VALKYRIE:
A LINE-STAGE PREAMPLIFIER
B y G ra y s o n K in g
P H O TO 1: The prototype (inside).
S
implicity and quality were my de­
sign goals when I built my first
from-scratch stereo component. This
preamp is the final result. My "recipe"
will come in handy for those who wish
to cook up their own Valkyrie. All the
ingredients are easy to find, including
the custom chassis and PC boards.
Use of high-performance op amps in the
signal path maintained simplicity and
achieved a level of quality that would
have been difficult, costly, and time con­
suming to attain with discrete compo­
nents. The main-gain stage (Fig. 1)
comes courtesy of Gary Galo and Walt
Jung ("POOGE-5: Rite of Passage for the
DAC960," TAA 2/92, p. 10; 3/92, p. 34).
Gary—whose articles, and particularly
this circuit, are doubtless familiar to
TAA readers—helped bring the Val­
kyrie to life. The AD811 has established
quite a reputation in the solid-stateaudio industry, and the addition of the
AD744 makes the Fig. 1 circuit arguably
one of the best op amp gain stages avail­
able for line-stage audio.
My final design incorporates one mi­
nor change from the Fig. 1 circuit. Figure
4 shows a lk shunt resistor in the
AD811's feedback loop, increasing the
gain of the 811 without affecting the
circuit's overall gain. Since the POOGE-
FIG U R E 1: Main-gain stage.
FIG U R E 2: Power supply.
The Circuit
10 The Audio Amateur 1/94
5 article, Gary has discovered that the
addition of this resistor audibly im­
proves performance.
With the specified values the circuit
offers a modest voltage gain of about 5
(14dB)—ideal for preamp gain in most
ABOUT THE AUTHOR
Grayson King was bom in Saginaw, Ml in 1971. His
fascination with electronics began early and he had a
working knowledge of Ohm's law and basic DC circuits by
age 9. He continued to produce flashing lights and noisemaker circuits until audio interests in high school added
practicality to his tinkering. His unique pastime eventually
led him to Clarkson University in pursuit of an electrical
engineering degree. He will graduate in May and hopes
to establish an audio-related career.
systems, since a gain pot performs best
in the middle to high end of its range.
For more gain, simply increase the value
of R35 (left) and R36 (right) to whatever
your amp (or ears) can handle.
The power-supply circuit (Fig. 2), a
Galo/Jung design ("Preamp Power Sup­
ply," TAA 4/90, p. 47), is available in kit
form from Old Colony Sound Lab
(#KG-5, $139). It doesn't fit in the Val­
kyrie chassis, so I designed my own PC
board. If you use the Old Colony kit in
another chassis, remember to add 1.5k
bleeder resistors (R5 and R6 in Fig. 2)
from positive and negative rails to
ground. Without them the supply won't
regulate properly at low currents.
The tape-output buffer is a simple
AD712 dual, FET-input, op amp oper­
ating at unity gain. A 100k resistor to
ground at the feedback loop keeps each
op amp from latching up in case of
clipping. I had enough space to house
four identical buffers, capable of driv­
ing a number of outputs, on my main
signal PC board. I am using three of
them now in my prototype, with space
for a possible fourth. Table 1 lists parts
for three buffers, but you can eliminate
one or two. Each unit requires an
AD712, two 0.15pF capacitors, two 100k
resistors, and two 4 7 5 0 resistors. A sin­
gle AD712 buffer should easily drive
two or three parallel outputs.
The ±15V supply rails are proximity
filtered at each op amp with 0.15pF
stacked metal film capacitors (Pana­
sonic V-Series). The main-gain-stage cir­
cu it also in co rp o ra tes a 2,200pF
electrolytic for each rail (Fig. 3). This
extensive proximity filtering allows for
better dynamics and lower noise.
The low DC offsets of the signal cir­
cuits make it possible to DC-couple their
inputs and outputs, and the resulting
lack of series capacitors in the signal
path allows for cleaner, more linear op­
eration. In fact, the only nonresistive ele­
ment in the entire signal path is a pair of
0.001 pF polypropylene shunt capacitors
prior to the main-gain stage (C33 right,
C34 left). Their purpose is merely to
filter ultrasonics that might affect signal
integrity. And with a cutoff frequency
around the megahertz range (depend­
ing on the output impedance of the
source), they will certainly have no ef­
fect on the audio signal's linearity.
