So i want to use a 3.2 ohm speaker with my 20watt amp. The amp is switchable between 8 ohm and 16 ohm. Can i put a 4.8 ohm resistor rated at 20 watts in series with the speaker to match the impedance?
Which resistor would be ideal for such tasks? Cheers
Hello robcoxon95.
The short answer is to use a moderate power chassis mount resistor. Here is a list of suitable components.
Chassis Mount Resistors | Electronic Components Distributor DigiKey
A more complex answer recognizes that speakers are not linear like a resistor. Instead, the resistance, or more appropriately called impedance, varies as we change the frequency. There are some frequencies where the speaker is hard to drive. There are others where the naturally vibrates. For example, if you “thump” the speaker it naturally vibrates (resonates) at the frequency.
Most amplifier are designed to handle this wide range of impedance variation. They have what is known as a high damping factor. This implies that the impedance of the amplifier is significantly lower that the speaker. Consequently, the speaker impedance is irrelevant. There are exceptions. The most interesting are the single ended vacuum tube designs. you can search the term 300B for some examples. IMO, these are works of art but a bit on the expensive side.
On the other hand, not all amplifiers are built to drive low impedance speaker to full power. The power supply and output devices may not be up to the task. You are correct to proceed with caution and ask questions such as this.
I do hope that you found this information useful. You may want to reply with the model number of your amplifier and others from this community may be able to provide a direct answer.
Welcome to the DigiKey community.
Best Wishes,
APDahlen
Welcome to the forum.
I recommend a 5 ohm resistor, it’s a common value in power resistors and an extra 0.2 ohms won’t make an audible, or maybe even measurable, impact on the amplifiers output.
If you want high fidelity you will need to use a non-inductive power resistor. Most wirewound resistors have enough inductance that they really damage the audio fidelity.
To handle 20W continuous power without fans you’ll want at least a 40W rated resistor.
Here’s the only in stock part meeting those specs I could find.
Keep in mind that the resistor will reduce the power going to the speaker by almost 2/3rd’s, most of the amplifiers power will be turned into heat via the resistor.
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That’s a good call, @PaulHutch.
I had forgotten about the impact of the wire wound resistors.
That brings my recommendation of 100 down to 1.
Sincerely,
Aaron