Amplifier Output Compensation Resistor to Replace Speaker

Sir Kit

Joined Feb 29, 2012
95
I have a small stereo amplifier based upon the TDA2030. Only one output is being used. Instead of connecting a speaker, I would like to drive an air core coil. Since the coil provides negligible resistance, I need to protect the amp's output with a series resistance. The input signal to the amp would be an audio wave from a function generator for test purposes. In other words, not music.

What value resistor should I place in series with the coil to compensate for the loss of the speaker's 8 ohm impedance? Given that the signal is continuous, should I provide a margin of error so the amp does not overheat.

If the amp is capable of 10W should the resistor be of that value or higher? Again to provide a margin of error.

Thank you for any suggestions.

AnalogKid

Joined Aug 1, 2013
9,636
Your question is missing a lot of detail, such as the resistance and inductance of the coil, the amplitude and frequency range of the signal, the power supply voltage of the amplifier, etc.

A coil does not automatically "provide negligible resistance". The resistance depends of the gauge and length of the wire in the coil. It is fairly common for the resistance of an 8 ohm speaker to be approx. 8 ohms.

If you are dissipating 10 W in a dummy load resistor, it should be rated for at least 20 W. It still will get too hot to touch safely, but it won't set paper on fire.

If your coil is relatively small, with a DC resistance that is significantly lower than 4 ohms, then you probably should have a resistor in series with it. The sum of the coil and the fixed resistor do not have to equal exactly 4 ohms; the amp has no problem driving a higher load impedance. Note that the larger the resistor is, the less power is developed in the coil.

ak

Last edited:

Sir Kit

Joined Feb 29, 2012
95
The coil, being of unusual design, has virtually zero resistance and impedance. The resistor would thus be the entire load.

The power supply for that IC is 12VDC so the signal out would be around 9-10Vpp.

To maintain maximum power to the coil, but without overloading the amp, what is the lowest value resistor I can use? I understand speaker coil impedance is not exactly the same as DC resistance.

AnalogKid

Joined Aug 1, 2013
9,636
The lowest number on the datasheet is 4 ohms. The amp's peak output current rating is 3.5 A. max operating voltage is 36 V. Whatever you choose must not exceed any of those values. 4 ohms at 12 V is safe electrically, but the chip will get very hot.

What is unusual about the coil?

ak

Audioguru

Joined Dec 20, 2007
11,249
A coil has inductance that increases its impedance. At a very low frequency its resistance dominates and at a high frequency its impedance dominates.

Audioguru

Joined Dec 20, 2007
11,249
Why are you using the minimum allowed supply voltage of 12V? Then the TDA2030 barely works.