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|17th August 2008, 02:30 PM||#1|
Join Date: Oct 2006
Stepped Attenuator giving unexpected behavior
I need help diagnosing and correcting a problem I am having with a DIY stepped attenuator.
I use a Digidesign 002 and ProTools LE for recording. Playback is through a Mackie HRS-120 powered subwoofer that picks off the low frequencies from each channel and then sends the rest of the signals to a pair of Mackie 624 powered monitors. Until now I have been taking the output signal from digi 002's monitor outputs, which have a volume control pot. It works well, but it is impossible to reset the volume to exactly the same level due to the absence of detents.
So I built a stepped attenuator for volume control with the intention of connecting it between the main outputs of the digi 002 and the HRS-120. The attenuator is a series resistor design with both balanced inputs and outputs. The input impedance is 6.3 kOhms, and the output impedance ranges from 0 to 6.3 kOhms over 19 steps. Inputs and outputs are balanced via 1/4" TRS connectors.
The attenuator checks out fine. Impedances match within <<0.2% over most of the steps. I also tested it at several frequencies using a signal generator and oscilloscope, and the attenuation results matched expectations at all conditions.
When I connect it into the system, however, results are not good. First, instead of being silent at the first position, there is sound from the monitors (mostly the right monitor). Also there is a gross imbalance in the sound level coming from the left and right monitors over the next 4 switch positions (with the right being louder). It isn't until the fifth switch position (about 100 Ohms output impedance) that the sound levels match.
To diagnose the problem, I swapped the left/right inputs and outputs from the attenuator to see if one pair of decks was bad. There was no change...the right speaker remained the louder. I replaced all of the cables in the signal path, and again no change. I swapped just the inputs and then just the outputs to the attenuator, and again no change. Only when I swapped the left/right outputs from the subwoofer did the louder sound shift to the left monitor. So the attenuator seems fine.
I then removed the subwoofer from the signal path and connected the attenuator outputs directly to the monitors, and they worked perfectly. Silent at position zero, and equal in left/right volume balance at each higher position. So the problem would seem to be with the sub....but the sub works fine when connected to the digi 002's monitor outputs???
Based on these results, there is an interaction between the attenuator and the subwoofer. What could be causing this behavior, and how can I fix it?
For further background... All other devices in the signal path have balanced inputs and outputs. Both the sub and monitors have input impedances of 20 kOhms. Also, I tried inserting a 56 Ohm resistor into each signal line of the attenuator output (after the stepped switch) to increase the output impedance to a minimum of 112 Ohms (based on the observation that the system worked well when the switch was in the 5th position where the output impedance had reached about 100 Ohms). Doing this did not correct the behavior.
Thanks to any who have read this far. And thanks in advance for any insight.
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