Oracle
Delphi IV turntable, SME 345 tone arm, Grado Reference hand-built cartridge.
The
support system for Oracle consists of a sand-filled plinth made from 1 1/8”
MDF with a rubberized mat placed on top of the sand and a ¾” MDF shelf on the
top surface. The mat is arranged in
such a way as to prevent any mechanical contact between the top shelf and the
sides of the plinth, i.e., the shelf rests solely on the sand. The plinth contains 40 lbs of sand. The Oracle is placed atop the plinth with three tiptoe
aluminum cones for additional isolation. The
hung-spring suspension system in the Oracle’s three towers provides excellent
damping and isolation from any vibration that may enter the Oracle’s base.
The minimal mass and dimensions of the Oracle’s platter/tonearm board
provide superior isolation from airborne acoustic feedback.
Other
Sources (not pictured): Theta Jade
Transport and Theta Generation V CD playback system; Marantz ST-55 Tuner.
Phono Stage:
The phono stage is a
classic topology using two Electro-Harmonix 12AX7EH triodes in cascade
(common-cathode stages), with a 6FQ7 cathode follower.
These three stages give an open-loop gain of approximately 2300 (67.3
dB). The Bias of each 12AX7 is
around 0.9 mA per section with >150V on the plates, and the 6FQ7 follower
runs @ 8mA, 200V per section. One
thing was apparent in the earliest listening sessions – a slight sacrifice of
tube life in favor of a slightly rich bias scheme pays huge dividends in the
texture, body, and palpability of the music.
The RIAA equalization is
the global-negative-feedback type. With
the EQ network connected, the 1- KHz gain is approximately 130 (42 dB).
Many designers have discussed the advantages and disadvantages of various
RIAA EQ topologies. One disadvantage of the feedback EQ topology is, of course,
potential instability at high frequencies.
HF stability is ensured in this design through the use of a series-RC HF
gain degeneration network that is connected effectively in parallel with the 1st-stage
plate load. Careful tuning of this
network gives a HF phase margin of >45 degrees with minimal attenuation of
the open-loop gain @ 10 KHz. This
stability is achieved with approximately 60 pF of load (cable) capacitance.
One often-neglected advantage of the feedback EQ topology is distortion
reduction at high frequencies, which is a desirable attribute in a phono stage.
Since the largest voltage swings in vinyl recordings typically occur at
the highest recorded frequencies (this is a result of the pre-emphasis curve),
the high-frequency negative feedback helps to reduce any distortion contribution
from the phono stage. Harmonic
distortion measures 0.015% at 1 KHz, 1V RMS out.
This is equivalent to 0.03% to 0.04% when the RIAA pre-emphasis is taken
into account. Due to the operating
points of the 12AX7’s and the 6FQ7 follower, the phono stage can swing 140V
p-p @ 1 KHz into a 47 K-ohm load before clipping!
Input overload margin at high frequencies is no problem.
The EQ network consists
of two capacitors (one Teflon, one Polystyrene) and one resistor to set the RIAA
time constants of ~3180 uS, 318 uS and 75 uS.
This is an unconventional network in that it only uses one resistor
instead of two. In this classic
tube design, the second resistor (1-10 Meg) normally limits the closed-loop,
low-frequency gain to a value slightly lower than the open-loop gain.
In a typical design, it would be selected to provide a turnover point of
approximately 50 Hz (3180 uS) and a low-frequency gain (10-20 Hz) that is almost
20-dB higher than the 1-KHz gain (just under 62 dB in this case).
However, critical listening favored the elimination of the second
resistor, which allows the closed-loop gain to reach approximately 64 dB @ 20
Hz. This represents a peak
deviation of around 2 dB from the RIAA curve at 20 Hz.
The largest time constant is therefore set by the intersection of the
closed-loop response, which rises with decreasing frequency @ 6 dB/octave, and
the open-loop low-frequency rolloff characteristic. The result is an effective time constant of >3180 uS.
This subtle low-frequency lift proved a favorable characteristic,
especially on Classical music.
The added gain
low-frequency gain made the choice of interstage coupling capacitors critical in
order to limit the phono stage’s gain at infrasonic frequencies where
turntable/tonearm/cartridge resonances reside.
