AUDIO
OUT
AUDIO OUT
L
R
By Jake Rothman
Analogue Vocoder – Part 5: Building the filters
also a single card containing a high-pass
channel and a low-pass channel called
the ‘HP/LP board’. These all plug into
a bus board with common busses for
power, two audio inputs and the two
audio summing lines. These boards are
unique to the Analogue Vocoder, they’re
not generic audio boards like the previous
mic pre-amp and driver boards. This
assembly is shown in Fig.1.
Jelly bean circuits
Fig.1. The heart of the vocoder with six band-pass filter cards and a high-pass/lowpass card. All plugged into the bus board. (No letters from lexicographical pedants
please. It can be ‘bus’ or ‘buss’! However, it’s always ‘busses’.)
TP2A TP+
TP2B
TP-
IC5
TPG
TP1B TP1A
C5
C4
TP5B
C2
C13
Card position: TP4B
C12
TP4A
C7
TP3A TP5A
C10
C11
TP3B
parts together and soldering-up will take
a long time, so you really need to pay
attention! There are six band-pass cards,
each containing two channels. There is
POT1
nitty-gritty of the project – the filter
cards, which are the heart of the
Analogue Vocoder. Getting all the many
POT2
T
his month, we get to the real
The full circuit of a band-pass card is shown
in Fig.2. Take care with the dual-channel annotation. Luckily, it’s just a large
number of ‘Jellybean’ parts, no weird expensive components. Note there are a few
improvements (differences!) compared to
the previous channel circuit given in the
December 2021 issue. First, R24 should be
180Ω and the decoupling capacitors C14,
C15 and C16 have been re-numbered in
a different order. Ceramic rail-to-rail decoupling capacitor C16 has become C30
because I had to move it to the other end
of the board where the NE5532 output
chips were. This was necessary because
5532s are fussier about supply decoupling
than TL082s. And another minor detail,
the buffer around IC3a got its input pins
mixed up – Fig.2 here is the correct version.
IC3
IC1
Right out
Vox in
+ve
Synth in
Left out
-ve
Gnd
Gnd
Gnd
Gnd
C1
C17
C15
C6
C27
C8
C3
IC
C21
C18
C22
C29
IC9
C28
C25
C26
Filter frequency:
T2
C19
IC6
C20
D4
C23
IC
Fig.3. Band-pass filter card overlay showing common components highlighted in red. (PCB design by Mike Grindle)
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Practical Electronics | March | 2022
Fig.2. Full circuit of
the band-pass PCB.
Note this contains two
complete channels.
Identical parts across all
six boards labelled in red.
V + 1 5 V
1 0 µF
2 5 V
0 V
0 V
1 0 µF
2 5 V
1 0
1 4
1 8
1 9
2
S p e e c h in
T P 1 A
F ilte r
