e011062.PDF

POWER SUPPLY

For the Valved RIAA Preampliﬁer and other applications

High Voltage Supply

330 V from 12 V

Design by T. Giesberts

Although this supply was primarily designed for use with the Valved RIAA

Preampliﬁer, we found that the inverter stage is useful in many other appli-

cations. With only a small modiﬁcation this circuit can be used to power

a 20 W PLCE (low energy) lamp from a 12 V car battery.

The Valved RIAA Preamplifier uses

two valves, just like the Valve Pre-

amplifier that was published in the

June 2000 edition of Elektor Elec-

tronics . Since the valves’ filaments

have again been connected in series,

the preamplifier requires two DC

supply voltages: 12.6 V for the fila-

ments and 330 V for the high voltage

supply.

In order to avoid the need to use

a custom transformer the circuit has

been designed to use a standard

15 V/3 A mains transformer. As we’ll

see later, the supply circuit consists

of two distinct sections: a conven-

tional 12.6 V filament supply and a

step-up converter which boosts the

12.6 V to 330 V. In other words, the

filament supply is also used to

power the inverter.

Since each section is built on a

separate PCB it becomes possible to

use them individually in other appli-

cations. This is especially useful in

case of the inverter, since it makes a

great camping light when used in

conjunction with a 12 V car battery

and a PLCE lamp. These lamps tend

to work very well off a 300 V DC

supply!

Elektor Electronics

1/2001

POWER SUPPLY

The 12.6 V supply

12V6

IC1

KA78T12

C10

As we can see in Figure 1 , this sec-

tion is a very basic circuit. The 3 A

ﬁxed-voltage regulator (TO-220 case)

is made to deliver a slightly higher

output (12.6 V) by adding an extra

diode (D1). The bridge rectiﬁer uses

6 A diodes and is followed by some

substantial smoothing capacitors

(C4, C5, C6). The bridge rectifier is

RF decoupled by C7-C10 and LED

15V

POWER

D 2

12V6

4x FR606

4x 47n

100n

10µ

63V

1N4148

3x 2200µ / 25V

000186 - 11

Figure 1. The 12.6 V supply incorporates the well-known diode ‘trick’, which causes the

output to increase by 0.6 V.

COMPONENTS LIST

12.6 V Supply

D2 functions as the power indicator.

Construction of the 12.6 V supply

shouldn’t cause any problems when

the PCB shown in Figure 2 is used.

The heatsink for IC1 (Fischer type

SK129 from Dau Components) is

placed directly onto the PCB, which

results in a compact module. It is

very important that an insulating

washer is used between IC1 and the

heatsink.

There are two PCB terminal blocks

(K1, K2) that provide the 12.6 V out-

put voltage. One of these supplies

the inverter and the other powers the

two in series connected filaments.

The third terminal block (K3) is for

the 15 V transformer, which should

be rated at least 50 VA.

switch mode supplies. We have used it before

in the ‘In-Car Audio Amplifier’, which we

published in 1994. The proper description of

this IC is a ‘regulating pulse width modula-

tor’, which sums up its function perfectly. A

special transformer is driven with an alter-

nating voltage by one or more switched tran-

sistors, with the driving voltage obviously

limited to a safe value. By varying the pulse

width of the signal, the amount of power is

controlled. The output at the secondary of the

transformer is rectified and fed back to the

PWM regulator in order to keep the output

stable. That completes the feedback loop of

the regulator.Since the SG3525A has been

described in depth before in Elektor, we will

limit ourselves to a brief overview of the

device. The regulator uses a reference voltage

of 5.1 V. Various internal circuits use this ref-

erence: error ampliﬁer, oscillator, PWM com-

parator and the current source for the soft

start. An extra delay circuit has been added

to give valve ampliﬁers enough time to warm

up before the HV supply is applied. Because

valve amplifiers generally have substantial

smoothing capacitors, the soft start period

has been increased and the value of 100 µF

for C5 is a fair bit higher than usual.

Resistors:

R1 = 1k Ω 2

R2 = 6k Ω 8

Capacitors:

C1 = 10µF 63V radial

C2,C3 = 100nF

C4,C5,C6 = 2200µF 25V radial

C7-C10 = 47nF ceramic

Semiconductors:

D1 = 1N4148

D2 = red high-efficiency LED

D3-D6 = FR606 (or similar 6A

diode)

IC1 = KA78T12 (3A)

Miscellaneous:

K1,K2,K3 = 2-way PCB terminal

block, lead pitch 5mm

For IC1: heatsink type SK129 63,5

STS, 3.5 K/W (Fischer) (Dau

Components)

Isolation material for IC1

PCB , order code 000186-2 (see

Readers Services page)

The 330 V inverter

This part of the supply (see Figure 3 )

is a push-pull-converter that uses an

old favourite of ours: the SG3525A.

