This is only a preview of the August 2014 issue of Silicon Chip. You can view 41 of the 104 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "Nirvana Valve Sound Simulator":
Items relevant to "The 44-pin Micromite Module":
Items relevant to "The Tempmaster Thermostat Mk.3":
Purchase a printed copy of this issue for $10.00. |
Build the
Valve So
Well, we know that there are lots of valve enthusiasts out there who
believe that valves are just better; much more musical and pleasant to
listen to than those sterile solid-state circuits with oodles of negative
feedback and vanishingly small harmonic distortion. Of course, valve
amplifiers do have drawbacks, like heat and fragility, but what if you
could get ‘valve sound’ from a solid-state state amplifier? Well now
you can, with our Nirvana Valve Sound Simulator.
I
T’S BEEN completely against the
grain but we have now designed a
solid-state circuit which deliberately
distorts. Our Publisher, Leo Simpson,
has had to be hand-cuffed, blind-folded, muzzled and otherwise restrained
from doing what he normally does –
driving us towards perfection. Now we
have taken another ‘path’ to produce
the ‘desirable and musical’ effects of
valve amplifier circuitry.
OK, OK, we know that if you want
genuine, true ‘valve sound’, the only
recourse is to use a valve amplifier.
But we are presenting another way
to musical nirvana which musicians
32 Silicon Chip
commonly follow; using a solid-state
amplifier with in-built valve circuitry
simulation. This way, it’s the valve
sound you have without using valves.
Our Nirvana Valve Sound Simulator
can be connected in series with any
solid-state mono or stereo amplifier.
It can be used by musicians or in the
home for normal music listening. It lets
you hear what valve sound is all about
so you don’t have to go to the expense
of replacing a perfectly good solidstate amplifier with a valve amplifier.
What does it do?
When a valve amplifier (sometimes
called a tube amplifier) is compared
objectively with a modern solid-state
amplifier, the results can be somewhat
uncomplimentary. The valve amplifier will typically have much higher
distortion, more noise, more hum
and certainly a less than straight-line
frequency response when driving real
loudspeakers. But the sum total of
those effects is what valve amplifier enthusiasts want: a mellower, softer and
(it’s claimed) more ‘musical’ sound.
Our Nirvana Valve Sound Simulator
does not add noise and hum but it will
produce the same effects on the signal
as a valve amplifier: softer symmetrical
siliconchip.com.au
Nirvana
By JOHN CLARKE
und Simulator
or asymmetrical clipping at the point
of overload, mainly even-order harmonic distortion similar to the effects
of a single pentode valve stage and a
frequency response similar to that of
a good quality class-AB valve amplifier with transformer coupling to the
loudspeaker.
We simulate the pentode valve stage
effect by using a FET source-follower
in the signal path. The soft clipping
effect is achieved in the same FET
source-follower stage and it is fully
adjustable for degree, asymmetry etc.
We also need to simulate the effect of
a valve amplifier’s output impedance
on the frequency response of a typical
loudspeaker. This is where solid-state
amplifiers have a big advantage over
valve amplifiers. Well-designed solidstate amplifiers usually have an output
UPPER BASS
RESONANCE
MID-BAND
IMPEDANCE HUMP
HIGH FREQUENCY
RISE
IMPEDANCE
LOWER BASS
RESONANCE
impedance which is less than onesixtieth of the nominal impedance of
a loudspeaker, ie, something less than
150 milliohms. By contrast, a valve
amplifier will typically have an output
impedance of about 2Ω, depending on
how much negative feedback is applied from the output terminals back
to the earlier stages.
The relatively high output impedance of the valve amplifier has two
effects when driving loudspeakers.
The first effect is a much lower
‘damping factor’ which is the ratio
of nominal loudspeaker impedance
to the amplifier’s output impedance.
For a solid-state amplifier, we expect
to see damping factors of 60 or more
and that means that the amplifier has
very tight control over the movement
of the loudspeaker cone. This leads to
less ‘boomy’ bass and lower distortion
of bass frequencies.
An equally important advantage of a
very low output impedance is a much
more linear frequency response from
all loudspeakers. This is because loudspeakers do not have a constant impedance, but one that varies widely with
frequency. So ideally, a loudspeaker
should be driven from a voltage source
and that means having a low output
impedance amplifier.
With the much higher output impedance of a valve amplifier, the considerable variations in a loudspeaker’s
impedance over the frequency range
means that the overall response will
be much ‘bumpier’ or less smooth.
