Oakley SRE330 User manual

Oakley Sound Systems
Stereo Ensemble – SRE330
Main Board Issue 2
Builder's Guide
V2. 0
Tony Allgood
Oakley Sound Systems
CARLISLE
United Kingdom

Introduction
This is the Builder's uide for the Stereo Ensemble Module SRE330 from Oakley Sound. This
document contains a basic introduction to the three circuit boards used to make the SRE330 rack
module and a full parts list for all the components needed to populate the board or boards.
For the User Manual, which contains an overview of the unit, the operation of the module and the
calibration procedure, please visit the main project webpage at:
http://www.oakleysound.com/sre330.htm
For general information regarding where to get parts and suggested part numbers please see our
useful Parts uide at the project webpage or http://www.oakleysound.com/parts.pdf.
For general information on how to build our modules, including circuit board population, mounting
front panel components and making up board interconnects please see our generic Construction
uide at the project webpage or http://www.oakleysound.com/construct.pdf.
The first prototype unit built into a Bryant Broadcast 1U 250mm deep 19” rack enclosure with the Schaeffer front
panel attached This one used an earlier issue 1 RPSU power module Issue 2 RPSU boards are green and use a
five way 0 156” connector to take power onto the SRE330 main board

Constructing the SRE330
The SRE330 project uses the SRE330 main board, the optional SREIO input/output board and the
RPSU +/-15V power supply module.
The SRE330 main board requires a well regulated and quiet +/-15V to +/-16V supply. The current
taken by the SRE330 varies slightly with modulation depth but the maximum current is around
+225mA and -180mA. Power is admitted onto the main board via a 5-way Molex 0.156” KK or 5-
way 0.156” MTA connector. Pin 1 is +15V, pin 2 and 3 are 0V, pin 4 is -15V and pin 5 is panel
ground. Panel ground is typically connected to 0V at either the power supply or the input/output
sockets. On the SRE330 main board Pin 5 connects only to the pot brackets and switches' metal
supporting tangs.
The SRE330 is a large board at 389mm wide and 153mm deep. It is secured to the front panel by
the pot brackets and to the lower panel of the case with three M3 screws and spacers. The board has
four copper layers, the top and bottom copper layers carry signals, the top middle layer carries
power supplies and some signals, and the bottom middle layer is solely designated to 0V. A four
layer design, although expensive to produce, gives better performance than standard two layer
board designs. It is, however, imperative that when soldering, and especially desoldering, that the
through hole plating used to line all the solder pad holes is not damaged. If you do need to desolder
a part then either use a proper vacuum desoldering tool or cut the component body out first and then
desolder one component leg at a time.
The 16-way ribbon cable used to transfer the balanced audio signals between the SRE330 main board and the
SREIO socket board
The SREIO board is an input and output socket board designed for this project. It allows the simple
interconnection between the SRE330 main board and the audio sockets via a single 16-way 0.05”
IDC ribbon cable. The SREIO board also features a electromechanical relay to cut the connection
between the main board's output circuitry and the output sockets at power on and power off. This
reduces the likelihood of damaging thumps on the audio output when power is turned on and off.
The anti-thump circuitry is powered from the main power supply module via a three way 0.156”
Molex or MTA connector. Pin 1 is +15V and pin 3 is 0V. Pin 2 can be used to 'ground' the sockets'

earth lug to the power supply's 0V if desired although this is not normally needed in most SRE330
builds.
The relay and power muting circuitry on the SREIO board
The SREIO board is 34mm high and 185mm wide. It is a two layer design. The sockets used are the
industry standard 1/4” TRS (Tip Ring Sleeve) socket, the Switchcraft 114BCPX. Various clones of
this socket are available but arguably Switchcraft still make the best version.
The RPSU is a power supply module designed specifically for the SRE330, although it is likely that
other future Oakley Sound rack projects will use this module too. It generates +/-15.3V at up to
around 1A given sufficient heatsinks and a suitable mains transformer. The SRE330 only needs less
than 250mA but even so adequate heatsinking must be used for the two power devices. Since best
performance for any electronic audio device comes from using a metal case it makes sense to use
the case itself as a heatsink. Details on how to do this are given later in this document.
The issue 2 RPSU wired for use with a Yamaha PA-20 external power supply