The chosen values give a cutoff frequency of ~12 Hz for the 1st
interstage and around 5 Hz for the output coupling. These two poles combine to give sufficient attenuation (2 dB @
10 Hz, 8 dB @ 5 Hz) of infrasonic frequencies with considerably less phase shift
than some of the more commonly employed “subsonic/ rumble” filters used on
popular phono stage designs. Of
course, this frequency contour is system and room specific. A higher-frequency and perhaps a steeper cutoff would be
necessary if not for the excellent acoustic isolation that results from the
combination of speaker/turntable placement and the suspension and support system
of the turntable itself (see SOURCE). In
addition, the loudspeaker system is fully capable of handling a degree of record
warp or other source-related infrasonic energy cleanly (some infrasonic energy
essential to the realistic reproduction of the recording, especially in
Classical music). The lowest octave
(15 to 30Hz) is relegated to the subwoofer drivers, each of which can deliver
>110 dB average 20 Hz sound level @ 1M with less than 1% THD.
The midbass driver’s excursion is electrically
limited below 10 Hz via the 0.22 uF TFT coupling caps at the high-level
stage’s full-range output (see High-Level Stage).
The outboard power supply
employs a Dynaco MKIII power transformer, a 5AR4/GZ34 rectifier feeding a 20 uF,
630-volt metallized polypropylene input filter capacitor, a 20H choke, and a
multi-section can electrolytic capacitor of 20/20/20/30 uF @ 500V.
The can is used as two, two-section caps with a 1K series resistor
between the first and second cap to make a pi-filter.
The high-voltage output (~510 volts) is fed into the phono stage main
chassis. The heater supply uses a
“brick” full-wave bridge rated @ 25A driving a 100,000 uF, 16-volt, computer
grade electrolytic capacitor. This
output (~7.5V, unregulated) is fed into the phono stage main chassis.
Heater regulation is
handled inside the main phono chassis by a Linear-Technology LT1083 adjustable
regulator. This regulator is fed by
the 100,000 uF, 16-volt filter capacitor that resides on the outboard power
supply (large blue can). Input
filtering to the regulator is done via two 14,000-uF, 16-volt caps with output
filtering of ~1000 uF for noise cleanup and stability.
Output ripple (the most critical parameter on heater circuits) is a low
75 dB below input @ 120 Hz. The
regulator’s input ripple is 140 mV, so output ripple is ~25 uV (0.025 mV).
In tube electronics, transparent & neutral sound starts with quiet
heaters! The heater circuit
is sufficiently isolated from the amplifying electrodes so heater regulator
noise is not considered an issue. However,
in order to minimize output noise, the LT1083 output is filtered with a 4700-uF,
16V capacitor here. The output
noise is specified as .003% of output voltage with a 10uF tantalum output cap,
which calculates to 0.189 mV for 6.3 volts.
Output noise should be considerably lower than this with the 4700-uF
value. Critical listening sessions have favored heater voltage
in the 6.4-volt range over lower values.
Inside the phono stage main chassis, the high voltage is split into three individual legs, each with pi-filter sections. The first leg feeds the first stage B+ (425 volts), the second leg feeds the second-stage B+ (425 volts), and the third leg feeds the cathode-follower B+ (375 volts). Each leg uses a combination of large electrolytic capacitors and large-value metallized film caps (100 uF, 630V polypropylene Solen and 360 uF, 500V polypropylene/ polyester Unlytic) for an unsurpassed low-noise supply. No regulation is employed on the high-voltage supply. For all their virtues, regulators generate broadband noise, which is very difficult to completely filter out once generated. Broadband noise on the B+ couples directly to plate outputs with only 5-6 dB of PSRR (power-supply rejection ratio) in typical common-cathode gain stages. This noise can be heard as “whiteness” in the background of most vacuum-tube consumer electronics. This artifact would be most apparent if regulators were employed in the B+ supply of low-level phono circuits. The passive power supply employed here should give the same blackness of background that is observed with battery-powered phono stages. However, the vacuum-tube circuit used here provides much greater musicality and higher overload characteristics than solid-state circuits.