I'm experiencing issues with the dScope III when connected to an (automotive) BTL amplifier (as are commonly used in car radio's). The problem seems to be that the outputs of these units have a DC voltage of about 6-7 Volts on both speaker wires (hot and cold wire) compared to the power- and input ground. Regardless of the analyzers' output GND setting (unbalanced, floating, etc.) the analyzer registers extreme levels of distortion, but only up until a certain level. At some point the input attenuator switches (audibly) and the measured signal becomes perfectly normal. When I decrease the level again the attenuator switches back at a lower point (there is some overlap/hysteresis) and the signal get's distorted again as soon as the attenuator switches range.
So there is a kind of "grey zone" in the input level; depening on whether I am increasing or decrasing the signal level the attenuator can be in 2 different ranges for the exact same input level. In this grey zone I am measuring the same signal at the exact same level with either ~0,01% distortion OR ~8% distortion, depening on the range the attenuator is in.
I assumed a defect attenuator so I've exchanged the unit for a different one from our supplier but the issue remained exactly the same. Both units tested OK using the self-test macro. We tried setting the input highpass filter (in the software) to DC block but that didn't help either. Only if we add two bipolar capacitors in series with the test leads (to block the DC) can we measure the entire level range without issues.
Since we have 2 units with the exact same issue I was wondering wether this was actually a "feature of the product" or if maybe I'm doing something wrong. Hoping you guys have experience.
So there is a kind of "grey zone" in the input level; depening on whether I am increasing or decrasing the signal level the attenuator can be in 2 different ranges for the exact same input level. In this grey zone I am measuring the same signal at the exact same level with either ~0,01% distortion OR ~8% distortion, depening on the range the attenuator is in.
I assumed a defect attenuator so I've exchanged the unit for a different one from our supplier but the issue remained exactly the same. Both units tested OK using the self-test macro. We tried setting the input highpass filter (in the software) to DC block but that didn't help either. Only if we add two bipolar capacitors in series with the test leads (to block the DC) can we measure the entire level range without issues.
Since we have 2 units with the exact same issue I was wondering wether this was actually a "feature of the product" or if maybe I'm doing something wrong. Hoping you guys have experience.
I believe the dScope does not use audio isolation transformers like the Audio Precision does so it's probably best to use DC blocking capacitors.
cheers
david
cheers
david
Hi David, thanks for your reply! Apparently dScope has jumpers inside the device that determine how it handles DC coupling. I did not have the time to check them yet but I post it here as soon as I did. If that isn't the cause then probably you're right and the only way to go is use the capacitors. Strange thing though for an audio analyzer (my opinion) to not be able to handle DC to some extend....
Update: in fact the dScope was in DC-Coupled mode so this was the cause of the issue. By default it is in AC-Coupled mode but I bought it as a used item the previous owner must have changed the setting. Switched back to AC-Coupled and now all works fine.