o u tp u t
D 1
1 N 4 1 4 8
R 7
100Ω
–
R 2
1
IC 1 a
T L 0 8 2
3
C 3
R 4
6
+
–
R 9
100kΩ
5
R 5
R 8
100kΩ
C 5
7
IC 1 b
T L 0 8 2
R ig h t o u t
2 0
R 6
C 1
S y n th in
1 6
1 7
V – 1 5 V
L e ft o u t
1 2
1 5
R 3
In p u t
s p e e c h
P le a s e n o te th a t th e r e
is n o C 1 6 in th is c ir c u it
8
1 1
1 3
C 4
R 1
6
4
9
C 3 0
4 7 0 n F
C 2
n
2
5
3
7
+C 1 5
R 3 5
33Ω
1
0 V
+C 1 4
R 3 4
33Ω
+
2
–
3
R 1 0
200kΩ
V R 1
1MΩ
D 2
1 N 4 1 4 8
C o n tro l
o u tp u t
V C A
b ia s
V +
R 1 3
2.2MΩ
R 1 4
100Ω
C 6
V –
R 1 2
47kΩ
R 1 1
100kΩ
6
1
IC 2 a
T L 0 8 2
T P 2 A
–
5
+
7
IC 2 b
T L 0 8 2
+
0 V
0 V
C 7
R 1 5
6
R 1 6
C 8
0 V
3
+
2
IC 3 a
T –L 0 8 2
1
In p u t
s y n th
0 V
+
C 1 0
2
R 2 5
12kΩ
( o p tio n a l)
R 2 3
22kΩ
+
R 2 0
R 1 8
10kΩ
1 1
4
R 2 4
180Ω
R 3 6
100Ω
+
2
1
R 2 8
12kΩ
3
7
6
0 V
R 4 5
100kΩ
R 4 2
C 1 7
2
3
–
R 3 8
R 4 0
1
IC 6 a
T L 0 8 2
C 1 9
6
+
–
5
R 4 1
C 2 1
7
IC 6 b
T L 0 8 2
R 4 4
100kΩ
+
2
3
R 4 6
200kΩ
V R 2
1MΩ
D 5
1 N 4 1 4 8
–
V C A
b ia s
V +
D 4
1 N 4 1 4 8
R 4 3
100Ω
R 3 9
R 3 3
2.2kΩ
( O m itte d
fo r s te re o )
R 3 0
560Ω
F ilte r
o u tp u t
C 2 0
7
IC 5 b
5 5 3 2
+
V –
T P 1 B
O u tp u t
r ig h t
R 2 7
4.7kΩ
R 2 6
10kΩ
C 1 8
R 3 2
2.2kΩ
–
5
8
6
0 V
In p u t
s p e e c h
C 1 2
R 2 9
IC 4 a
L M 1 3 7 0 0
5
T P 3 A
O u tp u t
le ft
C 1 3
R 3 1
R 2 2
R 3 7
R 7 2
100Ω
R 7 1
100Ω
–
1
IC 5 a
5 5 3 2
3
T P 4 A
V +
–
T P 5 A
T R 1
B C 5 5 9 C
C 1 1
R 2 1
R 1 9
7
IC 3 b
T L 0 8 2
5
R 1 7
3.3kΩ
D 3
1 N 4 1 4 8
C 9
2 2 n F
–
T P 2 B
C 2 7
R 5 0
100Ω
R 4 9
2.2MΩ
R 4 8
47kΩ
V –
R 4 7
100kΩ
6
1
IC 7 a
T L 0 8 2
C o n tro l
o u tp u t
–
5
+
0 V
IC 7 b
T L 0 8 2
7
+
0 V
C 2 2
R 5 1
6
R 5 2
C 2 3
0 V
3
+
2
IC 8 a
T –L 0 8 2
In p u t
s y n th
C 2 5
2
3
–
R 5 6
IC 9 a
5 5 3 2
+
5
R 5 3
3.3kΩ
D 6
1 N 4 1 4 8
7
IC 8 b
T L 0 8 2
0 V
+
T R 2
B C 5 5 9 C
R 6 2
12kΩ
( o p tio n a l)
1 1
R 6 0
22kΩ
R 5 4
10kΩ
R 6 1
180Ω
–
0 V
Practical Electronics | March | 2022
6
R 7 4
100Ω
R 6 7
1 6
1 2
1 3
C 2 9
R 6 5
12kΩ
C 2 8
1 0
R 6 3
10kΩ
6
–
5
R 6 6
IC 4 b
L M 1 3 7 0 0
1 5
R 5 8
T P 3 B
R 7 3
100Ω
+
1 4
1
R 5 9
100Ω
T P 5 B
T P 4 B
V +
C 2 6
R 5 7
R 5 5
1
C 2 4
2 2 n F
–
IC 9 b
5 5 3 2
+
7
O u tp u t
le ft
R 6 8
2.2kΩ
( O m itte d
fo r s te re o )
R 6 9
2.2kΩ
O u tp u t
r ig h t
R 7 0
560Ω
9
R 6 4
4.7kΩ
V –
0 V
47
Fig.4. Photo of band-pass board showing common components inserted. Note the
white 3.9nF capacitors are the filter capacitors. Note the white silkscreen label areas.
design by Richard Becker (ETI, September
1980). When I realised it wasn’t necessary it seemed silly not to do the mod.