Although the RPSU can be used with an internal mains transformer, I recommend that builders use
an external low voltage output line lump or wallwart type mains adapter. This keeps all the high
voltages away from your project and ensures your safety. An example of such is the Yamaha PA-20
which is available from all larger music stores. Later on in this document I will give details on how
to build your RPSU module to suit your chosen method of supplying power.
The RPSU is 150mm by 51mm in size. It is a two layer board and made from double thickness
copper to reduce unwanted voltage drops along the copper traces on the board. Unlike the issue 1
RPSU board there are no trimmers to set the voltages to exactly +/-15.00V. The SRE330 main
board's circuitry is not bothered whether the voltage is exactly 15.00V but only that it doesn't
change once the SRE330 main board has been calibrated. The RPSU board thus needs no trimmers
and produces a little over +/-15.3V when built as recommended.
The RPSU requires an AC supply of a minimum of 15V to work correctly. When used with a centre
tapped AC supply, such as that from the PA20 or internal mains transformer, the current taken is
approximately 0.4A (RMS). With a single phase supply, such as that from a standard AC output
wallwart supply, the current required will be in the order of 0.7A (RMS).
The three way power socket that will fit the Yamaha PA20 power supply

Parts Lists
For general information regarding where to get parts and suggested part numbers please see our
useful Parts uide at the project webpage or directly from http://www.oakleysound.com/parts.pdf.
The components are grouped into values, the order of the component names is of no particular
consequence. Component values given in this list supersede those shown on the schematic.
A quick note on European part descriptions. R is shorthand for ohm. K is shorthand for kilo-ohm.
So 22R is 22 ohm, 1K5 is 1,500 ohms or 1.5 kilohms. For capacitors: 1uF = one microfarad =
1000nF = one thousand nanofarad.
To prevent loss of the small ‘.’ as the decimal point, a convention of inserting the unit in its place is
used. eg. 4R7 is a 4.7 ohm, 4K7 is a 4,700 ohm resistor, 6n8 is a 6.8 nF capacitor.
SRE330 Main Board issue 2 Parts List
Resistors
All resistors should be 1% 0.25W metal film resistors except for R57 which can be 5% metal film
or carbon.
22R R40
75R R37, R36, R203, R257, R181, R227
120R R38, R39, R52
220R R34, R33, R29, R35
390R R151, R91, R87, R139, R136, R148, R78, R84, R53
510R R54
1K R56
2K2 R44
3K3 R200, R254, R224, R260, R178
3K9 R4, R24
4K7 R119, R162, R238, R189, R60, R211
6K8 R186, R121, R50, R108, R164, R191, R165, R62, R214, R51, R240, R107
10K R26, R14, R198, R13, R22, R184, R11, R12, R41, R95, R177, R43, R182,
R82, R167, R223, R176, R68, R190, R205, R222, R253, R199, R27, R196,
R23, R252, R83, R67, R71
11K R174, R220, R251, R185, R166, R197
15K R127, R175, R21, R66, R134, R231, R152, R90, R163, R76, R131, R70,
R98, R3, R103, R137, R135, R77, R244, R160, R45
22K R2, R6, R86, R20, R19, R16, R75, R132, R89, R17, R28, R155, R5, R94,
R74, R1, R73, R81, R97
30K R173, R172
33K R217, R256, R79, R246, R92, R242, R193, R250, R169, R180, R259, R15,
R55, R25, R202, R234, R226
36K R249, R261
39K R221, R239
47K R208, R209, R63, R122, R30, R216, R31, R245, R8, R18, R42, R192, R64,
R109, R49, R124, R7, R154, R48, R105, R158, R168, R207, R157, R150, R9

