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From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: Simple RIAA phono filter
Summary: Oh, yeah... that's only for dynamic output, isn't it.
Message-ID: <11375@milton.u.washington.edu>
Date: 19 Nov 90 08:47:28 GMT
References: <90311.114631MKUHN@auvm.auvm.edu> <11200@milton.u.washington.edu>
Organization: University of Washington, Seattle
Lines: 49

In article <11200@milton.u.washington.edu> whit@milton.u.washington.edu (John Whitmore) writes:
>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 

>	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).

	The rest of my original posting varied from 'inappropriate' to
'wrong' :-0 .
	As several folks have pointed out, the intrinsic properties
of a piezoelectric cartridge guarantee that there's 3 dB/octave
droop, and relatively good matching to the RIAA curve, without
much filtering.  In fact, it's only the 'flat' region in the RIAA
curve that requires treatment with a filter.  Something like this


Input ------^^^^^^^-----+---||---+------- Output
              R1        |   C1   |
                        +---^^^--+
                            R2   >
                                 >
                              R3 >
                                 |
                               GND

should do it.  
	R1 +R2 + R3 = Rcartridge (I think 1Mohm is a good guess)
        R2 * C1 = 1/(2*pi* 1.1kHz)
	( R1 + R2 + R3) =  4.4* ( R1 + R3)
	R3 = 47 kohm        

Note that R3 is the internal resistance of the AUX input socket; USE NO
RESISTOR, it's already part of the amplifier!
	If this is truly a mid-fi application, this set of equations
should give something listenable.  For hi-fi, give up!  Buy a good
cartridge (they start at $15 or so) and a cartridge preamp (those
used to be available as black boxes for circa $20).  Then put
the record player in a conspicuous spot in your home, and listen
covertly to CDs.


	John Whitmore