. I used them for years until I upgraded. They looked like this:

This was messy, had to be set up each time and the probe and mini grabbers would constantly slip off. I needed a dedicated set of dummy loads!


A dummy load is just a high wattage resistor with the desired resistance, usually 8Ω since most amps are rated for 8Ω output. Indeed, some folks buy resistors and make up their own dummy loads, which is fine. I preferred to buy purpose built dummy loads, like these from Parts Express:

These are rated at 100 watts. The construction of these makes mounting them on heatsinks simple. So that's what I did:



When heatsinked like this you can run hundreds of watts through these for a limited time, certainly long enough to make a power measurement. The dual banana jacks make safe, secure connections possible. These are inexpensive at Parts Express, and I had 4 of these mounted on heat sinks. These allowed me to run 2 of them in parallel for 4Ω measurements on each channel at once.

But as time went by I started to see some larger amps, and I decided it was time to upgrade the ol' dummy loads. Someone here showed a pair of 1,000 watt dummy load resistors of the same construction style as these. They got 2 of them at the auction site for around the $50 mark, and once I had seen them I knew they were going to be the last dummy loads I'd ever need.

Since they were so long I constructed them differently than the PE ones. I mounted them on top of some massive heat sinks I had, and put the dual banana jacks on the front:


What isn't shown here is that each dummy load has a cable run from the back of each set of banana jacks to a BNC cable that runs to the front of my distortion analyzer. The distortion analyzer output is run to my scope. But for basic level measurement you can just connect the BNC to your scope. It's just a safer way to make measurements, lowering the risk of shorts.

There's no single right way to do this, so think about it and create a solution that fits your needs. Many members here have shown amazing creativity with their dummy load designs. Here's a thread fellow member @rottalpha started:

https://audiokarma.org/forums/index.php?threads/show-me-your-dummy-load-s.757605/

 

[KingBubba]

I have a point contrary to the use of stabilized loads on amps. This is my story:

Around 1985, after having had my SX-1250 for 4 years, a problem arose. On an occasional basis, my receiver would go into protection. I finally got sick of it and took my Pioneer to the only repair shop in the Orlando area. Weeks went by and finally, I paid $90 to get my "fixed" unit back. I got it home and after the usual struggle with the beast, I fired it up. Almost right away it returned to it's usual behavior of shutting down. This trip to the shop return continued three times. Each time the said there was never anything wrong with my receiver. After the third time, I packed up my receiver and my speakers, I walked into the shop and began to setup my my stereo on the front counter. I fired it up and, once again, the receiver went into protection over and over. I made a believer out of the shop's techs. The next time I heard from them, I heard that there was a bad transistor in the protection card and to come get my receiver. The point here is that there are certain cases where a static load will not produce the needed results. It may always be useful to put a speaker load onto an amplifier outputs.

 

[Ray Gianelli]

I have a point contrary to the use of stabilized loads on amps. This is my story:

Around 1985, after having had my SX-1250 for 4 years, a problem arose. On an occasional basis, my receiver would go into protection. I finally got sick of it and took my Pioneer to the only repair shop in the Orlando area. Weeks went by and finally, I paid $90 to get my "fixed" unit back. I got it home and after the usual struggle with the beast, I fired it up. Almost right away it returned to it's usual behavior of shutting down. This trip to the shop return continued three times. Each time the said there was never anything wrong with my receiver. After the third time, I packed up my receiver and my speakers, I walked into the shop and began to setup my my stereo on the front counter. I fired it up and, once again, the receiver went into protection over and over. I made a believer out of the shop's techs. The next time I heard from them, I heard that there was a bad transistor in the protection card and to come get my receiver. The point here is that there are certain cases where a static load will not produce the needed results. It may always be useful to put a speaker load onto an amplifier outputs.

I always burn in repairs on speakers for at least a week before returning or selling anything. It also allows me to audition all different kinds of equipment without going into debt.

