8Ω speakers, how much is reactance vs Reactance?

Thread Starter

Wendy

Joined Mar 24, 2008
23,456
So I am thinking of doing a audio project it will need an 8Ω load and it got me to thinking, is a simple resistance a good enough load for an audio amplifier or does it need more?
My thought is to start off with a high wattage 10Ω resistor and put it in parallel with a lower wattage 40Ω resistor as a test load for this amplifier.
8ohms.png
 
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crutschow

Joined Mar 14, 2008
34,671
A resistive load is normally used for audio amplifier testing, but some added series inductance would be needed if you are concerned about amplifier stability under load.
 

Ian0

Joined Aug 7, 2020
9,986
Most amplifier designs will have a Zobel network on the output, so that the circuitry will always see 8Ω at high frequency, regardless of what load is connected.
Without it, the amplifier sees a load impedance that rises with frequency when a real loudspeaker is connected. As the voltage amplifier is a transconductance stage, its gain will rise with frequency, which a recipe for instability.

As for how much reactance, 1mH is a typical value for a bass speaker, which will add 6.28jΩ at 1kHz.
 

Thread Starter

Wendy

Joined Mar 24, 2008
23,456
While I was thinking about it I looked up 8Ω resistor on Amazon and was quite surprised to find out the resistors are already out there at very high wattages.
 

MrSoftware

Joined Oct 29, 2013
2,208
A resistive load would be a worst-case test for an audio amplifier, as far as load goes. Especially at lower frequencies, the voice coil moving through the magnetic field will create back-emf, which I believe you can more or less count as higher impedance at those frequencies. Plus the voice coil will have inductance. There's a YouTube channel Wilson Audio Labs, it's about testing audio amplifiers with equipment designed specifically for that, it's not super detailed but could give you some info and entertainment.
 

Audioguru again

Joined Oct 21, 2019
6,749
Many car amplifiers use speakers that are 4 ohms and 2 ohms for high power when the supply voltage is low (12VDC).
 
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Ian0

Joined Aug 7, 2020
9,986
A resistive load would be a worst-case test for an audio amplifier, as far as load goes. Especially at lower frequencies, the voice coil moving through the magnetic field will create back-emf, which I believe you can more or less count as higher impedance at those frequencies. Plus the voice coil will have inductance. There's a YouTube channel Wilson Audio Labs, it's about testing audio amplifiers with equipment designed specifically for that, it's not super detailed but could give you some info and entertainment.
At bass resonance there is a peak in impedance, but, apart from that, the impedance is equivalent to the voice coil resistance at low frequencies. The bass resonance never gives trouble because its frequency is so low in comparison to the dominant pole frequency that it can never send the amplifier unstable.
However, it can cause trouble with any overload protection circuitry, especially a simple voltage-dependent current limit, as the current is not in phase with the voltage.
[edit]not the dominant pole frequency, but the closed-loop -3dB frequency.
 
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Ian0

Joined Aug 7, 2020
9,986
While I was thinking about it I looked up 8Ω resistor on Amazon and was quite surprised to find out the resistors are already out there at very high wattages.
My dummy load uses six 47Ω 50W resistors in parallel. 7.83Ω is probably close enough!
 

MisterBill2

Joined Jan 23, 2018
18,926
I had an 8 ohm 50 watt WW resistor, so there are lots of low resistance high power resistors around. They often get used for emitter resistors when power transistors are run in parallel.
 

Thread Starter

Wendy

Joined Mar 24, 2008
23,456
At most they will probably get around 5 watts and that is assuming my amplifier has a half decent drive.
 
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