THE WORK OF OTT
Below is a "photo-schematic" detailing the layout, built as it might
be drawn from schematic. Included are oscilloscope captures that
show what half- and full-wave rectification look like. The letters
A, B and C in circles designate the test points for both the oscilloscope
and voltmeter probes. Some visitors have found the image a bit hard
to follow, so a "real" schematic, sans transformer, is now included below.
The Photo-Schematic above and a "real" schematic below. Please
note the addition of C4.
THE CONVERSION PROCESS
All audio circuits require DC power before amplification can occur.
Like an audio signal, the power that flows from a wall socket is also AC,
specifically, a 120 volt RMS sine wave. (Line voltage varies with location
and demand. RMS is the "effective" or equivalent power a sinusoid can deliver
when compared to its DC counterpart. Actual peak-to-peak voltage is 343
FROM LEFT TO RIGHT
The power cable is attached to a switch and a fuse. The transformer
converts 120 vac to 25 vac. Notice that there is no physical connection
between "primary" and "secondary" windings. The "connection" is made by
induction: the flow of current in a wire radiates a magnetic field into
the iron core (the vertical lines). (Guitar pickups "gather" noise — hum
from power transformers and buzz from light dimmers — also by induction.)
The ratio of turns between primary and secondary — in this case, 4.8 to
1 — will determine the voltage.
The conversion from AC to DC is called rectification. Logic would dictate
that 48 volts AC makes 48 volts DC. This is almost correct. I chose a 25
volt transformer because you can buy one at Ray’s Shack. A little sleight-of-hand
will squeeze out the necessary juice…
The oscilloscope insert to the right of the transformer displays a slightly
clipped sine wave of about 75 Vp-p. Diode D1 passes only the positive portion
of wave "A." Notice that diode D2 is reversed so that it passes only the
negative portion of wave "B." This circuit configuration is known
as a "voltage doubler."
Filter capacitors C1 and C2 smooth out the "ripple" that results from
rectification. (See the ‘scope insert to the lower right of the xfmr.)
Traces "A" and "B" were measured by first placing the ground lead at test
point "B," measuring "A." Then, leads were reversed so that ground
is at "A," measuring B." Notice that the frequency of the "A" and
"B" traces is double that of the "C" trace. This is full wave rectification.
Unfiltered, it yields a 120 Hz hum.
IT’S ALL RELATIVE
With the scope’s ground lead at test point "C," the wave form observed
at test point "A" will yield trace "C." (The polarity will be reversed
when observing test point "B.") This is what half-wave rectification
looks like. If the circuit did not continue beyond capacitors C1
and C2, the power supply would yield plus and minus 35 volts referenced
to "C," or, 70 volts when measured from "B" to "A."
The relationship of 35 volts DC to 25 volts AC has to do with the fact
that a sine wave is involved. Dividing 25 volts RMS by ".707" (35.36)
and then multiplying by 2 (70.72) yields the peak to peak value of the
sine wave. (The square root of 2 — 1.414 — divided by 2 yields .707, hence
R - M - S or root - mean - squared.)
Z1 and Z2 are 24 volt, zener diodes. When reversed biased (as shown)
a zener diode acts as a DC voltage limiter. Placing two in series
creates the 48 volt reference. R1 (2.7kW) sets the "knee" so that
the circuit will maintain regulation even if the line voltage drops to
92 volts. Increasing R1 will raise the knee and decrease regulation.
Decreasing R1 will overheat the zeners. A side-effect of regulation
is that it further reduces the ripple to almost a straight line.
Q1 is an NPN transistor in the "common base" configuration. It is a
general replacement type, either NTE or ECG 210. Its American counterpart
is a 2N6551. Raw 75 volts is fed to the collector (c)
which serves as the input. The reference voltage is connected to
the base (b) and
the output appears at the emitter (e)
six-tenths of a volt lower than the reference. This drop is typical of
a silicon junction device.
When the power line delivered 122 volts, 28.2 Vac appeared at the secondary.
This became 75 Vdc, rectified. The reference, 49.5 volts, was a bit higher
than expected and yielded 48.9 volts (at the emitter) with no load.
R3 serves as a current limiter (protection), but without it, the circuit
can easily drive a 1kW load without budging.