Moog Theremin T-Shirts

Theremin Pitch Antenna

Showing Posts 1 - 4 of 4
Posted: 1/19/2013 7:10:45 PM
RS Theremin
From: 60 mi. N of San Diego CA
Joined: 2/15/2005

threads posts

dewster, when I made the statement of my pitch drift being less than one musical note when set at 300 Hz over a room temperature change of 10 degrees F was not an exaggeration. Lev’s Antenna is all I have ever known and my conversation with other designers might be disqualified because we are comparing apples with oranges.

When you have drift over time at a consistent room temperature that is from unwanted internal warming which must be avoided. That is one reason my oscillator section can be separated from the main board if needed.

Lev’s Antenna is a hardened steel 16” spring coil as you know. This could be compensating or countering external room temp induced thermal drift by changing the pitch oscillator loading when the antenna expands or contracts ever so slightly? (Due to lack of formal electronic training I will use the word loading) I am sure the apples use another fundamental term. LOL

Do you see scientific bases for this thinking?

Maybe the antenna has more than one effect on the pitch other than originally recognized.

Christopher

I need an invitation from a So. California Classical Thereminist that I can set up the Theremin Phoenix so they can validate my theremin voice, response & research.

Posted: 1/20/2013 12:17:42 AM
RS Theremin
From: 60 mi. N of San Diego CA
Joined: 2/15/2005

threads posts

Hello Christopher, (-'

Thermal drift is interesting,  my pitch and reference oscillators are identical in design.. almost:

The tunable oscillator freq is controlled by its emitter current using a variable 500 ohm pot and this same oscillator has the Lev Antenna direct connection to its NPN collector.

The fixed reference oscillator transistor deliberately operates at a “lower current” with a 1K resistor and small parallel RF bypass capacitor mounted between the parallel tank to +V.

In early experiments I used a trimmer pot in place of the 1K off the tank to +V and noticed I could change whether the audio frequency rose or sunk in relationship to rising room temperatures. There is a balancing point between the two oscillators dancing together. Today I just use a 1K at the tank of the fixed oscillator as every mechanical component adds to the possibility of total failure at some future appointment.

I use transistors side by side off a tape reel hoping for matching batch characteristics.  I do use NPN transistors of a certain breed used by many of you.

The above may be an illusion but I don't think so as it works for me.

Posted: 1/20/2013 4:16:05 PM
dewster
From: Northern NJ, USA
Joined: 2/17/2012

threads posts

"Do you see scientific bases for this thinking?"  - RS Theremin

Not being an experienced materials engineer, I'm probably not the best person to ask.  But in general, Murphy's law being what it is, if anything can influence drift it will most likely make it worse rather than better.  Perhaps you've stumbled on a good combination of things, or perhaps you're not testing it rigorously enough to reveal any downsides?  Without testing everything is just wild speculation.

 

Posted: 1/20/2013 4:54:29 PM
RS Theremin
From: 60 mi. N of San Diego CA
Joined: 2/15/2005

threads posts

dew said: "Perhaps you've stumbled on a good combination of things"

Yes the good combination is always desired as I always say, "a theremin has more reasons to sound bad than good and to misbehave to boot." Having a happy theremin is very enjoyable.

"or perhaps you're not testing it rigorously enough to reveal any downsides?  Without testing everything is just wild speculation."

I once wanted to put my theremin design in a local hands on kids museum but I did not trust the circuitry for many reasons. Thermal drift over the course of the day of maybe a 30 degree F room temp change was one of them. I wanted the Null point to be set and stay there automatically. I did create a digital method of sampling the "audio freq" but my MPLab (nice products) PIC18F4XK20 programing got confused sometimes when on the wrong side of Null.

The next approach was to use human remote tuning on a 50' cable which was the best approach at that time. Separating each theremin aspect into individual functions is the direction I am evolving my design.

Christopher

Showing Posts 1 - 4 of 4

You must be logged in to post a reply. Please log in or register for a new account.