Impedance calibration process help needed

[carlosalmeida]

Hi there,



Doing some impedance measurements, i´ve found some points a bit confusing, and would like some advices and help from you...

When doing the audio interface calibration.....by the way, i´m using na RME Babyface 1st edition interface.......i could get almost the same gain in both channels, as you can see below. Babyface has the digital gain knob, so it´s easy to get the same gain in both channels.

But, after connect the impedance cable / jig, i´ve got a little difference between the channels gain, as yo can see below.. (the reference channel gain is lower than the measurement one)

So, my first doubt is: which could be the reason for that?? Maybe the sense resistor?? – I´m using a 100ohms 1W 5% resistor by the way..


By the way, my cable was built like this:

Advancing in the process, after doing the first impedance calibration process (open circuit), the result i´ve got tells me : “ measured was 92.464% of reference”

Most vídeos and tutorials i´ve seen shows that measurement about 99% at least.

Should i be worried about those 92.464%? What coulb influnencing the result? The gain diference between reference and measuring channels?

Thanks for help!

 

[John Mulcahy]

It seems odd to have such a gain difference. The only difference between the channels during the open circuit cal is the sense resistor in series with one channel, which suggests the input impedance of the inputs being used is very low.

 

[John Mulcahy]

From the spec the 1/2 inputs have only 1 k input impedance unbalanced. Looks like you would be better off using inputs 3/4 which have 1 M input impedance.

 

[carlosalmeida]

Hi John,

First of all, thanks for reply!

I´ve got the following specs from manual.
The input impedance is actually around 2KOhm and the first edition of babyface has only two analog servo balanced XLR inputs and an extra 1/4 unbalanced instrument input.

But dispite of this, should be ok to the keep the 92.4% measurement or would be necessary to increase this number?

 

[John Mulcahy]

Swap the connections around so the measurement input is the instrument input.

 

[carlosalmeida]

Swap the connections around so the measurement input is the instrument input.

I´ll try it....thanks

I´ll let you know as soon i get some results.