Most all the electronics in the signal path of the Vinyl Vault system are constructed using vacuum tubes and point-to-point silver wiring. The power supplies are overdone with a huge number of high quality caps and large-value (20H) filter chokes. The subwoofer part has gone over to the dark side with op-amp crossover and a solid-state 200wpc/400wpc (8/4 ohms) amp. The Pass Aleph 5 is the spare amp that is used when the Samps (Sam’s Amps) are undergoing modifications.
Below
are the phono stage on the left and two outboard power supplies on the right.
Power from the outboard
power supplies is provided to the phono stage and the preamp via two short snake
cables. Then, once the power is fed into the phono and preamp, even more
DC filtering is used for the left and right channels as well as for different
voltages. Compare all these caps to the one or two small electrolytic caps
found in most preamps and you can begin to imagine the inner detail of the music
and realize the complete blackness that the music in the Vinyl Vault system
emerges from.
High-Level Stage:
The high-level stage is a
simple circuit with no overall negative feedback.
It uses a common-cathode gain stage direct coupled to a cathode follower.
Distortion is minimized through use of the same tube type in the gain
stage and cathode follower – the inverse operation of these two stages
provides a degree of distortion cancellation (sometimes referred to as “predistortion”).
Optimizing the operating point for both stages in concert is vital in
obtaining the most transparent sound in this arrangement.
The tube is the Russian-made 6H30, which is one of the most rugged and
lowest-distortion medium-mu triodes available.
THD measures 0.03% @ 1V RMS with no negative feedback.
The CC gain stage idles @ ~18 mA per section and the cathode follower
idles @ ~22 mA/section.
Passive duties are
handled by a 24-step attenuator using Roderstein shunt resistors and a single
Vishay Bulk-Foil resistor in series. The
attenuator is wired in such a way as to place the switch contacts in the shunt
signal path only. There is no
selector switch in the high-level stage, thus completely eliminating switch
contacts from the signal path. All
other fixed resistors are Vishay Bulk-Foil type, which are the lowest-noise
construction available. Coupling
capacitors are Rel-Cap Polystyrene (separate film/foil) for the low-frequency
output and TFT Teflon (separate film/foil) for the full-range output.
The full-range output capacitor is 0.22 uF in order to prevent infrasonic
energy below 10 Hz from entering the three-way speaker system.
The full-range output capacitor is bypassed with a 0.01 uF TFT cap for
unsurpassed high-frequency transparency.
The outboard power supply
employs a Dynaco MKIII power transformer, a 5AR4/GZ34 rectifier feeding a 20H
swinging choke, and an R-C pi filter. It
was necessary to use a swinging choke to reduce the power supply’s output
voltage to approximately 290V (there was over 500V available from the old Dynaco
MKIII power transformer). The pi
filter consists of two multi-section (20/20/20/30 uF) cans (each is paralleled
for ~100 uF) connected via a 1K series resistor.
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.
As the photo shows, most
of the space in the main chassis is occupied by power supply filter capacitors.
The high-voltage filtering is a double-pi section.
The large white cans are 360 uF, 500V bypass capacitors, made by Unlytic
Capacitor Corporation (metallized polypropylene /polyester formulation). These caps in parallel with the large electrolytics comprise
a large-value filter capacitor that is essentially free of ESR, ESL and
dielectric absorption mechanisms throughout the audio frequency range.
The electrolytic caps on the main B+ are a combination of Mallory 1200uF,
250V (x2) and Nichicon 1000uF, 200V (x4, small black cans).
The total capacitance on the B+ is around 7000 uF; total for the
high-voltage supply is ~9000 uF. Smaller
(white) 10 uF metallized polypropylene caps are used on top of the Unlytics as
local bypasses for the B+ line that feeds the tube circuit.
The yellow tubular caps on top of the Unlytics are 1.0-uF polystyrene
(separate film/foil) caps. Four
smaller value polystyrene caps can also be seen nestled amongst the circuit
wiring for local bypassing at the plate-resistor B+ wiring node (gain stage) and
at the plate B+ wiring node for the cathode follower.
The high-level stage power supply largely sets the available size, depth,
and solidity of the system’s soundstage presentation.
Heater regulation is
handled by a Linear-Technology LT1083 adjustable regulator (Black finned heat
sink). This regulator is fed by a
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 (smallest blue cans),
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! Critical listening
sessions have favored heater voltage in the 6.4-volt range over lower values.
Power Amps:
The tube amps shown above
drive the main 10” 3-way speaker system.
These were designed and built by Sam Papadas. The amplifier circuit is based on the classic Williamson
design, circa 1946. This is among
the simplest and most elegant circuits available to achieve sufficient gain, low
distortion, and flexibility in a tube power amp design.
Here, it is implemented with a pair if KT-90 output tubes in triode
configuration with cathode bias. The
input/phase splitter stage is a 7308. The
driver stage is a diff-amp 6H30 standing on a ~5.1 K resistor.
Low-frequency stability
issues were encountered in the initial design, which were successfully addressed
by increasing the output stage cathode bypass capacitor from ~680 uF to ~4500 uF,
and staggering the cutoff frequencies of the 1st and 2nd
interstage AC coupling to give a >6:1 ratio. High-frequency stability is excellent, with typically less
than one cycle of ringing and <5% overshoot on 10-KHz square waves.
Output power is around 33 watts into an 8-ohm load with ~0.15 % THD.
The 3-dB bandwidth is excellent @ 120 KHz.
Global negative feedback is about 16 dB. Output impedance is 0.5 ohms nominal.
The power supply uses a
5AR4/GZ34 rectifier feeding a pi filter with an SCR 20uF, 630V metallized
polypropylene input capacitor. The
choke is 10H, rated @ 200mA. The
main filter capacitor after the choke is made from two 1100 uF, 450V
electrolytic can capacitors in series (black cans in photo).
This series combination forms a 550 uF, 900V capacitor.
Bypassing is handled by an SCR 20uF, 630V poly cap and a 0.47 uF, 600V RT
(Separate Film/Foil Polystyrene) capacitor.
A third 1100 uF, 450V can is used as the main filter cap for the driver
stage. This large value capacitor, coupled with the ~2.5 K dropping
resistor, ensures that no power supply ripple can be coupled from the large
Class-B swings of the output stage back into the Class-A driver stage.
The driver stage is bypassed with a Solen 39 uF, 630V metallized
polypropylene capacitor. The first
stage power supply uses a 1000 uF electrolytic capacitor bypassed with a 33 uF,
630V metallized polypropylene cap.