The second major change is that I’ve
phase reversed every other channel by
flipping the inverting and non-inverting
pins on the transconductance op amp
IC4b. This gives less phase cancellation
in the frequency response overlap regions
of the filters. When mixed together in
mono a clearer sound is produced. Normally, an extra inverting op amp stage is
used to do this, such as in the vocoder
Getting stuffed
On all seven filter boards (HP/LP and
bandpass) the VCA and full-wave rectifier components are identical in all the
filter channels, so when populating the
boards it’s best to fit these first, as listed
Band-pass channel
and frequency
R2, 5, 20, 29
R1, 19
R3, 21, 22
R38, 41, 56, 66
R37, 55
R39, 57, 58
Ch
(kΩ)
(kΩ)
(kΩ)
Freq (Hz)
C1, 2, 3, 4, 10,
11, 12, 13
C17, 18, 19, 20,
25, 26, 28, 29
(nF)
immediately below. The overlay for common components is shown in Fig.3 with
a photo in Fig.4. I do a run like this of
six band-pass boards. The HP/LP board
also has these components.
Then I add the eight main filter capacitors to the band-pass boards, where there
are four values covering all the frequencies. Most are 33nF. Finally, the remaining
components are installed which are
unique to each filter frequency and the
HP/LP board. These component values
are given in Table 1. Mike Grindle, my
PCB man, had the great idea of placing
white areas on the silk screen for writing the frequencies. You must label the
boards before building!
Components – bandpass
The following parts are common to all
six bandpass boards. Frequency-specific parts for the bandpass boards are not
listed, but are given in Table 1. (The parts
in Table 1 are 1% 0.25W metal film resistors, and 5% (preferred) or 10% 5mm
plastic film capacitors.)
R4
R6, 31
C5
C6
C7
C8
R15, 16
R40
R42, R67
C21
C27
C22
C23
R51, R52
(kΩ)
(kΩ)
(nF)
(nF)
(nF)
(nF)
(kΩ)
1
86 / 125
0 .2 2
1 .6
5 6
3 3 0
1 2
6 8
1 0 0
3 3 0
1 0 0
2 2
3 9 0
2
135 / 60
1 .5
1 5
3 9 0
3 3
7 5
4 7 0
6 8
2 2 0
6 8
1 5
3 9 0
3
190 / 230
1 .0
1 3
2 7 0
3 3
5 6
3 3 0
4 7
1 5 0
4 7
1 0
3 9 0
4
280 / 340
0 .6 8
5 .1
1 8 0
3 3
3 9
2 2 0
3 3
1 0 0
3 3 0
6 8
8 2
5
420 / 500
0 .4 7
3 .6
1 2 0
3 3
2 7
1 5 0
2 2
8 2
2 2 0
4 7
8 2
6
580 / 720
0 .3 3
2 .4
8 2
3 3
1 8
1 0 0
1 5
5 6
1 5 0
3 3
8 2
7
860 / 1250
0 .2 7
1 .6
5 6
3 3
1 2
6 8
1 0
3 9
1 0 0
2 2
8 2
8
1350 / 1600
0 .1 5
1 .1
3 9
3 3
8 .2
4 7
6 .8
2 2
6 8
1 5
8 2
9
1900 / 2300
1 .0
7 .5
2 7 0
3 .9
5 6
3 3 0
4 .7
1 8
4 7
1 0
8 2
10
2800 / 3400
0 .6 8
5 .1
1 8 0
3 .9
3 9
2 2 0
3 .3
1 2
3 3
6 .8
8 2
11
4200 / 5000
0 .4 7
3 .6
1 2 0
3 .9
2 7
1 5 0
2 .2
8 .2
2 2
4 .7
8 2
12
5800 / 7200
0 .3 3
2 .4
8 2
4 .7
1 8
1 0 0
1 .5
5 .6
1 5
3 .3
8 2
Table 1. Frequency-specific components for band-pass filters.