Be sure that these capacitors are lo­
cated on the input side of the volume
and balance pots so that changing your
volume doesn't affect the cutoff fre­
quency. The easiest way to do this is to
e­
U*
,2 2 0 0 u F
Hb_
0.1 SuF
0.15uF
Hb_
u-
15uF
o-
2200uF
“' G _
0.15uF
M W 1K
A V
>
4.02K
1
FIG U R E 3: Main-gain stage with filter capacitors.
-0 A U X
- 0
TUNER
O O
The Audio Amateur 1/94 11
FIG U R E 5: Complete schematic.
12 The Audio Amateur 1/94
T A B LE 1
V A L K Y R IE PRE A M P PAR TS LIST
ITEM(S )
(quantity)
DESCRIPTION
SUPPLIER
(and part number)_________________ (see sources)
TRl
IC1, IC2
IC3, IC4
IC5-IC7
REG1
REG2
BR1, BR2
D1-D4
Rl, R2
R3, R4
R 5 , R6
R7-R10
Rll, R12
R13-R20
R21-R26
R27, R28
R29-R34
R35, R36
C1-C5
C9-C12
C13, C14
C15-C18
C19-C32
C33, C34
HS1, HS2
HS3, HS4
FH1
SW1
SW2, SW3
RCA1-RCA3
RCA4-RCA10
POT1
POT2
18V+18V Transformer
D4022
BiFET Op -Amp
AD744JN
Video Op -Amp
AD811AN
Dual Op- Amp
AD712JN
Positive Regulator
LT1085
Negative Regulater
LT1033
Bridge Rectifier
PB61-ND
100V, 1A Diode
1N4002
1.00K
.5W, 1% Roederstein MK3
10.5K
.5W, 1% Roederstein MK3
1.50K
.5W, 1% Roederstein MK3
22m
.5W, 1% Roederstein MK3
10.0M
.5W, 1% Roederstein MK3
100K
.5W, 1% Roederstein MK3
4750
.5W, 1% Roederstein MK3
ioon
.5W, 1% Roederstein MK3
1.00K
.5W, 1% Roederstein MK3
4.02K
.5W, 1% Roederstein MK3
0.OlpF
100V, Metal Film P4713
47 0 OpF 35V, Electrolytic P6514
lOOpF
25V, Electrolytic P6779
22 0 0|lF 25V, Electrolytic P5717
P4527
0.15pF 50V, Metal Film
0 .001|lF 50V, Polypro..
P4551
TO-220 Heat Sink
267-1363
DIP Heat Sink
580 IB
Fuse Holder
270-364
DPDT Toggle
275-663
DP5T Rotary
948-7244
RCA Jack (pair)
Vampire CM1F
RCA Jack (pair)
274-852
100K Alps Balance Control
100K Alps Volume Control
Avel Transformer
Analog Devices
Analog Devices
Analog Devices
Old Colony
Old Colony
Digi-Key
Digi-Key
Michael Percy
Michael Percy
Michael Percy
Michael Percy
Michael Percy
Michael Percy
Michael Percy
Michael Percy
Michael Percy
Michael Percy
Digi-Key
Digi-Key
Digi-Key
Digi-Key
Digi-Key
Digi-Key
Radio Shack
Active Electronic
Radio Shack
Radio Shack
A11ied
Michael Percy
Radio Shack
Old Colony
Old Colony
1 .5 ft
25 ft
1 yard
1
1
1
1 8pack
2 lOpacks
8 “ long
2
4
1
1
1
14AWG Teflon Insulated Wire
20AWG Teflon Insulated Wire
19AWG Jumper Wire (Wonder)
18AWG Power Cord
278-1255
Cord Strain Relief
278-1636
1.5A Slo-Blo Fuse
270-1284
Rubber Feet
64-2342
Isolation Washers
MNY 1/4
1/4" Diameter Steel Rod
Shaft Couplers
6412K11
Aluminum Knobs
57F-2292
Kingl
Signal PC Board
Supply PC Board
King2
Chassis
King3
Sonic Frontiers
Sonic Frontiers
Sonic Frontiers
Radio Shack
Radio Shack
Radio Shack
Radio Shack
Old Colony
Hardware Store
McMaster-Carr
Newark
Nutronics
Nutronics
Sescom
mount the capacitors directly to the ter­
minals of the balance pot (Photo 1).
