Detective work help regarding strange impedance issue

[JMB]

I posted this on the Soundeasy forum also, but since it may not be exclusively a SE issue, I thought I would post this here, as well.

I have two set ups, one at home and one at my office. Both have dedicated Windows XP SP3 computers that run SE v19 without a problem. I have the ARTA software on both, as well. Both use the same jig. I have 3 different amplifiers. Same soundcard on each (Creative XFi Xtreme Gamer).

The problem is in testing a resistor (metal film - 20 ohm) I am getting a strange high pass effect on the office computer. This shows in doing a frequency response on the calibration setting, as well. Neither shows on the home computer. I switched out the jigs and amps and it is clearly an effect of the office computer. It does not show up when running LIMP in Pink Noise or using a Sine Wave Test. So it is isolated to the SE program only on my office computer.
My office has fluorescent lighting so I tested that computer at home (incandescent) and the problem followed the computer. I only switched out the computer and explored distances from the monitor thinking that perhaps this impacted things but it had no impact.
I switched out the leads, as well. At home, no problems regardless of which leads were used; in the office, all produced similar issues with a high pass function.

The settings are the same for SE on both computers. When I ran a direct loop from the output to the input using RMAA, it does not show up so the soundcard appears to be doing just fine. The only thing that I can think of from a hardware perspective is perhaps the output caps on the soundcard are having problems under load, but this is unlikely given the expected results obtained with Sine and Pink Noise tests.

I unloaded the virus program and still no change.

George (Soong), the computer is already streamlined with all of your recommendations .

I would welcome any guidance for further investigation. I am going to try to upload some screen shots.

Thnx, Jay

 

[colofan]

I am guessing here but I imagine they are not the same motherboard. Grounding schemes in the layout can have parasitic capacitance which could account for the difference.

 

[JMB]

So you are thinking that the soundcard may be experiencing parasitic capacitance? Not sure why that would impact Soundeasy and not LIMP nor why a loop with RMAA would not demonstrate the problem. Any ideas?

 

[bwaslo]

try swapping soundcards..

 

[JMB]

It's worth a shot. Thnx.

 

[mondogenerator]

I use a Soundmax card, admittedly not outstanding, but even in stereo mode the output is high passed at about 40Hz. Xo point for sub is variable, 40hz minimum. I believe there is some limitation in the software that prevents this highpass being bypassed.

Perhaps you have a similar issue.

 

[JMB]

I will try to switch out the soundcards this weekend. Mondogenerator, thanks for the input. I have the same software for the cards, which are both the same, on both computers so I don't believe that to be the issue. The settings are also the same. It is possible that one of the cards has a defect under load. Remember that the RMAA loopback showed no high pass effect.

J

 

[weltersys]

The 20 ohm resistor inductance may be working with the office computer's capacitance forming an LC filter.

 

[JMB]

Hi Weltersys, thanks for your input. I had thought of that but when I modeled the LC circuit, it was difficult to find values that looked reasonable to match the curve. In fact, there was never an actual match no matter what values there were. The inductance of the metal film resistor is really negligible.

bwaslo, I think that you hit the nail on the head. Much to my surprise, when I replaced the card with an old card that I had, the problem disappeared. I was thinking that since the problem was not seen with LIMP but was with Soundeasy, the card was not likely at fault. Both use the same jig and loop back circuit as reference but they obviously use the card's resources in different ways.

Thanks for all of the suggestions,

Jay