Say, for example, a valve amplifier has
an output impedance of 2 ohms and
the loudspeaker has a nominal output
impedance of 8 ohms. That means that
25% of the drive signal will be lost
within the amplifier itself.
But the effect is much worse because
the loudspeaker’s impedance varies
from less than 6 ohms to more than
30 ohms.
Fig.1 shows a bass reflex loudspeaker impedance curve. Typically, these
have a double hump at low frequencies, may rise to a second broad peak
at the mid-frequencies (depending on
the effect of the crossover network)
and then rise again at the high end,
due to the inductance of the tweeter.
By contrast, loudspeakers in sealed
cabinets have only one peak at the low
frequency end.
Any increase in impedance above
the nominal value (eg, 8 ohms) at a particular frequency will result in a boost
to the loudspeaker’s response, while a
reduction will result in a drop in the
response – see Fig.3. This diagram
depicts the effect on the frequency
response of four loudspeaker systems,
1kHz
10kHz
FREQUENCY
Fig.1: a typical bass reflex loudspeaker impedance curve. As shown, there’s a double hump at low frequencies, with
the impedance then rising to a broad peak at the mid-frequencies (depending on the effect of the crossover network)
and then rising again at the high end, due to the inductance of the tweeter.
10Hz
siliconchip.com.au
100Hz
August 2014 33
+15V
100 µF
100nF
LEFT IN
47pF
22k
8
3
IC1a
2
4
22k
VR1a
50k
INPUT
10 µF
Q1
2N5485
G
S
VR2
10k
CLIPPING
LEVEL
1M
1M
1.5k
1 µF
10k
2
MMC
RING
3
IC1: LM833
TIP
CON3
D
100nF
470pF
820Ω
+9V
A
TP1
–15V
–15V
ZD3
9.1V
100 µF
VR4
10k
10k
+9V
RIGHT IN
22k
8
IC2a
4
22k
SLEEVE
47pF
6
7
VR5
10k
λ
K
LED2
–PEAK
IC2: LM833
10 µF
Q2
2N5485
G
1M
1M
1.5k
CLIPPING
LEVEL
1 µF
10k
6
MMC
5
IC2b
620Ω
7
A
+15V
K
K
D1
1N4004
9–12VAC
INPUT
10Ω
CON1
S1
A
A
ZD1
15V
1W
470 µF
16V
4.7k
R5*
10Ω
K
D2
1N4004
A
λ
A
S
VR3
10k
VR1b
50k
A
TP2
470pF
820Ω
–15V
K
D
100nF
10k
620Ω
1
LED1
+PEAK
5
IC1b
35V
K
–15V
1
100 µF
100nF
270Ω
35V
K
A
ZD2
15V
1W
470 µF
16V
λ LED5
λ
K
K
LED4
–PEAK
λ
A
R6*
DC INPUT
+
0V
–
A
LED3
+PEAK
* SEE TEXT
CON2
R7*
R8*
K
–15V
SC
20 1 4
NIRVANA VALVE SOUND SIMULATOR
previously published in SILICON CHIP,
when driven by an amplifier with an
output impedance of 4 ohms.
As you can see, the main areas of
boosting occur at the two bass resonances and at the mid-band impedance hump. For example, with the
JV100 loudspeaker depicted at the
top of Fig.3, the boost is as much as
+3.9dB. Similarly, there is a broad
boost to the response of more than
+3dB from around 500Hz to 1.5kHz
and a smaller boost to the tweeter at
34 Silicon Chip
the high-frequency end.
By contrast, if the same loudspeaker
is driven by a solid-state amplifier with
a typical output impedance of less than
150 milliohms, there is no boost or cut,
as it should be!
The Nirvana simulates these loudspeaker frequency deviations with
a number of individually adjustable filters which are varied by the
“Loudspeaker Response” control. The
selection of a particular loudspeaker
for simulation requires choosing a
particular set of component values, to
be discussed later in this article.
The other control on the front panel
of the Nirvana Valve Sound Simulator
is for ‘Clipping Level’.