System C ontrol
The Valkyrie's chassis is designed for
two rotary switches to control input se­
lection. With the configuration shown
in Fig. 4 the tape outputs deliver the
source selection while the main output
delivers the monitor selection. For typi­
cal listening, the source is monitored:
you use the source switch to select the
component to which you listen, keep­
ing the monitor switch set to "source."
When recording, however, you may
choose to monitor a tape as it records
from your source selection. Since both
decks can be either sourced or moni­
tored, tapes can be recorded from 1 to 2
or from 2 to 1. The monitor switch also
has an EQ position so that the signal can
be run through an equalizer loop.
This sim ple sw itch ing netw ork
makes the preamp into a flexible system-control center—but beware: never
source a tape deck w hile that deck is
recording or m onitoring its source. This
would connect the deck's input to its
output, resulting in a very ugly feed­
back loop.
If you choose a different switching
network, the silkscreened front panel
labeling (Fig. 9) will not be accurate.
Since both front and rear panels are ver­
tically symmetric, however, you can
simply flip the panel over so the pre­
screened side faces the inside of the
chassis, leaving you an aluminum-col­
ored panel to label as you wish. If you
prefer a black anodized panel, you can
order blanks through Sescom.
C om ponents
Never underestimate the importance of
passive components: POOGE quality
parts allow a given circuit topology to
live up to its potential. All resistors
should be 1% metal film type. I used
Roederstein Resistas throughout, with
Holcos in some critical locations. Vol­
ume and balance pots are Alps 100k
(the big "black beauty" model). Rotary
switches are Grayhill 71 series. Use any
switch with sealed wiper action and
silver contacts. C ritical RCA jacks
should be quality devices. I chose Vam­
pire CM lFs for CD and AUX inputs as
well as main output. The rest of the
jacks are the basic Radio Shack goldplated type.
The toroidal power transformer I chose
is rated at 100VA (no, the Valkyrie is not
a power amp). This type of overkill keeps
transformer operation well within the lin­
ear region and basically eliminates stray
field emissions. You can easily substitute
a smaller toroid as long as it has two
floating 18V windings rather than a center-tap secondary. AveTs D4013 (60VA)
should be fine, but I don't recommend
using the 30VA model.
C onstructio n
The chassis layout (Fig. 6) combines the
isolation of an external supply with the
convenience of a single enclosure. A
midchassis shielding strip of 20AWG
steel divides the chassis into two distinct
sections and isolates the entire signal
path from any EM or RF signals that
might be present on the power-supply
side. The selector switches are located
near the back panel with shafts extend­
ing to the front, allowing signal wires to
be kept short and isolated from AC sup­
ply lines.
If you choose a more traditional chas­
sis layout, you'll probably need coaxial
cable for internal signal wiring. The
money I saved on coax I spent instead
on Cardas individually insulated multi­
strand wire, one of the best signal-carry­
ing conductors available for internal
The Audio Amateur 1/94 13
FIG U R E 6: Chassis layout.
17
17
17
FIG U R E 7: Panel drilling guide.
14 The Audio Amateur 1/94
wiring purposes. It's difficult to work
with, however, as individual strands of
magnet wire require a chemical strip­
ping process. I substituted a basic Tef­
lon-insulated 20A WG wire on the parts
list, but if you have the patience for
somewhat tedious construction you
may find Cardas wire worth the effort
and cost. You can obtain it through
Sonic Frontiers.
Figures 10-13 show the etching pat­
terns and stuffing guides for the PC
boards. Stuff and test the supply board
first. When you confirm that the supply
is working properly, stuff the signal
board and connect the supply rails be­
tween the boards with 14AWG wire.
Attach heatsinks to the AD811s before
installing them on the PC board. I got an
excellent pair of 8-pin DIP heatsinks from
Aavid E ngineering (also available
through Active Electronics). The LT
regulators on the power supply also re­
quire heatsinking, but Radio Shack TO220 units work fine in this case.
