(

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