51K R235, R229, R228, R243, R241, R233, R212, R215
56K R123, R102, R141, R113, R32, R10
100K R218, R46, R183, R128, R247, R69, R100, R99, R187, R111, R213, R236,
R248, R204, R219, R230, R130, R147, R145, R195, R194, R159, R142,
R171, R170, R206, R116, R232, R258
120K R161, R106, R125, R237, R188, R65, R47, R210
150K R225, R255, R201, R179
330K R110, R129, R104, R146
390K R144, R112, R149, R153, R156, R114
470K R138, R115, R61, R133, R58, R59, R126, R140, R143, R120, R117, R118,
R101
820K R88, R93, R96
1M R72, R80, R85
3M3 R57
Capacitors
10pF C0 2.5mm ceramic C14, C28
22pF C0 2.5mm ceramic C8, C21
33pF C0 2.5mm ceramic C169, C184, C195, C190
56pF C0 2.5mm ceramic C181, C122, C92, C155
100pF C0 2.5mm ceramic C25, C166, C52, C26, C138, C80, C189, C57, C2, C103, C3,
C75
220pF C0 2.5mm ceramic C112, C20, C29, C102, C27, C142, C165, C113, C101, C137,
C121, C15, C188, C136
1nF C0 2.5mm ceramic C93, C182, C156, C123
1n5 100V polyester C158, C97, C117, C185, C91, C128
2n2 100V polyester C139, C191, C111, C100, C167, C106
10nF 63V polyester C41
22nF 63V polyester C140, C192, C168, C84, C107
47nF 63V polyester C72, C82, C83
100nF 63V polyester C40, C60, C62
220nF 63V polyester C51, C58, C59
470nF 63V polyester C86, C172, C151, C87, C61, C150, C171, C116, C115, C70,
C50, C43
1uF 63V polyester C9, C5, C1, C149, C37, C131, C108, C39, C146, C143, C148,
C125, C32, C147
2u2 50V polyester C4, C10, C170, C180
100nF axial multilayer ceramic C68, C67, C76, C66, C187, C35, C63, C73, C53, C46, C54,
C79, C164, C56, C129, C193, C96, C95, C160, C176, C99,
C98, C183, C162, C130, C94, C77, C174, C78, C159, C44,
C124, C194, C45, C47, C186, C16, C18, C19, C17, C114,
C64, C22, C81, C85, C48, C13, C105, C24, C49, C12, C23,
C104, C141, C161, C36, C65, C55, C74, C175, C173, C163,
C157
2u2, 63V electrolytic C178, C177, C152, C88, C153, C89, C118, C31, C119

4u7, 50V electrolytic C109, C110, C30, C33, C144, C126, C145, C133, C132, C6,
C11, C127
10uF, 35V electrolytic C7, C135, C69, C71, C42, C134, C34
47uF, 35V electrolytic C154, C120, C90, C179, C38
Integrated Circuits
TL072CP dual FET op-amp U47, U20, U45, U6, U13, U10, U25, U37, U42, U50, U21,
U27, U14, U15, U26, U18, U9, U8, U31, U54, U22
OPA2134PA dual op-amp U48, U46, U3, U2, U1
NE5532AP dual audio op-amp U30, U34, U5
V571D or NE570 compander U35, U40, U4
V3102D BBD driver U44, U39, U52, U33
V3207D BBD U51, U32, U38, U43
LM13700N dual OTA U12, U24, U7, U19
LM2901N quad comparator U11
D 403DJ dual analogue switch U55, U23, U16, U17, U53, U28
78L09 +9V regulator 100mA U29, U36, U41, U49
The Belling BL3207 or Panasonic MN3207 may be used instead of the Coolaudio V3207.
IC sockets are recommended for the V571D, V3102D, V3207D and LM13700N even if you don't
socket any of the other ICs.
Discrete Semiconductors
BC550 NPN transistor Q9, Q8, Q14, Q10, Q15, Q12, Q13, Q11
BC560 PNP transistor Q5, Q7, Q4, Q6, Q3, Q2, Q1
1N4148 signal diode D16, D22, D24, D10, D25, D23, D20, D26, D18, D19, D15,
D11, D6, D12, D7, D21, D9, D17
1N5819 diode D13, D14
BZX55C2V7 2V7 zener diode D1, D2, D3, D4
BAT42 Schottky diode D5, D8
3 mm red LED PK
3 mm yellow LED HI, MD
3 mm green LED LO, ON
Trimmers
All trimmers are 6mm types. For example, Bourns 3386F.
5K NULL1, NULL2, NULL3, NULL4
20K T_LVL, S_LVL
50K FAST, SLOW, OFF1, OFF2, OFF3, OFF4
100K S_FRQ, T_FRQ, CLK1, CLK2, CLK3, CLK4