The V alky rie ch assis, av ailab le
through Sescom as "King3," comes
punched and silkscreened along with all
hardware, including the midchassis
shielding strip. To save some money,
you can order "King4" (no punching,
silkscreening, or hardware) and buy
20 AWG steel for the shielding strip from
a metal yard. Use a sheet-metal bender
(or a vise and a block of wood) to shape
it (Fig. 8). The notch in the top of the
panel is for routing power-supply lines
from the supply PCB to the signal PCB.
Use Fig. 7 as a guide for drilling your
chassis panels (punching is unneces­
sary for home construction).
All Sescom chassis have comer extru­
sions with nut rails for 4-40 hardware to
mount the PC boards (Fig. 6). The rear of
the supply PC board, however, does not
reach the rear nut rail, so it is fastened to
the bottom panel with 4-40 hardware and
Vie" spacers.
The rotary selector switches are lo­
cated in the rear of the chassis, requir­
ing shaft extensions to reach the knobs
on the front panel. Simply cut two 37'g"
sections of W'-diameter steel or brass
shaft and attach them to your switches
with shaft couplers. Just enough should
protrude from your front panel to at­
tach the knobs. You may find it helpful
to file a flat segment on the coupler end
of the shaft, but it won't be necessary on
the knob end, since the knobs have two
angled set screws.
The gain and balance pots mount diCon tinned on page 18
All Dim ensions Are in Inches
SOURCES
Aavid Engineering, Inc.
One Kool Path
Laconia, NH 03247
(603) 528-3400
FAX (603) 528-1478
Active Electronics
11 Cummings Park
Woburn, MA 01801
(800) 677-8899
Allied Electronics
Call for nearest distributor
(800) 433-5700
Analog Devices
Call for nearest distributor, such as
Newark
(617) 329-4700
FAX (617) 326-8703
Avel Transformer
47 South End Plaza
New Milford, CT 06776
(203) 355-4711
FAX (203) 354-8597
Digi-Key
701 Brooks Ave. S.
PO Box 677
Thief River Falls, MN 56701-0677
(800) 344-4539
FAX (218) 681-3380
Old Colony Sound Lab
PO Box 243, Dept. KNG
305 Union St.
Peterborough, NH 03458-0243
(603) 924-6371
FAX (603) 924-9467
McMaster-Carr
PO Box 440
New Brunswick, NJ 08903-0440
(908) 329-3200
FAX (908) 329-3772
Michael Percy
PO Box 526
Inverness, CA 94937
(415) 669-7181
FAX (617) 669-7558
Newark
7449 Morgan Rd.
Liverpool, NY 13090
(315) 457-4873
FAX (315) 475-6096
Radio Shack
Call for nearest distributor
(800) 627-7730
Nutronics
1703 Peoria Rd.
PO Box 1385
Springfield, IL 62705
(217) 544-9821
Sescom
2100 Ward Dr.
Henderson, NV 890154249
(800) 634-3457
FAX (702) 5654828
Sonic Frontiers
760 Pacific Rd., Unit #19
Oakville, ON L6L 6M5
Canada
(416) 847-3245
FAX (416) 847-5471
1. 25
FIG U R E 8: Midchassis shielding strip.
ACKNOW LEDGMENTS
Without the help of Gary Galo, the Valkyrie would not be
what it is today. I also thank Professor J.A. Svoboda, who
gave me design credits for my work on the Valkyrie. For
their patience during the hours I occupied their Macintosh
computers, thanks to Sheldon Stokes and the folks at
SUNY Potsdam's Distributed Computing Center. Finally,
thanks to those companies that generously supplied me
with parts for construction, especially Frank Miller at
Sescom, who made the custom chassis possible.
The Audio Amateur 1/94 15
7 > r
Vatf yru
^
AUX
TUNER
CD
I
----------- y
— ^
/ ------------------------- T A P E 1 ------------------------- --
SOURCE
1
J------------ E Q
\
„ --------------------- T A P E 2 --------------------..
4-
♦
1
+
*
-
+
+
M A IN O U T P U T
-T A P E
M O N IT O R
+
O U T P U T S ------------
+
1
/
s
-
/
-
V
/
SOURCE
\
'
X
POWER
/
S
-f
-
/■
\
s
/
B A LA N C E
V
VO LU M E
+
+
+
+
+
TAPE 2
TAPE 1
CD
AUX
TUNER
+
+
+
+
+
't g *
J h & s k
* —
+
1 .5 A F U S E
+
FIG U R E 9: Silkscreen patterns.
