2 ohm dummy load for QSC/ Behringer amps calibration

[diydidi]

Hi Guys. J
ust wondering what the hell you guys are using for the calibration of the "pos/neg current limits" on these amps?
Cheesh, a 2ohm load rated at 1200W min...
Service manual says to connect it to adjust for symmetrical clipping. My loads are all rated for 800 watts or so. Besides with my two 8ohm resistor banks, I wont get down to 2ohms.

[soundguruman]

If your hooking a Behringer up to a 2 ohm load...better buy a fire extinguisher.

[R.G.]

Fire safety aside, you just need to get enough watts and ohms.

Here's a decent start: 2 ohm 120W Resistors $5.95

Four of these in series/parallel gets you 2 ohms at 420W. Four of those clusters in series/parallel gets you two ohms, 1920W. So 16 resistors at $6 each, $96 for the resistor array.

A USA 120Vac hair drier is roughly 8 ohms/1.5kW. Four of those in parallel would work. For any precision, you'd need to strip out the resistance wire and shorten it as needed to get to actually 2 ohms. But I suspect that precision isn't needed as much as about the right resistance and at least enough power.

[km6xz]

The heating element in a water heater is about 16 ohms and they cost about $11 at Cosco. Being water cooled 8 of them would give 2 ohms at 36KW dissipation. If it is a digital amp, don't test for full power RMS, test with a 4-8 cycle burst or you will not get a value that makes sense in regard to the spec sheet.

[nickb]

I regularly use a 100ms 1KHz sine burst at 1Hz (or less) to keep the average power in the load manageable. Of course you'll need a storage 'scope..

[Chuck H]

+1 on the water heater elements. Using Ohms law it's easy to calculate, there isn't much derate with temperature (when used in a water medium) and the bath is available from your municipal water tap!

I've never actually done it myself but it occurred to me because I seem to fix a lot of water heaters and noticed the specs on the heating elements.

[Jazz P Bass]

Here is a water heater element resistance chart.

[diydidi]

The heating element in a water heater is about 16 ohms and they cost about $11 at Cosco. Being water cooled 8 of them would give 2 ohms at 36KW dissipation. If it is a digital amp, don't test for full power RMS, test with a 4-8 cycle burst or you will not get a value that makes sense in regard to the spec sheet.

Could you and NickB elaborate on testing digital amps in this way. I have an Arbitrary function generator and would like to explore this capability.

[tedmich]

check this guys rig!
Big Dummy Load « Synergetic Audio Concepts

[g1]

Not sure about the used motor oil part, is it conductive? Make sure to wear rubber gloves!

[tedmich]

hydrocarbons are totally non-conductive, like plastics. They are flammable though, thus the thermometer to see if a splosion is eminent.

I'd have used antifreeze, which I thought he was doing. The need to make durable insulation oil was what brought us PCB flavored hydrocarbon oils for the next few cancer enhanced millennia.

[nickb]

Could you and NickB elaborate on testing digital amps in this way. I have an Arbitrary function generator and would like to explore this capability.

Stan is talking specifically about testing (some) 'digital' amps. I think I know why the short burst but I'll let him speak for himself.

I'm talking about testing all amps. I often use a 100ms long sine wave burst at chosen frequency followed by 900ms of silence and loop it. Examine the output skipping the first few transient cycles using a 'scope to check clean and measure to pk-pk. Use pk-pk to determine power out. The average power dissipated in the dummy load is 1/10th of the continuous method. Of course, other duty cycles will work just as well. It's fairly arbitrary. Another advantage is you can see the effect of power supply sag. If you pick near the start of the burst and you measure max possible power out. Near the end and you measure max power after sag.