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Old 8th April 2005, 02:56 PM   #1
BWRX is offline BWRX  United States
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Default Stock SI spectrum analysis

Hello everyone. Last night I did some sonic impact testing with a dc bench supply, signal generator, oscilloscope, and dynamic signal analyzer. i used an 8ohm 20W resistor as the dummy load on the left channel, had the supply set to 12V, set my laptop to output a 1Vrms 1kHz sine wave to the SI input (verified with the oscilloscope), and hooked up the dynamic signal analyzer to the leads of the dummy load. i also set my camera up right in front of the signal analyzer and took shots of the screen for a bunch of settings. The first photo is with the sine wave off and the amp is just turned on (you know how the stock volume knob clicks when it's turned on, right after it clicks). The second photo is with the sine wave on and the amp is still only turned on. The third photo is with the output adjusted to 59mVrms. The fourth photo is with the output adjusted to 108mVrms. The photos after that are with the output adjusted in approximately 100mV intervals up to 3.793Vrms. My camera battery died or I would have kept going

http://www.personal.psu.edu/users/b/...kHzTHDtest.mov

At 1Vrms out it drew 83mA from the supply, 2Vrms - 137mA, 3Vrms - 210mA, 4Vrms - 300mA, 6Vrms - 538mA, 7Vrms - 689mA. 7Vrms was a little before the onset of clipping (volume knob a little before the 1 o'clock position). Higher output voltages resulted in a dramatic increase in current draw from the supply, and the noise spectrum from about 13kHz and up increased dramatically as well. The harmonics of the 1kHz tone were clearly defined on the screen and had a very sizable amplitude compared to the level of the fundamental.

So, from my observations so far, around 7W into 8ohms ( (Vrms*Vrms)/Load ) the distortion begins its ascent to the heavens. Anyone know how to determine THD from the series of images in the movie?
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Old 8th April 2005, 06:15 PM   #2
BWRX is offline BWRX  United States
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I just ran some calculations to find THD using dBV values I picked off the 2.805Vrms output photo (corresponds to approximately 1W into 8ohm load). Assuming I used the correct values (I can't be that far off), I calculated the THD to be 0.000007% at 1W into 8ohms. Assuming the noise (+N) is around the -90dBV level, the THD+N is practically the same value. Does this seem right?

I used the formula:
%THD+N=[sqrt(H2^2+...+H20^2+N^2)/sqrt(H1^2+H2^2+...+H20^2+N^2)]*100
and you just remove the N (noise power) terms for %THD.

H1=fundamental harmonic power level in Vrms
H2=second harmonic power level in Vrms
...
H20=twentieth harmonic power level in Vrms
N=noise power level in Vrms

Was I correct to assume that I can read a harmonic's dBV value from the photo, calculate its Vrms value, and use H1=(calculated Vrms value from dBV)^2/8 to get the harmonic's power level?
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Old 8th April 2005, 06:26 PM   #3
tiroth is offline tiroth  United States
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Something got screwed up there. I can assure you that neither your analyzer nor the SI has -143dB THD!

You could use this little javascript to figure it out easily.
http://www.anidian.com/audio/misc/calc.shtml
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Old 8th April 2005, 06:55 PM   #4
BWRX is offline BWRX  United States
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Thanks tiroth. I figured my calculations were incorrect because of the extremely low number

h1=8.96dB (fundamental)
h2=-80
h3=-63
h4=-85
h5=-77
h6=-87
h7=-77
h8=-79
h9=-70
h10=-75

Those are the values I got from the photo and are relative to 0dB.

Using the values I obtained from the photo and calculating the dB relative to the fundamental, the program you linked to returned a %THD of 0.02983268246740213. That looks much more realistic. Interesting considering the Tripath app note shows that their EVB has less than 0.02% THD at 1W (no load specified).

Do you (or anyone else) know what I did wrong in my calculations?
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Old 8th April 2005, 10:44 PM   #5
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Don't forget that the output caps on the SI are optimized for a 6 ohm load rather than an 8 ohm load. I would guess that changing the values for an 8 ohm load might improve your results slightly.

Then again, I have no idea about THD testing or anything else on what you're doing here.. Just taking a stab in the dark
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Old 8th April 2005, 11:46 PM   #6
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Default Re: Stock SI spectrum analysis

I want to use the video as a screen saver.
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Old 9th April 2005, 12:30 AM   #7
Pano is offline Pano  United States
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Nice work, BWRX.
What are you using for spectrum analysis? How far up can it go?