FIG U R E 10: Signal PC board, solder side.
Continued from page 15
rectly to the front panel, making it much
easier to attach their knobs. Old Colony
Sound Lab plans to offer compatible
models of both units, which will be re­
ferred to as Alps "Blue Velvets."
Use nylon isolation washers to isolate
the RCA jacks from the rear panel. The
18 The Audio Amateur 1/94
Old Colony ones specified in the parts
list require some modification for this
application: replace the 3/s" ID washer
with a Vi" ID version and cut or file the
3/s" shoulder on the shoulder washer to
about Vi6" (the thickness of the chassis
panel). Many quality RCA jacks come
with their own isolation washers, but
are usually pricey. The modified Old
Colony washers can be used on much
cheaper jacks at less critical points (tape
outputs and so on).
T ricks
To avoid ground loop distortion, incor­
porate a "star grounding system." In
SOURCE
FIG U R E 11: Signal PC board, stuffing guide.
this system, no ground bus should close
a complete loop on itself, and the circuit
ground must be connected to the chas­
sis at only one point (preferably near
the power supply). Therefore, input
and output jacks must be "floated" from
the chassis, using the isolation washers
mentioned above. I used the ground-tie
point at the left side of the supply board
to attach a grounding strip.
Wire pairs carrying AC signals, espe­
cially 60Hz supply lines, should be
twisted to minimize EM and RF interfer­
ence. (Some like to do this with signalcarrying wires, but the Valkyrie's signal
ground bus makes twisting impractical
except for pow er-supply lines.)
The power cord should be polarized
to minimize chassis leakage. To find
correct polarity, disconnect your fin­
ished Valkyrie from all other compo­
nents and turn it on. After it's warmed
up, use an AC voltmeter to measure the
sig n al b etw een your ch assis and
power-line ground. Turn your plug
around to switch polarities and measure
again. The lower reading indicates the
preferred polarity. Consider polarizing
the power cords on all your components
by using the same procedure.
You might prefer using a 16AWG
power cord rather than the 18AWG unit
specified in the parts list (I do). If so, pick
up a 16AWG extension cord and cut off
the receptacle end. You may have to
enlarge the hole in the chassis in order
to fit a larger strain relief.
Testing
Before plugging your Valkyrie into the
rest of your system, run some prelimi­
nary tests. You will need a digital mul­
tim eter and an oscilloscope. After
testing the power supply and then con­
necting it to the signal board, plug in
and power up. Check supply rails again
to confirm that they are at a steady +15V
with no measurable ripple, then meas­
ure all your offset voltages (DC output
voltages with inputs grounded). They
should all be less than lOmV in magni­
tude. Finally, test for output noise and
make sure it is basically unmeasurable.
If you have the necessary equipment
you can test for harmonic distortion, signal-to-noise ratio, and so on. Otherwise,
skip to the real test of performance—the
listening test.
Results
As soon as I'd tested my prototype I
went to Gary's house to audition it.
Excellent dynamics were unmistak­
able from the beginning, and soundstage was well defined. Gary praises
the Valkyrie's "precise imaging" and
its ability to capture ambience. Noise
was virtually inaudible even at full
volum e, and overall reproduction
was impressively accurate.
I couldn't find a weak spot in its per­
formance; that is, not until we plugged
The Audio Amateur 1/94 19
F IG U R E 12: Supply P C board, solder side.
in Gary's preamp, an extensively modi­
fied Adcom GFP-565 with a similar sig­
nal path to mine, but a superior power
supply in an external chassis. My proto­
type was quite close in performance, but
Gary's sounded slightly more spacious
and had a certain "pristine and clean"
quality, especially in the midrange.
20 The Audio Amateur 1/94
Overall, though, the Valkyrie exceeded
all my expectations, and I have installed
the prototype in many systems since
with consistently outstanding reviews.
Gary and I made a number of steadystate measurements on my prototype. We
measured THD with Gary's equipment,
which has a residual distortion of 0.01%.
It showed the same value when measur­
ing my prototype. S/N ratio was better
than 88dB relative to 2V (this would be
about 105dB relative to clipping). Slew
rate was so high that it couldn't even be
measured without removing the ultra­
sonic filter caps at the input.
