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> Amplifier
and Comparator Circuits > APP 1931
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App notes
> Automotive
> APP 1931

Mar 14, 2003

Keywords: low noise, op amp, phono, phonograph,
RIAA, preamp, opamp, operational amplifiers, op amps,
amplifier
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parts

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APPLICATION NOTE 1931

USB-Powered High Precision Hi-Fi Phono Preamp

Abstract: A
quad, low-noise, op amp and LDO are used to build a stereo
preamp with RIAA equalization to allow converting signals from
a record player (phonograph) to a linear output for digitizing
by computer sound cards etc.

A
phono preamp is needed for preamplifying and equalizing the analog
output of the good old record player. The former hi-fi
amplifiers or stereos included a phono input, but at this time
it is not very common because the CD player has a linear
output and doesn't need linearization. If you still want to
play your records or you want to archive LPs with a soundcard
on your computer you will need this phono preamp. This
application is easily powered by +5V the USB port supply. No
external power supply, dual supply, or wall cube is needed.

This design idea shows a 3.3V powered phone
preamplifier with RIAA equalization curve. The advantage is
the low power, low voltage
single supply solution. It is done with a MAX4478, a quad,
low-noise, rail-to-rail op
amp. A small linear
regulator, the MAX8877, regulates the noisy +5V from the
USB supply down to +3.3V (see Figure 1).

Figure
1. +5V USB power supply with +3.3V
output.

Additionally, the MAX4478 op amp offers a
very high power
supply rejection ratio of 120dB. This circuit is intended
for use the standard moving magnet type phono pickup (see
Figure 2).

Figure
2. One channel phono preamplifier with RIAA equalization
(courtesy of Rod Elliott. For more information, see: http://sound.au.com/project06.htm).

The
high value capacitors with 100µF, 22µF and 10µC could be
tantalum or standard electrolytic caps. The low value
capacitors with 82nF and 22nF should be 2.5% tolerance, if
obtainable, otherwise you may be able to measure a selection
of standard tolerance caps to find those which are closest to
the required value. Some deviation from the ideal RIAA
equalization curve will occur if these caps are too far from
the designated values (see my test). More important is
matching between channels—this should be as accurate as
possible.

Resistors should be 1% metal film for close
tolerance and low noise. This design is done with an active
low frequency and a passive high frequency filtering. Based on
the RIAA specification Table 1 shows the performance
with frequency. There is no rumble filter for frequencies
below 20Hz integrated. Please be careful with 50Hz-60Hz noise.
The application has a high amplification for low frequencies.
It is about 60dB (1000) for 20Hz.

As can be seen from
Table 1 or Figure 3, the accuracy is about 1dB. This
might be increased if a more accurate capacitor for the
high-frequency filtering is used. The gain at 1kHz is about
40dB (100) so a nominal 5mV cartridge output will give 500mV
output. This may be increased if necessary, by increasing the
value of the 100k resistor in the second
stage.

Table 1. Frequency Response

Frequency (Hz)

Test (dB)

Ref (dB)

Error

11,8

61,4

20

59,17

59,3

0,13

40

57,44

57,8

0,36

100

53,63

53,1

-0,58

200

48,52

48,2

-0,32

500

43,12

43,2

0,08

1000

40,83

40,0

-0,83

2000

38,41

37,4

-1,01

5000

33,23

31,8

-1,48

10000

27,82

26,3

-1,52

15000

23,85

22,8

-1,05

20000

21,53

20,4

-1,13

Figure
3. Frequency response vs. RIAA reference.

Related parts

MAX4478

SOT23, Low-Noise,
Low-Distortion, Wide-Band, Rail-to-Rail Op Amps

Free
samples

MAX8877

Low-Noise, Low-Dropout,
150mA Linear Regulators with '2982 Pinout

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samples

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Download,
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APP 1931: Mar 14, 2003

AN1931, AN 1931, APP1931,
Appnote1931, Appnote 1931

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