• on Audio/Video/Photography
  • Published in issue 1/2004 on page 0
About the article

RIAA Preamp with FETs

SRPP equaliser for moving magnet (MM) cartridges

RIAA Preamp with FETs
This preamplifier and equaliser is based around field effect transistors and is an updated version of the SRPP valve preamp we published in March and April 1987. Various changes are necessary due to the fact that the FET SRPP configuration offers a considerably higher gain and output impedance than its valve sibling. The Shunt Regulated Push-Pull (SRPP) topologyis frequently used in high-frequency applications, but is seldom seen in audio circuits. This is simply because SRPP works well with valve technology, but is not suitable for use with bipolar transistors or with operational amplifiers because the circuit does not employ negative feedback.As a result distortion is considerably increased. Using FETs, however, we can employ the SRPP topology as effectively as with valves.
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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.

R1,R3,R11,R13,R15,R23 = 47kOhm
R2,R4,R14,R16 = 100Ohm
R5,R17 = 12kOhm1
R6,R18 = 1kOhm33
R7,R19 = 68kOhm
R8,R20 = 470Ohm
R9,R21,R27 = 10kOhm
R10,R22 = 220kOhm
R12,R24 = 15kOhm
R25 = 274Ohm
R26 = 4kOhm87
R28 = 22Ohm
P1,P3 = 250Ohm preset
P2,P4 = 1kOhm preset
C1,C9 = 27pF
C2,C3,C6,C10,C11,C14 = 1µF MKT, lead pitch 7.5mm
C4,C12 = 220nF
C5,C13 = 82nF
C7,C15,C17,C20 = 100nF
C8,C16 = 100µF 40V radial
C18,C19 = 10µF 63V radial
C21 = 1000µF 63V radial
C22-C25 = 47nF ceramic, lead pitch 5mm
C26,C27 = 100nF 250 VAC (Class X2), lead pitch 15mm
L1 = suppressor choke 2 x 27mH (Epcos # B82721-K2401-N21)
B1 = B80C1500 bridge rectifier, rectangular case (80V piv, 1.5 A)
D1 = LED, low current
T1,T2,T6 = 2SK389-BL (Toshiba), (www.reichelt.de)
T3,T4,T7,T8 = BF245B (see text)
T5,T9 = BC550C
IC1 = LM317 (TO220 case)
K1-K4 = cinch socket, PCB mount (Monacor/Monarch # T-709G)
K5 = 2-way PCB terminal block, lead pitch 7.5m
TR1 = mains transformer, 2 x 12V / 1.8VA (e.g. Gerth # 304.24-2)
PCB, available from The PCBShop.
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