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Howto - Distortion Measurements with REW
Howto - Distortion Measurements with REW
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Old 25th September 2021, 05:27 AM   #981
phofman is offline phofman  Czech Republic
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IMO no need to fine-tune the distortions when measuring power amps, their output distortion is typically higher by orders of magnitude.
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Old 25th September 2021, 08:04 PM   #982
wtnh is offline wtnh  United States
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Default Victor's oscillator in a box

I posted this over in the Low-Distortion Audio-range Oscillator thread but thought it might be of interest to people here as well:

I decided to build a slightly more professional-looking enclosure for Victor's oscillator. Better shielding and improved connectors were the main goals. Similar to my stepped attenuator, I put it into a small 50x100x100 mm aluminum case. I designed the panel in KiCad and had it fabbed in aluminum by JLCPCB.

I replaced the level control with a 10-turn pot and vernier dial. This helps get consistent levels across multiple distortion runs with REW because the existing pot was a tad touchy. The RCA jacks were replaced with BNC connectors. I made provisions for a future differential output, probably consisting of an OPA1656 on a small piece of perf board (I don't have anything to test yet that has differential inputs).

Power comes from an external battery box with 4x 9Volt batteries.

It is taking its place alongside some of my other test equipment.


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Old 28th September 2021, 12:56 AM   #983
kozard is offline kozard
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Has anyone used a USB isolator? Such as with an E-MU 0404 USB or similar? I am trying to find ways of getting rid of the last power line frequencies from the E-MU 0404 USB single ended loopback. (Such as ADUM3160 based.)

I guess another possibility would be to use a balanced to single ended converter mounted right on the input terminals of the tested power amplifier.

Last edited by kozard; 28th September 2021 at 01:01 AM.
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Old 28th September 2021, 01:16 AM   #984
trobbins is offline trobbins  Australia
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I've used an ADuM4160 based isolator board, which needs a battery supply to power the USB to EMU interface. It does suppress mains loop related hum artefacts, although not to the same level as a laptop obviously (which is what I use now if I need to mitigate those artefacts). I have noted a few ebay variations of the board over the years - using different voltage regulation circuitry. I had one ADuM4160 fail on me once, but it was fine once the chip was replaced.
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Old 28th September 2021, 01:55 AM   #985
kozard is offline kozard
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With your E-MU 0404 USB and laptop do you still get mains related artifacts in your loopback test? Single ended loopback? I have some singled ended and I just wonder how much is to expected with the E-MU 0404 USB and laptop.


I also noticed that I get a little less mains related artifacts when testing my amplifier if my laptop is on AC power plugged into the same outlet as the power amplifier. It is a little higher when the laptop is on battery.
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Old 28th September 2021, 02:06 AM   #986
trobbins is offline trobbins  Australia
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I only do single-ended measurements. If there are two devices powered from the mains then yes there are typically some artefacts showing - so given I normally use a PC at my bench then there are artefacts as the EMU is powered from the mains and typically so is the DUT.

I made up a battery feed for the EMU, which I use with a laptop, to then allow an item of mains powered equipment to be tested without any mains born hum/noise (unless it's radiated ingress). An advantage of other more modern USB soundcards is of course that they don't need a custom battery supply.

Single ended connections to an amp means I have to be careful to make the soundcard input and output gnds the same within the DUT.
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Old 28th September 2021, 02:36 AM   #987
kozard is offline kozard
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However I am seeing the opposite which is odd. I am getting less artifacts when the laptop is on mains power plugged into the same outlet as the DUT. I get a little more artifacts on battery power. The E-MU is on battery power in both cases (power bank with 18650 inside).


It isn't enough to get very excited about but it seems odd.
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Old 28th September 2021, 07:33 AM   #988
phofman is offline phofman  Czech Republic
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Why not using the balanced inputs on the E-MU for measurements? The cold line to ground at the DUT output will reject the common-mode ground-loop noise.
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Old 28th September 2021, 11:37 AM   #989
kozard is offline kozard
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I am using the balanced input along with the balanced attenuator for the measurements.

The input to the amplifier DUT is still single ended from the left output of the E-MU 0404 USB. The mains artifacts are still there and are very close to the same levels seen in the E-MU loopback test. So I am assuming that they are primarily coming from the single ended output of the E-MU 0404 USB.
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Old 29th September 2021, 11:33 AM   #990
Morde is offline Morde  Finland
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Howto - Distortion Measurements with REW
Quote:
Originally Posted by wtnh View Post
... I made provisions for a future differential output, probably consisting of an OPA1656 on a small piece of perf board (I don't have anything to test yet that has differential inputs).
See this: Low-distortion Audio-range Oscillator

You can have diff. output natively from the Victor's oscillator PCB.
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