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Howto - Distortion Measurements with REW
Howto - Distortion Measurements with REW
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Old 31st May 2019, 02:06 AM   #21
xrk971 is offline xrk971  United States
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Howto - Distortion Measurements with REW
REW also permits frequency sweeps as Chede showed here for his SissySIT amp. It lets you see the various harmonic distortion levels as a function of frequency, plus the main amplitude response flateness and phase. Very nice info - thank you, Chede.

Click the image to open in full size.
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Old 31st May 2019, 02:53 AM   #22
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Howto - Distortion Measurements with REW
This is a manly-man's dummy load: 200w and 4ohms. I have a voltage divider connected to the differential +ve and -ve inputs on the ring and tip (respectively) of a 6.35mm TRS plug so that it can go into the balanced input of a Focusrite 2i4.

These monster wire-wound rheostat style dummy loads are $15ea on eBay, in case you are interested. I will be using these to test a 200w into 4ohms Class D amp (80Vpp). Definitely need a voltage divider on this one.

Howto - Distortion Measurements with REW-200w-4r-dummy-load-rig-jpg
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File Type: jpg 200W-4R-Dummy-Load-Rig.jpg (363.3 KB, 426 views)
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Old 31st May 2019, 08:31 AM   #23
chede is offline chede  Germany
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Howto - Distortion Measurements with REW
Quote:
Originally Posted by xrk971 View Post
Hi Chede,
Thanks so much for trying this out. Your Focusrite 2i2 shows excellent performance on the loopback. I just ordered the 1kHz ppm oscillator - seems essential to reach the 0.0008% THd figure that you got.

I’m curious if you used the balanced differential input to the Focusrite or just single ended using and XLR to RCA adapter?
Hi X,

actually, I just used the oscillator in REW, set to - 3dB with 24 bit dither. For signal out, just a simple Cordial patch cable with TS plugs, going from Focusrite 2i2 output either directly into the Focusrite input, or via the attenuator box. That box, albeit basically only a resistor divider, adds a bit of 2nd harmonic distortion, however (don't have a measurement picture with me here in the office now).
So, it's Focurite as signal generator, set to 0.707 V output via the master volume. Since it's TS, only single-ended connections. Into the amps I go via a TS - RCA adapter.

Laptop used is a Lenovo Thinkpad S440 with SSD hard disk and 8 GB RAM, if that might be some contributing factor also.

Best regards,
Claas
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Old 31st May 2019, 08:40 AM   #24
chede is offline chede  Germany
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Howto - Distortion Measurements with REW
Quote:
Originally Posted by xrk971 View Post
Your SissySIT amp measurements look great too. The profile on that amp must sound wonderful. I see a forest of noise from the mains though. You might consider an SLB to help smooth some of that out.
Hi X,
yeah, the PSU-related noise is really interesting (measurements here: SissySIT).
The biggest spectral lines are at 50 Hz (mains frequency) and its odd harmonics. That would be radiated noise from the transformer and nearby equipment / mains wiring, at least for the fundamental, not completely sure about the odd-order harmonics. I would think your SLB would mostly be effective in bringing down 100 Hz component and its harmonics ?

Interesting is the 30 Hz sub-harmonic. Can't recall the mechanism that would produce a 30 Hz sub-harmonic, as seen on the right channel, since there is no 60 Hz anywhere in our electrical system. On the left channel, it is 25 Hz, though ...


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Old 31st May 2019, 12:03 PM   #25
xrk971 is offline xrk971  United States
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Howto - Distortion Measurements with REW
Hi Chede,
That mains spectrum of 50Hz, and 100Hz from the rectifier and it’s harmonics are really something. I saw same thing in my M2 and had a thread on it. I think it’s EMI from the transformer that is picked up by the amp itself. I moved the trafo outside of the amp case and it went away partially. Replacing the linear PSU with a SMPS takes it away too. I think there are some strong EMI noise radiated from those trafos.

The 30Hz puzzles me too as I don’t see how you can generate 30Hz from any difference frequency of 50Hz and it’s harmonics? Maybe you have something in your vicinity that has a 60Hz inverter (maybe a neighbor has one)?

Click the image to open in full size.

Here was my forest of noise from the M2 a few years ago:
Click the image to open in full size.
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Old 2nd June 2019, 01:43 AM   #26
Eric is offline Eric  United States
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Howto - Distortion Measurements with REW
Thanks for the writeup on this, xrk!

Quote:
Originally Posted by xrk971 View Post
These monster wire-wound rheostat style dummy loads are $15ea on eBay, in case you are interested.
You can get similar ones rated at 120w for $6 each from MPJA. They have a number of ohmic values. I purchased four 16R resistors and put a wall switch between two pairs, so I have both a 4R at 480w capacity and 8R load at 240w capacity with the flick of a switch.

Now I'll add a voltage divider to it so I can run the output into my PC for analysis.
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File Type: jpg DummyLoad.jpg (43.9 KB, 110 views)
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Last edited by Eric; 2nd June 2019 at 02:06 AM.
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Old 2nd June 2019, 02:13 AM   #27
xrk971 is offline xrk971  United States
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Howto - Distortion Measurements with REW
Thanks for the tip Eric! That’s a nice setup you have there. Great idea to use wall switch for changing between 4ohms and 8ohms! What amps are you testing that make 480w!!?
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Old 2nd June 2019, 02:22 AM   #28
Eric is offline Eric  United States
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Howto - Distortion Measurements with REW
Ha- I was more interested in 4R than 480w. I made this when I was testing my big Aleph-X amps, but they "only" produce 150w into 4R. When you drive real power into these resistors, they get rather warm and turn a bit yellow in color until they cool back down again...

Now I'll add a pair of small resistors and an RCA jack!
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Old 2nd June 2019, 07:20 AM   #29
xrk971 is offline xrk971  United States
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Howto - Distortion Measurements with REW
I bought the Akitika 2ppm 1khz excitation source and assembled it in about an hour. Very nice projext and super easy to build. I tried it out on the new Yarra-Melbourne preamp.
Howto - Distortion Measurements with REW-yarra-test-2ppm-osc-jpg

Here is 2.0Vpp into a 10kOhm load (about 0.00088% THD and mostly 2nd and 3rd harmonic):
Howto - Distortion Measurements with REW-yarra-melb-0-707vrms-10kohm-fft-jpg

The loopback with just the 2ppm source gave me 2.4ppm (0.00024% THD)
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File Type: jpg Yarra-Test-2ppm-Osc.jpg (374.5 KB, 370 views)
File Type: jpg Yarra-Melb-0.707Vrms-10kohm-FFT.jpg (234.3 KB, 580 views)
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Old 2nd June 2019, 10:01 AM   #30
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That is a phenomenal set of measurements!!
Thanks for giving them X!

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