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From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: Simple RIAA phono filter
Summary: RIAA two-pole filter curve and its implementation
Message-ID: <11200@milton.u.washington.edu>
Date: 15 Nov 90 23:18:25 GMT
References: <90311.114631MKUHN@auvm.auvm.edu>
Organization: University of Washington, Seattle
Lines: 67

In article <90311.114631MKUHN@auvm.auvm.edu> MKUHN@auvm.auvm.edu (Marty Kuhn) writes:
>Ok, here's the problem:
>   I've hooked up a (ceramic-cartridge) turntable to my boom-box's AUX input.
>This works fine, execpt that records sound much 'tinnier' than they should.
>I would guess this is due to the RIAA equalization curve that records are
>cut with.  What I want is a simple filter to de-equalize the signal to match
>a standard AUX-type input.  This should preferably be a simple R-C passive
>filter, so that I don't have to build a separate power supply 


	The RIAA playback curve is a three-pole curve with corners
at 50 Hz, 500 Hz, and 2200 Hz.  It is flat to 50 Hz, -3dB/octave to
500 Hz, flat to 2200 Hz, and -3dB/octave from 2200 Hz on up.  In
normal filter-specification manner, the piecewise linear curve I just
described is the ASYMPTOTE of the actual curve (which is more easily
implemented).
	National Semiconductor application note AN-64 gives an example
of an RIAA preamp; I'll try to show it here.


Input    C1    |\ U1
------+--||----| \
      |        |  >------+-------- output
      >  +-----| /       |
   R1 >  |     |/        |
      >  |               |
      |  +--^^^^-+-^^^^--+
    GND  |   R2  |  R3   |
         |       |       |
       +-+-+    --- C2   |
       |   |    ---      |
       >   >     |       |
       >   >     +--^^^^-+
    R4 >   >R5   |   R6  |
       |   |     +--||---+
       |  ---
       |  --- C3    C4
       |   |
      GND GND

R1: match the transducer (circa 1Mohm)
R2: zero ohms (read the article; it's there for a reason)
R3: 1.2Mohm
R4: 100 kohm
R5: 180 ohm
R6: 100 kohm
C1: coupling (value not critical); 0.1 uF
C2: .003 uF
C3: 15 uF
C4: .001 uF


Their example uses LM381, an audio 'op amp'; if you use a general purpose
op amp, be aware that the (+) input should NOT remain connected only
to the capacitor C1; you'll need a resistor to ground as well (10 Mohm 
should suffice).

	You'll need two channels, for stereo; dual op amps suitable for
this include LM381, TL072, NE5532.  Similar (simpler) circuits exist
for dedicated audio units (if you can find LM382, an RIAA preamp with
it will only require 4 capacitors and one resistor, per channel).
You'll also need 10 to 30V of power supply (this should already
be available inside the unit you're using).

	Dontcha wish you hadn't asked?

		John Whitmore