TP2A TP+
TP2B
TP-
IC3
IC1
C3
C15
D2
IC2
D1
D3
C8
C9
T1
Filter frequency:
C21
C18
C22
C28
C14
IC4
C30
IC9
TP1B TP1A
5
C4
3
C2
3
C1
IC5
TPG
POT1
POT2
TP5B
C6
C13
Card position: TP4B
C12
TP4A
C7
TP3A TP5A
10
11
TP3B
Right out
Vox in
+ve
Synth in
Left out
-ve
Gnd
Gnd
Gnd
Gnd
T2
IC6
C20
C27
D5
D4
D6
IC7
C24
C23
IC8
Fig.5. Overlay of band-pass board, frequency-specific parts highlighted – green odd channels, blue for even. (PCB design by Mike Grindle)
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Practical Electronics | March | 2022
Also, 7 off female straight 10+10 pin double row header sockets for the bus board
are required. Rapid part no. 22-5140.
Manufacturer, Oupiin 2044-2*10G00SA.
Tayda alternative 1690.
Freaky filter values
The filter frequencies for each channel are
different, so most of the components associated with these have different values. Be
careful here to avoid errors, which can be
difficult to detect unless you can measure
frequency response. Because the vocoder
Fig.6. Completed band-pass board (note that since this prototype was made, a couple is effectively a parallel processor, it can
work deceptively well with a few filter
of capacitors have been repositioned).
channels missing so it can be difficult to
The quantities listed are for one bandpass board – you need
detect errors by listening. These filter values comprised one
to multiply quantities by the number of boards used, normally
of the most tedious component lists I’ve ever typed, so I had
six. All parts available from author (see AOShop ad on page 53).
to resort to a spreadsheet, as shown in Table 1. You can download the table from the March 2022 page of the PE website.
Semiconductors
The next construction stage is to put in the filter components
IC1-IC3, IC6-IC8
TL082 JFET input dual op amp or
shown in the overlay in Fig.5 (it’s quite crowded, but you can
equivalent (eg LF353 or TL072)
download it and Fig.8 to view/print them magnified from the March
IC4
LM13700 dual transconductance
2022 page of the PE website). Fig.6 shows a completed board.
op amp
IC5, IC9
NE5532 low-noise dual op amp or
High-pass / low-pass board
equivalent, such as LM833
The common components and circuitry is the same as the
TR1, TR2
BC559C small-signal PNP (can be
band-pass channels, but the topology of the high-pass and
any general-purpose centre-base
low-pass filters is different, it’s composed of standard caspin-out – eg, BC212)
caded Sallen and Key second-order sections, rather than
D1-D6
1N4148
multiple-feedback band-pass. The circuit for the LP/HP
board is shown in Fig.7. the overlay in Fig.8 and a photo of
Resistors
the completed board in Fig.9.
All 1% 0.25W metal-film
R8, R9, R11, R19
R10, R46
R7, R14, R36, R43, R50,
R59, R71-R74
R13, R49
R12, R48
R17, R53
R18, R54
R23, R60
R24, R61
R26, R63
R25, R28, R62, R65
R30, R70
R34, R35
R32, R33, R68, R69
VR1 and VR2
Capacitors
C9, C24
C14, C15
C30
Miscellaneous
Terminal pins
IC sockets
PCB plug
100kΩ
200kΩ
100Ω
2.2MΩ
47kΩ
3.3kΩ
10kΩ
22kΩ
180Ω
10kΩ
12kΩ (R25 and R62 not normally
used), R24 and R65 have to be reduced to compensate.
