If I want to use a consumer stereo amplifier to drive a purely resistive load, rather than a speaker, what factors should be considered avoid damage to the output stage?
How can I maximize power output without choosing a too small value resistor?
Richard Keller
The rated minimum load resistance - assuming it's a solid-state power amplifier.
By using the rated minimum load resistance, of course!
Chris
Just select R load that is equal to what the amp is rated for. 4 or 2 ohms for example.
And make sure the load can handle the power and also, many consumer amps will not operate at max level for long periods. You may want to check that in the spec's of the amp.
Other then a dummy load, what could be a purely resistive load?
Keep in mind that the "rated audio power at rated resistance" of the amplifier could mean absolutely nothing. You can probably count on sustaining of no more then 1/10 of so-called rated power.
Vladimir Vassilevsky DSP and Mixed Signal Design Consultant
-- An incandescent lamp? news:aje0g5pg8caolcojp3f8hgg2ajmlslq3s0@4ax.com JF
Incandescent lamp is very much nonlinear resistance. It is a tough load to drive with an audio signal, actually good for amplifier reliability testing.
VLV
Isn't the loading also dependent on frequency in accord with speaker impedance?
What if the single audio frequency I want to apply is in the lower end of the audio range, say 60Hz, and steady not intermittent like music?
Do I need a higher value resistor to compensate?
Richard Keller
"Richard Keller"
** Speaker impedances are all over the place - some are even resistive at all audio frequencies. ** No problem.But running at full power continuously will likely oveheat the transformer.
Running at or near half power will overheat the output transistors.
.... Phil
"Vladimir Vassilevsky"
** A very cynical and false view.** A very pessimistic view, completely without justification.
Domestic stereo amps can normally deliver their rated power figures for long periods, probably indefinitely if only one channel is driven.
..... Phil
That depends on the rating, and on the type of amplifier.
The standard used in the U.S. for rating home-audio power amplifiers, for many years (starting in 1974, believe) was set by the FTC. The rule required that the amplifier be able to deliver and sustain full rated power into a resistive load (usually 8 ohms although it can also be the load "for which the amplifer was primarily designed") with no more than 2% total harmonic distortion.
The rule also required that this test be performed after a warmup period, in which the amp was operating at a lower output power level. My recollection is that the original standard was for a preconditioning for an hour at 1/3 of the rated power output level... this is actually pretty close to a worst-case for traditional Class AB audio amplifier, as it results in high heat dissipation in the final amplifier stage. The current standard requires preconditioning at 1/8 of the full rated power level.
After preconditioning, the actual maximum-power test requires that the specified power level and distortion limits must apply, at all frequencies being tested, for a minimum of five minutes of operation.
You *should* be able to count on a home audio amp delivering its rated power into 8 ohms, although if you want to run this continuously you may need to provide for additional cooling (e.g. fan airflow onto the heatsink fins). If you run it in a closed cabinet with no external venting, it'll almost certainly overheat... might shut down safely, might self-destruct.
Many home-audio amps can deliver greater power into lower loads than they're rated for. How much you can get, into what resistance, is going to depend a lot on the amp topology, and on details such as the powerline voltage and on whether you're providing additional cooling.
With car-audio amplifiers and "active speaker" amplifiers, or amps not rated according to the U.S. standards, it's anyone's guess what you'll get. The power "ratings" are often very inflated and unrealistic.
--
Dave Platt AE6EO
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