Pots
All pots 16mm Alpha or Alps types.
50K linear TRI_RATE, SIN_RATE
50K logarithmic TRI_DEPTH, SIN_DEPTH
50K dual gang linear OUT_LEVEL, BALANCE, INPUT
Seven Alpha pot brackets. Seven knobs to suit.
Switches
SPDT ON-OFF-ON toggle switch MODE
SPDT ON-ON toggle switch TYPE, MONO
Interconnects
5 way Molex 0.156” header POWER
16 way 2 x 8 IDC box header* IN1
* If choosing to hand wire the sockets rather than using the optional SRIO board then you do not
need to fit this IDC header. Instead you will wire the sockets to the appropriate solder pads where
the header would normally be fitted.
Miscellaneous
Mounting hardware for the three mounting holes. For the Bryant Broadcast or Holt Broadcast 1U
aluminium cases the following hardware can be used.
M3 hex threaded 6mm spacers (3 off)
M3 CSK 16mm screws (3 off)
M3 hex nut (3 off)
M3 shakeproof washers (6 off)
M3 flat washers (3 off)
The shakeproof washers go between the bottom panel and the hex spacer, and between the flat
washer and the top nut. The flat washer goes up against the top surface of the PCB.

SREIO Board issue 1 & 2 Parts List
Important Note
The 1/4” sockets are fitted on the underside of the board. They should be fitted last and soldered
from the top side of the board.
Resistors
All resistors 1% 0.25W metal film resistors.
220R R2, R3
2K2 R5
3K9 R6
10K R4
39K R7
Issue 1: R1 can either be a 0R resistor or simply a wire link made from a resistor lead clipping.
Issue 2: There is no R1 but you need to fit three wire links to positions LK1, LK2 & LK4. The
position LK3 must be left empty.
Capacitors
100nF axial multilayer ceramic C1
2u2, 63V electrolytic C2
22uF, 35V electrolytic C3
Discrete Semiconductors
1N4004 diode D1
BC550C NPN transistor Q1, Q2
Interconnects
Switchcraft 114BPCX 1/4” socket IN_A, IN_B, OUT_LEFT, OUT_RI HT
16 way 2 x 8 IDC box header IN1
3 way Molex 0.156” header PSU
16 way 2 x 8 IDC socket 2 off for SREIO to SRE330 interconnect.
16 way 0.05” IDC ribbon cable Cut to length to fit between SREIO and SRE330 boards
Miscellaneous
Axial leaded ferrite bead L1, L2, L3, L4, L5, L6, L7, L8
12V or 15V DPDT relay* RELAY
* This relay is an industry standard type such as TE Connectivity part number 8-1393792-8.

RPSU issue 2 Parts List
Resistors
All resistors 1% 0.25W metal film resistors.
270R R3, R4
3K R1, R2
R5 and LK are not fitted.
Capacitors
100nF axial multilayer ceramic C1, C4, C5, C6
10uF, 35V or 50V electrolytic C2, C3, C7, C8
1000uF, 35V or 50V electrolytic C9, C10, C11, C12
C9 to C12 are radial types and have standard wire ended leads. Lead spacing is 7.5mm. Do not get
'push-fit' types as their pins would be too large to fit into the PCB. Ensure they have a ripple rating
of at least 750mA and that their height doesn't exceed your chosen case once they are fitted into the
board.
C13 is not fitted.
Discrete Semiconductors
1N4002 or 1N4004 D1, D2, D3, D4, D5, D6
1N5401 D7, D8, D9, D10
D11 and AC LED are not fitted.
D9 and D10 do not need to be fitted if you are using a single phase wall wart or line lump, although
they won't cause any problems if they are. For full wave rectification, that is, if you are using a split
output line lump or an internal transformer with twin secondaries, then D9 and D10 are required.
Integrated Circuits
LM317T 1A variable regulator U1
LM337T 1A variable regulator U2
Ensure that both devices are TO-220 types and not any surface mounting or TO-3 packages. I much
prefer the devices that have a thicker mounting (dual gauge) tab.
Do not fit solder U1 and U2 into the board just yet. They are only to be soldered once the board is
fitted to the base panel of your case. See the section on mounting the RPSU board later in this
document.