560Ω
33Ω
2.2kΩ (see Stereo panning below,
4.7kΩ for mono channels)
1MΩ Side adjust/vertical trimmer
0.1-inch or TO5 spacing.
22nF 20% 5mm ceramic/plastic film
10µF 25V tantalum bead or radial
electrolytic
470nF 20% X7R ceramic
0.1-inch single-sided 8 off.
8-pin DIL 8 off.
male right-angle header 10+10 rows,
Rapid part no. 22-0815, TruConnect.
(Alternative: Tayda part no. 3420)
Practical Electronics | March | 2022
Component list for HP/LP board
Semiconductors
IC1-IC3, IC6-IC8
IC4
IC5, IC9
TR1, TR2
D1-D3
TL082 JFET input dual op amp or
equivalent (eg LF353 or TL072)
LM13700 dual transconductance
op amp
NE5532 low-noise dual op amp or
equivalent, such as LM833
BC559 small-signal PNP (can be
any general-purpose centre-base
pin-out – eg, BC212)
1N4148
Capacitors
All are 5mm plastic-film 5% (preferred) or 10%, unless indicated with a ‘*’.
C1, C8, C9, C13
470nF
C8, C27
47nF
C2, C3, C5, C6, C14, C15, C17,
C18, C29
15nF (9 off)
C10
150nF
C11
33nF
C12, C20, C31, C32
22nF 20% ceramic
or plastic-film*
C30
3.3nF
C21, C22, C24, C25, C28,
C33, C34, C36, C37
2.2nF (9 off)
C23, C26, C35, C38
27pF ceramic
C39, C40
10µF 25V tantalum
or electrolytic*
C41
470nF 20% ceramic X7R*
49
V + 1 5 V
Fig.7. Circuit of the highpass/low-pass board.
1 0 µF
2 5 V
0 V
C 1
4 7 0 n F
R 2
180kΩ
0 V
1 0 µF
2 5 V
C 2
1 5 n F
R 1
22kΩ
3
+
2
IC 1 a
T –L 0 8 2
R 6
180kΩ
1
1 0
1 4
1 8
1 9
S p e e c h in
T P 1 A
+
IC 1 b
T–L 0 8 2
6
R 5
27kΩ
7
R ig h t o u t
2 0
F ilte r
o u tp u t
C 8
4 7 n F
R 1 2
100kΩ
R 1 1
100kΩ
2
3
R 1 3
200kΩ
R 9
62kΩ
R 8
39kΩ
D 2
1 N 4 1 4 8
–
C o n tro l
o u tp u t
T P 2 A
V R 1
1MΩ
C 9
R 1 5
2.2MΩ 4 7 0 n F
R 1 7
100Ω
V –
R 1 6
100kΩ
V C A
b ia s
V +
D 1
1 N 4 1 4 8
R 1 0
100Ω
5
S y n th in
1 6
1 7
R 7
180kΩ
L e ft o u t
1 2
1 3
1 5
C 5
1 5 n F
R 4
39kΩ
P le a s e n o te th a t th e r e
a re n o C 4 , C 7 , C 1 6
a n d C 1 9 in th is c ir c u it
8
1 1
C 6
1 5 n F
C 3
1 5 n F
R 3
180kΩ
6
4
9
C 4 1
4 7 0 n F
V – 1 5 V
In p u t
s p e e c h
2
5
3
7
+C 4 0
R 8 2
33Ω
1
0 V
+C 3 9
R 8 1
33Ω
R 1 4
100kΩ
6
1
IC 2 a
T L 0 8 2
–
+
7
IC 2 b
T L 0 8 2
5
+
0 V
0 V
C 1 0
1 5 0 n F
R 1 8
390kΩ
R 1 9
390kΩ
6
2
