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Howto - Distortion Measurements with REW
Howto - Distortion Measurements with REW
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Old 30th May 2019, 08:55 AM   #11
xrk971 is offline xrk971  United States
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Howto - Distortion Measurements with REW
For the Solo there is not a TRS balanced input. Just XLR, so you will need an XLR to RCA adapter. However, I have noticed that sometimes using the balanced input gives better noise performance. That is, connect your dummy load across the pins 2 and 3 of the XLR and leave off the ground (drain) pin 1. You cannot do this with a XLR to RCA adapter as those have the RCA GND to pin 1 and bypasses the differential input feature.

Click the image to open in full size.

On the 2i4, the XLR jack has a central hole that allows you to use a stereo TRS 1/4in plug. You can use a blank plug (designed tonsolder your own wires) and connect your dummy load to the tip and ring, leave the sleeve not connected.

The differential mode input seems to have better results from extraneous noise. Another tip, sometimes the PC power supply adds noise or causes hum or ground loops. You can try disconnecting PSU from laptop and run in battery mode.

These are all the sorts of things that require trial and error to see what works best for you. I highly recommend and independent 1kHz excitation source though.
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Old 30th May 2019, 09:26 AM   #12
mondogenerator is offline mondogenerator  England
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This is a very handy thread.

I've been wondering how to get started measuring using REW or some other software.

Its appealing since I have been looking at low cost options that don't rely on me manually reading off harmonic series values from the DSO at work, and then typing into a calculation sheet to give a % value.

Seeing as I have been looking at cheap solutions, the following questions occurs to me?

Can you tell us, what advantage the use of a Ext USB SFX card has over something of similar price - picoscope 2200 scope?

Off the top of my head the Pico is 8 bit ADC, and only 10Mhz (I can't recall the kS/s data rate) so perhaps amplitude resolution is limited (though they do use some form of ETS to increase the resolution nearer 12bit)

Now a soundcard (focusrite, behringer, alesis all the basically the same card IIrC) is 24bit depth, much less BW.

What would be interesting is how to/what level loopback THD is measured in REW using a typical budget USB sound card set up, such as yours (or mine, when I get round to buying a new card)

I imagine the Picoscope measurement floor is perhaps higher, discriminating 0.25% THD and 0.05% may be impossible.

whether it necessary to make that discrimination is something I'm not sure about.
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Old 30th May 2019, 11:49 AM   #13
xrk971 is offline xrk971  United States
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Howto - Distortion Measurements with REW
I have tried the Behringer UCA202 - it wasn’t bad THD wisenin loopnbekemm was a good but ultimately it was the higher noise floor.
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Old 30th May 2019, 01:52 PM   #14
6L6 is offline 6L6  United States
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Howto - Distortion Measurements with REW
Fantastic writeup!! My sincere thanks - I've been looking for a simple solution like this, I was using some Mac only software and an old computer (that still had analog mic input!) and that machine bricked after a too-new OS update.

Tried a loopback of the software generator though my Focusrite 2i2 getting decent results. Need to attach a power amp and see what can be seen.
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Old 30th May 2019, 02:47 PM   #15
Russellc is offline Russellc  United States
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Howto - Distortion Measurements with REW
Quote:
Originally Posted by xrk971 View Post
That’s a nice oscillator for $79. I might just get one. I use this function all the time.
Thanks, Anand.

Russellc, sure - I will show this all hooked up. The funny little black connector is a 3.5mm female TRS jack. That let’s me use a 3.5mm male to RCA to go to Focusrite input via a RCA to 1/4 in guitar plug.

The banana and female spade let me connect to the amp outputs. Banana for back panel on amp and spade for directbto the PCB amp outputs - useful when amp is on bench and not even installed on a chassis (most of the time my amps have no chassis).

Much appreciated will look forward to additional photos.

Very much appreciated!

Russellc
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Old 30th May 2019, 03:07 PM   #16
danny_66 is offline danny_66  Belgium
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Excellent guide for an excellent tool !
I didn't knew you also used RTA for amplifier measurements.

