1
\$\begingroup\$

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.

\$\endgroup\$
  • \$\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
4
\$\begingroup\$

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.

\$\endgroup\$
  • \$\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
  • 1
    \$\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
0
\$\begingroup\$

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.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.