Measuring Frequency Response - Where to connect o-scope leads?

[Paul Birkeland]

I use ARTA and an M-audio Mtrack plus MKII.  Arta is free (though we own a license) and the soundcard is less than $100.

You'll want a loading resistor to load the Stereomour.  An 8 ohm 10W is fine, but there are 8 ohm 50W loading resistors just for this purpose.

You'll need some 1/4" male to RCA female plugs for the sound card.  You'll need two mono RCA cables: 1 to feed what comes out of the sound card into the amp and the second to feed what's coming out of the amp back into the sound card.  I also recommend a 100 ohm linear pot to pad down what comes out of the amp so it doesn't overdrive the soundcard (for distortion measurements, you don't need all the signal to get accurate results).  Some clip leads and spare RCA jacks can help set all this up.

Your current scope seems adequate to measure the voltage coming out of the amp. 

[Deke609]

Thanks again, PB.  I really appreciate all the time you've taken to respond helpfully to my various posts (and many thanks to Doc and PJ too).

I already have some of the parts that you mention: I built pairs of switchable 2/4 and 8/16 ohm 50W dummy loads for the amp (and they work!); a 100 ohm stereo pot and some rca connectors.

I will look into getting the soundcard and other parts I'm missing tonight.

cheers and thanks,

Derek

[Deke609]

You'll need some 1/4" male to RCA female plugs for the sound card.

Mono to mono, or stereo to dual rca?

Thanks,

Derek

[troplin]

Thanks for the software recommendation. I looked into Room EQ Wizard - impressive set of functions.  But I do not have a loudspeaker system - I'm using the SII with Audeze LCD4 headphones, so I don't think the mic approach will work.  Not 100% sure of that though.  Maybe I could directly plug the output of the amp into my laptop.  But I think I'd like to go the traditional testing equipment route, provided it's not prohibitively expensive, so that in the future I can poke around with different components and sub-assemblies to see what they are doing -- e.g., low pass and high pass filters (something I've just been reading about and am fascinated by).

You don't have to go through speakers and microphone to use Room EQ Wizard, you can just feed the speaker level output back into the "line in" of your soundcard.
I've done that with my S.E.X. 3.0 and it worked well.

[Paul Birkeland]

Mono to mono, or stereo to dual rca?

Mono.

[Deke609]

Thanks PB.

The M-audio M-track plus MKII is no longer made, so I ordered what I am fairly certain is a functional equivalent: the "Focusrite Scarlett-2i2 Gen2 USB Audio Interface" - it has the same inputs/outputs and specs as the M-audio and, except for color, looks identical. And it is reported to play nicely with my OS, Windows 10 64 bit.

Tobias - thanks. As you can see, I've decided to mirror PB's Arta + USB audio interface setup - b/c I know it works, and if I mess it up it should be easy for PB to correct me   

cheers,

Derek