So far I've used REW for measurements of my speakers,
besides the normal sweep measurements I use RTA white noise to get better <500hz in room measurements.
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Old 30th May 2019, 03:31 PM   #17
xrk971 is offline xrk971  United States
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Howto - Distortion Measurements with REW
Here is a photo of a typical measurement setup. The dummy resistor is connected to the amp outputs via the banana plugs. The dummy load is connected to a voltage divider which is connected to the Focusrite input via a 3.5mm to RCA cable and RCA to XLR adapter. The DVM (yellow Fluke 101) used for measuring the AC volts is connected directly across the dummy resistor with alligator clips. The output RCA of the Focusrite is connected to the amp RCA in.
Attached Images
File Type: jpeg 5A577D91-8B43-47E7-BB4D-0C4F116E6FA6.jpeg (310.0 KB, 369 views)
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Old 30th May 2019, 04:00 PM   #18
xrk971 is offline xrk971  United States
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Howto - Distortion Measurements with REW
Quote:
Can you tell us, what advantage the use of a Ext USB SFX card has over something of similar price - picoscope 2200 scope?

Off the top of my head the Pico is 8 bit ADC, and only 10Mhz (I can't recall the kS/s data rate) so perhaps amplitude resolution is limited (though they do use some form of ETS to increase the resolution nearer 12bit)
These tools are for different purposes. One is to really see transient features, the other is to record high resolution (in amplitude) but lower frequency. Can you use REW with the USB OScope to get the FFT?

The new Behringer UC2 for $38 really makes “cost” a moot point. Based on reviews I would imagine it will work just fine. Even at $99 for the Focusrite Solo, the cost is not unreasonable as the unit in itself is an excellent DAC for playing music as a source.
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Old 30th May 2019, 09:27 PM   #19
chede is offline chede  Germany
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Howto - Distortion Measurements with REW
Thank you for pointing me to REW as distortion analyzer software, xrk971 !

Because I already had the Focusrite 2i2 and a self-built attenuator box (-6 dB to -30 dB in 6 dB steps), and making use of the opportunity that today is a holiday in our part of Germany, I could do some quick measurements of my SissySIT.

The results I have posted here:

SissySIT

I also did some measurements of my Wayne BA 2018 linestage and a quick check of my Aikido tubed linestage, but I need to do a write-up of these measurements first before showing the pictures.


I find REW easier to use than ARTA, which I have used so far. For me (WIN7) the ASIO drivers worked better than the Java drivers - might be because I had used ARTA before and drivers were still loaded at some Windows interface point or another ...

Here is the self-noise of the Focusrite 2i2:


Howto - Distortion Measurements with REW-fr-2i2-self-noise-96-khz-190530-png

And here the loop-back with the level control of the Focusrite optimized for lowest distortion - that give near to the lowest noise as well. Digital output level is - 3dBFS, analog output level is 0.707 V. These settings I have used for all my distortion measurements so far:


Howto - Distortion Measurements with REW-loopback-1-gen-3-db-default-level-png

Note also the small 50 Hz hump (European mains frequency). These measurements were done with my laptop and the Focusrite on the sofa, totally battery-powered, no electrical outlet nearby, and no significant current draw in the whole room. I believe some of the 50 Hz noise I will show on some preamp measurements later is probably pick-up from other equipment nearby or the mains cables in the wall.


Here is the switchable attenuator I built, also using green protection LEDs against overvoltage:

Howto - Distortion Measurements with REW-attenuator-2i2-jpg

The design of the attenuator I picked up from Dan Joffe. He started it all in this thread:
Distortion Measurement with Soundcard and ARTA software

The 1 kHz oscillator (post #12 in the thread above) had already been mentioned in this thread at #9. On the Akitika website, towards the bottom of the page with the 1 kHz oscillator, is a link to a document describing the attenuator. That's what I referred to to build it.

1 kHz Oscillator


All the best, Claas
Attached Images
File Type: png FR 2i2 Self-Noise 96 kHz 190530.png (89.4 KB, 456 views)
File Type: png Loopback 1 - gen -3 dB old default level.png (59.3 KB, 454 views)
File Type: jpg Attenuator and 2i2 small.jpg (421.0 KB, 437 views)
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Old 30th May 2019, 11:46 PM   #20
xrk971 is offline xrk971  United States
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Howto - Distortion Measurements with REW
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?

I recently tried the ASIO driver and it works fine on Win10.

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.
Cheers,
X
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Last edited by xrk971; 30th May 2019 at 11:54 PM.
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