If you want to delve more into valve
sound, here are some interesting sites:
(1) http://spectrum.ieee.org/consumer-electronics/audiovideo/the-coolsound-of-tubes
(2) http://spectrum.ieee.org/consumer-electronics/audiovideo/the-coolsound-of-tubes/distortion
siliconchip.com.au
+15V
100 µF
35V
–15V
47k
LEFT OUT
4
6
7
IC3b
5
NP
11
RIGHT OUT
10
10 µF
150Ω
47k
8
IC3c
9
150Ω
10 µF
SLEEVE
VR6b 10k
100k
LOUDSPEAKER
RESPONSE
2.2pF
C2L*
R2L*
C2R*
2
3
C1L*
IC3a
R1R*
1
R2R*
13
C3R*
12
C1R*
1M
IC3d
14
1M
LOWER BASS
RESONANCE
HIGH
FREQUENCY
RISE
R3L*
C5L*
+15V
4
6
5
IC4b
7
100 µF
35V
11
C7L*
C4R*
R4R*
8
13
C7R*
100k
IC4c
UPPER BASS
RESONANCE
C6R*
IC4a
10
IC4: TL074
1
9
1M
2
3
R3R*
C5R*
–15V
UPPER BASS
RESONANCE
R4L*
LOWER BASS
RESONANCE
HIGH
FREQUENCY
RISE
1M
C6L*
CON4
100k
2.2pF
IC3: TL074
C3L*
C4L*
TIP
NP
–15V
VR6a 10k
R1L*
OUTPUT
RING
12
IC4d
14
100k
MIDBAND HUMP
* SEE TEXT
MIDBAND HUMP
2N5485
LED1–5
D1, D2
A
ZD1–3
K
A
S
K
K
A
G
D
Fig.2: the complete circuit of the Nirvana Valve Sound Simulator. The input signals from CON3 are amplified by IC1,
then distorted and clipped by JFETs Q1 & Q2. IC2 provides an indication of clipping symmetry while IC3 & IC4 act as
parametric equalisers to adjust the frequency response to match that of a typical valve amplifier driving loudspeakers.
(3) http://en.wikipedia.org/wiki/Tube_
sound
In use, the Nirvana Valve Sound
Simulator connects between the
preamplifier outputs and the power
amplifier inputs of a solid-state amplifier. In amplifiers with a tape loop you
can use this facility, while for a musician’s (eg, guitar) amplifier, it would be
connected into the effects loop.
As shown in the photos, the unit
is housed in a compact case and can
be powered from an AC plugpack.
siliconchip.com.au
Alternatively, balanced DC supply
rails could be obtained from existing
equipment. The socket for the AC supply is accessed from the rear, as are the
3.5mm stereo input and output sockets.
Circuit details
Refer now to Fig.2 for the circuit
details. Each channel uses six op amps
(all in four ICs) and a JFET, and both
channels are identical.
The input signal is applied via
CON3, a stereo 3.5mm jack socket. If
only a mono signal is required, then
a mono jack plug can be used to apply signal to the left channel only.
This will connect the ring terminal to
ground and so prevent signal in the
right channel.
The following circuit description
is for the left channel signal path. As
shown, signal is applied via the tip
connection of CON3 and is reduced
by a factor of two, using two 22kΩ
resistors, so that line-level signals will
not necessarily cause clipping in the
August 2014 35
4.2dB
3.9dB
3.6dB
3.3dB
3.0dB
2.7dB
2.4dB
2.1dB
1.8dB
1.5dB
1.2dB
0.9dB
0.6dB
10Hz
Speaker Simulation
JV100
24°
21°
18°
15°
12°
9°
6°
3°
0°
-3°
-6°
-9°
100Hz
1kHz
10kHz
4.4dB
24°
4.0dB
21°
3.6dB
18°
JV80
3.2dB
15°
2.8dB
12°
2.4dB
9°
2.0dB
6°
1.6dB
3°
1.2dB
0°
0.8dB
-3°
0.4dB
-6°
0.0dB
10Hz
-9°
100Hz
1kHz
10kHz
3.0dB
16°
14°
12°
10°
8°
6°
4°
2°
0°
-2°
-4°
-6°
-8°
2.7dB
2.4dB
JV60
2.1dB
1.8dB
1.5dB
1.2dB
0.9dB
0.6dB
0.3dB
0.0dB
-0.3dB
10Hz
3.6dB
3.3dB
3.0dB
2.7dB
2.4dB
2.1dB
1.8dB
1.5dB
1.2dB
0.9dB
0.6dB
0.3dB
0.0dB
10Hz
100Hz
1kHz
10kHz
21°
18°
8-Inch Woofer & Piezo Horn
15°
12°
9°
6°
3°
0°
-3°
-6°
-9°
100Hz
1kHz
10kHz
Fig.3: these curves simulate the wide deviations from a flat frequency response for four loudspeakers previously
published in SILICON CHIP, caused by the interaction of the varying loudspeaker impedance with the typical 4-ohm
output impedance of a valve amplifier. The amount of boost can be seen on the left-hand vertical axis (in dB) while the
deviation in phase is shown in the dotted curves and the corresponding right-hand vertical axis (in degrees). These same
effects can simulated with the Loudspeaker Response control of the Nirvana Valve Sound Simulator.
following JFET stage if op amp IC1a is
set for minimum gain.