 

[NAD80]

The best in dummy loads. Looks great.

 

[gadget73]

Interesting failure mode that the amp would act up only with speakers and not the dummy load. Must have been some odd impedance swing thing causing the part to go flukey.

 

[WntrMute2]

I built this as an experiment. Still working on the whole o-scope thing. Have a Hickok scope and this - now need a signal generation and some adaptors/leads.

 

[Bill Ferris]

In the mid seventies, when I was first employed as a bench tech, I realized, If I was to effectively test audio amplifiers/receivers, that I would need a multi range audio dummy load.(side note: my first employer, was super frugal ! So I was on my own to purchase my desired test equipment, as he, though a engineer, didn`t seem to understand why I would need some pieces of test equipment to help me be more productive in repairs !)
So I bought and built a Heathkit 4X8 ohm 50 watt per binding post dummy load that was readily available at that time.
It had straps to configure the individual 8 ohm load resistors in to series or parallel combinations, and worked pretty well for me until larger power amps started showing up for repair, and my desire to run both channels at the DUT`s rated power level.

I had already installed a 120 volt 6" box fan underneath with 2" standoffs to help cool the 4 50 watt load resistors, but it became apparent that as SS amplifiers wattages being brought to me for servicing, that were increasing, and that the Heathkit, even with the box fan modification would need to be, either internally upgraded or replaced, and being poor, I elected to upgrade the Heathkit`s internal 8 ohm load resistors with 320 watt non inductive ones, which barely fit in the chassis.

I found out real quick, that it was best to use SS hardware & 10ga. copper wire crimped with bare ring connectors to secure the replacement resistor`s tab`s connections to the Heathkit`s front panel binding posts, as power soaking high watt audio amplifiers would melt, or weaken any lead based solder connections after a while.
I used that modified Heathkit load bank, until Sencore(no affiliation) offered me, to 'beta test ?" & possibly purchase, in the late eighties their recently new audio servicing test equipment, the PA 81 Audio Analyzer, which is a very well built, versatile & handy, but pricey unit when purchased new.
I really liked the, now no longer in production, PA 81, but couldn`t justify the price, and lack of my increasing needs of higher power capacity at the time, so I sent it back to Sencore.
A few years later, as my audio repair business picked up with 5 local music stores providing me a buttload of weekly repairs, and the Kilowatt + Crest, Crown, Hill, etc. amps started showing up for repairs, I realized I could now justify the investment in Sencore`s PA 81, and their newly released PA 82(also out of production) companion higher wattage(1 K watt per channel) supplement dummy load that connected to the PA 81`s power amp input binding posts.
Haven`t regretted the purchase for my main bench.
A few years ago Audio Classics had a used attractively priced PA 81 without it`s offboard 12 volt power supply "brick", so I bought it and purchased a universal 60 watt laptop selectable voltage SMPS "brick" for my secondary repair bench.

My only complaint with the PA 81, which I modified to get around it, was the active nature of the PA 81, which was really designed for servicing SS amplifiers, was, if I was testing a tube power amp, the PA 81 monitored DC off set of the amp`s output signal being fed to it, and any very low frequency pulse. pop, etc. from any amp connected to the PA 81 would promptly disconnect it`s load resistors !!

Great while testing a SS power amp. to save it from further damage, but not good for a tube power amp being tested at full power, and say rocking a driver tube in it`s socket causing the power amp to put out a LF thump/noise which caused the PA 81 to think that it should promptly disconnect the load resistor, not knowing that the amp being tested was a tube amplifier, and not a good thing to do at all !!
The first time it happened to me, I thought I was going to soil myself, and quickly hit the reset button !!

When I complained to Sencore about this undesirable, and dangerous to the tube amp`s output transformer/s situation when testing any tube amps, they were, "like who repairs tube amps anymore?!?!"

