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(
AUGUST 1959
LICATI ES
TRANSISTORIZED
HI-FI P R EAMPLIFIER
This TI Application Note describes a three-transistor
hi-fi preamplifier for use with a magnetic cartridge. The
recommended circuit should provide lower hum, more
freedom from microphonics, a small er package, and
greater r eliability than a compar abl e vacuum -tube
rreamp. The primary features of this design are a fre
quency response of +1 db from 30 to 15,000 cps, less
than 1 % total harmoni c distortion, RIA A equalization ,
and separate bass and treble tone controls. The rated
output is I-volt peaks into a 2K-ohm load. The power
supply volLage is not critical and may be obtained from
1) the high-voltage B supply in any vacuum-tube power
amplifier, 2) a low-voltage supply from an all-transistor
hi-fi system, or 3) a battery power source. These speci
fications should be acceptable to all but the most criti cal
audiophile.
The circuit diagram of Fig. 1 shows the preamp with
proper equalization for a G-E magnetic cartridge. Other
cartridges can be used by changing three resistors. The
necessary conditions for RIAA equalization are:
Rl = 22 R2, R2 = 100 R3, R3 = Ll5
(where L is the inductance of the cartridge in mh )
If a selector switch is placed at Point X, the volume
and tone controls can also be used for tuners, tape
players, etc. For good performance, the output impedance
from such equipment should be 25K ohms or less, and
the input signal should have a flat frequency response
with an amplitude of 0.5 to 1.5 volts.
In Fi g. 2, the family of solid curves shows the response
of the preamp to signals applied at Point X with the tone
controls in various positions. The dashed curve presents
the output response of the preamp with tone controls in
the flat position and a signal from a G-E magnetic car
tridge tracking an RIAA test record.
All specifications can be realized by using TI type
2N185 transistors with other components of the tolerances
noted. Although the fr equency response may vary slightly
with the selection of individual transistors, it will in all
cases be better than ± 2 db from 30 to 15,000 cps when
used with a magneti c cartridge source.
To achieve the same performance with different power
supplies, the proper connections must be used: Alternate
No. 1 for use with the B supply from a vacuum-tube
power amplifier , and Alternate No.2 for use with either
a transistorized power amplifier or a battery power
source.
TEXAS INSTRUMENTS INCORPORATED SEMICONDUCTOR - COMPONENTS DIVISION
-
J252 PRIN T ED IN U.S.A..
TINOTES
5 @
~f
5v
o-±-t -INPUT
%
A
10K RI lOOK 200K 10~f
@20v 15 K
+
f? ~ 2NI8S
~
R2 IK ~ 10K 10K
200 >--@S~f
R3 100
0 1100~' ~ 0-_ '1!> 50v
~ 0.01
B B+ ALTERNATE NO. I
A 0
0 B
* 6 .8K
,~~IOO~f + @50v
0.02 POINW
!? ,,~O~f -:;
~ 2NI85 + ", 20v
33K VOLUME ~ 50K
* 200~f 0.01
IK - @5v BASS + lOOK
>-- 10K
r---100 4 .7K
to-30 ± 5 volts
:.t lO+
ALTERNATE NO.2
IK A ,
I
47K 6.8K
10 ~f @20v
~1+OU.g, 10 ~f
I 22K @,,5v 1 F 1 * + 2NI85
0.005
~ TREBLE 10K
UT
f:0.05 * 10K IK , F-200~ 3.3K +@5v IK
~
FO.QIS* 47
B
Al l resistor valu e s are in ohms and
shou ld have 5 % tolerance
POWER- SUPPLY CONNECTIONS
All capacitor values are in /If
*10% tolerance capacitors
B
6 1 I
4r .<:I
~ 2 l
.. ...... Z 0 0 Il. FLAT en ~ -2
-4 f -6
-B I
r 20
Figure 1
PREAMP UNEQUALIZED, INPUT FROM AUDIO OSCILLATol
PREAMP EQUALIZED, INPUT FROM G-E CARTRIDGE
TRACKING RIAA TEST RECORD
+- BASS
I 1
I TREBLE ~
1000 FREQUENCY- cps
Figure 2
FULL BOOST
FULL
J. ~ 10000 20000