IC1a’s gain can be varied between
1.2 and 13 by potentiometer VR1a
which sets the signal clipping level in
the JFET stage. When VR1a is set for
minimum gain, the input signal needs
to reach 1.66V RMS before clipping
Main Features
•
•
•
•
•
•
•
•
Simulates the frequency response of a valve amplifier when driving loudspeakers
Provides mainly even-ordered harmonic distortion, ie, second, fourth, sixth etc
Input level control sets distortion threshold and clipping
Soft clipping on overload
Clipping indicators for positive and negative signal excursions
Clipping symmetry can be adjusted
One of four different loudspeaker responses can be used or design your own
Can run from a 9-12VAC supply (eg, a plugpack) or a ±12VDC to ±45V DC dual supply
(eg, from existing equipment)
36 Silicon Chip
occurs and when VR1a is set for maximum gain, the input signal only needs
to reach 109mV RMS before clipping.
Following IC1a is the JFET amplifier stage, Q1. This is configured as a
source follower (similar to a bipolar
transistor emitter-follower or a valve
cathode-follower). The JFET produces
harmonic distortion similar to that in
pentode valve stages (predominantly
even harmonics) and it also produces
soft signal clipping when overloaded.
The signal from IC1a is fed to the
Q1’s gate via a 100nF capacitor, while
the signal output is taken from Q1’s
source. Trimpot VR4 adjusts Q1’s
operating current and this varies the
symmetry of clipping, ie, whether the
siliconchip.com.au
signal clips symmetrically or whether
it clips the positive or negative signal
swings more severely.
IC2a drives the positive and negative
clipping indicators. It compares the input and output signals of Q1. When the
signals differ, such as when Q1 is clipping, the output of IC2a swings high or
low to drive LED1 (positive clipping)
or LED2 (negative clipping). For this
indication to be accurate, IC2a’s gain
needs to be carefully adjusted to be
equal to the gain of Q1, using trimpot
VR2 (or VR3 in the right channel).
Loudspeaker simulation
The output signal from Q1 is then
fed to the loudspeaker simulator section which comprises op amps IC3b,
IC3a, IC4b & IC4a (the equivalent functions in the right channel are provided
by IC3c, IC3d, IC4c & IC4d).
IC3b can be regarded as the main
op amp and its feedback network is
modified by op amps IC3a, IC4a & IC4b
which can each be regarded as singlefrequency equalisers, much like those
used in gyrator-based graphic equalisers. The difference is that we have no
slider controls to vary the individual
equalisers. The maximum gain at high
frequencies is set by ‘high-frequency
rise’ components R1L and C1L and
the overall gain is set by VR6a, the
Loudspeaker Response control.
IC3a is the equaliser providing the
simulated lower frequency impedance peak in a bass-reflex loudspeaker
system. IC4b adds the upper bass peak
for bass-reflex systems and the main
peak in sealed systems. In the latter
case, IC3a is effectively disabled and
has no effect on the overall frequency
response.
Finally, IC4a provides a mid-band
impedance hump that may be present
with some speaker systems.
So each of the three equalisers
boosts a defined frequency band about
a certain centre frequency.
By selecting the values of the
capacitors and resistors, we can set
the required tuning frequency and
shape of the boost. We have designed
the speaker impedance simulation
circuitry using LTSpice (see www.
linear.com/designtools/software/).
This SPICE simulation program from
Linear Technology can be used with
Windows or Mac operating systems.