So I looked into modifying the internal high current rated load DC sense relay inside the PA 81..
It looked like I could remove the PA 81`s chassis` front panel`s grounding post(sometimes needed for operating the active dummy load in a strong RFI/EMI environment according to the manual), and install a mini SPST switch to selectively connect/disconnect the relay`s low amperage 12 volt actuating coil, and mark the front panel to which position to use. based on the type amp being tested..
I really like the PA 81, with it`s 250 watt selectable 2~32 ohms load + open, and selectable signal type twin L/R metering with auto ranging & hold with line level RCA jacks, L/R monitoring speakers, and thermostatically controlled internal cooling fan.
Way more sophisticated for the average hobbyist`s needs, but is a great reliable, and time saver bench device, for my audio servicing requirements.

 

[avionic]

Doubles as a hot plate..

 

[Bill Ferris]

View attachment 1492125 Doubles as a hot plate..

I see that, but I assume that you don`t use plastic coffee cups, though, I`m sure that the plastic used can withstand coffee temps, I would be careful if you are high power testing into those loads that a plastic coffee cup doesn`t start looking like it`s sagging, or altering it shape !! HA !!

 

[avionic]

I see that, but I assume that you don`t use plastic coffee cups, though, I`m sure that the plastic used can withstand coffee temps, I would be careful if you are high power testing into those loads that a plastic coffee cup doesn`t start looking like it`s sagging, or altering it shape !! HA !!

The photo was staged.. No liquids ever came close to electronic circuit. LOL Oh, BTW it was a ceramic cup.

 

[Bill Ferris]

The photo was staged.. No liquids ever came close to electronic circuit. LOL Oh, BTW it was a ceramic cup.

Well then Sir. I again assume no risk of "unauthorized electrical current path`s" were forth coming, if there might be a undesired meltdown of a plastic cup, whilst testing high powered audio amplifier`s !!
Very good then avionic, carry on ..
Just concerned !!

 

[arts]

In the mid seventies, when I was first employed as a bench tech, I realized, If I was to effectively test audio amplifiers/receivers, that I would need a multi range audio dummy load.(side note: my first employer, was super frugal ! So I was on my own to purchase my desired test equipment, as he, though a engineer, didn`t seem to understand why I would need some pieces of test equipment to help me be more productive in repairs !)
So I bought and built a Heathkit 4X8 ohm 50 watt per binding post dummy load that was readily available at that time.
It had straps to configure the individual 8 ohm load resistors in to series or parallel combinations, and worked pretty well for me until larger power amps started showing up for repair, and my desire to run both channels at the DUT`s rated power level.

I had already installed a 120 volt 6" box fan underneath with 2" standoffs to help cool the 4 50 watt load resistors, but it became apparent that as SS amplifiers wattages being brought to me for servicing, that were increasing, and that the Heathkit, even with the box fan modification would need to be, either internally upgraded or replaced, and being poor, I elected to upgrade the Heathkit`s internal 8 ohm load resistors with 320 watt non inductive ones, which barely fit in the chassis.

I found out real quick, that it was best to use SS hardware & 10ga. copper wire crimped with bare ring connectors to secure the replacement resistor`s tab`s connections to the Heathkit`s front panel binding posts, as power soaking high watt audio amplifiers would melt, or weaken any lead based solder connections after a while.
I used that modified Heathkit load bank, until Sencore(no affiliation) offered me, to 'beta test ?" & possibly purchase, in the late eighties their recently new audio servicing test equipment, the PA 81 Audio Analyzer, which is a very well built, versatile & handy, but pricey unit when purchased new.
I really liked the, now no longer in production, PA 81, but couldn`t justify the price, and lack of my increasing needs of higher power capacity at the time, so I sent it back to Sencore.
A few years later, as my audio repair business picked up with 5 local music stores providing me a buttload of weekly repairs, and the Kilowatt + Crest, Crown, Hill, etc. amps started showing up for repairs, I realized I could now justify the investment in Sencore`s PA 81, and their newly released PA 82(also out of production) companion higher wattage(1 K watt per channel) supplement dummy load that connected to the PA 81`s power amp input binding posts.
Haven`t regretted the purchase for my main bench.
A few years ago Audio Classics had a used attractively priced PA 81 without it`s offboard 12 volt power supply "brick", so I bought it and purchased a universal 60 watt laptop selectable voltage SMPS "brick" for my secondary repair bench.

