Is it bad to start an amp without a load?

"Ok, so when testing a 40wpc stereo audio amp, does each 8 ohm speaker input get connected to a separate 8 ohm, >40watt dummy load? do you test o with both speaker inputs connected the dummy loads or one speaker at a time?"

To be clear they are are speaker outputs. This will help you keep a clear notion of the flow of the power.

Each speaker output channel, left and right, needs to have its own, separate dummy load resistor(s) connected to the respective channel.

The speaker outputs remain separate, each speaker output is connected to it own dummy load resistor(s). Both channels need a dummy load connected to them during testing.

jegor said:

Created for testing amplifiers.

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2 x Papst 80x80mm

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Is that a heat sink salvaged from a car amplifier? You've actually given me an idea. I have several dead car amps here that I'm using for parts donors. No reason one or two can't donate a nice big finned aluminum case to build a piece of test gear. My current dummy load has good power capacity, but its an odd resistance and they are inductive resistors.

gadget73 said:

Is that a heat sink salvaged from a car amplifier? .

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Nope, from old server PSU.

The Dale rules.

Ok, but to be clear on the connections and testing protocol requirements. Do both channels need to be connected to their respective dummy loads AND operating or can I just test one channel at a time? Also, I'm still not clear on where to connect the leads on the amp side or how to determine the appropriate ohm rating for the dummy load. My MC240 amp has options for 2,4,8, 16 ohm terminals.

Perkinsman said:

Ok, but to be clear on the connections and testing protocol requirements. Do both channels need to be connected to their respective dummy loads AND operating or can I just test one channel at a time? Also, I'm still not clear on where to connect the leads on the amp side or how to determine the appropriate ohm rating for the dummy load. My MC240 amp has options for 2,4,8, 16 ohm terminals.

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I suggest to have both channels loaded. Use whatever impedance you want to test. Match load with intended output.

nashvillebill said:

...This is clipping--the amp can't deliver any more current so the tops and bottoms get clipped off the signal.

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FWIW, isn't necessarily current, could be voltage too...and generally is at higher impedance. At lower impedance it may be current limited, most will be.

Sorta six of one half-dozen of another though. The amp is "all in" when it's clipping, regardless of the reason...as you know.

Perkinsman said:

Ok, but to be clear on the connections and testing protocol requirements. Do both channels need to be connected to their respective dummy loads AND operating or can I just test one channel at a time? Also, I'm still not clear on where to connect the leads on the amp side or how to determine the appropriate ohm rating for the dummy load. My MC240 amp has options for 2,4,8, 16 ohm terminals.

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If you look at the MC240 schematic, there are multiple taps on the output transformers. Labeled 16 ohm, 8 ohm, 4 ohm and com. If you don't have a schem you should find one soon. Check my thread on my rebuild. I linked to a high res version I created from my original that can be printed to D sized.

With the MC240, you attach the speakers to an appropriate tap. IE, if you have an 8 ohm speaker you attach to Com and 8 ohm. If you have a 4 ohm speaker, you attach to Com and 4 ohm. Although you can switch the amp to two channel mode, to run each side independently, you can't disable one side or the other. Each side will need a load if you are going to power it up or you risk damaging the transformer / tubes on the unloaded side. So if you buy the resistors we talked about the other day, 8 ohm/100watt, you need one for each side and you need to attach them to the com/8ohm terminals as if they were speakers.

To do the testing that is being discussed here, you need a scope and a signal generator. I don't have these and won't be buying them unless I have issues to diagnose. I'm using a load resistor on initial heat up because I'm worried that one or both output transformers may be shorted and I'd rather risk a $5 resistor than a speaker. I wouldn't leave the resistor under power for all that long after reading this thread. It should be fine but folks here are building large dummy loads to comp for the heat. I suspect I'll be able to cook with these if I leave them under load too long.

DC said:

The formula for resistors in parallel is Rtotal = R1×R2/(R1+R2). An 8 Ohm and 4 Ohm resistor in parallel has an impedance of 2.67 Ohms.

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I learned it as:
1/R total = 1/R1 + 1/R2 + ... for as many resistors as are in that particular circuit.

GuyK said:

I learned it as:
1/R total = 1/R1 + 1/R2 + ... for as many resistors as are in that particular circuit.

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Yeah, I should have said: The formula for two resistors in parallel is Rtotal = R1×R2/(R1+R2).

The reciprocal formula works with multiple resistors, of course.