The circuit file for this loudspeaker
simulation (Valve Simulator.asc) is
available on our website. You can
siliconchip.com.au
Parts List
1 double-sided PCB, code
01106141, 129.5 x 100mm
1 front-panel artwork, 132 x 27mm
OR 1 front-panel PCB, code
01106142
1 rear panel artwork, 132 x 27mm
1 ABS instrument case, 140 x
110 x 35mm (Jaycar HB-5970,
Altronics H 0472)
1 9-12V 50mA AC plugpack (optional, see text)
1 PCB-mount DC socket (CON1)
1 3-way PCB-mount screw terminal
block, 5.08mm pitch (CON2)
2 3.5mm PCB-mount stereo jack
sockets (CON3,CON4)
1 SPDT PCB-mount toggle switch
(S1) (Altronics S 1421)
1 16mm dual-gang 50kΩ linear
potentiometer (VR1)
1 16mm dual-gang 10kΩ linear
potentiometer (VR6)
4 10kΩ horizontal trimpots (VR2VR5)
2 knobs to suit potentiometers
2 DIL8 IC sockets (optional)
2 DIL14 IC sockets (optional)
4 No.4 x 6mm self-tapping screws
4 PC stakes (GND,GND,TP1,TP2)
1 100mm length of 0.7mm tinned
copper wire
Semiconductors
2 LM833 op amps (IC1,IC2)
2 TL074 quad op amps (IC3,IC4)
2 2N5485 JFETs (Q1,Q2)
2 3mm high-intensity red LEDs
(LED1,LED3)
2 3mm high-intensity blue LEDs
(LED2,LED4)
1 3mm high-intensity green LED
(LED5)
2 15V 1W zener diodes (ZD1,ZD2)
1 9.1V 1W zener diode (ZD3)
2 1N4004 1A diodes (D1,D2)
change the values and set the loudspeaker simulation curve yourself if
you wish. Otherwise, we have a table
that produces impedance curves for
some typical loudspeakers.
Power supply
Power for the circuit can come from
an AC plugpack (9-12V) rated at 50mA
or more. Alternatively, positive and
negative DC supply rails from existing
equipment can be used. In the latter
case, power is applied via CON2.
Resistors R5, R6, R7 & R8 are used
Capacitors
2 470µF 16V PC electrolytic
5 100µF 35-63V PC electrolytic
2 10µF 16V PC electrolytic
2 10µF 16V NP PC electrolytic
2 1µF monolithic ceramic
4 100nF MKT
2 470pF ceramic
2 47pF ceramic
2 2.2pF ceramic
Selected capacitors
JV100 simulation: 2 x 330nF, 2 x
150nF, 2 x 47nF, 2 x 22nF, 2 x 6.8nF
& 2 x 1nF MKT, plus 2 x 470pF
ceramic
JV80 simulation: 2 x 270nF, 2 x
100nF, 2 x 56nF, 2 x 22nF, 2 x 6.8nF
& 2 x 1nF MKT
JV60 simulation: 2 x 120nF, 2 x
82nF, 2 x 22nF, 2 x 12nF, 2 x 6.8nF &
2 x 1nF MKT, plus 2 x 470pF ceramic
8-inch woofer with piezo horn
simulation: 2 x 270nF, 2 x 100nF, 2
x 33nF & 4 x 4.7nF MKT
Resistors (0.25W, 1%)
8 1MΩ
2 1.5kΩ
4 100kΩ
2 820Ω
2 47kΩ
2 620Ω
4 22kΩ
1 270Ω
4 10kΩ
2 150Ω
1 4.7kΩ
2 10Ω
Selected resistors
JV100 simulation: 2 x 22kΩ, 4 x
12kΩ, 2 x 10kΩ
JV80 simulation: 2 x 33kΩ, 4 x 10kΩ
JV60 simulation: 2 x 22kΩ, 4 x 12kΩ,
2 x 10kΩ
8-inch woofer with piezo horn
simulation: 2 x 10kΩ, 4 x 8.2kΩ
Power supply resistors
R5-R8: see text & Table 1
when the external supply is 15V or
more. They provide the voltage drop
for 15V zener diodes ZD1 and ZD2.
Table 1 on the following page shows
the resistor values required for various
supply voltages.
Construction
The construction is straightforward
with all the parts mounted on a PCB
coded 01106141 and measuring 129.5
x 100mm. This is housed in a small
instrument case measuring 140 x 110
x 35mm (W x D x H).
August 2014 37
47pF
A
VR1 50kΩ
LED5
LED1 LED2
A
10 µF NP
10 µF NP
R4R
C6R
C5R
C4R
R3R
IC3
R1R
TL074
C3R
1M
2.2pF
A
C7R
IC4 TL074
1M
C3L
A
150Ω
47k
47k
R4L
C7L
C6L
C5L
1M
R3L
C4L
1M
R2L
C2L
R1L
100 µF
A
GND
620Ω
620Ω
1M
820Ω
22k
47pF
100 µF
10k
IC2
LM833
10k
470pF
100nF
22k
S1
10k
IC1
LM833
10k
4004
470pF
820Ω
4004
4.7k
10Ω
VALVE SIMULATOR
14160110
01106141
C 2014
D2
100nF
22k
R6
100nF VR3 10k
VR2 10k
D1
100k
100 µF
1 µF
100nF
22k
15V
1W
15V
1W
R8
1 µF
ZD1 ZD2
100k
TP2
2N5485
Q2
1.5k
1M
1.5k
R7
R5
+
2N5485
Q1
150Ω
100k
100 µF
TP1
470 µF
470 µF
100k
1M
1W
R
10 µF
100 µF
9.1V
10Ω
10 µF
VR5 10k
270Ω
+
R
VR4 10k
The PCB is fastened into
the case using four selftapping screws which
go into integral corner
pillars.
