Sound card FFT chart on a blameless amplifier

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#44
jan.didden said:
I was just going to suggest that! Great minds think the same, haha!

Not sure you need a xformer - it should be possible to chose your ground points and scope ground to get rid of ground noise.
After all, your amp also has to work without that xformer!

Jan
Click to expand...

Hi Jan,

I know I have to make the ground right in order to make whole thing work along with all other equipments, otherwise, this is just a peice of junk. But the transformer will make my life easier when doing the trouble shooting. After all, I just don't have much experience in the ground loop thing, a lot to learn.

Also, I think I know exactly what is wrong in my build. I just powered the amp up using a pair of batteries and all the hamonics are gone. I followed the tips Mr. Self mentioned in his book when laying the board, but I think I just not fully understood why. I need to digest some more books and articles before I can make this right.:h_ache:

Cheers!

Richard
 
#46
banana said:
It's not fair to compare with battery, which is free from mains harmonics.
Anyway, grounding optimization is essential for noise measurement. Does your Soundcard have balanced in/out?
Click to expand...

Got it, I'll try to isolate the computer using the isolation transformer.

The sound card is ASUS Xonar STX, which is a PCI-E card. No balance input and output.

BR,

Richard
 
#47
boborich said:
Hi Banana,

Not sure you would mind if I call you babana, but this is all I have to address you. I'm sorry if this offended you, didn't mean to do so..............
Click to expand...
You are boborich and he is banana. Both are unique in this Forum and thus never introduces any ambiguity.

I tend to capitilise the first letter as a sign of respect and since in UK convention we use the capital first letter for names, i.e. Boborich or Banana.

In my view we should all use the Forum username when specifically addressing, or referring to another Member.
 
#48
I have mentioned grounding and showed the right approach in Post #22 😉

Transformer is not really required if no ground loop arranged (your measurement system is connected to the amplifier at a single point).
My card has got balanced XLR inputs/outputs, however I'm using RCA adapters, utilizing just one side of them.
 
#50
vzaichenko said:
I have mentioned grounding and showed the right approach in Post #22 😉

Transformer is not really required if no ground loop arranged (your measurement system is connected to the amplifier at a single point).
My card has got balanced XLR inputs/outputs, however I'm using RCA adapters, utilizing just one side of them.
Click to expand...

Hi Vzaichenko,

Thanks, and I do read your post 3 times, but that is not my case. I only have input to my sound card, and the output is not utilized. I think the groud loop is caused by the earth connection on the mains.

Again, I'm still trying to connecting the dots by more experiments and book digestings. Not quite sure I got everything you said😀

Thanks for the inputs.

Richard/boborich
 
#51
AndrewT said:
You are boborich and he is banana. Both are unique in this Forum and thus never introduces any ambiguity.

I tend to capitilise the first letter as a sign of respect and since in UK convention we use the capital first letter for names, i.e. Boborich or Banana.

In my view we should all use the Forum username when specifically addressing, or referring to another Member.
Click to expand...

Agree, otherwise will cause confusions.

boborich
 
#52
boborich said:
Hi Vzaichenko,

Thanks, and I do read your post 3 times, but that is not my case. I only have input to my sound card, and the output is not utilized. I think the groud loop is caused by the earth connection on the mains.

Again, I'm still trying to connecting the dots by more experiments and book digestings. Not quite sure I got everything you said😀

Thanks for the inputs.

Richard/boborich
Click to expand...
Try using a Pin1 loop tester. Properly known as a "Hummer".
 

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#53
boborich said:
Hi Jan,

Thanks for your comment! I think I know why the waveform is triangle. It is the result of Cap charge/discharge time difference, the discharge time T =RC, where R is the load, is slower than charge time. Please correct me if this is not correct.

Also I poked around with some trouble shooting, and I found this. I disabled the XD circuit which draws quit some power, now the amp works like a normal blameless circuit. The bias is still at 26mv Vre, and the power comsumption is now 210W. I got this FFT chart on the compuer with nothing attached at idle:
attachment.php


With nothing changed, when I just put the scope probe on the positive rail, I got this on the compuer FFT chart:
attachment.php


Obvirously, the PSRR of my amp is not good at all, right? The noise were injected throught the ground, right?

Any thoughts on this?

Thanks!

Richard
Click to expand...
 
#55
boborich said:
Hi Vzaichenko,

Thanks, and I do read your post 3 times, but that is not my case. I only have input to my sound card, and the output is not utilized. I think the groud loop is caused by the earth connection on the mains.

Again, I'm still trying to connecting the dots by more experiments and book digestings. Not quite sure I got everything you said😀

Thanks for the inputs.

Richard/boborich
Click to expand...

In case you don't use the card's output, your case is much easier 🙂
Anyway, pay attention to the way your standalone waveform generator's ground is connected. Test / calibrate the measurement system first with no amplifier connected.
 
#57
ostripper said:
(below) is the real "baseline" for a typical "blameless" (with either TPC or TMC)
.... transitional 2 pole feedback compensation.
The amp has input pair noise dominant at low output but drops to <10ppm -100W (2-below).

If you are short of this , it is either layout or the shortcomings of the testing rig.

PS - this was a true test on a 2700 series AP. The forums "Honeybadger" is
pretty much the same circuit as the test amp (EF2 + blameless).
I'm sure a (EF3 +blameless) or Vzaichenko's non-switching EF would halve
these AP tests - especially with more load/wattage.

OS
Click to expand...

Hi OS,

Thanks for the note, will reference these measurements once I fixed the ground issue I have right now.

I checked your Honeybadger and this is very similar to mine, with the input differential cascoded. The cascode bias in my design is provided by a zenar rather than resistor deviders. The reason I used cascode arrangement is simply to reduce the power disapation, I'm using +-70V supply and the 2sa1349 can't handle the 3ma current.

I guess I just sucked in laying the board...

BR,

boborich
 
#58
xrk971 said:
I have a related question - how can I use a USB sound card to make THD measurements of my amp? I tried making a voltage divider to put the voltage across a dummy resistive load into the inputs. However, I am getting a strange oscillation with the Behringer UCA202. It's a swing to full input (saturates) every 3 seconds. Is this because of some sort of USB power ground loop? If anyone has a link to an amp measuring setup with a USB Sound "card" (dongle) that would be most appreciated.
Thanks,
X
Click to expand...

I did just that with my Behringer soundcard (although not with an active amp). Let me know if you need more info.
 
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