SI Distortion Spectrum
I got a couple of these when they first appeared hoping to use them for portable speaker measurements. They work for most but not all of the measurement needs. The distortion is higher that the best speakers, the output impedance is higer than I would like and I have a few other complaints, but they are convienent. I have not had the drive to hassle modifing them. The packaging is too odd.
Here is a high res distortion spectrum of a stock Sonic Impact powered from a 15V 2A switching supply. Source 500 Hz at just below clipping junk. Load 47 Ohms.
http://pws.prserv.net/demian/demian/SonicImpactDistortionSpectrum.JPG
I looked at the spectrum out to 2 MHz with another analyzer and there really isn't a lot there. The LP filters do seem to work.
The measured distortion on a conventional analyzer isn't very low at around .21 -.25% And its dominated by noise and higher harmonics.
Hope this helps.
I got a couple of these when they first appeared hoping to use them for portable speaker measurements. They work for most but not all of the measurement needs. The distortion is higher that the best speakers, the output impedance is higer than I would like and I have a few other complaints, but they are convienent. I have not had the drive to hassle modifing them. The packaging is too odd.
Here is a high res distortion spectrum of a stock Sonic Impact powered from a 15V 2A switching supply. Source 500 Hz at just below clipping junk. Load 47 Ohms.
http://pws.prserv.net/demian/demian/SonicImpactDistortionSpectrum.JPG
I looked at the spectrum out to 2 MHz with another analyzer and there really isn't a lot there. The LP filters do seem to work.
The measured distortion on a conventional analyzer isn't very low at around .21 -.25% And its dominated by noise and higher harmonics.
Hope this helps.
More on SI distortion
I thought I would look for the distortion vs. level distortion. Here are the numbers, 500Hz, 8 Ohm, 400 Hz to 30 KHz-
8.1V .54%
4V .8%
2V .47%
1V .63
.5V .28%
.25V .34%
.125V -50 dB (I had to switch to Sinad, with the low signal)
.062V -44 dB
.031V -45 dB
.015V -47 dB
The distortion is really pretty high by conventional terms at all levels. And its visible as waveform distortion in the monitor output. There are speakers that are significantly better.
-Demian
I thought I would look for the distortion vs. level distortion. Here are the numbers, 500Hz, 8 Ohm, 400 Hz to 30 KHz-
8.1V .54%
4V .8%
2V .47%
1V .63
.5V .28%
.25V .34%
.125V -50 dB (I had to switch to Sinad, with the low signal)
.062V -44 dB
.031V -45 dB
.015V -47 dB
The distortion is really pretty high by conventional terms at all levels. And its visible as waveform distortion in the monitor output. There are speakers that are significantly better.
-Demian
Re: SI Distortion Spectrum
My first question is why would you use a load of 47 ohms? The output filter is designed for 6 ohms, and works fairly well for both 4 and 8 ohm loads. You said you measured the distortion (THD+N) with a conventional analyzer up to 30kHz but at what output power? I haven't been able to test the stock or modified SI on a signal analyzer with a bandwidth up to at least 1MHz, but my THD calculations were on par with (actually slightly worse for higher power) the SI specs for a bandwidth of 50Hz-20kHz.
Also, the link to the picture does not work. Is that an error with my browser or is the address incorrect. Sometimes the internet at my apartment is very tempermental. I'm interested in seeing the results you obtained.
1audio said:
My first question is why would you use a load of 47 ohms? The output filter is designed for 6 ohms, and works fairly well for both 4 and 8 ohm loads. You said you measured the distortion (THD+N) with a conventional analyzer up to 30kHz but at what output power? I haven't been able to test the stock or modified SI on a signal analyzer with a bandwidth up to at least 1MHz, but my THD calculations were on par with (actually slightly worse for higher power) the SI specs for a bandwidth of 50Hz-20kHz.
Also, the link to the picture does not work. Is that an error with my browser or is the address incorrect. Sometimes the internet at my apartment is very tempermental. I'm interested in seeing the results you obtained.
