This is a simple DIY Speaker dummy load that was put together very quickly. After constructing an AMP6 T-Amp kit, and while testing the maximum power output using 4.7 ohm 50W ceramic power resistors the hot resistors had begun to scortch a cutting mat they were resting on and also melting the wire insulation of test leads which had fallen on one of the hot resistors. So rather than test with unsecured hot resistors, it was necessary to build a properly enclosed speaker dummy load that would not be a fire hazard.
Rather than just using a load resistor across the output of an amplifier, it is often nice to also be able to hear output during testing. The schematic below shows two dummy loads, built into one enclosure. With a speaker connected, the amplifier will see either a 4.3 or 7.4 ohm load while the attached speaker will receive less than about 1% of the output power. You can check the net load for yourself by following the basic electronics formulas. With a simple setup like this you can test at high power without risking damaging speakers, going deaf or driving other people crazy. If a speaker is not connected to the dummy load, the amplifier will see either a 4.5 or 8 ohm load.
The two dummy speaker loads are housed in the same type of aluminum baking pan that I used for my 50EH5 Single Ended Tube Amplifier. To help promote cooling, I mounted the 50W resistors to heat sinks that had been salvaged from dead computer power supplies. The 4.5 ohm resistor is attached to the larger heat sink. The aluminum enclosure also helps dissipate some heat from the resistors. Since the resistors are cooled somewhat, it is ok to apply more than 50W of power for a very SHORT period of time, but don't get carried away.
Guitar Amp Builder Notes — Af Power Amplifier Dummy Load
I used binding posts to allow for easy connection to and from the dummy load. The binding posts also make it easy to add a small capacitor or inductor if you want to try different load scenarios when testing an amplifier.
NEW - Matt shares with us his build of a fantastic looking dummy speaker load box. The dummy speaker load box is configurable for 2-ohm, 4-ohms, 6-ohm, 8-ohm, 16-ohm and 32-ohm loads.I've been building audio amplifiers for years. And until now I've always used just a bare wirewound resistor with some banana plugs as a test load. Well, no longer! I now have a very nice dummy load:
It's a simple thing, really, but I thought others might get some inspiration and/or at least be interested in the sources for the bits and pieces I used.
Reactive Load For Amplifiers Teaches Lessons About Inductors
The binding posts are used to connect to the amplifier. I added RCA jacks in parallel, to allow a monitor connection to the audio analyzer (or other test & measurement apparatus).
I used some nice big 8 ohm 50W (or 100W when heatsinked) power resistors that I found cheap at Surplus Sales of Nebraska (www.surplussales.com). The specific item is the (RWF) 12M59-8-L on this page: http://www.surplussales.com/Resistors/NonInductive/Res-NonInd_list.htm It's priced well - I bought 10 pcs. for $45.00. These are nice metal-tubbed power resistors made by Milwaukie resistor.
I used a selector switch from Idec to allow selection of 4 ohms (two resistors in parallel), 8 ohms (4 resistors in series-parallel), or 12 ohms (one in series with two in parallel). Really, the only reason for the 12 ohm load is that I use this to measure Zout of an amplifier, by measuring the output voltage difference form 8 to 12 ohms. The switch is a little odd... it is a DPDT switch but has three positions. In the center position, one pole is open, and the other closed. It's a little hard to understand how all this works, but I'll leave that to you to figure out :)
Dummy Loads And O Scopes
You can buy this switch direct from Idec or from distis like Allied Electronics. They aren't cheap, at around $25 each, but they are nice. By the way, I think this particular switch series (called L6) is being discontinued, but there is another model (LB1S-3T6).
The enclosure is a chassis from Landfall Systems. This chassis works really well to use with an extruded heatsink. The chassis top plate is the extrusion. Landfall can make one of these for you, or something different in a similar size.
The extrusion came from Heatsink USA. They are one of the only vendors I know that will supply smaller cuts of heatsink extrusion for a reasonable price. This particular heatsink is the 7.28 wide extrusion. I ordered it cut to 10 long (which they do an excellent job of, by the way - a very clean cut with no need to do any cleanup). This extrusion works particularly well with the Landfall chassis, as the outer fins line up correctly with the mounting holes in the chassis.
Audio Dummy Load
I drilled counterbored holes through the top of the extrusion to mount it to the chassis, and drilled blind holes in the back and tapped them #8-32 to mount the power resistors. Use plenty of grease with these resistors as they are not exactly flat on the back. This is pretty easy to do if you have a drill press. Landfall might be willing to do this for you for a small fee...
Landfall also had the extrusion and chassis black anodized for me, then engraved the front with the legends you see in the photo above.
It came out beautifully. It should be able to handle 400W per channel on the 8 ohm setting, with less (200W @ 4 ohms and 125W @ 12 ohms) for some period of time. Of course it will get hot - the heatsink theta-JA is about 0.4 degrees C per watt, so it will be at about 100C with 100W/channel continuous (it would probably take an hour to heat up that much due to it's thermal mass). Far more than I'll probably ever need.A dummy load is a piece of equipment used in place of a speaker when working on or testing amplifiers. It is designed to safely dissipate the entire power of the amplifier. Dummy loads are useful for determining maximum amplifier output power and for burn-in of an amplifier under full power for several hours or days to insure reliability before the amplifier is sold to a customer, or returned after a repair.
