I have found several guides on how to measure THD via a voltage divider using ARTA. However, my sound card input is single ended and the output of my amp is bridged (BTL). How would it be possible to connect them together?
I'm really not sure that's possible. Not without jumping through a number of hoops, anyway.
What you could do, on the other hand, would be to hunt down a used "pro-sumer" external USB sound card / interface (with at least one XLR or combo input), something like a Focusrite Scarlett Solo or 2i2, or a Tascam US-122, or their correspondents from Presonus, Behringer, M-Audio etc. I'm quite sure some of those can be had even within $30-40 or so.
A balanced input, as can be found on those, could be "uneventfully" connected to the (attenuated) output of a bridged amp. Although you'll probably want to also be using a dummy-load.
What you could do, on the other hand, would be to hunt down a used "pro-sumer" external USB sound card / interface (with at least one XLR or combo input), something like a Focusrite Scarlett Solo or 2i2, or a Tascam US-122, or their correspondents from Presonus, Behringer, M-Audio etc. I'm quite sure some of those can be had even within $30-40 or so.
A balanced input, as can be found on those, could be "uneventfully" connected to the (attenuated) output of a bridged amp. Although you'll probably want to also be using a dummy-load.
He is talking about his output being btl and how to hook it up to his single ended input. Can't see the problem ,but that was his question, take the btl output from amp through a voltage divider, feed it to mic in or line in, or am I'm wrong?
You are wrong. To measure a symm output you will need a symm divider (2-channel divider) and some symm input. Most sound cards offer symm input, so I think the direct way is looking for an appropriate soundcard. I do this with a cheap EMU-tracker USB-soundcard.
The trick there is, the two outputs are opposite polarities (ie. when one goes "up", the other one goes "down"), so connecting them together will yield (in theory) 0V AC, and whatever DC is halfway between the amp's supply rails 🙄
You could measure the THD for each side independently. However its possible the differential distortion is less than either if the distortion is partly common-mode.
However the differential distortion cannot be more than the worst of each side's distortion(*), so you get an upper bound. If the signal inversion and splitting and filtering is properly balanced all should be well.
(*) strictly this assumes balanced operation, ie the common mode signal << differential mode.
However the differential distortion cannot be more than the worst of each side's distortion(*), so you get an upper bound. If the signal inversion and splitting and filtering is properly balanced all should be well.
(*) strictly this assumes balanced operation, ie the common mode signal << differential mode.
@Krohn, It indeed seems like I have to get a card with a differential/balanced input.
@Mark, On my scope I can see that the individual outputs are way different than the differential output. I do not think I will get reliable measurements that way.
Does anybody know a good and inexpensive soundcard with balanced input? Otherwise I will have to keep an eye out for something like an E-MU on ebay.
@Mark, On my scope I can see that the individual outputs are way different than the differential output. I do not think I will get reliable measurements that way.
Does anybody know a good and inexpensive soundcard with balanced input? Otherwise I will have to keep an eye out for something like an E-MU on ebay.
Which part of the world do you live in?
Let's see, in no particular order:
Focusrite Scarlett 2i2
M-Audio Fast Track (the newer, 3-knobs-on-top version) / Fast Track Pro / Fast Track Ultra M-track 2x2 / 2x2M
Presonus Audiobox iOne / iTwo / USB 96 / 22VSL
Behringer U-Phoria UMC22 / UMC202HD
Lexicon Alpha
Mackie Onyx Artist / Producer
Steinberg UR12
Plenty to choose from on the used market, and for ridiculously low prices (especially for some of the older models).
Let's see, in no particular order:
Focusrite Scarlett 2i2
M-Audio Fast Track (the newer, 3-knobs-on-top version) / Fast Track Pro / Fast Track Ultra M-track 2x2 / 2x2M
Presonus Audiobox iOne / iTwo / USB 96 / 22VSL
Behringer U-Phoria UMC22 / UMC202HD
Lexicon Alpha
Mackie Onyx Artist / Producer
Steinberg UR12
Plenty to choose from on the used market, and for ridiculously low prices (especially for some of the older models).
Thanks for the list, I see quite some options on Marktplaats (dutch Ebay)
I can even get the Behringer U-Phoria UM2 for 33€ new. Would that be a suitable option?
It could be done with balanced voltage divider and instrumentational amplifier.
Its just important to provide path for input base current of instrumentational amplifier to not rail stick. (divider center point connected to power supply gnd).
Then you need to play with attenuation, gain, and maybe to add zener protection on output.
If you like to do gadgets..... otherwise buy balanced card.
Its just important to provide path for input base current of instrumentational amplifier to not rail stick. (divider center point connected to power supply gnd).
Then you need to play with attenuation, gain, and maybe to add zener protection on output.
If you like to do gadgets..... otherwise buy balanced card.
Attachments
I made an external, low noise low distortion balanced amplifier to solve the same problem. It also has ground tied input dividers (to prevent high CMV overload). This external circuit is useful even for standard power amplifier with grounded load measurements, because it prevents ground loops and induced hum. One of my soundcards is a modified SB USB X-Fi HD. I can measure up to 40Vrms.
It is true, with TPA3255 class D, the single ended output is one magnitude worse than BTL configuration. Obviously the single ended noise and distortion is the same on both outputs and is cancelled in bridge mode.
The used EMUs are excellent value for money and play in the same ballpark as the more expensive focus rites.
I would try to use this approach:
AMP_OUT -- R1-R2 --NODE -- R3-R4 -- AMP_OUT
R1=R4
R2=R3
or at least make R1/R2 = R4/R3
with R1>>R2 and R3>>R4.
Essentially, there are two identical voltage dividers in series.
Measure with a single ended probe with the HOT lead placed between R1 and R2 and the cold lead placed at NODE.
In BTL mode and the conditions for R1-R4 listed above, NODE is at midpoint potential between the AMP OUTs, and this should be ground. Make sure to carefully balance the resistors.
AMP_OUT -- R1-R2 --NODE -- R3-R4 -- AMP_OUT
R1=R4
R2=R3
or at least make R1/R2 = R4/R3
with R1>>R2 and R3>>R4.
Essentially, there are two identical voltage dividers in series.
Measure with a single ended probe with the HOT lead placed between R1 and R2 and the cold lead placed at NODE.
In BTL mode and the conditions for R1-R4 listed above, NODE is at midpoint potential between the AMP OUTs, and this should be ground. Make sure to carefully balance the resistors.
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