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trwh 31st August 2002 03:52 PM

Running amps unloaded
Why should there be any problems running a solid state amplifier with no load on the output? I know tube amps need loading at all times, I think to prevent voltage spikes being fed into the output stage due to ringing in the transformer.

Some people may remember reading about my difficulties with the Velleman K4005 amp - I have since been told that the amp kept overheating because I was conducting my tests with the unit unloaded. Is this poor design or a common phenomenon? I run my P3A amp in “mute” mode (i.e. no speakers connected) for hours on end, and it doesn’t bat an eyelid. Can someone explain this more fully to me?


skaara 31st August 2002 03:56 PM

There should be absolutly no problems running solid state amp without load. tubes are the other story..

trwh 31st August 2002 04:12 PM

Yeah, 's what I thought. The technician I spoke to at my eletronics supplier justified the Velleman amp's problem by saying many professional amplifiers have circuits to simulate minimum loading. I don't remember seeing any of these circuits, and I have run several PA amps with only one channel loaded or not loaded at all (i.e. in a redundant amp in a large rack), with no difficulties. Does anybody have any further thoughts?

hugobross 31st August 2002 05:21 PM

I've never had any problems with unloaded amps before; but I've heard somewhere that older amps may have problems. I don't know if they're referring to tube amps or solid state amps.


LesW 31st August 2002 09:05 PM

Running Amps Unloaded
Running valve amps in an unloaded state can generate quite large voltages within the transformer windings, breaking down the insulation.

When I worked with auto electrics, running an alternator without a battery load usually burnt out the stator and the explanation the Lucas technicians gave was that the harmonics generated a voltage increase greater than the insulation could withstand.

Of course, transistor amplifiers are quite safe, or should be, without a load.



BeanZ 2nd September 2002 02:58 PM

The circuit in solid-state amplifiers to simulate a load in unloaded conditions is the output Zobel network. It is a resistor and a capacitor in series and put across the output of the load.


AuroraB 2nd September 2002 04:36 PM

The Zobel network is not really there to simulate the normal load, but rather to insure a proper load at freqs above 20-30 KHz, to improve HF stability.
Loudspeakers can sometimes represent a very high impedance at out-of-band higher freqs.

mcp 2nd September 2002 05:22 PM

There should not be any problems with running solid state power amps without any load connected.

But in reality, sometimes its better or even necessary.

I have comes across power amps that are marginally stable. One particular brand comes instantly to mind, a well respected name surprisingly.

With this particular amp, with a scope, one can observe clearly what resembles like little water droplets riding on the output sine wave when no load is connected.

Connect a resistive load or a speaker, it disappears.

Maybe it is due to manufacturing faults, or perhaps the design leaves a lot to be desired.

Whatever the case may be, if a solid state power amplifier needs to be connected to a load to be stable, I don't think it is a very good design, regardless of how it sounds. Sound engineering comes first.


BeanZ 2nd September 2002 05:23 PM

It in fact performs both functions. The damped inductor protects against capacitive loading and the Zobel network protects against inductive loading (i.e. high frequency ringing from an underdamped load).


trwh 2nd September 2002 05:28 PM

Thanks for the replies.

I am annoyed that the Velleman K4005 amp I built enters thermal runaway when unloaded. It seems this is not just a problem with my kit - I sent the unit back to my supplier to be checked, they tested it OK, justifying the problem by saying many amps require a speaker load for stability.

Interestingly, Velleman provides spaces on the PCB for a Zobel network, but does not include the components. I tried mounting some I had spare (10 ohm resistor, and 100nF cap), but this did not help its stability. I am now left with an amp that self destructs should the speaker lead get disconnected - ironic since the kit has protection from almost any other fault (shorted speaker lead, reverse supply polarity, etc.)

Thanks again,

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