At the Null Point the
Volume is Maximum, green LED-1 bright, breaking out of Null more negative
spikes (0 to 500 hz) quiets the sound.
|
This wave shape sample is taken from the Out-2B
TRS 3.5 mm stereo jack on the Phoenix Volume Control board. It will not sound
good but we
are only interested in the wave shape.
Sounds
like this .mp3 80k
Ugly but perfect for Volume Control
The
display reveals the amplitude of the signal arriving at the sound
card without any distortion like the flattening of the peaks by
overdriving. Pot-1 controls the signal level to the
Out-2B jack
and turned up too
high will introduce unwanted distortion.
Above
is real-time
but the wave shape can also be recorded into Audacity
and spread out to view.
Try
and match my frequency. The
image is Screen
Captured by
holding the "Alt"-
key and pressing "Print
Screen"
on your keyboard to transfer the image to the clipboard,
then paste the image into a Photoshop file and save as a .gif
image file. I
may ask you to send me this graphic file for troubleshooting.
Download this
Excellent Theremin Pitch
Tuner v1.3
Also download his
Spectrum Analyzer v2.9
Freeware
Do not pass the volume signal
through the ground loop filter when viewing it, this will invert and distort it.
|
An insulated volume loop will give better performance than
touching a raw metal.
Instead of coating
the outside of the volume loop with insulation, use rubber insulated wire
on the inside of the
loop made of plastic or metal
and connect this
to the volume antenna terminal.
Use two MPS-A42
Transistors on the
oscillator section for Q1 & Q2 and one near the 555 on the main board
Q3. Different oscillator transistors effect the wave shape
and thermal drift behavior.
|
Setting
up the Spiky Waveform for
PWM |
| Individual downward spikes
build up to quiet the volume, the higher the frequency the more
downward spikes
to the point it shut off the Vactrol's internal LED which mutes the
sound. |
|
This
is the Black
Art of Oscillators
looking for a spiky waveform.
You need to bypass the
10x
gain of U1b when using
the Volume Control spiky signal. The Volume loop connects to Ant-
(not Ant2) Method
#1 is most likely the method you
will use. Do not have any wires
in the ABCD terminal.
|
| The red LED-2
is an excellent tuning aid as you learn to understand its
response.
Method #1:
The Volume Loop connects to Ant- (not Ant2). A good spiky waveform is created
by connecting a 100k (47k)resistor from terminal B to
Ant2.
If the signal near Pot-1 sampled at the left side of C15-1uf is
pointing Down use this method. If Up go to Method #2
Set Pot-1 at a half
or less, do not overdrive the Q3 transistor circuit
triggering the 555 or the square wave out at pin 3 of the
555 will have a poor positive to negative pulse width driving the Vactrol.
R12-1M should be changed to 47k. |
| Method #2:
Loop connects to Ant- (not Ant2). A good spiky waveform is created
by connecting a 100k (47k) resistor from terminal B to
Ant2. If the signal near Pot-1 sampled at the left side of C15-1uf
is pointing Up use this method.
If the spike
is Down go to Method #1
Remove the Op U1 chip and fold out pin #7 so
it does not insert back into the socket. Pin #7 of U1 is located just
below the 00 of the R11 resistor. Then install a jumper bypass wire
from the center tip of Pot-1 over to TP-1. This prevents U1b from
inverting the signal. Pot-1
should be set halfway or more so you get the proper drive and
response from the 555 driving the Vactrol. |
|
Edit
8.04.13 Which
ever method is used a negative going 1 volt p-p spike must be
found at the right side of C1 near Output-2. Otherwise the Volume
Control will respond with no shading, just on/off. |
On the Volume Control board start with
Pot-1 and
Pot-2 set
1/2 way. Only Pot-1
controls the drive to the 555.
Both Pots control
the signal intensity at the Out-2B Jack, you don't want to overdrive the
sound card input which will flatten the peaks of your sample or
make it look square, peaks with flat tops. Use
a stereo cable to feed the volume oscillator signal from Out-2B
on the Volume Control board into your sound card direct with no hum loop
filter in the path which will invert the signal. View your wave shape in Audacity.
Do
not
install any Mute jumper wire
used at the
T5 terminal near the Vactrol until
everything is dialed in. Shorting across
T5 Mutes the sound.
Note: Some sound cards
might invert the displayed image when it is actually in the correct
direction. If the wave shape is correct and really reversed (the broad side is at the bottom) the
volume response will reveal this with a quick volume On/Off with no volume shading.
Do not use a
ground plate on the board used for Volume Control.
You must
have a reasonable waveform before you can start fine tuning the
frequency response of your setup.
The wave form graphic on
this page was
recorded in Audacity then spread out in the display. If you have
an oscilloscope monitor the Out-2B for quicker reference. It is important
that at around 100 hz the frequency top side of the wave form is wide while the
negative going spike is narrow. Volume shading
requires this spiky wave shape to perform effectively.
If the volume oscillator
signal is at Null then a flat line is what you would see in
Audacity while sampling the Out-2B signal on the volume board. The green
LED-1 will be On while the red
LED-2
will be Off. The Vactrol outputs maximum volume
at this point if an audio signal was passing through the Vactrol.
As the volume oscillators signal frequency increases with your hand approaching the
volume loop the
negative spikes get closer together with more of them, each negative spike
pushes the
volume quieter as the average spike count increases.
The Op Amp amplified signal drives the
volume board Q3 transistor which
drives the 555 IC for Pulse Width Modulation
output at pin 3. The 555 is used as a Schmitt Trigger to square up
and strengthen the signal. The output pin 3 of the 555 drives the Vactrol
internal LED and the two other LED's. C9-470 uf capacitor smoothes the 555 pulsing to average the current
through the Vactrol internal LED and LED-1.
The green
LED-1
is lit when the Vactrol can pass the maximum audio signal volume as it is
at minimum resistance. If one of the LED's never seems to light or
flash at power up, it probably is
mounted in the wrong direction.
LC Tuning can be done by only using the red
LED-2
on the Volume & Pitch board. Normally if it is lit the 555
is getting the heterodyning signal even if it is out of hearing range.
Observing LED-2
you can properly adjust the L1
oscillator coil without ever listening to Out-2B.
If the LED-2
is off and with the oscillators at the Null
Point, moving your hand near L1
or the antenna will turn the LED-2
on.
The Pitch board is tune to
~900 kHz and the Volume board to ~750 kHz.
While adjusting L1,
passing over the Null Point the
LED-2 will quickly turn off then rapidly back on. Carefully adjust the
LED-2 to be off
at the Null Point.
If the red
LED-2
is On the sound is most likely attenuating or off because the volume
oscillator is producing audio spikes and not at the Null point. Only
red LED-2 can be substituted for any type
or color of LED you like. LED-1
is critical as its forward voltage needs to stay below 2.5v for the
volume control shading to be correct.
You want to find a balance
between the Null Point set by your hand distance from the loop and the
maximum quieting response when turning Pot-1 clockwise with your hand
closest to the loop. Experiment with this as volume shading can be quite
good. It is all about using the proper wave shape. Also adding Modification
R32-2.2k enhances shading even more. You could
use a switch
to flip R32 in or out to
select the desired shading response.
.