This regulator is an industry stan-

dard part that is used in many

000186-2

C10

Figure 2. The heatsink just ﬁts on the PCB, which results in a nice compact 12.6 V module.

1/2001

Elektor Electronics

POWER SUPPLY

* see text

zie tekst

12V

R13

2 Ω 2

* voir texte

R11

R12

* siehe Text

C10

C11

100n

220µ

25V

100k

47mH

10n

TR1

C14

BAT85

330V

30mA

100n

100µ

25V

R14

2µ2

450V

OSC OUT

BUZ11

C12

INV IN

OUT A

IC1

R15

4n7

SYNC

SG3525A

SHUTDOWN

OUT B

47mH

ETD29

BY329-1000

R16

C13

C15

330V

30mA

COMP VREF

N I IN

DIS

CSS

2µ2

450V

2µ2

450V

BUZ11

470k

12V6

12V

R10

40µH

5V1

2A T

C16

C17

C18

10n

100n

100µ

16V

100µ

16V

1000µ

25V

1000µ

25V

1000µ

25V

10k

BC550C

000186 - 12

Figure 3. The main parts of the inverter are the integrated regulator (IC1), transformer and bridge rectiﬁer.

We’ve used a standard ETD29

type former with N27 core mater-

ial for the (home wound) trans-

former. The switching frequency

has been kept relatively low

(30 kHz) in order to save on

smoothing capacitors at the pri-

mary side. Furthermore, three of

them have been connected in par-

allel, which splits the current

between them. This design can

deliver a power of about 30 W.

The oscillator frequency can be

set with P2 within a wide range

(±7 kHz) to compensate for the

tolerance of C4 (1 nF MKT),

although the exact frequency isn’t

critical. To facilitate maximum

power transfer, the dead time has

been kept to a minimum by con-

necting the discharge output

directly to CT and by keeping the

value of C4 as small as possible.

The reference voltage is decou-

pled by C3 and fed to the non-

inverting input of the error ampli-

ﬁer by R5. The output of the error

ampliﬁer (COMP) is also the input

of the PWM comparator, which

determines the pulse width. C2

limits the bandwidth and pro-

vides stability. The 330 V output

voltage is fed to the inverting

input of the error amplifier via

potential divider P1/R1/R2/R3,

000186-1

C14

TR1

R11

R10

+330V

C13

C10

C17

C16

+330V

C15

R16

R15

C18

Figure 4. The PCB for the inverter is also tidy and compact.

Elektor Electronics

1/2001

POWER SUPPLY

where R4 determines the open loop

gain. P1 is used to adjust the value

of the output voltage. The range has

purposely been made fairly large

(theoretically 270 V to 370 V), which

gives the inverter plenty of scope for

use in other applications. Slightly

lower or higher output voltages can

be obtained by varying the number

of secondary turns proportionally

(e.g. 273 turns would give 300 V).

Keep in mind that if you use too

many turns you can still get the cor-

rect output voltage, but at a reduced

efficiency because the output is

peak-rectified. The surplus energy

will then be lost and dissipated in

the transformer.

The modest circuit around T1 pro-

vides a delay of about 45 seconds

between the application of the 12.6 V

supply and taking the shutdown

input low; this has to be below 0.6 V

to enable the inverter. C6 is charged

slowly by the potential divider of

R8/R9/R10, which causes the voltage at the

base of T1 to rise slowly and causes it to con-

duct. D5 causes C6 to discharge quickly when

the supply is switched off.

R11, R12, R13 and C7-C11 are used to

decouple the supply to the PWM regulator.

R14/C12 reduce the spikes that are caused by

the fast switching of transistors T2 and T3.

The alternating voltage at the secondary is

rectiﬁed by four fast soft-recovery diodes (D1-

D4). Smoothing is carried out by 450 V radial

electrolytics (C13, C14, C15). These are fol-

lowed by low pass ﬁlters that reduce the rip-

ple of the switching frequency even further.

We’ve assumed that the inverter will be used

with a stereo ampliﬁer so we’ve provided two

supply outputs, each with its own ﬁlter net-

work (L2/C14, L3/C15). For the inductors

we’ve used the 2200R-series from Newport

Components, but the board will also accept

the 8RB and 10RB series from Toko (available

from Cirkit).

