Amp needed for measuring speakers with laptop

[JohanH]

Hello all,

I am looking for a small DIY amp that I will use to hook up to my laptop or PC soundcard.
The reason for this is that the built in amps are not good enough (I think) to use when measuring with software as Speakerworkshop etc.

Power should probably be somewhere around 2 to 5 watts.; mono is ok. Distortion should be low etc.

My question also is: should it be constant-voltage or constant-current based?

For ease of project I would prefer a chip-amp, but I'm not sure if they are suitable for measuring loudspeakers.

Thanks very much in advance for your input.

Johan

[SY]

I mounted an LM1875 into a small Bud-box just for that use. Powered off an 12VAC wall-wart. A volume control is a good idea. My total cost was about $10.

You want constant voltage- the CC is simulated with a precision series resistor.

[JohanH]

Hello Sy,

thanks very much for your answer.

How does it cope with low impedances? I am asking because I used to have a ESL with an impedance of 0,8 ohm at about 8000 Hz that would bring most measuring equipment to its knees.

Although I don't own it anymore I would like to be able to measure loads like ESL's in the future.

Johan

[scone]

The lm1875 would have serious trouble coping with a .8 ohm impedance. All of the other national audio opamps would as well. Even a parallel design (using a 4780 or something) would crack under a load like that. Not have I've ever tried this, but looking at the spec sheets this would seem to be the case.

You might be able to do something with a series power resistor to raise the overall impedance level as seen by the amp. This is terrible for efficiency, and I'm not sure what it would do to the test results. Perhaps others can comment. It's quite possible that you'll have to settle for testing "normal" 4+ ohm loads with a simple amp based on the 1875, and will have to use a fancier one for your electrostatic experiments.

[JohanH]

After some searching through datasheet I found the TDA2003.
Supposedly it will be able to drive loads down to 1.6 ohms, so that's probably close enough for my purposes.
It does however have a poor PSRR so it would need a battery or regulated PSU.
Also some experimenting might be needed to get a decent frequency response coupled with good stability.

And a plus: there's a Velleman kit that's cheapish so I could build it.
So for a quicky it looks ok.

Johan

[SY]

At a certain point, you might just shrug and buy an extra power amp secondhand.

0.8 ohms? That's a speaker which needs some serious redesign.

[richie00boy]

0.8 ohms is plain stupid. However, if you measure using the pseudo constant current method (large series resistor) then it won't matter even if your speaker is 0.00001 ohms as far as hurting your amp is concerned. The issue will merely be getting a readable/reliable measurement.

[JohanH]

Hello Sy,

don't get me started on redesigning the ESL: its high-voltage multiplier was directly connected to the mains. Sounded nice though.

The reason for wanting to build an extra (small) amp is that I don't want to have to haul my spare integrated amp everywhere for testing loudspeakers.

But then again, maybe you're right and I am going overboard by wanting to have yet another amp,

Johan