CON4
L
R2R
ZD3
OUTPUT
CON3
L
C2R
INPUT
GND
CON2
+V 0V –V
1M
CON1
9V to 12V
AC in
2.2pF
VR6 10kΩ
LED3 LED4 C1L
C1R
Fig.4: follow this parts layout diagram to build the PCB. Resistors R1-R4
and capacitors C1-C7 in the filter networks are selected from Table 2, while
the power supply resistors (R5-R8) are selected from Table 1 (see text).
Table 1. Dropping Resistors For External Dual Supply Rails
Supply Voltage
R5
R6
R7
R8
±45VDC
2.7kΩ 1W
2.7kΩ 1W
2.7kΩ 1W
2.7kΩ 1W
±40VDC
2.2kΩ 1W
2.2kΩ 1W
2.2kΩ 1W
2.2kΩ 1W
±35VDC
1.5kΩ 1W
1.5kΩ 1W
1.5kΩ 1W
1.5kΩ 1W
±30VDC
620Ω 1W
–
620Ω 1W
–
±25VDC
390Ω 1W
–
390Ω 1W
–
±20VDC
220Ω 1/2W
–
220Ω 1/2W
–
±15VDC
10Ω 1/2W
–
10Ω 1/2W
–
±12VDC
10Ω 1/2W
–
10Ω 1/2W
–
Note: a dash (–) means that no component is installed.
Before installing any of the parts,
you need to use Table 2 to select the
required values for resistors R1-R4
and capacitors C1-C7 to simulate a
particular speaker. These values depend on the speaker load that is being
simulated, as explained earlier.
Basically, Table 2 shows the values
required to simulate various loudspeaker loads. In other words, you can
simulate the sound of a valve amplifier
driving one of these types of speakers.
If you don’t have a preference, we
suggest using the JV80 values. Alternatively, you can determine your own
component values based on LTSpice
Table 2: R & C Values For Vented, Sealed & Piezo Horn Loudspeakers
HF Rise
First Impedance Peak
Second Impedance Peak
Midband Hump
Loudspeaker
VR6
Setting
C1
R1
C2*
C3*
R2*
C4
C5
R3
C6
C7
R4
JV100 (8-ohm)
5.6kΩ
470pF
22kΩ
330nF
22nF
12kΩ
150nF
6.8nF
10kΩ
47nF
1nF
12kΩ
JV80 (8-ohm)
5.6kΩ
–
–
270nF
22nF
10kΩ
100nF
6.8nF
10kΩ
56nF
1nF
33kΩ
JV60 (4-ohm)
3.9kΩ
470pF
22kΩ
120nF
22nF
12kΩ
82nF
6.8nF
10kΩ
12nF
1nF
12kΩ
8-inch speakers, with
piezo horn (8-ohm)
3.9kΩ
4.7nF
8.2kΩ
270nF
33nF
8.2kΩ
100nF
4.7nF
10kΩ
–
–
–
Note 1: R & C numbers show an ‘L’ suffix for the left channel components and an ‘R’ suffix for the right channel components on the circuit and
PCB layout. Note 2: * denotes no component for a sealed enclosure. Note 3: VR6 setting shown is for 4Ω output impedance amplifiers. VR6 is
set to a lower resistance for lower output impedance. Note 4: a dash (–) means that no component is installed.
38 Silicon Chip
siliconchip.com.au
3-way screw terminal block CON2
is necessary only if you are using an
external split DC supply.
Now for the two potentiometers
(VR1 & VR6). Before fitting them, cut
their shafts to suit the knobs using a
hacksaw and clean up the ends with
a file. It’s also necessary to file away a
small area of the passivation layer at
the top of each pot body, to allow an
earth wire to be later soldered in place
(see Fig.4).
The pots are then fitted to the PCB,
noting that VR1 is 50kΩ and VR6 is
10kΩ. Push them all the way down onto
the PCB before soldering their pins.
