I've looked around for a while, and my question appears to be very specific and I was hoping that someone who knew more about audio amplifiers could help.

I'm doing some research assistance where sound sources with impedances from 20-40 Ohms are going to need to be driven at 20W. Based on some Googling I've done, this appears to be very specialized. Even very expensive amplifiers are only rated with 4 to 8 Ohm load speakers. The amplifiers need to be suitable for audio (passband from maybe 100Hz to about 25kHz), and I've found nothing so far.

My question boils down to this: does anyone know where I might find such an amplifier, or whether I need to construct my own? I'd like to spend less than $100 on the amplifier, which may be impossible.

  • \$\begingroup\$ You can also use a pair of more modest amplifiers in bridge mode to double the available voltage, without having to worry about their drive current rating, as each amp will see a 10-20 ohm load. \$\endgroup\$ – Brian Drummond Feb 18 '16 at 17:16
  • \$\begingroup\$ @Brian: answers.yahoo.com/question/… I did a quick search on how to bridge two separate amplifiers and found this immediately. Do you think it applies here? \$\endgroup\$ – xcnmoore Feb 18 '16 at 17:41
  • \$\begingroup\$ If you pick the wrong amplifiers it might. Bridging two channels of a stereo amp (shared ground) would be fine : you just need a way to invert the signal to feed the second amp. \$\endgroup\$ – Brian Drummond Feb 18 '16 at 19:31

An ordinary audio power amplifier should be able to handle your application. For example, an amplifier rated for 100 watts into 8 ohms has to supply an output voltage of about 28 VRMS. This voltage, with a 40 ohm load, will develop 20 watts. With a 20 ohm load, it could provide 40 watts. There should be no problem using a higher impedance load with typical audio power amplifiers. The only drawback is that you need an amplifier with a higher power rating than you need for your load.

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  • \$\begingroup\$ I'd Like to add on that it needs to NOT be a Class-D amp (i.e. He should find one that's a Class-AB). But I like your answer otherwise. \$\endgroup\$ – Dave Feb 18 '16 at 17:12
  • \$\begingroup\$ @Dave: What's wrong with using a class-D amplifier? \$\endgroup\$ – Dave Tweed Feb 18 '16 at 17:32
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    \$\begingroup\$ @DaveTweed, In a Class-D amplifier, the load is part of the LC filter on the output of the switching transistors. Changing the load will result in changes to frequency response. Moving a few % up or down doesn't do much harm, but 20-40 ohms is around 300-500% different. At those limits the high frequency response of the filter will actually pass more harmonic content and may destabilize the amplifier. \$\endgroup\$ – Dave Feb 18 '16 at 17:39
  • \$\begingroup\$ @Barry Exactly 20W into 40 Ohms for an amp rated at 100W into 8 Ohms no less! Thank you. I did some basic calculations and was thinking that maybe a regular amplifier would work, but wasn't sure if I was missing some intricacies of the circuitry. \$\endgroup\$ – xcnmoore Feb 18 '16 at 17:47
  • \$\begingroup\$ @Dave Thanks! I'll avoid Class-D and look for Class-AB. \$\endgroup\$ – xcnmoore Feb 18 '16 at 17:48

Shouldn't be a problem, You can drive a 40 ohm source with an an amp specified for 8 ohms... you just need to make sure you have enough voltage. V^2/R = 20 Watts.

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