SMALL CIRCUITSCOLLECTION

24 Elektor Electronics 7-8/2002

In the ‘Audio Limiter (for DVD)’ circuit, the peak val-ues of the audio signals are used to reduce thedynamic range of the sound. A possible disad-vantage is that the entire audio spectrum is usedto determine the level, so that peak levels in thelow or high frequencies may lead to suppression of,for example, voices in the mid frequency range. Ifwe divide the spectrum into three ranges and foreach range a separate window comparatordefines the signal level then the signals in onerange will have a smaller influence on the othertwo ranges. It is the intention of this filter, there-fore, that the notorious ‘breathing’ of the limiter isreduced.The filters proposed here are standard 3rd ordertypes with crossover frequencies of 200 Hz and2.5 kHz. IC1a/IC2a form the low-pass filters for thelow range, IC1b/IC2b are the high-pass filters forthe high range, and IC1c/IC2c and IC1d/IC2d thehigh- and low-pass respectively for the mid range.The crossover frequencies are not simply the cor-ner frequencies of the filters, but these frequen-cies have been calculated such that the curvescross when the attenuation is 0.25 dB. This waythe detected amplitude remains approximatelyequal across the entire audio spectrum. The realcorner frequency with a 3rd order Butterworth is aratio of 1.6 further away than the –0.25 dB point.The curve shows what this looks like in practice.

005

-20

+3

-19

-18

-17

-16

-15

-14

-13

-12

-11

-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

+0

+1

+2

dBr

20 20k50 100 200 500 1k 2k 5k 10k024073 - 11Hz

6

7

1IC3.B

8

9

14IC3.C

4

5

2IC3.A

10

11

13IC3.D

6

7

1IC4.B

8

9

14IC4.C

4

5

2IC4.A

10

11

13IC4.D

6

7

1IC5.B

8

9

14IC5.C

4

5

2IC5.A

10

11

13IC5.D

K11

2

3

4

5

6

7 8

9

10

11

12

13

14

R1

46k4

R2

46k4

R4

73

k2

R5

28

k7

R6

49

9k

2

3

1IC1.A

6

5

7IC1.B

C1

15n

R3

45k3

C3

2n2

C2

39nC6

1n

C5

1n

C4

1n

R10

8k25

R11

8k06

R7

34

k0

R8

13

k3

R9

23

2k

13

12

14IC1.D

9

10

8IC1.C

C10

6n8

R12

7k87

C12

1n

C11

18nC9

27n

C8

27n

C7

27n

R13

46k4

R14

46k4

R16

73

k2

R17

28

k7

R18

49

9k

2

3

1IC2.A

6

5

7IC2.B

C13

15n

R15

45k3

C15

2n2

C14

39nC18

1n

C17

1n

C16

1n

R22

8k25

R23

8k06

R19

34

k0

R20

13

k3

R21

23

2k

13

12

14IC2.D

9

10

8IC2.C

C22

6n8

R24

7k87

C24

1n

C23

18nC21

27n

C20

27n

C19

27n

+8V –8V

IC1

4

11

C25

100n

C26

100n

IC2

4

11

C27

100n

C28

100n

+12V

–12V

+12V

–12V

C29

100n

C31

100nIC5

3

12

IC4

3

12

C30

100n

+8V

–8V

IC3

3

12

024073 - 11

IC1, IC2 = TL084

REF –

REF +

IC3 ... IC5 = LM339

Filter for Audio Limiter (for DVD)

At the crossover from the mid to high range, the high-pass fil-ter has a little more damping and it appears therefore that thecrossover point has shifted slightly. This is of no real conse-quence in practice.The connection to the audio limiter is made with a 14-pin DIL-connector to the socket for the comparator of this limiter. Thisfilter utilises the same DIL connector (K1) so that the connec-tion can be made with a short length of ribbon cable. Thepower supply for the comparators is also connected throughthis ribbon cable to supply power for the filter. The power for

the opamps, however, has to be taken from the power supplywith three separate wires. The increase in current consump-tion of the limiter is about 15 mA.In addition, a couple of small changes have to be made to thelimiter: R19 and R38 (both 3k3) have to be replaced with 47-Ω resistors. Otherwise the input impedance of the filters willaffect the level of the input voltage. The PCB shown here isunfortunately not available ready-made.

(024073-1)

SMALL CIRCUITSCOLLECTION

257-8/2002 Elektor Electronics

(C) ELEKTOR

024073-1

C1

C2

C3

C4

C5

C6

C7

C8

C9

C10

C11

C12

C13

C14

C15

C16

C17

C18

C19C20

C21

C22

C23

C24

C25

C26 C27

C28

C29

C30

C31

H1H2

H3 H4

IC1 IC2

IC3

IC4

IC5

K1

R1

R2

R3

R4

R5

R6

R7

R8

R9

R10R11

R12

R13

R14

R15

R16

R17

R18

R19

R20

R21

R22R23

R24

02

40

73

-1+

-0

(C) E

LEK

TOR

02

40

73

-1

COMPONENTS LIST

Resistors:R1,R2,R13,R14 = 46kΩ4R3,R15 = 45kΩ3R4,R16 = 73kΩ2R5,R17 = 28kΩ7R6,R18 = 499kΩR7,R19 = 34kΩ0R8,R20 = 13kΩ3

R9,R21 = 232kΩR10,R22 = 8kΩ25R11,R23 = 8kΩ06R12,R24 = 7kΩ87

Capacitors:C1,C13 = 15nFC2,C14 = 39nFC3,C15 = 2nF2C4...C6,C12,C16,C17,C18,C24 = 1nFC7,C8,C9,C19,C20,C21 = 27nF

C10,C22 = 6nF8C11,C23 = 18nFC25-C31 = 100nF

Semiconductors:IC1,IC2 = TL084IC3,IC4,IC5 = LM339

Miscellaneous:K1 = 14-way DIL connector (2 off)14-way flatcable

The joysticks used in games and modelling contain two poten-tiometers with a resistance of about 100 kΩ, which turnthrough 60 to 90 degrees. In fact only one third to one quar-ter of the total resistance is used in these potentiometers. Thediagram shown here should be used when making your ownjoystick with ordinary potentiometers that turn through 270degrees. The values for R1 and R2 are given as guidelines onlyand their optimal value should be found through trial anderror. It will be easier if you temporarily replace R1 and R2 with

a combination of a fixed resis-tor and a preset, since it canbe a time consuming job todetermine the correct values;this is because each of theresistors affects the other.

(024064-1)

006R1

39k

R2

39k

P1

25klin.

024064 - 11

Joystick Replacement