The two 3.5mm jack sockets (CON3
& CON4) can go in next, followed by
PC stakes for TP1 & TP2 and at the two
GND positions (one to the right of VR1
and one to the left of CON3).
Installing the LEDs
simulation, as explained earlier.
You also need to decide on the
power supply that you will be using
and select resistors R5-R8 from Table 1
if using an external split DC supply (ie,
one with positive and negative supply
rails). This could come from a power
amplifier or preamplifier, for example.
Alternatively, resistors R5-R8 are
not required if using an external
9-12VAC plugpack supply.
Fig.4 shows the parts layout on the
PCB. Begin the assembly by installing
the resistors. Table 3 shows the resistor colour codes but you should also
check each one using a DMM before
mounting it in place.
Follow with the IC sockets, diodes
D1 & D2, zener diodes ZD1-ZD3 and
trimpots VR2-VR5. Take care to ensure
that the diodes and zener diodes are
orientated correctly and note that the
IC sockets all face in the same direction
(ie, pin 1 at top left).
The capacitors are next on the list.
Table 4 shows the codes used on the
smaller ceramic and MKT types. Be
sure to orientate the polarised electrolytic types correctly and note that
the two 10µF electrolytics at top right
are non-polarised (NP).
Switch S1 and power socket CON1
are necessary only if using the AC
plugpack for the supply. Conversely,
Table 3: Resistor Colour Codes
o
o
o
o
o
o
o
o
o
o
o
siliconchip.com.au
o
o
No.
8
4
2
4
4
1
2
2
2
1
2
2
Value
1MΩ
100kΩ
47kΩ
22kΩ
10kΩ
4.7kΩ
1.5kΩ
820Ω
620Ω
270Ω
150Ω
10Ω
4-Band Code (1%)
brown black green brown
brown black yellow brown
yellow violet orange brown
red red orange brown
brown black orange brown
yellow violet red brown
brown green red brown
grey red brown brown
blue red brown brown
red violet brown brown
brown green brown brown
brown black black brown
The five LEDs are installed with
their leads bent down through 90°, so
that they later protrude through matching holes in the front panel. First,
check that the anode (longer) lead is
to the left (lens facing towards you),
then bend both leads down through
90° exactly 8mm from the rear of the
plastic lens. This is best done by folding them over a cardboard strip cut to
8mm wide.
Once that’s done, install each LED
so that its horizontal leads are exactly
4mm above the PCB. In practice, it’s
just a matter of pushing each LED
down onto a 4mm-thick spacer (eg, a
cardboard strip) before soldering its
leads. Use a green LED for LED5, red
Table 4: Capacitor Codes
Value
1µF
100nF
470pF
47pF
2.2pF
µF Value IEC Code EIA Code
1µF
1u0
105
0.1µF
100n
104
NA
470p
471
NA
47p
47
NA
2p2
2.2
5-Band Code (1%)
brown black black yellow brown
brown black black orange brown
yellow violet black red brown
red red black red brown
brown black black red brown
yellow violet black brown brown
brown green black brown brown
grey red black black brown
blue red black black brown
red violet black black brown
August 2014 39
brown green black black brown
brown black black gold brown
06/24/14 11:19:31
Valve Sound Simulator Spectral Response
+9
-10
+8
-20
+7
-30
+6
-40
+5
-50
+4
Amplitude Variation (dBr)
Spectral Power (dBV)
0
-60
-70
-80
-90
-100
+1
-3
-4
-140
-5
1k
2k
Frequency (Hz)
5k
10k
20k
Fig.5: spectrum analysis of the output signal (1kHz input),
showing strong second harmonic distortion along with
third, fourth, fifth and sixth harmonics at lower levels.
LEDs for LEDs1&3 and blue LEDs for
LEDs2&4.
The PCB assembly can now be completed by earthing the pot bodies to
the GND PC stake next to VR1. That’s
done using a length of 0.7mm-diameter
tinned copper wire (see Fig.4 and
photos). You can straighten the tinned
copper wire by clamping one end in a
vice and then stretching it slightly by
pulling on the other end with pliers.
It can then be bent to shape so that it
contacts the GND stake and soldered.
Minimum Loudspeaker Response
-1
-130
500
Intermediate Loudspeaker Response
0
-2
200
Maximum Loudspeaker Response
+2
-120
100
-6
20
50
100
Before installing the PCB assembly
in the case, you have to drill a number
of holes for the front and rear panels.
