Page 7
TECHNICAL FEATURES
2
1
3
V1
1/2 5687
V1
V2
Right
Inputs
Right
Balance
Vo
lume
Input
Selector
Mono
Power
Right
Outputs
1
2
4
5
3
2
1
50K
stepped
atten.
5
4
8
5
4
8
+160V +250V
+250V
+6V
+163V reference
Muting relay
control
100K
lin
Lundahl
500H
choke
Power Supply
(unregulated)
40 second
Delay
4 second
Delay
Line voltages
available:
100V
,
110V
,
120V
,
220V,
230V
,
240V
50 or 60Hz
Features:
- Electrostatically-
shielded transformer
- High-speed rectifiers
- High-temperature
(105°C) electrolytics
7
9
6
V2
1/2 5687
Left
Inputs
Left
Balance
Left
Outputs
1
2
4
5
3
2
1
50K
stepped
atten.
+160V
100K
lin
Lundahl
500H
choke
In
s
LA-1 Line Amplifier
Simplified Schematic
Artemis Labs
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
high-order distortion products - the worst-sound-
ing ones. The Artemis Labs LA-1 uses a single 5687
medium-mu, low resistance triode as the amplify-
ing device for each channel. The 5687 has excellent
linearity. 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 significant
advantages to the LA-1 design. A good-quality
audio choke is essentially a perfect current source,
allowing the triode to operate into an infinite-
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 firm 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 single-
triode amplifying stage. This stabilizes the gain and
makes the frequency response less sensitive to load
-
ing. The inherent distortion of the 5687 with choke
loading is already very low, so the high-order har
-
monics 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,