My only complaint with the PA 81, which I modified to get around it, was the active nature of the PA 81, which was really designed for servicing SS amplifiers, was, if I was testing a tube power amp, the PA 81 monitored DC off set of the amp`s output signal being fed to it, and any very low frequency pulse. pop, etc. from any amp connected to the PA 81 would promptly disconnect it`s load resistors !!

Great while testing a SS power amp. to save it from further damage, but not good for a tube power amp being tested at full power, and say rocking a driver tube in it`s socket causing the power amp to put out a LF thump/noise which caused the PA 81 to think that it should promptly disconnect the load resistor, not knowing that the amp being tested was a tube amplifier, and not a good thing to do at all !!
The first time it happened to me, I thought I was going to soil myself, and quickly hit the reset button !!

When I complained to Sencore about this undesirable, and dangerous to the tube amp`s output transformer/s situation when testing any tube amps, they were, "like who repairs tube amps anymore?!?!"

So I looked into modifying the internal high current rated load DC sense relay inside the PA 81..
It looked like I could remove the PA 81`s chassis` front panel`s grounding post(sometimes needed for operating the active dummy load in a strong RFI/EMI environment according to the manual), and install a mini SPST switch to selectively connect/disconnect the relay`s low amperage 12 volt actuating coil, and mark the front panel to which position to use. based on the type amp being tested..
I really like the PA 81, with it`s 250 watt selectable 2~32 ohms load + open, and selectable signal type twin L/R metering with auto ranging & hold with line level RCA jacks, L/R monitoring speakers, and thermostatically controlled internal cooling fan.
Way more sophisticated for the average hobbyist`s needs, but isView attachment 1492079 View attachment 1492080 View attachment 1492084 View attachment 1492090 a great reliable, and time saver bench device, for my audio servicing requirements.

Bill,what are the shims between the scope and the analyzer for?

 

[avionic]

Bill,what are the shims between the scope and the analyzer for?

Rubber feet of scope are removed... They are laying in front of the Sencore in the photo..

 

[Bill Ferris]

Bill,what are the shims between the scope and the analyzer for?

Just a easy nearby place to store them.
With the use of only my right hand, I sometimes need little blocks of wood, those shims, balls of putty, spring loaded clamps, baby C-clamps, helping hands, etc. to help me to deal with my limitations, especially if my caregiver/house keeper is not around.
I`m sometimes forced to be creative with nonstandard applications of common items to help me get the job done.
Thanks for your interest Art.

 

[Bill Ferris]

Rubber feet of scope are removed...

Nope, scope feet are installed, but the handle is removed..

 

[gadget73]

Ever dabble in liquid cooling the dummy load to get more capacity from it? Heathkit's "Cantenna" RF dummy load was a 50w resistor in a 1 gallon paint can full of oil. It was rated up to 1kw for very brief periods but several hundred watts sustained. I've never had the need, but I've considered it as an option if my dummy load won't get the job done and I need more power in a pinch.

 

[Bill Ferris]

Ever dabble in liquid cooling the dummy load to get more capacity from it? Heathkit's "Cantenna" RF dummy load was a 50w resistor in a 1 gallon paint can full of oil. It was rated up to 1kw for very brief periods but several hundred watts sustained. I've never had the need, but I've considered it as an option if my dummy load won't get the job done and I need more power in a pinch.

No need for liquid cooling needs for any audio work gadget73.
I don`t take in amplifiers that would require that sustained level of load heat dissipation anymore.