C 1 1
3 3 n F
–
3
7
IC 3 b
T L 0 8 2
5
1
IC 3 a
T L 0 8 2
T R 1
B C 5 5 9 C
0 V
R 3 0
12kΩ
( o p tio n a l)
In p u t
s y n th
C 1 3
4 7 0 n F
R 2 3
180kΩ
R 2 4
180kΩ
C 1 4
1 5 n F
R 2 2
1MΩ
T P 4 A
R 2 8
22kΩ
3
+
2
5 5 3 2
–
4
R 2 9
180Ω
R 2 7
100Ω
R 2 6
R 2 5 27kΩ
39kΩ
1 1
1
+
2
3
T P 3 A
IC 4 a
L M 1 3 7 0 0
7
R 4 3
5.1kΩ
C 2 1
2 .2 n F
C 2 2
2 .2 n F
R 4 1
100kΩ
C 2 4
2 .2 n F
+
3
–
R 4 5
27kΩ
R 4 4
39kΩ
C 2 5
2 .2 n F
R 4 6
9.1kΩ
C 2 3
2 7 p F
T P 1 B
+
6
IC 6 b
T L 0 8 2
–
R 5 9
68kΩ
6
2
C 3 0
3 .3 n F
3
–
IC 8 a
T L 0 8 2
1
–
5
+
C 3 3
2 .2 n F
R 6 2
10kΩ
C 3 4
2 .2 n F
R 6 4
5.1kΩ
3
+
C 3 1
2 2 n F
2
R 6 3
5.1kΩ
R 6 5
3.9kΩ
0 V
50
IC 9 a
5 –5 3 2
R 6 6
2.7kΩ
C 3 5
2 7 p F
R 6 9
180Ω
R 6 7
100Ω
T P 3 B
–
3
V R 2
1MΩ
D 5
1 N 4 1 4 8
IC 7 a
T L 0 8 2
C o n tro l
o u tp u t
T P 2 B
C 2 8
R 5 5
2 .2 n F
2.2MΩ
R 5 7
100Ω
V C A
b ia s
V +
R 5 6
100kΩ
V –
R 5 4
100kΩ
6
1
–
IC 7 b
T L 0 8 2
5
+
R 6 0
3.3kΩ
0 V
+
T R 2
B C 5 5 9 C
1 3
T P 5 B
T P 4 B
R 6 1
10kΩ
V +
1 1
R 7 8
100Ω
R 7 3
R 7 5
9.1kΩ
C 3 6
2 .2 n F
1 6
–
+
1 5
6
C 3 7
2 .2 n F
R 7 4
9.1kΩ
IC 4 a /(b )* *
L M 1 3 7 0 0
1 2
1 4
7
+
0 V
7
R 7 0
12kΩ
( o p tio n a l)
1
2
R 5 3
200kΩ
D 6
1 N 4 1 4 8
IC 8 b
T L 0 8 2
R 6 8
22kΩ
R 5 1
100kΩ
C 2 6
2 7 p F
0 V
In p u t
s y n th
C 2 7
4 7 n F
R 4 9
62kΩ
0 V
R 5 8
68kΩ
R 5 2
100kΩ
O u tp u t
r ig h t
0 V
D 4
1 N 4 1 4 8
7
R 4 8
39kΩ
C 2 9
1 5 n F
F ilte r
o u tp u t
R 5 0
100Ω
5
1
IC 6 a
T L 0 8 2
2
R 4 2
5.1kΩ
R 4 7
9.1kΩ
R 4 0
2.2kΩ
R 3 7
R 3 6
6.2kΩ
3.9kΩ
R 3 2
4.7kΩ
R 3 1
10kΩ
7
IC 5 b
5 –5 3 2
6
8
6
R 3 9
2.2kΩ
+
5
V –
In p u t
s p e e c h
C 3 2
2 2 n F
R 3 5
180kΩ
C 1 7
1 5 n F
5
O u tp u t
le ft
C 1 8
1 5 n F
R 3 4
180kΩ
0 V
C 2 0
2 2 n F
R 3 3
100Ω
–
1
IC 5 a
R 3 8
100Ω
R 2 1
10kΩ
V +
C 1 5
1 5 n F
T P 5 A
0 V
+
+
R 2 0
3.3kΩ
D 3
1 N 4 1 4 8
C 1 2
2 2 n F
–
1 0
R 7 1
10kΩ
O u tp u t
le ft
+
5
6
IC 9 b
5 5 3 2
–
7
R 7 7
6.2kΩ
R 7 9
2.2kΩ
R 8 0
2.2kΩ
O u tp u t
r ig h t
9
R 7 2
4.7kΩ
V –
C 3 8
2 7 p F
R 7 6
3.9kΩ
0 V
Practical Electronics | March | 2022
C25
C27
C24
C26
IC9
C29
C37
C33
C32
C23
C38
C34
C39
IC4
C41
C6
C2
C1
C40
D2
D1
D3
C12
IC1
IC2
Filter frequency:
C16
C8
C5
