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Topic: Ready made load resistor (Read 4607 times) previous topic - next topic
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Ready made load resistor

Dear colleagues,

I like to measure my amplifiers. I have a home made load resistor, but would like to get a proper one with nice connections and maybe a cooling fan to allow for testing of higher powered amplifiers.

Is there a place where I can buy a ready made audio load resistor that has ready connections for speaker cables? (I am not particularly good at soldering and would prefer an assembled piece.)

Many tanks for you answers.

Best,

MM

Re: Ready made load resistor

Reply #1 –
What kind of connection?  Google found this one with ring terminals.

Here  are cables with alligator clips on one end and a banana plugs on the other that could be used with a bare power-resistor.

If the resistor is over-rated enough you shouldn't need a fan, although you might want a heat sink.    I doubt you'll find one with a fan unless you find some kind of multi-purpose audio tester, simply because it's cheaper & more convenient to make something that doesn't require external power.

Re: Ready made load resistor

Reply #2 –
Quote from: mmrkaic on
Dear colleagues,

I like to measure my amplifiers. I have a home made load resistor, but would like to get a proper one with nice connections and maybe a cooling fan to allow for testing of higher powered amplifiers.

Is there a place where I can buy a ready made audio load resistor that has ready connections for speaker cables? (I am not particularly good at soldering and would prefer an assembled piece.)

Many tanks for you answers.

You can now find 2, 4, and 8 ohm 50 and 100 watt power resistors on eBay with leads and small heat sinkes attached for attractive prices. Cursory measurements suggest that they have fairly low inductance and temperature coefficients, which are the two biggest failings of cheap power resistors.  To run them at full wattage, find some simple hardware and thick sheet aluminum. Of use common shop clamps.

https://www.ebay.com/itm/50W-100W-Variation-Watt-Power-Metal-Shell-Wirewound-Resistor-0-1-500-R-J-5/192260280620 or equivalent of which there are several.

Re: Ready made load resistor

Reply #3 –
Since no-one has pointed this out, I feel it's worth a comment: a loudspeaker isn't a simple resistive load, it's a lot more complex and harder for an amplifier to drive. Depending what you are trying to measure, the resistor load will not represent reality and the results you get will tell you almost nothing about the amplifier's true performance with a loudspeaker connected.

This is not a reason to not go ahead, just a caveat.

Re: Ready made load resistor

Reply #4 –
Quote from: antz on
Since no-one has pointed this out, I feel it's worth a comment: a loudspeaker isn't a simple resistive load, it's a lot more complex and harder for an amplifier to drive. Depending what you are trying to measure, the resistor load will not represent reality and the results you get will tell you almost nothing about the amplifier's true performance with a loudspeaker connected.

This is not a reason to not go ahead, just a caveat.

Trouble is, handling that situation on the test bench is far more complex as the attachments show. 

What most audiophiles would find surprising is that in general, pure resistive loads are tougher for modern amplifiers to handle than typical speaker loads.

Sll it took to upgrade amps so they handled reactive (speaker) loads well were some tweaks to power transistor processing in the semiconductor factory, while resistive load handling takes a fair amount of iron and copper or a switchmode power supply. 

Switchmode power supplies significantly but not fully come down to similar semiconductor processing issues, and they have comparable costs at this time. Of course superstitious audiophiles hate them as a rule and few mainstream amps and AVRs have them. But the more pragmatic pro market has accepted them pretty well.

Re: Ready made load resistor

Reply #5 –
Don't bother trying to produce a heatsink to dissipate the heat from the resistor, just immerse it in water. This is a trick that I learned back in my audio repair days.

Re: Ready made load resistor

Reply #6 –
Quote from: pdq on
Don't bother trying to produce a heatsink to dissipate the heat from the resistor, just immerse it in water. This is a trick that I learned back in my audio repair days.

If you are going to immerse it in water, notice that the right voltage and wattage electric water heater element will have a low varation as it warms up, and be non-inductive. Not to mention waterproof and willing to accept a lot of power by amplifier standards as long as you keep it submerged in water.

Re: Ready made load resistor

Reply #7 –
A 1500W 120V oil-filled radiator has a resistance of 9.6 Ohms. I have no idea what its inductance would be.

Re: Ready made load resistor

Reply #9 –
Quote from: pdq on
A 1500W 120V oil-filled radiator has a resistance of 9.6 Ohms. I have no idea what its inductance would be.

BTW, makes the point that oil can be used to cool things that don't get along with water immersion, which is most regular electronic parts and anything uninsulated with more than a few volts between the conductors.  Electrolytic corrosion is very real!

With oil comes a non-trivial amount of messiness.

Re: Ready made load resistor

Reply #10 –
Many thanks for your helpful answers. If possible I would like to buy a load resistor in housing with heat sinks and speaker (banana) terminals. What I have now is a resistor of the kind that some of you linked -- a bare bones thing with ring terminals. As someone suggested, I use cables with alligator clips on one end and banana cables on the other to connect the amplifier. The contraption does not instill much confidence, probably the worst is that the contact surfaces are tiny and slippery.

Re: Ready made load resistor

Reply #11 –
Quote from: mmrkaic on
Many thanks for your helpful answers. If possible I would like to buy a load resistor in housing with heat sinks and speaker (banana) terminals. What I have now is a resistor of the kind that some of you linked -- a bare bones thing with ring terminals. As someone suggested, I use cables with alligator clips on one end and banana cables on the other to connect the amplifier. The contraption does not instill much confidence, probably the worst is that the contact surfaces are tiny and slippery.

If you google audio dummy loads, you might find some audio dummy loads for gutar amps. Some of them might apply, but they run $100s each, and generally come in just one impedance per unit so you might need 3.

Plan B might be to learn how to solder, strip wire, and cut and apply heat shrink tubing. For detailed instrcutions, see google.

If money and convenience are an issue, you can find all of these things for attractive prices on eBay and Amazon, for example.

Re: Ready made load resistor

Reply #13 –
Heathkit made dummy loads for Ham radio operators, that were placed in a paint can and filled with oil. Don't remember what kind of oil.
Kevin Graf :: aka Speedskater

Re: Ready made load resistor

Reply #14 –
Quote from: Speedskater on
Heathkit made dummy loads for Ham radio operators, that were placed in a paint can and filled with oil. Don't remember what kind of oil.

Regular household mineral oil may have a little perfume and that can cook off pretty quickly.

Motor oil is generally pretty ugly smelling.

Both can permeate seals and generally make a mess. They cool with good effectiveness, but if you really crank the power they can boil and build up pressure in potentially inasthetic ways.

I built several audio  loads this way, and found to my dismay that the resistors I used  had immense positive temperature coefficients. IME this is a bigger practical problem then modest amounts of inductance.

Re: Ready made load resistor

Reply #15 –
Many thanks for your answers. I have realized that it will be cheaper to learn soldering and other necessary techniques to make an enclosed load resistor, than to buy one for more than $3k.

Re: Ready made load resistor

Reply #17 –
Quote from: Speedskater on
So Heathkit suggested, but did not supply mineral oil. They also selected a sealed resistor.

https://www.orcadxcc.org/content/cantenna_va7jw.pdf


The amp load resistors I've built lately were based on metal-cased resistors that were mounted on heat sinks and forced-air cooled using fans obtained from PC sources.  I previously built a few in 1-gallon paint cans but tired of messing with oil.