[sdiy] Rick Jansen Moog filter revision -> problem solved! -> next problem PSU!

[Rutger Vlek]

Hi Alan, thank you! The email got through (but not the first time!). I think the DIY list has some problems. Or maybe it's just me ;). A couple of days I responded to Tim Stinchcombe's email but that only made it to him through direct email and not via the list (and that was what I meant to do). The problem with the filter is solved :). I found out that I made a mistake when I drew the schematic in Eagle PCB from Rick Jansen's original. I put a 1M resistor between the expo converter and the ladder instead of an 1K. Now I got it fixed it respondes nicely to incoming control voltages. Next in my to-debug-list is the resonance cv control. Before I can do this I must assure that a proper CV is available, which means that I have to get the DAC's running properly first. The DAC's are controlled by PIC's that are running the MIOS (operating system) and MBSID V2 beta 6 software (www.ucapps.de). I intend to modify the DAC driver to output dual 12-bit CV control for the filter frequency and a single 8-bit CV for the resonance (that is shared between 2 filters -> no need for stereo resonance differences). A big issue at this stage however is the PSU and grounding. Something seems to be really wrong there. The PSU unit is built according to this schematic: http://members.home.nl/r.j.vlek/psu2.jpg. Voltage outputs are alright now (the problem with blowing fuses is fixed. I found out that the body of a 7912 regulator carries the input signal and was shorted to ground through the heat sink). However, the problem is a very unclean ground (at least I think that is the problem). These are the anomalies I found: - When I use the ground (in this case the grounded heat sinks) close to the PSU as reference for my oscilloscope and measure the ground elsewhere it carries some sort of positive part of a sine wave. I see the same thing when I look at the voltage outputs (+5V, -12V) of the PSU (referenced to the heat sink ground), however I don't see it at the +12V output. I have not attached any external grounds and I have a floating ground system at the moment (no connection to earth). The PSU is part of my main PCB but does not carry high voltage. Transformers (single 9V and dual 15V) are in the same metal 19" case on a seperate PCB. The voltages 2x15V AC and 1x9V AC are fed to the PSU on the main PCB. The case is electrically isolated from the ground in my circuit. Any thoughts? I'm assuming a grounding issue and not regulation issue since I think I have quite decent caps for ripple rejection (4700uF for the +5V rail that draws about 1.5A max and 2200uF for both the +12V and -12V which draw about 0.5A each. Kind regards, Rutger

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