Artemis Labs LA-1 Schematic
Technical Features of the
2
1
3
V1
1/2 56 87
V1
V2
Rig ht
Inp uts
Right
Balan ce
Volume
Input
Selec tor
Mono
Power
Rig ht
Out put s
1
2
4
5
3
2
1
50K
steppe d
atten.
5
4
8
5
4
8
+160V +250V
+250V
+6V
+163V reference
Muting relay
contro l
100K
lin
Lundah l
500H
choke
Power Supp ly
(unre gulat ed)
40 se cond
Delay
4 sec ond
Delay
Lin e v olt ages
ava ila ble :
100 V,
110V,
120 V,
220 V,
230 V,
240 V
50 or 60H z
Fea tur es:
- E lec tro stati cal lyshi eld ed trans for mer
- H igh -sp eed r ect ifi ers
- Hig h-t emper atu re
(10 5°C ) e lectr oly tic s
7
9
6
V2
1/2 56 87
Lef t
Inp uts
Left
Balan ce
Lef t
Out put s
1
2
4
5
3
2
1
50K
steppe d
atten.
+160V
100K
lin
Lundah l
500H
choke
Input Co ntrol Gain Muting & Ou tput Low-G ain R egulati on Power Supply & Ti me De lays
LA-1 Li ne Ampl if ier
Simplified Schematic
Artemis Labs
Artemis Labs LA-1
Single-triode gain stage with choke loading
Experience has shown that the best audio quality
is achieved with the minimum number of active
devices in the signal path. Each amplifying device
has its own “transfer characteristic” and when these
are multiplied together in a complex design, the
result is an overall transfer characteristic with highorder distortion products - the worst-sounding
ones. The Artemis Labs LA-1 uses a single 5687
medium-mu, low resistance triode as the amplifying
device for each channel. The 5687, originally
designed by Tung-Sol for military and industrial
uses, has excellent linearity, and in the mil-spec version, excellent unit-to-unit consistency. Its low plate
resistance allows it to drive heavy loads without the
use of a cathode follower stage.
The use of an inductor (choke) load instead of a
conventional plate resistor offers several signicant
advantages to the LA-1 design. A good-quality
audio choke is essentially a perfect current source,
allowing the triode to operate into an innite-resistance load, giving the lowest distortion. Unlike
semiconductor constant-current devices, an inductor is an energy-storage device. This allows the plate
voltage to swing 100% above the B+ plate supply
voltage, doubling the “head-room” and allowing a
lower B+ supply voltage to be used, reducing power
consumption. This large headroom permits output
of over 40 volts rms before clipping.
The audio chokes used in the LA-1 are made by
Lundahl, a Swedish rm that uses a proprietary core
material that has low distortion. Two windings are
used on a common “C-core”, which reduces pick-up
of hum and noise.
Modest feedback stabilizes gain without hurting
sonic quality
About 4dB of feedback is used across the singletriode amplifying stage. This stabilizes the gain and
makes the frequency response less sensitive to loading. The inherent distortion of the 5687 with choke
loading is already very low, so the high-order harmonics that can be generated by excessive feedback
are minimal.
Can drive 8K ohm (aggregate) or higher loads
Despite the lack of a cathode-follower output stage,
the LA-1 can drive loads as low as 8K while still
meeting specications. This is important when
driving certain solid-state ampliers or pro audio
equipment which can have input impedances as low
as 10 or 15K ohms.
High-Quality Conductive-Plastic Balance pots
Since a stepped attenuator allows only discrete level
steps, the balance controls are conductive plastic
pots by Spectrol, which have a minimum impact on
the sound. By using a separate pot for each channel,
the default position (fully clockwise) permits full
gain from the preamp.
Each channel individually voltage regulated
Each channel has its own MOSFET source-follower
low-gain regulator. This insures there is no coupling
between the two channels through the power supply.
The source follower regulator gives good regulation
without the “transistor sound” typical of high gain
regulators using op-amps. Protection resistors and
diodes help prevent burn-out of the MOSFETs.
DC heater supply
A well-ltered DC heater supply helps keep hum low
and allows the use of tubes that would otherwise
have too much hum in AC-heated systems.
Delayed high-voltage with delayed muting relay
The high voltage plate supply is applied to the
tubes after a delay of about 40 seconds. This gives
the tubes ample chance to warm-up without the
chance of “cathode stripping” (degradation of the
cathode by bombardment by positive ions before it
is hot enough to build-up a protective space charge
around the cathode.) This helps extend tube life. A
muting relay shorts the audio outputs until about 4
seconds after the high voltage is applied. This allows
start-up transients to die away.
No troublesome integrated circuits
All time delay and regulation circuits use timeproven, simple discrete semiconductor devices.
True voltage regulation minimizes noises and
changed operating conditions due to power line
shifts. Since the impedance of the power supply,
as seen by the amplier, is low and at, the bass
response is even and solid.
Cool-Swap™ tube connections let tubes run cooler,
provides built-in spare
Many dual triodes, such as the 12AX7 and 5687,
have split heaters, allowing them to run from either
6.3V or 12.6V. This heater set-up allows only one
of the two triodes to be heated. In the Cool-swap
conguration used in the Artemis Labs LA-1, one
tube is used for each channel, each with one half
heated. The right channel uses triode #1 and the left
channel uses triode #2. Since the main failure mode
in modern tubes is cathode wear-out, the unused
triode in each tube is essentially a spare. By swapping the two tubes, the unused spare is brought into
use. Thus Cool-Swap essentially doubles tube life,
and since only half the tube is heated, the tube runs
cooler, an important factor in the normally hotrunning 5687. A side benet is reduced cross-talk
between channels, since each channel has its own
tube.
Critical audio circuits point-to-point wired with
military-style terminal boards
Conventional ber-glass PC boards are used for
the power supply and for the rear panel jacks and
switching. All critical circuits in the audio path
use point-to-point wiring on military-style terminal
boards using silver-plated turrets. This technique
minimizes the sonic effects of PC boards and allows
easy component replacement.
High-speed rectiers and electrostatically-shielded
power transformer
Power supply noise is kept to a minimum by the
use of a custom power transformer with electrostatic shielding between the primary and secondaries. High speed rectiers are used for both the plate
and heater supplies.
Available mains voltages (wired at factory): 100V,
110V, 120V, 220V, 230V, 240V, 50 or 60Hz
Virtually all the power systems of the world can
be accommodated by the LA-1 transformer connections. These are set at the factory or can be re-wired
by a qualied technician.
Artemis Labs products
are distributed by:
AYDN Vacuum Tube Audio
679 Easy Street, Unit E
Simi Valley, California 93065 USA
www.aydn.com
Tel: +1 818 216-7882
email: info@aydn.com
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