These measurements tell us that the
preamp is functioning well, but numbers
say little about overall performance.
Your two best pieces of test equipment
attached to your head will, I believe, con­
firm my claims for the Valkyrie.
The Valkyrie reveals many details that
might not have been audible previously,
including imperfections in the recording
process. Some people find this impartial
accuracy disagreeable, especially if they
are used to the "euphonic" sound from
some equipment. A euphonic sound,
while pleasing, is largely the result of the
second harmonic distortion component.
Certain circuit topologies produce distor­
tion consisting entirely of even harmonics
(especially second), which add a "sweet"
sound to the signal. The result is generally
pleasant to hear, but masks a portion of
the true musical performance. If you wish
to hear sound that reproduces the original
as closely as possible, the Valkyrie is likely
to bring to your ears much that you've
been missing.
□
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j
I___________________________________________________________________I
The Audio Amateur 1/94 21
LETTERS
POOGE 5.5 CORRECTION
ONE ERROR IN my Pooge 5.5 article
("More DAC960 Modifications/' TAA
1/94, p. 22) has been brought to my
attention. In Table 1 on p. 23, the correct
Digi-Key part num ber for the 4.7pF
Panasonic ECQ series polyester capaci­
tor is EF1475, not EF1474 as published. I
thank reader Glenn Moore for bringing
this to my attention.
G a r y A. G a l o
Potsdam, NY 13676
VALKYRIE REVISITED
IN THE VALKYRIE Preamplifier article
(TAA 1/94, p. 10), the parts list omits
capacitors C6-C8. I assume they are the
same as C l to C5, i.e., O.OlpF, 100V film.
Also, the stuffing guide of the signal
board has the markings on the four elec­
trolytic capacitors backwards: plus
should be minus.
L ester Jo h n so n
Oshkosh, W I54901
Grayson King responds:
Lester Johnson points out an important error
in my Valkyrie article. The polarity labeling
o f the electrolytic capacitors in Fig. 11 is
indeed the opposite o f what it should be. Any
readers who have already built their
Valkyries with capacitors oriented as in the
figure may not have noticed the error, even
upon listening to the preamp. They would,
hoioever, notice a discernible improvement
upon correction o f the error.
Mr. Johnson also highlights one o f two
typographical errors in the article I'd noticed
since its printing. C6, C7, and C8 are miss­
ing from the parts list. They should be
0.01/uF, the same as C1-C5. Another error is
the labeling o f resistor R38, which should be
R30, in Fig. 11.
F IG U R E 1: Revised printed circuit board for Mr. Eaton's headphone
amplifier.
42 The Audio Amateur 3/94
Since I, along w ith m ost Audio
Amateur authors, am human and make
mistakes in my work, I speak for all o f us
when I thank those readers who inform
Audio Amateur o f article errors. Without
this sort o f editing, many technical errors
would remain in print, noticed only by a
select and silent Jew.
IMPROVED DESIGN
UPON READING EARLE Eaton's article,
"Build This High-Quality Headphone
Amplifier" (TAA 3/93, p. 20), I was struck
by what I believe is a potentially danger­
ous situation on the power supply PCB.
The AC line connections to the trans­
former are very close to the secondary
connections, risking either a solder bridge
short circuit or, worse still, a flashover
from primary to secondary during a light­
ning strike.
Simply rotating the transformer 180°
and relocating the pads for the AC line
FIGURE 2: Component-side view for the revised amplifier.
Mr. Galo's design
by a more sophisti­
cated external supply.
You can carry this further
(indeed, some may say, to
absurd lengths) by construct­
ing
external,
separate,
smoothed, and regulated ±24V
supplies for each channel. This quiet
supply is introduced to the separate
signal boards for each channel and re­
regulated at each stage (line driver, tape
driver, and, in my case, at each stage of
the phono amp and at the balanced line
drivers). Noise? I can hear none at more
OUT
FIG U R E 1: Bernabei’s circuit suggestions.
indeed true for any kind o f circuit and need
not be emphasized.
Kalm
an
R u b in s o n
New York, NY 10021
Grayson King responds:
I thank Mr. Rubinson for his interest in my
work and fo r his worthwhile suggestions,
but I believe some o f his comments need to be
clarified.