Miscellaneous
0.156” Molex KK 5-way header PS1
0.156” Molex KK 3-way header PS2 – if using the SRE I/O board.
0.156” Molex KK 5-way housing 2 off for power cable to main board
0.156” Molex KK 3-way housing 2 off for power cable to SREIO board
0.156” Molex crimps 16 off for power cables
Antisurge 20mm fuse* F1, F2
20mm fuseholder PC mount* F1, F2
4-way screw terminal 5mm POWER, SWITCH
* If you are using a single phase wall wart supply, then fuseholder F1 and its associating fuse does
not need to be fitted. F2's fuse rating should then be a 1A antisurge or time lag type. If you are using
an internal transformer or split output line lump supply then both fuseholders are fitted and the fuses
are both 800mA antisurge or time lag types.
2 off TO-220 insulator For mounting of U1 and U2 to panel
2 off TO-220 insulating bush For mounting of U1 and U2 to panel
Heatsink paste For mounting of U1 and U2 to panel if using mica plates
Suitable power switch
Suitable power socket
A suitable length of 24/0.2 insulated cable for all power connections.
Mounting hardware for the four mounting holes. For the Bryant Broadcast or Holt Broadcast 1U
aluminium “Joggle” cases the following hardware can be used.
M3 hex threaded 6mm spacers (4 off)
M3 CSK 16mm screws (4 off)
M3 hex nut (4 off)
M3 shakeproof washers (8 off)
M3 flat washers (4 off)
The shakeproof washers go between the bottom panel and the hex spacer, and between the flat
washer and the top nut. The flat washer goes up against the top surface of the PCB.
For mounting the power devices, U1 and U2, you will also need the following parts.
M3 10mm screws (2 off)
M3 hex nut (2 off)
M3 shakeproof washers (2 off)
A shim plate may also be required if the lower panel of your case is particularly thin and you wish
to use countersunk screws.
Please see later in this document for more details on how to mount the power devices.

Mounting the Pots and Switches
A close up of the single gang Triangle Modulation Depth pot with its solder bracket
If you are using the recommended Alpha pots then they can help support the PCB with the addition
of the specially manufactured pot brackets. However, given the large size of the SRE330 PCB it is
also necessary to utilise the three additional mounting holes at the rear edge of the board. These
holes are sized to take an M3 screw and can be used with suitable hex spacers to attach the PCB
directly to the lower panel of your case.
Alpha pots are labelled with an A, B or C prefix. For example: B50K or C10K. Alpha and ALPS
use the key; A = logarithmic, B = linear and C = reverse logarithmic. So a B50K is a 50 kilohm
linear pot. You can use 47K in place of a 50K pot.
When constructing the board, temporarily fit the pot brackets to their pots by the nuts and washers
supplied with the pots. Now fit the pot and pot bracket assembly into the appropriate holes in the
PCB. Solder only the three, or six for the dual gang pots, pins that connect to the pot. Do not solder
the pot bracket at this stage. When you have soldered all the pot pins you can fit the board
temporarily to the front panel. Ensure that the PCB is at right angles to the panel, the three dual
gang pots should hold it so, and then solder each of the brackets.
If you have used an enclosure, such as the Bryant and Holt “Joggle” 19” cases, that does not allow
you to access all the pot bracket pins once the board is fitted up to it, then only solder the pins you
can easily reach from the board's underside. Once these have been soldered remove the board from
the front panel and solder the ones along the front edge.

Now remove the board from the front panel if you have not done so already. It's time to fit the
switches. The PCB mountable switches should fit quite tightly into their holes on the board. Ensure
that you have the three position switch, all the others just have two positions, up and down, fitted in
the location marked MODE. Make sure the switch body is flat against the board. Now solder all the
pins on each of the switches including the two securing pins to the front. You may need to trim the
switches' pins if they stick out too far from the underside of the board.
These switches are Multicomp's two position 1MS1T2B4M7RE and three position MS3T2B4M7RE They look
identical so make sure you click through their positions to verify which is which before you solder them in place