And I know about and bought and used the Heathkit mineral oil filled Cantenna during the eighties for my customer`s, and my own CB linear powered station repair/performance confirmation, on both SS and sweep tube units.

Though my heart was audio repair, I, while waiting for my Medical Malpractice lawsuit to be concluded, I would repair, or attempt to repair whatever might cross my bench, as my SS disability check of $267.00 a month would only go so far, especially buying cigarettes, imported beer, pot, gas, rent, etc.

As a related alternate high powered dummy load after my HVAC was replaced in 1995, I kept the heat strip from it, that was with both heat strip elements used, was rated at 10Kw, though I toyed with it, for some reason that escapes me now, I was not happy with some aspect of using it, and kept on using my Sencore PA81/82 active dummy load package.

I realize now that the PA 82 is max rated at 2,500 watts per channel for a limited duration(10 minutes, I think), instead of the lower rating that I quoted earlier, and is not currently in use on either bench.

 

[gadget73]

If you've only got a 50w dummy load and need to work on an amp that makes 300w, its a quick and dirty solution to the problem. My own dummy load is good for 4 channels, 100w, 8 ohm. I can make that into a single channel 400w but if I ever wanted to run both channels on my Phase Linear 700 at full power into a load, it wouldn't suffer the abuse without a dunk in a water bath I expect.

 

[Livin4]

...I really liked the, now no longer in production, PA 81, but couldn`t justify the price, and lack of my increasing needs of higher power capacity at the time, so I sent it back to Sencore.
A few years later, as my audio repair business picked up with 5 local music stores providing me a buttload of weekly repairs, and the Kilowatt + Crest, Crown, Hill, etc. amps started showing up for repairs, I realized I could now justify the investment in Sencore`s PA 81, and their newly released PA 82(also out of production) companion higher wattage (1 K watt per channel) supplement dummy load that connected to the PA 81`s power amp input binding posts.
Haven`t regretted the purchase for my main bench.

Yeah, I think I had the same boss, couldn't understand the need for good test equip.

I did the Sencore loaner as well, had it for a couple of months. Also had the SG80. I was in the process of boxing them up to send back and he (my boss) decided that the gain in my productivity was enough to justify the cost, I think it was ~$6k back then for both. Several years later, I took another job, and since my old boss couldn't find a decent audio tech, he ended up selling the SG80 and PA81 for ~$3k. When I got back into audio as a hobby, I searched and acquired the whole set the SG80 mainly for the IF/RF capabilities, and then a PA81, and finally a PM82.

Little more info on the Sencore equipment, the model # for the loads are PA81 and PM82, the PM82 is basicly x10 load for the PA81, and they are extremely rare, I've only seen 3 or 4 run thru the bay in the last 10 years, 1 of which I bought. The PM82 is able to handle (per mfg) 2 channels at 1kw continuous and 2.5kw intermittent when used with the PA81. And if you bridge the 2 channels, 2k continuous and 5k intermittent.

The PA81 can easily handle 150w/ch continuous for 10 min by itself, if I need to run that hard longer, I add in the PM82. Both are great units but do have the "vintage" electronic issues, dirty pots and switch contacts, both somewhat easy to clean as compared to some of the audio units I have worked on.

Edit. Some more info, the PA81 has BNC outputs for the scope, as well as line level inputs for testing pre-amps.

 

[Bill Ferris]

Edit. Some more info, the PA81 has BNC outputs for the scope, as well as line level inputs for testing pre-amps

Yes, used the line level RCA jacks a few times over the years, and has R/L volume controlled amplified speakers too.
But I rarely used the scope connections, as my Tek auto signal leveled THD+N/IMD distortion analyzer`s custom made input cables were 3/4" dual banana terminated to plug directly(stacking) into the PA 81/82 input banana jacks, or at the power amp`s output banana jacks, again stacking the load wires.