Do you still have a turntable and a lot of old vinyl laying around that you
would like to listen to and your stereo does not have a phono input or a good
built-in phono pre-amp? Or you don't have a stereo at all and your computer is
your stereo? Would you like to hear the best from your vinyl? Most audiophiles
believe that vinyl sounds better than digital audio at 16 Bit
44.1 KHz, that is if you have a high quality stylus & cartridge,
turntable and a good phono preamp that accurately tracks the
RIAA equalization curve with an Op-Amp that
has a high enough slew rate that it does not muddy up the high frequencies.
Vinyl has the capability to produce frequencies approaching 60 KHz. This
extra bandwidth was used to make the CD4 Quadraphonic records where the
additional channel information was modulated on a 38 KHz subcarrier with
+/-19 KHz sidebands, so it is no wonder that vinyl has a more polished
sound for the top end than CD's. Many people notice a harsh, rough, brittle,
or crisp, almost too crisp sound that digital audio produces as a result of
the quantatizing error. As you approach the highest reproduceable frequency,
22.05 KHz, you have sampling inter-modulation distortion. The modulating
frequency is the difference between the signal being sampled and
22.05 KHz and the percent of modulation increases as the sampled signal
approaches 22.05 KHz. To alieveate this problem for digital audio you
would need to use a 24 Bit 96 KHz sampling. This would make
the digital audio files 3¼ times larger than what is used for todays
digital audio. This is the next step in digital audio and the Super CD refered
to 2496 or 9624 format and would take up over 2 gigs uncompressed for 74 min.
Until the recording industry decides on a standard format we are not likely to
see this anytime soon. Right now Sony and others are battling it out to have
their format adopted. So enjoy your vinyl!
Without having to drag out a receiver with a phono amp you can use this
pre-amp to run the signal into the back of your soundcard to play your
records. You could also apply digital noise reduction to remove surface noise
like pops and clicks caused by scratches or dirt. Given that many titles on
vinyl are out of print some would like to be able convert their vinyl to
digital to play on their PC or burn the WAV files to a CD or make MP3's out of
them.
The schematic below shows the pre-amp circuit. A Signetics NE/SE5532AN op-amp
is used for the preamp circuit while the high pass rumble filter uses a Texas
Instruments TLO72. You could also use the Signetics amp for the rumble filter
but it is the more expensive of the two. You will want to get the 5532AN
version as the one with the "A" suffix has guaranteed low noise
performance. The low noise version may be hard to find so use tne 5532N if it
is the only one available. It still offers good noise performance along with
high end clarity, much better than what is used in the todays receivers. In
one brand of receiver the 4558DX is used and does not have a high enough slew
rate for the use as a phono pre-amp. If your receiver uses a dual op-amp 8 pin
DIP package you could just replace it with the 5532 and improve the
performance that way although you will not get as low of noise as the feedback
components are of a higher resistance and reactance.
A rumble filter is included to help reduce the large signal swings of
inaudible noise caused by mechanical vibration or acoustical feedback from
high volume levels. This is important because it robs available headroom
causing clipping during A to D conversion. Reducing signal input to compensate
will reduce the signal to noise ratio of the digital signal. The pre-amp has a
maximum gain of 60db at 50 Hz or lower and rolls off @ 6db/octave
with a gain of 22db @ 20 KHz. With a gain of a 1000:1 @ 50 HZ or
lower you can see why you need to attenuate these unwanted signals.
You can raise the low end roll off frequency by changing the .27µƒ
capacitor to a lower value. The chart below lists the approximate roll off
frequency for each capacitance value.
The .01µƒ, .0033µƒ, 24KΩ, and
the 300KΩ form the feedback network that preforms
the RIAA equalization. It is recomended that
you definitely use polystyrene capacitors and if possible match them between
the channels. Some digital voltmeters have capacitance testing so if you have
one use it to select capacitors. It is also recomended that you use 1%
resistors or match them also. Matching of the 300Ω
resistors or using 1%ers will guarantee a good match in gain between the
channels. Matching the 33µƒ capacitors will guarantee that the low
end roll off frequency are close to equal although this is less critical.
Matching the 47KΩ resistors and the 150pƒ
capacitors will ensure that cartridge loading between the channels will be
balanced.
The .27µƒ capacitors in the rumble filter should be polystyrene also
but if you can't find them you can use any High-Q low ESR capacitors like
mylars although mylars have poor temperature coeffeceints (tending to drift in
value depending on temperature). If you want you can match them also along with
the 11KΩ & 62KΩ resistors.
For perfect match in signal response between left & right channels you can
match all components between the both channels. The only non-critical
compoments are the 100KΩ, 100Ω resistors & 100µƒ
capacitor as these are for all pass DC isolation. The only reason to do this is
if the program material you want to listen to has any spatial processing so exact
phase and amplitude response will equal between the left & right channels.
This will help preserve acoustical image of the signal.
If you can't find the larger 33µƒ & 100µƒ tantalum
capacitors then use electrolytics although try to get a tantalum for the
33µƒ as they have good temperature coeffeceints and good high
frequency shunting.
In the schematic "FB" designates Ferrite Beads. They are used to
filter out any radio frequency interference picked up by the wiring from
the cartridge to the pre-amp.
The power supply provides a regulated ±15V to the op-amps using
half-wave rectification for each regulator. Using a single winding with a
18VRMS output instead of a 36VRMS
center-tapped one for full-wave rectification allows easier transformer
selection. Each regulator probably draws no more than 25mA of current with
circuit load so a 75mA transformer is more than adequate, and you could
probably get by with a 50mA one. Finding a 120VAC to 18VAC adapter that plugs
into the wall would be ideal as it puts the transformer far enough away from
the pre-amp and cartridge so as to not cause 60 Hz interference. If you
do put the transformer into the same box as the pre-amp you need to wrap a
layer of heavy copper foil around it and solder it as to create a short
circuit to dampen the magnetic field that the transformer eminates.
You need to put the pre-amp in a metal box or line the inside of the box with
heavy copper foil. The box also needs a ground screw for the turntable ground.
For complete isolation place the power supply in one box and the pre-amp in
another box but place the 1µƒ capacitors in the preamp box and not
the power supply box. Connect the two boxes with 3 wires +15V,Ground,-15V.
If you do use the 5532 op-amp for the rumble filter you can allow for greater
signal voltage swing by increasing the supply voltage supply to ±18V by
replacing the voltage regulators with 18 volt versions 78L18 & 79L18. You
will also need to increase the transformer voltage to 22VRMS but DO NOT go above 25.2VRMS as the
rectified and filtered voltage will be greater than 35V and will damage the
voltage regulators.
Frequency @ -3db
Capacitance
~20 Hz
.27µƒ
~24 Hz
.22µƒ
~27 Hz
.18µƒ
~32 Hz
.15µƒ
~38 Hz
.12µƒ
If you wish to add a headphone amp for walkman style headphones with a 16Ω impeacance the following circuit should do it. You will need to use a 100mA Transformer and replace the 220µƒ capacitors with 470µƒ ones in the power supply. The output will also drive an 8Ω load @ 70-100mW for small speakers depending on what power supply voltage is used. If you use the ±18V supply for more output power you will want to replace the regulators with 78M18 & 79M18 "M" series for added power handling capability. the "L" series is fine for the ±15V supply as long as you mount them on a heat sink. It wouldn't hurt to put a heatsink on the 5532 headphone amp also.