The reason I ask is that it would be nice to find a way to look at the results of modifying the output filtering. Having a look up to ~3Mhz would should show how well the filters are working.

It would also be nice to to look at the 100Khz to 1Mhz range to see what is going on with the "spread spectrum" switching. Unfortunately, this rage above what most audio analyzer do, and below the radio spectrum analyzers.

Can you measure in that range? Sure would be nice to see how much ultrasonic power is getting through the filters. It could help filter design to know what frequencies are present, and what can be done about them.

Just for fun, hook up your o'scope in XY mode across one channel output and look at the phase. It sure goes through some wild gyrations over the audio band.

My measurements and calculations showed that the amp was capable of about 5W RMS at the unset of "ringing"*. Once the amp starts to really clip, power draw goes way up, as you note, and the waveform tops flatten out completely. That's why the chip draws so much more power.

Thanks again for the cool analysis. With the right tools and the right guys, this amp will have no secrets left!

*digging around in the Tripath docs, what appears as "ringing" on the tops of waveforms is a "feature" of the amp. It's the Tripath version of soft clipping. Supposed to be inaudible. Hmm...
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Old 9th April 2005, 01:24 AM   #8
BWRX is offline BWRX  United States
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Quote:
Originally posted by panomaniac
Nice work, BWRX.
What are you using for spectrum analysis? How far up can it go?

The reason I ask is that it would be nice to find a way to look at the results of modifying the output filtering. Having a look up to ~3Mhz would should show how well the filters are working.

It would also be nice to to look at the 100Khz to 1Mhz range to see what is going on with the "spread spectrum" switching. Unfortunately, this rage above what most audio analyzer do, and below the radio spectrum analyzers.

Can you measure in that range? Sure would be nice to see how much ultrasonic power is getting through the filters. It could help filter design to know what frequencies are present, and what can be done about them.
I'm using a Hewlett Packard 3561A dynamic signal analyzer. Unfortunately, it can only go up to 100kHz. It would be nice to see what's going on up there, but I doubt the university has any that can handle that frequency range.

Quote:
Originally posted by panomaniac
My measurements and calculations showed that the amp was capable of about 5W RMS at the unset of "ringing"*. Once the amp starts to really clip, power draw goes way up, as you note, and the waveform tops flatten out completely. That's why the chip draws so much more power.

Thanks again for the cool analysis. With the right tools and the right guys, this amp will have no secrets left!

*digging around in the Tripath docs, what appears as "ringing" on the tops of waveforms is a "feature" of the amp. It's the Tripath version of soft clipping. Supposed to be inaudible. Hmm...
Yessir, the clipping could easily drive the chip into thermal protection mode if allowed to play for long enough, especially without any heatsink. I have to take their inaudible "ringing" claim with a grain of salt, but I can't hear anything above 17kHz anyway so as long it's just that inaudible "ringing" on the waveform peaks and doesn't distort the audible waveform then it won't bother me much My speakers are ~90dB so I never have to turn it up that loud anyway.

And thank you for sharing your modding experiences as well. That's what makes DIY hobbies so much fun.
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Old 9th April 2005, 02:03 AM   #9
BWRX is offline BWRX  United States
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i just took a few photos of the output up to 100kHz with a 1Vrms sine wave input. i will do intermodulation distortion testing but i can't find any appropriate resistors at the moment and i don't have enough cables to hook up two signal generators and the signal analyzer. any other suggestions/requests for what frequencies i should test at other than 1kHz?

i also wanted to let everyone know that the LT1083 is an excellent low dropout regulator. today it was tested for load transient response and line transient response and the output voltage deviation for both tests was about twice as good (half as much deviation) as the dc bench supplies available in the lab! certainly more than adequate power for a single SI board.
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Old 9th April 2005, 04:00 PM   #10
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Quote:
Originally posted by BWRX


I'm using a Hewlett Packard 3561A dynamic signal analyzer. Unfortunately, it can only go up to 100kHz. It would be nice to see what's going on up there, but I doubt the university has any that can handle that frequency range.
If you're at Penn State I am sure you can find an HP3585A there -- 20Hz to 40MHz -- many of the rf analyzers start at 9kHz.

the inaudible (perhaps) 20kHz to 200kHz region is important for examining amplifier stability., at least in analog land.

you might also find that using an analog 1kHz "notch filter" ahead of the analyzer input will allow you more precision in your measurements.
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