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Old 15th July 2011, 09:53 AM   #11
askii2 is offline askii2  India
diyAudio Member
Join Date: May 2011
Location: Pune
Originally Posted by coluke View Post
Well, I hope he's not going to see exactly this, of course (x-over residuals from an LM358-based unity gain follower without pull-down resistor - SVF-based auto tuning/nulling notch, stable notch depth ~100dB, post-notch gain +40dB).

Originally Posted by dchisholm View Post
Yes, if you're at all serious about making measurements then drop the LM358. The TL07x series outperform it across the board at about the same cost, and the NE5534 beats the pants off it while still falling in the "low cost" category.
LM358 is definitely off . I had read about cross-over distortion. Wasn't aware of how exactly it looks like as a difference wrt. the original.

Originally Posted by dchisholm View Post
On the other hand - if this notch filter is used as a front-end to a spectrum analyzer or wavemeter that already has a resolution of -60 dBc (or much better), then you are starting to get into the range where your measurement capability is comparable to state-of-the-art performance.
This is definitely a good approach

Originally Posted by dchisholm View Post
My fear would be that the switched-capacitor filter will add corruption to the signal and limit your measurement floor far short of where you're trying to get.
Has anyone on this forum used switched-capacitor filters for audio work? Are they so bad?

Originally Posted by dchisholm View Post
For background information, you should probably take a look at Dick Moore's page, "About THD Analyzers", as well as his home-constructed notch filter at "Active Twin-T Notch Filter". The definitive article for DIY THD analyzer construction is Bob Cordell's 1981 series, "Build a High-Performance THD Analyzer (Part1, 2, and 3)", available on the Cordell Audio website at < - Home >.
A lot of excellent reading stuff. Need to digest it over a period of time. It'll be helpful in building my own state of the art THD measurement tools. Thanks.

Last edited by askii2; 15th July 2011 at 09:54 AM. Reason: Corrected quoted references
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Old 21st August 2011, 11:24 AM   #12
askii2 is offline askii2  India
diyAudio Member
Join Date: May 2011
Location: Pune
Update - Surprisingly I found crosstalk to be the most prominent problem of my amp. While I was trying THD+N measurements as outlined above, these aren't necessary now. With a high crosstalk, It's obvious that my off-the-shelf bought amplifier needs a replacement. So now I'll immediately start working on building a my own amplifier. Read more below.

To start with I installed ARTA (free, so can't post any measurements or graphs here) on my laptop. Then to determine my measurement capability, I did ARTA's THD+N measurement by using a loopback from line out of an external USB DAC to laptop's line in. THD+N number shown by ARTA hovered around 0.3%. Since it varies a lot between 0.2% to 0.35% most of the times and shoots up to 20% at regular intervals, I concluded that 0.3% is my measurement floor. The shooting up is because of clock noise of the DAC, so that can be ignored.

If I plugged in my amplifier from line-out to line-in, ARTA's THD+N doesn't show any observable difference. So I concluded that my amplifier had to have a THD+N lower than 0.3% and can't be measured by my present setup.

I then decided to take the shorter path of measuring THD+N - {getting rid of original signal by connecting load between the two channels with one channel grounded} instead of the more difficult notch measurement. As a trial step before this, I connected two 5Ohm 20W resistances to two channel output (output and ground each), then shorted one channel input and connected ARTA signal generator to the other channel. Measurements on the channel with ARTA signal generation coincided with my previous measurements. To my surprise a spectrum measurement on the shorted channel showed a 35-40dB rise at the fundamental frequency when volume knob was turned to 12OClock position. To confirm this I repeated by connecting a speaker on the shorted channel. The speaker played the tune (and music as well when I used a CD player in place of the ARTA signal generator).

I guess this 35-40dB can be concluded as the cross-talk. The 35-40dB level stays constant with volumes higher than 12)Clock. That's a really high cross-talk and will significantly affect stereo image. At this point I have completely lost faith in my present off-the-shelf bought amplifier. Need to build my own state of the art amplifier right-away .
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