The accompanying panel artworks
(Fig.7) can be copied and used as drilling templates or you can download
them (in PDF format) from the SILICON
CHIP website and print them out.
alve
NirvanalaVtor
Simu
SILICON
CHIP
Power
Clipping
Level
-
+
L
Peak
+
R
200
500
1k
Frequency (Hz)
2k
5k
10k
20k
Fig.6: this graph shows the frequency response of the unit
when set to simulate driving JV60s, with the Loudspeaker
Response knob in three different positions.
On the front panel, you will need to
drill (and ream) a 5mm hole for switch
S1, 3mm holes for LEDs1-5 and 7mm
holes for the pot shafts. The two stereo
jack sockets on the rear panel require
6mm holes, while the DC power socket
requires a 6.5mm access hole.
Once that’s done, print the artworks
from the website onto photo paper
and attach them to the panels using
silicone sealant. The holes can then
be cut out with a sharp hobby knife.
Alternatively, you can purchase
a PCB-based front panel (blue with
white labels) with pre-drilled holes
from the SILICON CHIP Partshop.
After that, it’s just a matter of fitting the panels to the PCB, sliding the
assembly into the case and securing
the PCB to the four corner mounting
pillars using No.4 self-tapping screws.
The assembly can then be completed
by pushing the knobs onto the pot
Final assembly
06/24/14 11:04:52
+3
-110
-150
Valve Sound Simulator Frequency Response
Loudspeaker
Response
shafts. Reposition the end pointers of
the knobs if necessary, so that they correctly point to the fully anti-clockwise
and fully clockwise positions.
Testing
If you haven’t already done so, insert
the four ICs into their sockets, taking
care to orientate them correctly. Next,
apply power and check that the power
LED lights. If that checks out, check
the supply voltage between pins 8 & 4
of both IC1 and IC2 and between pins
4 & 11 of IC3 and IC4. This should be
around 30V DC if you are applying
12VAC via CON1. Alternatively, you
can apply ±12V DC or more via 3-way
screw terminal block CON2.
Note that you will only get around
25V (ie, ±12.5V) if using a 9VAC supply. Regardless, there should be about
9.1V across ZD3.
Assuming these supply voltages are
Fig.7: these two artworks
can be copied and used as
drilling templates for the
front & rear panels. They
can also be downloaded
as a PDF file from the
SILICON CHIP website.
Power
9-12VAC
Output
40 Silicon Chip
Input
siliconchip.com.au
all correct, follow this step-by-step
procedure to adjust the unit:
Step 1: connect a DMM set to volts between TP1 and a GND stake and adjust
VR4 for a reading of 5.8V. Similarly,
adjust VR5 for a reading of 5.8V at TP2.
This gives more or less symmetrical
clipping for both Q1 and Q2.
Step 2: apply a low-level 1kHz signal
to both the left and right inputs and
adjust VR2 & VR3 so that the positive
and negative peak LEDs in both channels are off. You will find that there’s
a ‘dead spot’ in each trimpot’s setting
range where both LEDs are off. Set each
trimpot to the middle of its dead spot.
If the LEDs do not extinguish with
this adjustment, try reducing the signal
level using VR1 or at the signal generator (note: if you don’t have a signal
generator, it’s easy to find a virtual
instrument online).
Step 3: increase the signal level so
that the clipping LEDs begin to light.
When that happens, readjust trimpots
VR4 & VR5 to give symmetrical clipping, so that both the red and blue
clipping LEDs light at the same time
(ie, for the positive and negative signal
excursions).
Finally, note that the input and output sockets can be linked to RCA connectors via adaptor cables (ie, 3.5mm
stereo jack plug to RCA). For mono use,
a mono 3.5mm jack plug can be used
in which case only the left channel
will be supplied with signal and the
right channel input will be grounded.
A mono plug could then also be used
for the output since the right channel
SC
will not have any output.
The rear panel carries access holes for the input and output sockets and for
the power socket. Note how the metal bodies of the two pots are earthed to
the GND stake using a length of tinned copper wire.
Fig.8: the output
of the unit (green)
compared to the
input (yellow) at
1kHz. The signal
level is set below
clipping and the
distortion residual
(blue) is primarily
second harmonic.
This can be clearly
seen as the residual
is at twice the
fundamental frequency, ie, 2kHz.
Fig.9: the same traces as in Fig.8 but with more input
signal, causing clipping. The effects of soft clipping and
the frequency response shaping filter are evident.
siliconchip.com.au
Fig.10: the input signal is still being clipped here but
now we have adjusted VR4 & VR5 to give asymmetrical
clipping, resulting in a different type of distortion.
August 2014 41
|