Electronics > Projects, Designs, and Technical Stuff

OSH Audio THD instrument and live BOM

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rhb:
What I really wanted to call attention to was his methodology. not copying the circuit.

asgard:
I will have to do a little thinking about two of the things in that article, first the use of a CdS cell/LED pair to substitute for the JFET, and the other is the use of two adjacent 1N4148 diodes to somehow thermally stabilize the circuit.  I suppose that involves why a series reference diode also needed to be there.

asgard:
for what it is worth this is the oscillator module prototype.  The fact is that for whatever design changes I want to make in the future, the cost is mitigated by the modular design method employed here.  That lowers the bar to adoption for this instrument to what I hope is a manageable level.

The main point of this thread was not any specific circuit design decision, but an overall management strategy using what I hope is a useful spreadsheet tool that allows for project visibility from parts purchases to fabrication and assembly.  This instrument is intended as an example of a multi-assembly project employing such a tool.

rhb:
Very nice. My interests lean a lot in the RF direction. I eventually want to assemble a collection of modules that can be used for prototyping a design.  For that, having everything in  metal boxes with standard connectors, dimensions, etc is desirable.  So very much in the same vein as your efforts.

But I'm still buying test gear so I haven't got to the PCB and BOM yet.

I suspect that to really understand Jim's paper you should review the detailed physics of the devices.  Williams, Pease and Widlar were renowned for their understanding  of subtle details that less thoughtful people just blundered by and which were the difference between a circuit working and not working.  I also think the inclusion of the A2 output trace in the figures is a strong hint.

asgard:
These two pictures are the notch filter module and a view of the EAGLE image for the main board (the rev A board is coming back from fab sometime later this month, so the pretty parts picture is not available yet).  This picture is after I decided to allow for a seventh module to contain the track-and-hold circuit and another bypass toggle associated with it.  I am breadboarding the track-hold now and will test it externally for now, but allowance is made for the next (rev B) run to modularize it.  It might have to be done with SMD parts to fit the form factor, but specifying the parts for the ultra-stable construction I want is way more problematic in the 0805 or 0603 sizes I typically use.

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