Rat Tube (valve) Amp Dummy Load Box / Line Out Box
Reactive loads are good for testing amplifiers using global negative feedback, because they can sometimes cause a marginally stable amp to oscillate, when it won't oscillate into a purely resistive load. It is always a good idea to test into a reactive load while looking at the output on the scope and adjusting all the knobs to make sure there is no setting that can cause oscillation. If the amp does oscillate, it may need less negative feedback, or phase compensation of the loop. Click here to read a paper on designing reactive loads. You must use a non-reactive load, however, when testing an amplifier for power output, because the power measurement is only accurate into a purely resistive load. If you are using a reactive load, the actual output power is equal to the voltage multiplied by the current, multiplied by the cosine of the phase angle between them. Highly inductive or capacitive loads can fool you into thinking you have a higher output power than you really do, because, even though the voltage is higher, the current is not in phase with it, so the real power output cannot be accurately determined by measuring the voltage, squaring it, and dividing by the specified impedance, you must multiply by the cosine of the phase angle. A purely resistive load will have the voltage and current in phase, so the cosine of the phase angle is equal to 1, and can be ignored.
The following schematic shows how to build a variable impedance resistive dummy load: Note that this design is for informational purposes only - you are on your own if you choose to build and use these circuits and we will take no responsibility if you blow up your amp! Also, we unfortunately do not have the time to assist builders in component selection, layout, modification, or questions regarding the use of these circuits - the info is offered free of charge and is worth every penny.
You can buy this switch direct from Idec or from distis like Allied Electronics. They aren't cheap, at around $25 each, but they are nice. By the way, I think this particular switch series (called L6) is being discontinued, but there is another model (LB1S-3T6).
The enclosure is a chassis from Landfall Systems. This chassis works really well to use with an extruded heatsink. The chassis top plate is the extrusion. Landfall can make one of these for you, or something different in a similar size.
The extrusion came from Heatsink USA. They are one of the only vendors I know that will supply smaller cuts of heatsink extrusion for a reasonable price. This particular heatsink is the 7.28 wide extrusion. I ordered it cut to 10 long (which they do an excellent job of, by the way - a very clean cut with no need to do any cleanup). This extrusion works particularly well with the Landfall chassis, as the outer fins line up correctly with the mounting holes in the chassis.
Audio Dummy Load
I drilled counterbored holes through the top of the extrusion to mount it to the chassis, and drilled blind holes in the back and tapped them #8-32 to mount the power resistors. Use plenty of grease with these resistors as they are not exactly flat on the back. This is pretty easy to do if you have a drill press. Landfall might be willing to do this for you for a small fee...
Landfall also had the extrusion and chassis black anodized for me, then engraved the front with the legends you see in the photo above.
It came out beautifully. It should be able to handle 400W per channel on the 8 ohm setting, with less (200W @ 4 ohms and 125W @ 12 ohms) for some period of time. Of course it will get hot - the heatsink theta-JA is about 0.4 degrees C per watt, so it will be at about 100C with 100W/channel continuous (it would probably take an hour to heat up that much due to it's thermal mass). Far more than I'll probably ever need.A dummy load is a piece of equipment used in place of a speaker when working on or testing amplifiers. It is designed to safely dissipate the entire power of the amplifier. Dummy loads are useful for determining maximum amplifier output power and for burn-in of an amplifier under full power for several hours or days to insure reliability before the amplifier is sold to a customer, or returned after a repair.
Rat Tube (valve) Amp Dummy Load Box / Line Out Box
Reactive loads are good for testing amplifiers using global negative feedback, because they can sometimes cause a marginally stable amp to oscillate, when it won't oscillate into a purely resistive load. It is always a good idea to test into a reactive load while looking at the output on the scope and adjusting all the knobs to make sure there is no setting that can cause oscillation. If the amp does oscillate, it may need less negative feedback, or phase compensation of the loop. Click here to read a paper on designing reactive loads. You must use a non-reactive load, however, when testing an amplifier for power output, because the power measurement is only accurate into a purely resistive load. If you are using a reactive load, the actual output power is equal to the voltage multiplied by the current, multiplied by the cosine of the phase angle between them. Highly inductive or capacitive loads can fool you into thinking you have a higher output power than you really do, because, even though the voltage is higher, the current is not in phase with it, so the real power output cannot be accurately determined by measuring the voltage, squaring it, and dividing by the specified impedance, you must multiply by the cosine of the phase angle. A purely resistive load will have the voltage and current in phase, so the cosine of the phase angle is equal to 1, and can be ignored.
The following schematic shows how to build a variable impedance resistive dummy load: Note that this design is for informational purposes only - you are on your own if you choose to build and use these circuits and we will take no responsibility if you blow up your amp! Also, we unfortunately do not have the time to assist builders in component selection, layout, modification, or questions regarding the use of these circuits - the info is offered free of charge and is worth every penny.
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