AudioXpress 2006 06.pdf

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Mo d i f yi n g the Class ic Sc ot t A m p
JUNE
2006
US $7.00 • Canada $10.00
Back
The Audio Technology
Authority
Build This
250W
Horn Speaker
Combines
Tube Amp
Style
and
Function
Workbench
Test Gear
for your
Servo Sub
Design
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>
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Review of
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Constructing
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tubes
Mad Katy
A 125W per channel KT88 Amplifier –
Part 1
Meet Miss Katy, a stereo tube amp featuring KT88s, who seems to
possess the perfect temperament to faithfully reproduce music.
ad Katy is a 250W (stereo or
monoblock) tube amplifier
on a 17˝ × 22˝ chassis; weight
is about 70 lbs (
Photos 1-6
).
It has excellent sonic fidelity, transpar-
ency, and solidness up to clipping level,
achieved by circuit features designed to
minimize high-order distortion, reac-
tive load sensitivity, and compressive
gain modulations. Ruggedness and reli-
ability are such that it can be used as a
musical instrument amp.
To provide 250 sustained watts (1%
THD) from a 70 lb. tube amp required
PHOTO 1:
Mad
By Dennis Colin
M
the high efficiency
of push-pull Class
AB topology (and
the power-to-
weight efficiency
of toroidal trans-
formers). How-
ever, the potential
anomalies of Class
AB operation
have been avoided
by means of some
of the circuit fea-
tures (
Fig. 1
).
PHOTO 3: Left side.
PHOTO 4: Top view.
Katy
.
PHOTO 5: Rear view.
PHOTO 2: Front view.
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6/06
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TOPOLOGY AND DESIGN FEATURES
1. Forward (non-feedback) linearizing
of output stage transfer function with
complementary-transfer driver (
Figs. 1
and
2
).
2. “Partial Triode” KT88 mode: 67%
screen tapping maintains triode-like
characteristics (
Fig. 3
), but with twice
triode power output.
3. Negative feedback (from KT88
plates to 12AT7 driver input) includes
neither the output transformer nor the
12AX7 input stage. With phase shift
thereby minimized, stability and tran-
sient response are uncompromised (
Fig.
4
), and distortion is minimized with
(real-world) reactive loads.
4. 6SN7 cathode-follower driver buf-
fers are direct-coupled to the KT88
grids. The low-impedance drive without
coupling caps provides wide bandwidth
FIGURE 1: Topology. Forward linearizing: 0.27% THD, 1kHz, 100W, mostly 2nd and 3rd harmonic.
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audioXpress
June 2006
7
and smooth, anomaly-free overdrive
onset and recovery (
Fig. 5
).
5. Fully-balanced (differential) circuit-
PHOTO 6: Right input.
ry throughout
provides 113dB
“A” weighted
dynamic range.
6. Damping
factor is ad-
justable f rom
4, through es-
sentially infin-
ity, to a small
negative out-
put resistance.
PHOTO 7: JJ/Tesla.
KT88.
FIGURE 3: KT88 characteristics.
FIGURE 2: Voltage transfer functions.
Source = 1kHz linear triangle wave,
level for ±28.3V (200W Pk, 4Ω), 67W
average (100W if sine).
KT88 V
g–g
driven
from 50Ω source
through Hammond
1650T as step-up.
Each V
g
=0 to –120V.
Output voltage
across 4Ω load.
Expanding curvature
is transfer function
of output stage
(4
×
KT88, 67% UL,
HV
≈
+500V, Bias =
50mA/KT88)
Normal
amplifier input
=
±1.095V
pk
= 0.632V RMS
(0.775V if sine
w/same peaks)
KT88 V
g–g
(0 to
–120V each V
g
)
compressive
curvature (from
12AT7) compen-
sates output stage
Output voltage
across 4Ω load
(with sine wave,
THD = 0.27% @
100W)
G-2548-2
G-2548-3
8
audioXpress
6/06
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This is provided by a local current feedback loop that doesn’t
compromise stability or transient clarity.
7. Non-feedback bias stabilizer: This, along with extremely
solid power supplies, keeps high-power signal compression to
less than 0.1dB (
Fig. 6
).
8. Servo DC transformer balance circuitry; no adjustment
needed, no saturation even with one output tube inoperative.
9. Plitron toroidal output transformers deliver 112W per
channel at 20Hz, 1% THD, and a BW –1dB of 12.5Hz to
39kHz (
Fig. 13
).
10. Ruggedness: output can be shorted during sustained
10dB overdrive, without blowing even a fuse.
1. Switchable RCA or balanced XLR inputs.
2. DACT precision step attenuators.
3. Polarity-inverting switches, useful for bridged monoblock.
4. Bias-indicating LED comparators.
5. Six power supply fuses with corresponding status LEDs.
6. Ultra-clean power supplies using five toroidal transform-
ers. All tube heaters and HV are surge-limited. Power-on HV
has 20 second delay.
FIGURE 4: Tone bursts. 200W peak,
HV FET s w i t c h
average power stated. 4ΩΩ load.
provides “soft” (inau-
20kHz 100W sine • 10kHz 68W triangle
dible) standby/active
3kHz 1-cycle 100W sine • 80Hz 100W sine
40Hz 100W sine (envelope delay due to 6Hz –3dB rolloff)
switching.
7. AC line is fed
through an EMI fil-
ter receptacle, inrush
current limiter, and
voltage transient sup-
pressor.
Overall, the ampli-
fier (schematics,
Figs.
7
,
12
,
17
(in part 2)
)
appears complex,
with a fairly large
parts count. Howev-
er, the audio path is
straightforward and
relatively simple, as it
should be. Much of
the complexity is due
to the “housekeep-
ing functions” and
six very solid power
supply voltages (four
regulated).
Materials cost was
about $2300. In the
interest of economy,
you could use the
basic amp with sim-
pler supplies and
without the “house-
keeping” functions.
You could use 6550
G-2548-4
output tubes, but
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June 2006
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