• on Audio/Video/Photography
  • Published in issue 7/2011 on page 36
About the article

Measurement Filter for Class D

Measurement Filter for Class D
The aim of the present design is to create a filter that has less distortion at 20 kHz and which can cope with higher voltages, so that it also becomes suitable for more powerful Class D power amplifiers. The theoretical values have remained the same as those in the original circuit, and the circuit itself has hardly changed. The biggest improvement is in the way the inductors are made.
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Gerber file

CAM/CAD data for the PCB referred to in this article is available as a Gerber file. Elektor Members can exclusively download these files for free as part of their Membership. Gerber files allow a PCB to be produced on an appropriate device available locally, or through an online PCB manufacturing service. Elektor recommends its business partner Eurocircuits as the company of choice for its own prototypes and volume production in selected cases.

Resistors
R1,R3 = 1kOhm, 5%, 7 W (Tyco Electronics ER581K0JT)
R2 = 59Ohm, 1%, 0.6W*
P1 = 100Ohm, 10%, 0.5W, 24 turn trimpot (Vishay Spectrol M64W101KB40)

Capacitors
C1,C14,C18 = not used
C2,C5,C11,C13 = 1nF, 1%, 500V, silver mica (Cornell Dubilier CD19FD102FO3F)
C3,C8,C12 = 120pF, 1%, 500V, silver mica (Cornell Dubilier CD15FD121FO3F)
C4 = 6.8pF, 1%, 500V, silver mica (Cornell Dubilier CD15CD(6.8)DO3F)
C6,C15 = 270pF, 1%, 500V, silver mica (Cornell Dubilier CD15FD271FO3F)
C7,C9,C17 = 680pF, 1%, 500V, silver mica (Cornell Dubilier CD19FD681FO3F)
C10 = 180pF, 1%, 500V, silver mica (Cornell Dubilier CD15FD181FO3F)
C16 = 220pF, 1%, 500V, silver mica (Cornell Dubilier CD15FD221FO3F)
C19 = 5-57pF PTFE trimmer, 250V, (Vishay BCcomponents BFC2 809 08003)

Inductors (incl. materials)
L1 = 1.15mH, 85 turns 0.8 mm enamelled copper wire*
L2 = 689µH, 65.5 turns 0.8 mm enamelled copper wire*
L3 = 557µH, 59 turns 0.8 mm enamelled copper wire*
L4 = 802µH, 71 turns 0.8 mm enamelled copper wire*
L1-L4 core = RM14 core set, 160nH (AL) N41 (Epcos B65887E160A41), e.g. RS Components #212-6772
RM14 12-pin coil former (Epcos B65888C1512T1), e.g. RS #212-6839
RM 14 clamp, stainless steel spring (Epcos B65888A2002X, 2 per coil), e.g. RS Components #647-9323
RM 14 isolation washer. base (Epcos B65888B2005X), e.g. RS Components #180-121
0.8 mm enamelled copper wire (Pro Power ECW0.80). Nearest US equivalent: AWG20

Miscellaneous
K1,K2 = BNC socket, PSB mount, angled, 75Ohm (Tyco Electronics 1-1478032-0)
Locking washer, BNC/TNC (Tyco Electronics 1-1634817-0)
Nut, BNC/TNC (Tyco Electronics 1-1634816-0)
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