1. To achieve a gain o f 11 (20.8dB),
change R31 and R32 to 220Q as Rubinson
suggests, but change R35 and R36 (not R29
HRFI
NO PAIN, NO GAIN
SINCE I BUILT a preamp with a linestage based on the AD744/AD811 com­
bination described by G alo/Jung
("POOGE-5," TAA 2/92, 3/92), I have
several comments about the Valkyrie
preamp designed by Grayson King and
published in TAA 1/94 ("Valkyrie: A
Line Stage Preamplifier," p. 10). I com­
mend this project to anyone who wishes
an extremely high-quality, yet simpleto-construct, line level preamp. Con­
sidering the number and cost of parts, I
cannot im agine anyone being d is­
pleased with the results.
1. If you need more gain than the pub­
lished circuit provides, change R31/R32
to 220Q and R29/R30 to lOkD for a gain
of 11 x 20.8dB (thanks to W. Jung).
2. Instead of cascading the selector
and monitor switches, set them in paral­
lel to designate the monitor switch as a
tape selector. In so doing, you can wire
the tape selectors) to preclude switch­
ing tapel to tapel or tape2 to tape2. This
eliminates the possibility of the feedback
loop against which King warns. As a
bonus, only one set of switch contacts—
instead of two —is in series with the
monitored signal line or the recorded
signal line.
3. Another configuration change is to
replace the balance control with a pair of
stepped gain/trim controls. This permits
a full balance function but also increases
the accuracy of fixed balance corrections
allow ing repeatability. I mark the
amount of the imbalance on the jacket or
booklet of my recordings. This configura­
tion also permits the gain of both channels
to be reduced simultaneously for the
comparison of signal sources with differ­
ent output levels.
4. Mr. King refers to the improve­
ments which are presumably wrought in
than 3" away from the speaker
diaphragm. Same for crosstalk.
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The Audio Amateur 3/94 45
and R30) to 10k. The equations for achieving
any gain are as follows:
Left Channel:
Right Channel:
1 + R35/R33 = gain
1 + R36/R34 = gain
1 + R29/R31 = gain/2
1 + R30/R32 = gain/2
2. The parallel designation o f monitor
and tape selector switches that Rubinson
suggests has advantages and drawbacks. On
the positive side, it makes recording while
listening possible and eliminates one set o f
switch contacts in the signal path. On the
negative side, the sixth input from the equal­
izer loop becomes impossible.
Also, contrary to Rubinson's claim, either
network is prone to the feedback loop men­
tioned in my article. But at the same time,
either network can eliminate the potential for
feedback simply by sacrificing the flexibility
o f recording both tape2-to-tapel and tapelto-tape2. This may not be obvious at first,
but those who take the time to work out their
own network will reach an understanding.
Also, for those who choose the network in my
article, adding a fifth ("CD direct") position
on the monitor switch connected to the CD
input allows direct listening in which the
signal passes through only one set o f switch
contacts rather than two.
3. Rubinson's idea for a stepped attenua­
tor type balance control is excellent. Also, the
further upgrade to a stepped attenuator vol­
ume control would result in an even greater
increase in quality.
4. The last o f Rubinson's suggestions,
involving a preregulator and multiple local
regulators, may well be his best, though it is
by far the most difficult to implement. At the
very least, it requires a new signal PC board
design and the addition o f many expensive
regulator components.
In the meantime, an easier and less expen­
sive mod fo r Valkyrie builders is simply to
replace the bridge rectifiers in the supply
with high-speed switching diodes. It will cost
you very little, and, according to Gary Galo,
you may hear a noticeable improvement.
SNUBBER STRATEGY
RICK MILLER'S SIDEBAR about para­
sitic RF oscillation in power supply rec­
tifier circuits (TAA 1/94, pp. 26-27) is
interesting, but I'm not sure that a semi­
conductor diode's reverse recovery time
is the correct parameter to consider, or
that a particular diode part number —
even a "soft recovery" unit—will cure
the problem in all cases. This effect is
well known to designers of high-fre­
quency, switching mode power sup­
plies, and is discussed in manufacturers'
application notes that may be accessible
to TAA readers (cf. references, below).
