How to - Distortion Measurements with REW

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T

triplej

Member
Joined 2011
bohrok2610 said:
When measuring a tube (or valve) amp with a soundcard some test interface is usually needed to lower the voltages to levels suitable for the soundcard.
E.g. Pete Millett's test & measurement interface:
https://www.diyaudio.com/community/threads/test-measurement-interface-for-soundcard.155405/
Or Jan Didden's Autoranger:
https://www.diyaudio.com/community/threads/autoranger-for-soundcards.299635/
Click to expand...
thanks b, yes input level definitely an issue but in my case it was my VTL compact 100 amp that was VERY unhappy.
 
M

matulo_g

Member
Joined 2004
@JohnPM

Is there any chance that in REW will be possible to invert one of the two outputs from generator?

It could be useful to make stereo SE output as a "quasi balanced".

Martin
 

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lykkedk

lykkedk

Member
Joined 2002
Hi all...

So i bought this one here :: AIYIMA D03 - We are throwing a party in august (wife getting 50 ... sshhhhh! o_O )
I would like to be able to have all kind's of streaming including bluetooth for the guests that day.
When party is done i will proberly use the Aiyima for my lab.

- Well nevermind that, i will ofcause try to measure it when i hopefully recieve it next week.
I'am just not sure if i should take special care when connection it to my measurement rig, which is a custom diy balanced Behringer, nothing special through except that it can handle +10vrms input (no voltage dividier needed)

Is it just like any other normal amp. (chipamp. Class A poweramp. etc... ) ? - I just throught i read somewhere that a special setup must be used when measuring such
class D amplifier. -I'am sure the amp. is plain SingleEnded configured (at least the in/out -put's are!)

The documentation is not that easy to understand, but i attached them here for ref. (the sloa068a.pdf is good reading btw...)

Jesper.
 

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jjasniew

jjasniew

Member
Joined 2002
lykkedk said:
Is it just like any other normal amp. (chipamp. Class A poweramp. etc... ) ?
Click to expand...
Many class D are BTL output, which has given me more headache when trying to measure. You need an ADC with true differential inputs, which I dont have. (I just ordered a Scarlet solo to solve that one, plus get a better ADC instrument all in one shot).

Unsure if you can just measure one side of the BTL to ground - since the other is just the negative of it - and call it good. Will the spectrum be the same as going across both outputs with load and measurement connections?

Also there's the matter of signal level. Do you use 1 Watt, 8 Watts - what's a reasonable level for seeing the amplifier distortion character that you'd actually listen to, versus ultimate power performance comparitive specmanship?

Then there's the signal level db down from FS of the ADC, such that the ADC is giving its best performance (the level that you'd use in the "loopback performance test" should be about the same from the amplifier through any attenuation, at 1W / 8W / whatever output power)
 
lykkedk

lykkedk

Member
Joined 2002
Nice...

I will read some more regarding the BTL output.
I have this Behringer which i wrote before, accepting balanced input.

So i guess i'am good... I just need to make some TRS plugs with connection only to T & R (I use T as "+" and R & S as "GND" in the configuation i use now for best performance - having the possibilty to disconnect the GND directly on the Behringer)

Thank's

Jesper.

Modded-Behringer-1.jpg


MagicBus-Board.JPG
 
fubar3

fubar3

Member
Joined 2011
I've been reading the REW manual trying to build a working knowledge of this fine software.

First thing I noticed is a problem with installing on Linux. There are two command-line steps that degrade security.

1. chmod 777 REW_linux_5_19.sh
2. sudo ./REW_linux_5_19.sh

Item 1 gives read-write-execute permission to all users including bad actors. It would be better to use symbolic notation:-
chmod u+x REW_linux_5_19.sh .. only you or the person installing has permission

It would be good to have a Linux expert review the software and installation procedures. I am not an expert.
 
R

RickRay

Member
Joined 2014
Paid Member
Those are some excellent articles and I will forward them to the guy I know wanting to buy an audio interface for measurments.

The actual comparison I am looking for was a table with about 15-20 audio interfaces listed and a bunch of specs compared. I should have printed it and filed it away in my measurement folder.
 
NickKUK

NickKUK

Member
Joined 2019
fubar3 said:
I've been reading the REW manual trying to build a working knowledge of this fine software.

First thing I noticed is a problem with installing on Linux. There are two command-line steps that degrade security.

1. chmod 777 REW_linux_5_19.sh
2. sudo ./REW_linux_5_19.sh

Item 1 gives read-write-execute permission to all users including bad actors. It would be better to use symbolic notation:-
chmod u+x REW_linux_5_19.sh .. only you or the person installing has permission

It would be good to have a Linux expert review the software and installation procedures. I am not an expert.
Click to expand...

Most linux installs shouldn’t need root (sudo) given the app user space drivers.

Right person, right access, right time, right context and continuously assessed. As the cyber security folks would say.
 
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