Distortion Spectrum
I used a 47 Ohm resistor for convienence. I just replaced the image with a new one with a 4 Ohm load and more documentation.
http://pws.prserv.net/demian/demian/SonicImpactDistortionSpectrum.JPG
And here is a picture of the spectrum to 5 MHz
http://pws.prserv.net/demian/demian/SISpectrum.JPG
The link works on two computers here, but there can always be a hidden issue. Let me know if there is still a problem.
The distortion is about -70 dB for 2nd and 3rd harmonic. Or around .3% at 7.5 V RMS into 4 Ohms or 14 W. Measuring at 12 W gives essentially the same results.
At 3 watts the distortion drops by 6 dB to -76 dB
At 1.5 watts the 2nd harmonic came back up to -70 dB
At .3 Watt the second harmonic has risen to -65 dB
The measurements on the distortion analyzer, a Boonton 1120, seem to be pretty consistant with these measurements.
My guess is that the second harmonic comes from a mismatch between the two output sections (balanced output . . .)
It may also contribute to the perceived sonic character.
-Demian
I used a 47 Ohm resistor for convienence. I just replaced the image with a new one with a 4 Ohm load and more documentation.
http://pws.prserv.net/demian/demian/SonicImpactDistortionSpectrum.JPG
And here is a picture of the spectrum to 5 MHz
http://pws.prserv.net/demian/demian/SISpectrum.JPG
The link works on two computers here, but there can always be a hidden issue. Let me know if there is still a problem.
The distortion is about -70 dB for 2nd and 3rd harmonic. Or around .3% at 7.5 V RMS into 4 Ohms or 14 W. Measuring at 12 W gives essentially the same results.
At 3 watts the distortion drops by 6 dB to -76 dB
At 1.5 watts the 2nd harmonic came back up to -70 dB
At .3 Watt the second harmonic has risen to -65 dB
The measurements on the distortion analyzer, a Boonton 1120, seem to be pretty consistant with these measurements.
My guess is that the second harmonic comes from a mismatch between the two output sections (balanced output . . .)
It may also contribute to the perceived sonic character.
-Demian
I can see the images now. The distortion spectrum looks very similar to what I saw on an analyzer as well.
In the switching spectrum image it looks like the carrier frequency is about 750kHz. Is that right? I believe one of the Tripath application notes states that the switching frequency varies from 200kHz for a full scale input to 1MHz for a low input level.
Thanks for sharing your findings with us
In the switching spectrum image it looks like the carrier frequency is about 750kHz. Is that right? I believe one of the Tripath application notes states that the switching frequency varies from 200kHz for a full scale input to 1MHz for a low input level.
Thanks for sharing your findings with us
I will look closer at the spectrum now that I know what to look for.
Looking at the balanced output brings a special challenge with unbalanced test equipment. I have a wideband differential preamp that may work, but usually they resolve one problem and bring another like distortion and noise.
-Demian
Looking at the balanced output brings a special challenge with unbalanced test equipment. I have a wideband differential preamp that may work, but usually they resolve one problem and bring another like distortion and noise.
-Demian
Not very pretty
The amount of hash and spikes cannot be good for sonics. The two halves may not be very well matched. The amount of dc on the outputs is all over, some -100 mv another channel +100 mv.
The measurements shown before all showed a 6 - 8 dB drop in noise after beefing up the on board filter caps. And the switch mode supply used may be adding a little noise of its own.
The 5Mhz spectrum looks really bad. What is the overall level of distortion? All I see is noise.
George
The amount of hash and spikes cannot be good for sonics. The two halves may not be very well matched. The amount of dc on the outputs is all over, some -100 mv another channel +100 mv.
The measurements shown before all showed a 6 - 8 dB drop in noise after beefing up the on board filter caps. And the switch mode supply used may be adding a little noise of its own.
The 5Mhz spectrum looks really bad. What is the overall level of distortion? All I see is noise.
George
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