IC3
C11
C17
C7
C3
3
T1
C14
TP1B TP1A
C4
C9
IC5
3
C10
C18
C15
C13
LP/HP
TP2A TP-
POT1
TP+ TP2B
TP5A TP4A TPG TP4B TP5B
C19
POT2
TP3B TP3A
Right out
Vox in
+ve
Synth in
Left out
-ve
Gnd
Gnd
Gnd
Gnd
C36
T2
C21
C20
IC6
C22
C28
D5
D4
D6
IC7
C31
C30
C35
IC8
Fig.8. Component overlay of HP/LP board. (PCB design by Mike Grindle)
Resistors
All 0.25W 1% metal-film
R1, R22
1MΩ
R2, R3, R6, R7, R23, R24,
R34, R35 ( 8 o f f )
180kΩ
R4, R8, R25, R44, R48
39kΩ
R5, R26, R45
27kΩ
R9, R49
62kΩ
R10, R17, R27, R33, R38, R50,
R57, R67, R73, R78 ( 1 0 o f f )
100Ω
R11, R12, R14, R16, R51,
R52, R54, R56, R41
100kΩ
R13, R53
200kΩ
R15, R55
2.2MΩ
R18, R19
390kΩ
R20, R60
3.3kΩ
R30, R70 (not usually used) 12kΩ
R21, R31, R61, R62, R63, R71 10kΩ
R32, R72
4.7kΩ
R28, R68
22kΩ
R29, R69
180Ω
R36, R65, R76
3.9kΩ
R37, R77
6.2kΩ
R39, R40, R79, R80
2.2kΩ
R42, R43, R63, R64
5.1kΩ
R46, R47, R74, R75
9.1kΩ
R58, R59
68kΩ
R66
2.7kΩ
04/01/2022 12:09 f=3.64 /Users/mg/Documents/PCBs/Jake/Vocoder/HP LP/HP LP.brd
Ω
VR1 and VR2 1MΩ side-adjust/vertical trimmer 0.1-inch or
TO5 spacing.
Tuning for smoke
The power-rail decoupling resistors
(R34/R35, R81/R82) also provide protection and confine shorts to the card.
Since there is a possibility of inadvertent short circuits during testing, these
resistors should be mounted a few millimetres above the board (Fig.10). Burnt
resistors cost a penny, a burnt PCB a lot
more! On one PCB a small ball of solder
got lodged between two pins under IC2’s
turned-pin chip socket connecting the
negative rail to ground, (pin 3 to 4) and
it took ages to find. It was the first time
I’ve seen it (Fig.11).
Stereo panning
On the band-pass boards, two summing resistors for the left (R32 and R68) and right
(R33 and R69) mix busses are provided for
each filter channel. Normally only R32 and
R69 are inserted. This flips the channels
alternately to left and right mixing busses
as shown in Fig.12. However, for the mono
Fig.9. Completed HP/LP board. Note this was an earlier version with a few extra
component positions for experimental purposes. All the boards supplied by PE will
exactly conform to the overlays.
