• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

How to measure frequency response with a PC?

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zafira1981

Member
Joined 2004
How to measure frequency responce with pc

Hi people,

I'm trying to measure the F.R. of my tube amp (http://www.diyaudio.com/forums/showthread.php?threadid=97002) and I cant make TrueRTA work.

I tried with NVIDIA SoundStorm sound card an Sound Blaster Audigy 2 ZS. I cant calibrate the sound card to have a plain frequency response to start measurements.

I don't know what is my mistake.

Sound Controls (both cards tested):
recording: line input selected, at MAXIUM
playback: wave and master at maxium, line input muted (To avoid oscillations)
Loop cable conected between line in and line out.
I also checked and no "audio effect" is ON.

Any clue? Any other program to use?
 
Z

zafira1981

Member
Joined 2004
Gold_xyz said:
Hi :D

Try an oscilloscope/spectrum analyzer from this site
http://digilander.libero.it/hsoft/

Link download : http://digilander.libero.it/hsoft/VA.zip
(with continous spectrum :up: )
Click to expand...


Thanks for your reply!!!!

I've downloaded the program. I'm trying to calibrate it. Wave generator is configured as Sweep. After a few loops, the graph shows the frequency response of the sound card:

An externally hosted image should be here but it was not working when we last tested it.


Is there any way to "flat" it to correct the response of the sound card?

thanks
Jorge.
 
S

Svein_B

Member
Joined 2005
I often use ARTA, the speaker measurement program also for amplifier measurements.

- The Spectrum analyser is nice for measuring harmonic distortion, and it has built in tone generators.
- Compensation files can be loaded.
- FR measurement is OK, but most PC's fall short here with a limit of 20KHz. The same can be said for usefulness of any soundcard based oscilloscope.


Svein B.
 
C

Conrad Hoffman

Member
Joined 2007
conradhoffman.com
Zafira, I like VA quite a lot. If your sound card can sample higher than 44.1khz, you can test higher than 20khz. My inexpensive E-mu 0202 USB box supposedly can go to 192khz, though I haven't pushed it that far. Tests at 40khz (96khz sample) and more seem fine. I notice that your level is -60 db in the graph you attached. That's way down there- check your levels and 0dB setting. You might also find it interesting to use the pink noise source and set the analyzer for 1/3 octave or finer measurements. Handy when you move beyond the amp to the speakers.
 
kevinkr

kevinkr

Administrator
Joined 2004
kta-hifi.net
Paid Member
I believe if you are using line in and out and doing a loop back test the line input needs to be unmuted - that is where the signal you are trying to measure is connected to.

There should be no oscillations with loopback connection and both input and outputs enabled.

Note that depending on level settings and sound card you may clip either the input or output, this should be fairly obvious if you get high thd numbers.. I generally run at no more than 90% of digital FS to get good results.

I use Arta and Audio Tester.
 
Z

zafira1981

Member
Joined 2004
Well I did a measurement. I don't know if I'ts OK.

EF86+12AT7+2xKT88-PP at 60W RMS power output (Vo=13.4 , RL = 6 ohms)
obviously, the value of the DB in the graph is wrong.

10K/100R feedback resistor (40dB :eek: ) , no feedback capacitor (I dont have a good square source to adjust it)
An externally hosted image should be here but it was not working when we last tested it.


THD
An externally hosted image should be here but it was not working when we last tested it.



what do you think?

I will do the same test with 1W RMS.
 
Brian Beck

Brian Beck

Member
Joined 2004
The regularly-spaced bumps in the frequency response above about 500Hz are highly unlikely to be from your amp. This looks like a digital artifact to my eyes. It suggests that there is a delayed version of the signal being added to the signal, creating the characteristic picket-fence response peaks and dips. This is probably due to a digital filter (which uses tapped delays) some where in your measurement chain. It would be good to get rid of it by subtracting out the loopback response. Have you done that?
 
SY

SY

Ex-Moderator
Joined 2002
SYclotron.com
I saw the last two or three posts before reading the first one and immediately said to myself, "Audigy 2Zs." That card, though having very good hardware, has terrible drivers and interface. This ripple is an issue of settings (I think that's covered in some bit of Audigy literature), but even moreso, an issue with that card and users who don't want to become experts in Audigy quirks.

I switched to an M-Audio 192 about a year ago and the hours I used to spend wrestling with the mixer settings have gone away completely.
 
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