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.

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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.

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chede

Member
Paid Member
2016-01-09 8:59 pm
In the Düsseldorf suburbs
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
 

chede

Member
Paid Member
2016-01-09 8:59 pm
In the Düsseldorf suburbs
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: https://www.diyaudio.com/forums/pass-labs/326536-sissysit-79.html#post5807668).
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 ... :confused:


Best regards, Claas
 
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)?

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Here was my forest of noise from the M2 a few years ago:
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Thanks for the writeup on this, xrk!

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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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.
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Here is 2.0Vpp into a 10kOhm load (about 0.00088% THD and mostly 2nd and 3rd harmonic):
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The loopback with just the 2ppm source gave me 2.4ppm (0.00024% THD)
 

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BamboszeK

Member
2012-06-04 8:02 pm
I will be using these to test a 200w into 4ohms Class D amp (80Vpp). Definitely need a voltage divider on this one.
Class D amplifiers can't be tested directly. Amplifiers in that class has high out of band switching noise. It can severly affect measurment results, overloading ADC in analyser. External filter is required to remove high frequency noise. For example, Audio Precision AUX-0025.
Focusrite Scarlett 2i2 is really good performer for its price. I'm using Audient iD4 which is another affordable and good performing option. Here's results for Audient iD4, loopback with 6.3mm balanced cable. 24 bit / 44.1kHz, Blackman-Harris 7 64k, 32 avg, 20Hz-20kHz BW
To get best performance it is important to pay attention to input/output levels. Those devices usualy perform best at -10 to -16dBFS levels.



 

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Bamboszek,
Thanks for showing results on your Audient - that looks great for a loop back. Maybe one thing we will see on this thread are the different loopbacks for various audio interfaces? I just ordered the $38 Behringer UC2 to see how well that stacks up. Can never have too many USB I/O’s. :D

Thank is for the heads up on the need for extra out-of-Band switch noise filtering to prevent aliasing the data. I don’t want to spend a fortune on an AP filter. Could one be made with a CLC or two properly designed to filter steeply above 100kHz?
 
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BamboszeK

Member
2012-06-04 8:02 pm
Thats right, Audio Precision filter is a bit pricey, especially for hobbyst use. Of course it is possible to design DIY version but it might be challenging. To keep distortions down you will probably need to wind your own aircore inductors or do some research on ferrite core distortion characteristics. This Texas Instruments application note might give you some insight - http://www.ti.com/analog/docs/litab...eNumber=sloa107&docCategoryId=1&familyId=1976 and this thread Passive Filter for Class D Amp Testing
Having a small database of cheap interfaces measurments is great idea. Some measurments could be found in this thread ad ASR forum - Audio Interface Suitable for Measurements | Audio Science Review (ASR) Forum



 
LKA, that's very nice - what is the audio interface you are using here?

Here is my loopback with the Akitika 2ppm 1kHz sine generator. Measurement in a different room and seems a little off as there is a 100Hz comb around the 1khz today - not sure where that came from. I am getting only 0.00036%THD today. This is 1.0Vpp into 10kohms (single ended input pins 1 & 2) on XLR "Line input" setting of Focusrite 2i4 in ASIO mode.

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Aridace

Member
2018-06-23 5:19 pm
Bamboszek,
Thanks for showing results on your Audient - that looks great for a loop back. Maybe one thing we will see on this thread are the different loopbacks for various audio interfaces? I just ordered the $38 Behringer UC2 to see how well that stacks up. Can never have too many USB I/O’s. :D

Thank is for the heads up on the need for extra out-of-Band switch noise filtering to prevent aliasing the data. I don’t want to spend a fortune on an AP filter. Could one be made with a CLC or two properly designed to filter steeply above 100kHz?

An elementary filter like this one would do (but requires minor mods for non-ideal coils).
 

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