Practical Electronics | March | 2022
channels (the HP/LP board and both the
channels on the lowest-frequency bandpass board), all four resistors are inserted.
(For the HP/LP board these resistors are
R39, R40, and R79 , R80).
Fig.10. The power supply decoupling
resistors R34 and R35 should be
mounted off the board in case they get
hot under fault conditions.
Fig.11. Short circuits can occur in the
oddest places. This is a reconstruction
of what occurred on one of my boards.
In reality, it was worse, the solder ball
was behind the pins so couldn’t be seen
and a Model 1000 Tracer had to be used
to find it.
51
Fig.12. (above) To get a stereo effect, channels are panned
alternately left and right by omitting resistors R33 and R68.
Fig.13. (Right) For mono channels, all four resistors are inserted
and their value increased to 4.3kΩ.
In mono, to keep the relative levels the
same on the band-pass board, the summing resistors are increased from 2.2kΩ to
4.3kΩ as implemented in Fig.13. Note that
because the low-pass and high-pass filters
have lower gain relative to the band-pass
filters, their resistors are kept at 2.2kΩ.
Bus board
All the filter cards are plugged into a bus
board, which is similar to a sophisticated
Veroboard where the tracks don’t come off.
This is shown in Fig.14. Note how the connectors are double pin to reduce contact
resistance and give increased mechanical
stability (Fig.15). Extra connectors are also
placed on this board to feed in the power and connect to the drive amplifier and
mixer boards described last month. There
are also additional pins to the summing
busses for feeding in dry signals from the
microphone and synthesiser if required.
Testing 1-2-3
As with all electronics, test in minimum-sized sections; get one channel
card working at a time. If you connect the
whole lot in one go, it’s guaranteed not to
work. First, do a basic visual check that
all the polarised components such as the
op amps are in the right way. Then, apply
power to check for heating of R34 and R35
caused by excess current. Follow this with
checks for DC offsets on the op amp test
pins (TP1A/B) along the top of the board.
100Ω resistors are connected in series with
the pins to stop oscillation when long test
leads are connected. Now you’re ready for
the audio signal tests next month.
Trimming
Pricier vocoders (such as the Richard Becker design mentioned above) have typically
five preset adjustments per channel. I reduced these to the absolute minimum of
one for the VCA control voltage offset on
each channel. VR1 and VR2 are trimmed
to the point where the carrier signal (usually a string sound) is just backed off from
breaking through. This point gives optimum
linearity for best speech intelligibility.
The band-pass filters’ Q can vary a lot
with capacitor tolerances, since the multiple feedback band-pass filter equation
assumes the two frequency determining
capacitors are exactly equal. If cheap 10%
types are used it is possible for their values
to be up to 20% apart. When this happens,
the resulting gain on individual channels
can occasionally peak up excessively. If any
frequencies dominate the output mix, the
values of the filters’ input resistors R1 and
19 or R37 and R55 can be increased. The
levels here can be checked on test points
TP1A and TP1B. If you want to add extra
presets this is one area where it could be
beneficial. Alternatively, one could use
close-tolerance polystyrene capacitors.
Fig.14. The bus board – note a few summing resistors have been added to allow for
auxiliary inputs.
52
However, they may be difficult to mount
on the boards vertically since they are often
only available in axial form. An example
is shown in Fig.16.
Next month
In the next article we’ll cover the final construction and testing. Also, I’ll present a
triple-rail ultra-low-noise power supply
suitable for the Vocoder and other audio
systems, such as mixers, that use many
op amps and/or require phantom power.
Fig.15. Connector plugs are doubled up to
improve mechanical rigidity. Eight pins are
used on the 0V rail for low impedance.
Fig.16. There is just about enough room to
use 1% polystyrene capacitors for the filters,
if you can get them. These Philips 424
series were excellent audio filter capacitors,
but because the foil material was the
neurotoxic metal lead, they were banned.
Practical Electronics | March | 2022