L1 ﬁlters the input supply and F1 protects

the input supply from overload, which is

important when, for example, a car battery is

COMPONENTS LIST

330-V converter

Resistors:

R1,R2 = 120k Ω

R3 = 4k Ω 7

R4 = 470k

Ω

R5 = 1k Ω

R6 = 15k

Ω

R7 = 6k Ω 8

R8,R9 = 680k Ω

R10 = 330k

Ω

R11,R12,R13 = 2 Ω 2

R14 = 100 Ω

R15,R16 = 82k

Ω

P1 = 100k Ω preset H

P2 = 10k

Ω

preset H

Capacitors:

C1 = 1nF ceramic, lead pitch 5mm

C2,C9 = 10nF ceramic, lead pitch

5mm

C3,C7,C10 = 100nF ceramic, lead

pitch 5mm

C4 = 1nF MKT, lead pitch 5mm

C5,C6 = 100µF 16V radial

C8 = 100µF 25 V radial

C11 = 220µF 25V radial

C12 = 4nF7

C13,C14,C15 = 2µF2 450V radial,

lead pitch 5mm, diameter 10mm

C16,C17,C18 = 1000µF 25V

radial

Inductors:

L1 = suppressor coil 40µH 3A,

type SFT10-30 (TDK)

L2,L3 = 47mH, e.g., 2200R series

type 22R476 (Newport Compo-

nents)

Semiconductors:

D1-D4 = BY329-1000 (Philips)

D5 = BAT85

T1 = BC550C

T2,T3 = BUZ11

IC1 = SG3525A(N) (ST Micro-

electronics)

Miscellaneous:

K1 = 2-way PCB terminal block,

lead pitch 5mm

K2,K3 = 2-way PCB terminal

block, lead pitch 7.5mm

F1 = fuse 2AT (time lag) with PCB

mount holder

TR1 = ETD29 (Block) *

primary: 2 windings 11 x (3 x 0.5

mm parallel) ecw

secondary: 1 winding 300 x 0.3

mm ecw

PCB, order code 000186-1 (see

Readers Services page)

* see text

1/2001

Elektor Electronics

POWER SUPPLY

the field has the smallest possible

offset, which improves the perfor-

mance of the transformer.

If you make the secondary wind-

ing very carefully, it is possible to

use 0.4 mm wire, which reduces its

resistance (resulting in better effi-

ciency). But note that a sloppily

wound 0.3 mm winding can fill the

former almost completely. The trans-

former doesn’t have an air gap!

And ﬁnally…

Once both boards have been popu-

lated and tested, they can be

mounted with a 15 V transformer in

an enclosure. It might appear easiest

to mount the supply and Valved

RIAA Preampliﬁer in one enclosure,

but to obtain the best quality, and to

keep interference to a minimum, we

would recommend that the supply

and the preampliﬁer are mounted in

separate enclosures. If you do decide

to mount them in one enclosure, you

should at least have a metal screen

between the supply and preampliﬁer

sections, and the distance between

the boards should be made as large

as possible.

When we tested for interference

suppression in our lab we weren’t

disappointed. When the Valved

RIAA Preampliﬁer was used in con-

junction with this power supply we

measured the 60 kHz component at

–90 dB, which is below the noise

level of a typical phono signal. The

30 kHz component caused by the

ﬁeld of the inverter was at a level of

–110 dB. Both measurements are rel-

ative to an output signal of 200 mV.

(000186)

Figure 5. This shows an exploded view of the transformer parts.

used as source. For an even cleaner supply

you should thread both 330 V cables through

a large ferrite bead, which reduces common

mode interference.

from each other. The laying of sub-

sequent windings is made easier by

the tightly placed insulating foil. The

ends of the secondary windings can

be covered with the supplied insu-

lating sleeve, which reduces the pos-

sibility of shorts between the wind-

ings.

The two primary windings consist

of 11 turns of three strands of 0.5 mm

enamelled copper wires, which are

wound in parallel (next to each

other) as if it was one conductor. The

ﬁrst winding is made between pins

3 and 11. This should cover the for-

mer with one layer of 0.5 mm copper

wire. A layer of insulating foil is

placed tightly across this, after

which the next primary is wound

between pins 2 and 12. This too is

covered with a layer (or two) of insu-

lating foil. Both primary windings

have to be wound in the same direc-

tion to make sure that they are more

or less identical. This ensures that

Construction of the inverter

The PCB for the 330 V inverter is shown in

Figure 4 . Because of the high voltages pre-

sent, the layout is such that there is a mini-

mum separation of 3 mm between the HV

tracks and the earth plane. It is for this rea-

son that the wire link between the cathodes of

D1 and D2 is routed away from the low-volt-

age section (otherwise it could have been a

track between the diodes).

Populating the board is simply a matter of

carefully going through the parts list and sol-

dering the components in place. The only

part that could cause problems is transformer

TR1. But this isn’t as complicated as it might

appear, since we can use a transformer kit

supplied by Block (EB29 — see Figure 5 ).

This also contains a pre-cut insulating foil,

which is used to isolate the three windings

Elektor Electronics

1/2001

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