Mounting the LEDs
The choice of how you mount the LEDs is up to you. The suggested panel design uses the four
LEDs used in the signal meter going up at a forty five degree angle and has the power LED on the
central horizontal axis of the panel in line with the pot shafts. It does, however, make it easier if you
just fit all the LEDs in a straight line just above the board's top surface. Whichever way you chose
you must remember that LEDs are diodes and should be fitted the correct way around or they will
not light up. Check the data sheet for the devices you have bought to see which lead is the cathode.
The cathode should be soldered to the square pad on the PCB.
The key about mounting an LED to any front panel is to make sure it won't push into the unit when
force is applied to it from the front. There are several ways to do this, LED clips, glue (no don't do
this), or just relying on the stiffness of the leads. For the second SRE330 prototype I mounted the
LEDs onto a bit of stripboard. Stripboard, or Veroboard, is a laminated paper based printed circuit
board with copper strips on one side and perforated by a 0.1” grid of holes into which you put the
component leads through. You can cut it to size by using a knife to score along the holes on both
sides and then snapping the board along the score. The rough edges can be filed down to make a
nice tidy little board useful for prototyping. Or fitting LEDs.
The power on LED
The power on LED is fitted to small piece of stripboard secured very firmly by a two way right
angled board to board connector. The right angled connector is soldered first to the stripboard and
then to the SRE330 main board. The LED is poked through, making sure the leads are correctly
polarised, and then soldered in place only when the front panel is fitted and the LED is at the correct
depth into the panel's hole. The photo shows how the right angle board to board connector fits into
the main board and the stripboard with the black plastic part up against the main board. The copper
strips are vertical to allow the LED to connect with the board to board connector.
The four LED signal meter was done similarly and, although fiddly to make, proves to be very
sturdy and effective. This time the right angled board to board connector was soldered up against

the stripboard to allow for the lowest LED to fit into the board and have the meter to sit centrally on
the panel. Again solder the LEDs only once the panel has been fitted and the LEDs are pushed into
their panel holes. Care must be taken to ensure that the board to board connector does not foul the
resistor, R39, just behind it.
Using stripboard to make the LED connections
The stripboard's copper strips go vertically Care must be taken to ensure that the lower edge of the stripboard is
properly filed down so no copper tracks can touch the SRE330 main board surface

The Power Supply
Safety Warning
The RPSU supply module has been designed to work with isolated low voltage AC inputs only.
Connection to any alternating current supply is done at your own risk. Low voltage is classified as
being less than 25V with respect to the ground potential. Voltages above this level can, and often
are, lethal to living creatures.
Oakley Sound Systems will not advise on building or modifying this board to allow for direct
connection to the mains, or other high voltage sources, further to what is provided in this document.
Please do not ask me for additional information pertaining to direct mains connections or using
internally mounted transformers as I will not give it.
For safety and legal reasons I cannot recommend powering this board from any other supply
than low voltage AC output mains adapters.
Oakley Sound Systems are not liable for any damages caused by the misuse of this product. It is
your responsibility to use this product safely. If you have any doubt about installing a safe power
supply, then please do not attempt to do so.
Single Phase AC output wall wart supply
These are the most common AC output power adapters but increasingly they are getting harder to
find particularly at higher output currents. Most wallwart supplies produce DC (direct current)
voltages which means they are not suitable for use with the RPSU. Ensure that when you buy a
wallwart type adapter it does actually output an AC (alternating current) voltage. Some DC output
adapters are confusingly called AC adapters simply because they plug into your AC mains supply.
The SRE330 requires around 650mA or 0.65A from a single phase 15V AC supply. That means
you need to find one with at least this capability. A 15V 1A device would be perfect, but a 15V
0.5A would not. An 18V AC supply would also work but again at least 0.65A is required.
Quite often you will find power supplies not rated in amps but instead given an overall maximum
power rating, or wattage. The maximum amount of current that can be taken is worked out by
dividing the power rating in watts by the voltage output. The problem is that you don't often know
the exact voltage the device is producing since it does vary a lot from what it says on the device.
For example, a 15V supply may well be producing 18V even when at full load. Even so a 15W 15V
AC device should work fine. A 10W 15V one however, although in theory should work, in practice
it may not.
Ultimately, the proof of whether it works is twofold. It must firstly produce the correct voltage so
the RPSU can actually create a stable +/-15V when driving the SRE330. And secondly, the adapter
must not get overly warm in use. If you've bought what should be a good adapter but it gets hot or
hums loudly when powering the SRE330 then it is not suitable. Another solution must then be
sought.

