Temperature Generated Distortion

[colin.hepburn2]

Hi All

Some Notes from the man of sonic link audiocables


The Journal of High Performance Audio Construction

Editor: Graham Nalty
(MA Mech. & Elect. Sciences Cambridge)

Temperature generated distortion

Temperature generated distortion occurs in
semiconductor amplifiers as a result of
temperature variations within transistor
junctions. If a semiconductor circuit is
switched on, current will flow through the
transistors. As there will be a voltage drop
across the transistor terminals, electrical
power will be dissipated and this electrical
power will be converted into heat. heat
always flows from a hot body to a cold
body, so the heat generated in the transis-
tor junction will flow through the body of
the transistor and through the leads to heat
the air and everything else around it.
Gradually the body of the transistor will
get hotter until an equilibrium is reached
in which the heat flow from the transistor
exactly equals the heat generated at the
junction. The greater the power dissipated
in the transistor, the greater the flow of
heat and the higher the equilibrium tem-
perature of the junction and the body of

the transistor.
As a transistor changes temperature impor-
tant electrical characteristics, including the
gain, will change. So the gain of the tran-
sistor when it is dissipating I watt will be
different from the gain at 2 wails
dissipation.
If the transistor is subjected to a varying
input signal, such as is found in music, the
power dissipation of the transistor will vary
in relation to the voltage of the music sig-
nal. the rate at which heat is dissipated
from the junction through the transistor
body will depend on the previous ambient
temperature of the transistor body. If we
take a simple musical example of a high
frequency woodwind note being played
before and after a loud drum roll, we
would see that the transistor body temper-
ature after the drum roll would be higher
than before. When the woodwind note is
played after the drum roll, the temperature

of the transistor body is higher so that the
heat dissipated from the woodwind note
dissipates more slowly, resulting in a high-
er junction temperature during this wood-
wind note. As the temperature is different,
the gain which the transistor applies to the
electrical signal representing the wood-
wind note will be different before and after
the drum roll. If the second woodwind
note is played during the drum roll, the
gain of the woodwind note will vary dur-
ing the drum roll. The resulting distortion,
which can easily be calculated mathemati-
cally, will tend to blur the sounds of both
drum and woodwind. Such blurring can
often be heard in modest quality equip-
ment and can be very noticeable on large
orchestral climaxes when distortions from
other effects (dielectric absorption in cables
and capacitors) add to the effect.
Temperature generated distortion can be
audibly reduced by improved heat sinking.
In simple cases the change from a T092
transistor to a TO 126 orTO220witha met-
al tab can reduce temperature generated
distortion enough to be audible. Mounting
the transistor on a heat sink further
reduces temperature generated distortion.
Other ways to reduce temperature gener-
ated distortion with audible result include
replacing a two transistor Darlington out-
put stage with a Darlington transistor
mounted on the heatsink or changing the
Vbe multiplier transistor used to set the
output current to a power type thermally
coupled to the heat sink near the output
devices (T0220 types are very good here).
But it is important to ensure that the heat
variations drained from one transistor do
not have a low thermal resistance path to
other transistors in the circuit. For exam-
ple, if a number of IC regulators share the
same (fairly small) heatsink, the removal
of the heatsink may improve sound quality
by eliminating crossmodulation of heat
between the different ICs. Ways of reduc-
ing temperature generated distortion
include:
' larger heat sinks
• separate heat sinks for different parts of
the circuit
• insulating pads of lower thermal resis-
tance.

Colin

[slowhands]

Balderdash!