MKUHN@auvm.auvm.edu (Marty Kuhn) (11/08/90)
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 for it, but if
the signal loss would be too low, a simple single-transistor per channel
active filter would be OK. Incidently, this circuit does not have to be
super-accurate; this is not rec.audio.high-end here!
Sorry if this is sort of a basic question, but I don't know how the RIAA
equalization curve is defined. What is the RIAA curve anyway?
Anyway, thanks in advance for your help...
-- Marty
(Insert random quotes, jokes and clever ASCII pictures here)whit@milton.u.washington.edu (John Whitmore) (11/16/90)
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
francis@cs.ua.oz.au (Francis Vaughan) (11/16/90)
|> 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 |> Umm, No. Most crystal/ceramic cartriges are built with a natural RIAA curve in them. Of course it will probably be a pretty lousy match to the legit curve, and you may have other problems. Might be worth looking at impeadances. Maybe put 10k across the input and see what happens. If your boom box has an attempt at a graphic equaliser you can play with that. Francis Vaughan.
dunphy@col.hp.com (Dan Dunphy) (11/17/90)
Normally you only need an RIAA preamp/filter for a moving magnet cartrige. This curve is required becauuse the MM cartiige is a velocity device, in other words, the highest signal occurs at the zero crossing of the groove wiggle. The output is usually only a few millivolts. A ceramic cartrige is an amplitude sensitive device, and would require a different type of compensation if any.
whit@milton.u.washington.edu (John Whitmore) (11/19/90)
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