The recommended option: The Yamaha PA-20
This is a linelump supply and features a fixed 17.5-0-17.5 volt AC output at 0.94A maximum. This
means it gives us two AC outputs with a centre tap or mid point reference voltage. So unlike a more
common single phase AC adapter output with two leads, this one has three. This means you need to
use the Oakley RPSU in full wave rectification mode. That is, both fuses must be fitted and all four
of the big rectifier diodes.
The PA-20 is made for Yamaha products and they are available from Yamaha spares departments
as well as many music shops, eg. Thomann. These are CE approved and connect to the mains via
your local mains connector. They will be different types depending on the country you need them
for. It comes with a handy three way plug at the low voltage end that you can use with an
appropriate socket. If you wish you can ditch their connector and use your own.
In the UK the line lump’s part number is V9812300 and the is around £35. If you are buying these
direct from Yamaha and, for some reason, are asked why, the probably best thing to say it is for a
M 12/4 mixer.
Once rectified, smoothed and regulated the Yamaha PA-20 can deliver up to 0.52A continuously
into both 15V rails.
You should fit both fuses and both should be anti-surge types and rated at 800mA, often written as
800mAT on the fuse body.
The European version of the PA-20 Other country's units are similar but will have the local mains connector
fitted

rounding Your Case
If you are powering your SRE330 project with an internal mains transformer then you will need to
earth your case directly. This is covered in more detail in the section “Using a Mains Transformer”
later in this document. If, however, you are powering your case from a wallwart or linelump power
supply, the 0V reference point in the SRE330 will be floating with respect to the mains earth. The
0V will then only be 'tied' to mains earth or ground when you connect your audio cables to your
studio system.
It is a good idea to connect the metal casing of your project to 0V. This helps keep unwanted signal
interference to a minimum. There are variety of ways that the case can be connected to 0V and I
have found the best way is to use the sleeve connections of the input and output sockets. Linking R1
on the issue 1 SREIO board, or LK4 on the issue 2 SREIO board, connects all the sleeve
connections to the 0V that comes from the SRE330 main board. The sleeve connection on each
socket is directly bonded to the threaded bush of the 114BPCX socket. Since these are securely
fixed to the case this should mean that the case is also now connected to 0V. To ensure a good
connection I use toothed shakeproof washers between the sockets and the inside surface of the rear
panel.
The rear panel's mounting screws should then allow the rest of the case to also be connected to 0V.
However, this does depend on the type of case, whether it is painted, how it is constructed and so
on. It is worth therefore measuring the resistance between parts of the case. If you have no
connection, ie. you have a resistance over 10K, between the lower panel and the rear panel it will be
worth fitting a wire link in the LK position on the RPSU board. This should tie the RPSU's 0V to
the lower panel via the top right hand mounting screw on the RPSU and all the pot brackets on the
SRE330 main board via pin 5 of the PS1 connector. How effective it is will depend on how well
connected the top right mounting screw is to the bottom panel and how well the pots make contact
with the front panel. You may need to scrape any paint from the relevant areas to allow the
mounting screw or pot nuts to make good contact.
One has to a little careful when using the word 'ground'. I sometimes talk about local ground and 0V
as being the same thing. This is technically incorrect but it is used a lot. I worked at Marconi in the
80s and Soundcraft in the early 90s, and ground and 0V were used interchangeably even by
seasoned engineers. We'd talk about chassis ground, dirty ground, signal ground and clean ground.
They'd all be connected to 0V somewhere in the system but the term ground was in common usage.
round, when used in this way, is then a local common reference connection tied to the 0V of the
unit's power supply. It is not the same as mains earth. Indeed, it may not even be tied to mains earth
in the unit in question.
Strictly speaking, electrical ground is mains earth and historically it was solely referred to as that,
but usage, incorrect or not, has meant a shift in the meaning. Ideally, we should call our common
reference connection within our unit as 0V and not use the term ground.

Linelumps and Wallwarts: Wiring Diagrams
The input power wiring will depend on the type of wallwart or linelump you will be using.
Standard AC output wallwart
Single phase, two wire, wallwarts or linelumps need to use half wave rectification so the Oakley
PSU can generate both positive and negative supplies simultaneously. They only need the terminal's
AC1 and 0V1 wired to the power socket. AC2 and 0V2 are left unused.
D9, D10 and F1 are not needed to be fitted to the RPSU although it will do no harm if they are
fitted.
Connecting to a wallwart with single phase AC output
The front panel switch is a single pole single throw (SPST) switch which simply connects S1R and
S1S together when switched on.
Only F2 needs to be fitted and it should be rated at 1AT, ie. a one amp anti-surge or time lag fuse.