Hi all

Does anyone know how to check the amp speaker output using software? Not just the power, but the signal?

On another thread there is a discussion about replacing caps. Someone mentioned that everyone seemed to use their "ears" to test the sound and said that there should be a way to test the speaker output before and after making modifications.

Good point. Other than your ears, how do you know you actually improved anything? Can you really remember what it sounded like before you started swapping components? Probably not.

So, that go me thinking about oscillisopes, until I saw their prices. That in turn got me thinking about all of the computer based sound analysis tools that are available for pretty cheap (like the kids are using for home studios).

Does anyone know how to check the amp speaker output using software? If I could do that, I could write the file to hard disk and then compare it after I made changes. What software hardware setup would be required? Thinking out loud, I would probably want to build a dummy 8 ohm load or leave the speakers connected so as to have something to soak up the power.

Any ideas?

Thanks
-DEL

Sounds like you're on the right track. I have seen analyzer software that interfaces with the compute thorugh the sound card. You will indeed need to soak up the power, and you may even want to use a reactive load. Capacitive and inductively reactive and both at the same time.
cheers,
Douglas

Hi Del,

Speaker workshop can be used to do exactly what you describe. I built a test jig for testing amps. It is basically a load bank with a resistor ladder divider in parallel which supplies the voltage input to the sound card. You have to be careful to design your resistor ladder so that the amplifier output is divided down enough so that it does not harm your sound card! Sound cards typically have a usable input voltage of 0 ~ 2v. Any more and it will fry.

The software allows you to do frequency response graphs and tests for harmonic distortion. While this is all well and good, you were very accurate in saying that you need your ears. I have found it very difficult to do an objective measurement of what "sounds" good. People have been arguing about this for the last century, and I don't see a resolution coming any time soon. The absolute best way to determine what cap sounds the best is to make a test setup where you can quickly A/B test the amplifier. In other words, change the cap on one channel only, and then have the ability to quickly change from the original to the modified channel while you are listening. Most people "forget" what something sounds like within a few minutes, so even going back to bench and changing a cap and coming right back is too long. I know that my own audible memory is very short, and I'm never 100% confident that I can tell a difference unless I can immediately do an A/B comparison.

Mark

You can use RMAA to test your amplifier with your PC sound card. It requires two cables that you can buy or build yourself in 15 minutes. The software is free for download.
It will tell you bandwidth, harmonic distortion, signal-to-noise ratio and intermodulation distortion.

You can use RMAA to test your amplifier with your PC sound card. It requires two cables that you can buy or build yourself in 15 minutes. The software is free for download.
It will tell you bandwidth, harmonic distortion, signal-to-noise ratio and intermodulation distortion.


RMAA is another good one. You will still need a power soak and divider if you intend to test at full volume however.

Mark

Wow,

Thanks for the answers. I didn't really expect such good info so quickly.

Now, where can I find plans for a divider and power soak?

Thanks
-DEL

Wow,

Thanks for the answers. I didn't really expect such good info so quickly.

Now, where can I find plans for a divider and power soak?

Thanks
-DEL
Hi dlowrey,
The divider is just two 1W resistors per channel. This is required to test your amplifier at anything above 1W output level and up to approx. 20W. Use a 910 Ohm resistor and a 100 Ohm resistor for a (approx.) 10:1 divider. To test for higher power you need a 50:1 divider or so, to avoid saturating the sound card input.
See here: http://en.wikipedia.org/wiki/Voltage_divider
For the dummy load you can use 8 x 1 Ohm 25W resistors in series, properly attached to a heatsink (per channel), and possibly with an inexpensive 80mm fan blowing on the heatsinks to cool them down..

Hey Gigapod,
Thanks for the info.

-DEL

Gigapod hit it right on. Also, Parts Express (link on this site) sells a single 8 ohm 100 watt resistor that makes an excellent power soak.

Mark