46 The Audio Amateur 3/94
Mr. Miller deserves credit for showing
that it can afflict simple, line-frequency
power supplies as well.
The oscillation is a kind of "ringing"
that occurs when the (voltage-depen­
dent, nonlinear) diode capacitance res­
onates with stray inductance, which
may be internal or external. Amateur
radio operators w ill recognize this
behavior as a kind of parametric ampli­
fier, where the "pumping frequency" is
120Hz and output occurs at harmonics
of the parasitic resonance frequency.
For instance, from the oscillograms
published in his article, it looks as
though Mr. Miller's test circuit rings at
multiples of about 16kHz. Ironically,
certain design practices often advocated
for audio-equipment power supplies,
such as low-inductance filter capacitors
and low-resistance transformer wind­
ing, are likely to exacerbate this problem
because they virtually guarantee a high­
er, high-frequency resonant circuit pass­
ing through the rectifier. So, what to do?
As in the case of Class A/B audio
power amplifiers, the standard solution
recommended in most recent literature
is a series-RC "snubber" network to
damp the oscillation, connected either
across the rectifier network's output (i.e.,
across the filter section's input) or in
parallel with each diode. Unfortunately,
choosing the component values for opti­
mum damping is tricky.
The snubber network's capacitance is
usually about two to ten times the aver­
age diode capacitance and its value is
not too critical, but the network's resis­
tance, which determines the Q of the
damped resonant circuit, is very sensi­
tive to the amount of inductance and
resistance elsewhere in the rectifier cir­
cuit, and is difficult to predict I suggest
that readers study the references for
information about how to make some
estimates, then be ready to experiment
In low-power circuits, an alternate
solution might be to abandon capacitorinput filters in favor of choke-input
designs or, at least, to put enough induc­
tance ahead of the filter to lower the par­
asitic resonance frequency so ordinary
filter capacitors can cope with it. You
REFERENCES
1. Power Transistors, Power Electronics Semiconductor
Department General Electric Company, 1985.
2. Rectifier Applications Handbook 3rd ed., Motorola
Incorporated, 1993.
3. Schottky Diode D esigner’s Manual, 1st ed.,
International Rectifier Corporation Manual No. SDM-1,
1992.
might also include a small resistor in
series with each diode to damp the oscil­
lation and lim it cu rren t pulses. Of
course, this restricts the rate at which fil­
ter capacitors can charge, but because
the current pulses trigger oscillation,
some compromise is necessary.
A final strategy m ight be contain­
ment: Build your power supply in an
RF-shielded enclosure separate from the
audio circuitry, with well-decoupled
(AC) input and (DC) output lines, then
just let it ring.
Sc o t t M a r o v ic h
East Palo Alto, CA 94303
Rick Miller replies:
Because all AC-powered supplies use diodes
to produce the DC voltage to run our audio
equipment, it makes good sense to pick the
best diode for the job, in this case, the one
that sounds best. As o f this date, high speed
and soft recovery parameters are the most
important specifications for best sound, and
have the lowest RFI.
I have used snubber capacitors across
diodes to reduce the amount o f RFI from the
diode as Gary Galo shows in his POOGE
articles. Comell-Dubilier has an excellent
application note on how to design an RC
snubber. Gary is not using a series resistor
in the snubber because o f the circuit's low
impedance. To quote from the application
note, "choose a resistor value no larger than
the characteristic impedance o f the circuit." I
measured the RFI output with and without
different value resistors and the lowest RFI
was without a series resistor.
I have always liked choke-input designs,
which might help lower RFI levels. With
everything that we have learned about diodes
and RFI to this point, it is apparent that every
step taken to clean up RFI is audible. The
point o f diminishing returns o f filtering and
decoupling has not been fully reached yet.
Because o f this, I do not think that letting a
supply "ring” in a box is a good way to go.
POOGE PROJECT
I AM THE ow ner of a PO O G E'd
DAC960 enabled by your tw o-part
series ("POOGE 5: Rite of Passage for
the DAC960," TAA 2/92, p. 10 and TAA
3/92, p. 34). I am by no means an engi­
neer, or otherwise well-schooled in elec­
tronics, but am competent in following
directions. The modified Philips unit
w orked w onderfully from the first
moment I completed the modifications.
Thank you for your work.