Etherwave questions

Posted: 3/29/2008 3:42:45 PM

Joined: 3/8/2006

I found a schematic for the RDS theremin which is a copy of the Etherwave.
The schematic is very much like the one in the Hot Rodding and Etherwave pdf except in two places!
In the pitch tuning circuit C11 is a 100uF cap going to ground, in the Hot Rodding pdf it is 47uF going to +12V.
Here is a cropped pic:
I have always thought it was an error in the Hot Rodding manual....
Is the RDS correct or the Hot Rodding manual?
I also noticed that C13 has been changed to 1uF from the 22uF value of the Hot Rodding manual.

Last question, some pics I have seen of the circuit board show 4 volume and only three pitch inductors with the middles ones perpendicular to the others.
Notice the small L12 inductor in one pic.
Does anyone have the values of these inductors and why they were changed?
Is this a newer board or and older one?

Sorry for the long winded post but curiosity is killing me!

Posted: 3/29/2008 5:27:05 PM

From: Colmar, France

Joined: 12/31/2007

C11 is a buffer capacitor for the control voltage (0 to -12V) arriving on >5. Its value is not critical. And for RF concerns, it doesn't matter if it is connected to ground or to +12V.

Almost the same thing for C13. The dimension of it is more or less arbitrary.

Concerning the coils, the answer is simple: The pitch antenna needs theoretically a 40 mH coil. But such big coils have also a big capacitance between their layers which may decrease sensitivity and linearity of the pitch antenna. Thats why it is better to replace it by two or more smaller coils in series, i.e. 20 + 10 + 10 mH or 10 + 10 + 10 + 10 mH.

Same thing for the oscillator coils: There are Etherwave designs with a single 100 uH adjustable coil and others with 47 uH adjustable plus 33 uH fixed... Perhaps these differences are also due to availability problems for some specific parts.

The orthogonal position of the antenna coils is to prevent inductive and capacitive coupling.
Posted: 3/29/2008 5:42:52 PM

Joined: 3/8/2006

Thanks, Thierry.
That answers my questions.
I know that the variables were replaced by a fixed and a variable and why.
What I did notice in that area is that L14 looks like a molded inductor but L12 and L14 are small varnished chokes.
Parts availability issues again?
We are deep into a digital and surface mount era and I fear it will be harder and harder to get old style through hole parts...
I do mess around a abit with surface mounts but it is hard to work with those little itty bitty things.

Now I am off to lay out pcbs for an Etherwave WITH the add ons like mute switch, tuner and headphone amps, and CV outputs.


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