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Howto - Distortion Measurements with REW
Howto - Distortion Measurements with REW
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Old 14th September 2019, 09:27 AM   #281
29285 is offline 29285  Europe
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Quote:
Originally Posted by JohnYang1997 View Post
Tpa6120a2 composite amplifier
8ohm 2Vrms 500mw and noise floor.
Impressive numbers! Could you pls describe your test setup?
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Old 14th September 2019, 09:34 AM   #282
JohnYang1997 is offline JohnYang1997
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Quote:
Originally Posted by voltwide View Post
Impressive numbers! Could you pls describe your test setup?
Pre distortion calibrated(using add harmonic function in rew) without the amplifier in the signal chain. Then measure the distortion with the amplifier with and without load.
Dac: topping dx3pro
Adc: motu 828es
load: 8ohm 20W resistor
Single ended all the way through

To validate the method, I measured Jds atom, jds o2, dx3pro and with various modifications, as well as iterations of my own designs with 8ohm 32ohm load. I can say the incremental improvements from each step is clearly differentiated in this setup. If measured on APx555, I believe the measurements will be even better.
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Old 26th September 2019, 07:48 AM   #283
xrk971 is online now xrk971  United States
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Howto - Distortion Measurements with REW
I just found out how to measure distortion on a Class D bridge tied load (BTL) amp where both wires are antiphase and live. You need to use a symmetric divider, say 9k1-2k-9k1 across the dummy load and put the Focusrite balanced XLR input across the middle 2k but bypassed with a film cap on each wire.

More here:
TPA3255 Reference Design Class D Amp GB

Setup:
Click the image to open in full size.

Here’s the divider:
Click the image to open in full size.

It’s an abbreviated one (only 20dB) used by Voltwide here:
Click the image to open in full size.

I used an asymmetric divider previously with DC coupled balanced inputs and it got very high (artificial) H2.

Here is the data from asymmetric divider:
Click the image to open in full size.

To symmetrical divider:
Click the image to open in full size.
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Last edited by xrk971; 26th September 2019 at 07:51 AM.
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Old 26th September 2019, 08:42 AM   #284
tcpip is offline tcpip  India
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I guess this is the kind of approach one would use for any BTL amp? A Modulus-686 is in the pipeline, and I fully intend to measure HD and noise.
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Old 26th September 2019, 09:09 AM   #285
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nothing wrong with this test for any BTL amp. TI is the only brand I can comment on, and these chips produce some common mode output distortion that is cancelled when driven in BTL mode.
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Old 14th October 2019, 11:47 PM   #286
xrk971 is online now xrk971  United States
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Howto - Distortion Measurements with REW
I received one of Victor's ultra low distortion 1 kHz oscillators - it should have levels immeasurable to the Focusrite, so what we will see will basically be the limit of the Focsurite. I used 4x 9v batteries to give it a clean source of 35v. At the lowest output setting of 490mVpp and fed into to internal 50kohm input impedance of the Focusrite 2i4, I am getting the following:
Howto - Distortion Measurements with REW-victors-1khz-osc-490mvpp-50k-fft-jpg

The noise floor is amazing - I have never measured anything this quiet, and it is not even inside a case yet.

There is a 3rd order peak that is rather high and stays constat during the time averaging process, so probably part of the Focusrite fron end.

Howto - Distortion Measurements with REW-victors-1khz-osc-photo-01-jpg

Howto - Distortion Measurements with REW-victors-1khz-osc-photo-02-jpg

Very nice unit prebuilt for 50 Euro. Highly recommended.
Attached Images
File Type: jpg Victors-1kHz-osc-photo-01.jpg (426.2 KB, 268 views)
File Type: jpg Victors-1kHz-osc-photo-02.jpg (399.0 KB, 271 views)
File Type: jpg Victors-1kHz-osc-490mVpp-50k-FFT.jpg (234.0 KB, 283 views)
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Old 23rd October 2019, 02:58 AM   #287
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Howto - Distortion Measurements with REW
In case anyone is interested in Victor's ultra-low distortion oscillator, just send him a PM @vicnic.

https://www.diyaudio.com/forums/members/vicnic.html
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Old 23rd October 2019, 04:07 AM   #288
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Originally Posted by xrk971 View Post
In case anyone is interested in Victor's ultra-low distortion oscillator, just send him a PM @vicnic.
I am getting oscillators and notch filters from him. I'm super excited.
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Old 3rd December 2019, 02:05 AM   #289
poseidonsvoice is offline poseidonsvoice  United States
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Howto - Distortion Measurements with REW
The Focusrite Scarlett 2i2 was recently tested. The 2i2 unit is a good unit especially if you use the low distortion oscillators (Akitika and Victor’s) mentioned in this thread.

Here are the 2i2 measurements:

Focusrite Scarlett 2i2 Audio Interface Gen 3 Review | Audio Science Review (ASR) Forum

I think it did quite well overall.

Best,
Anand.
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