This report aims to detail the electronic
operation of a preamplifier design developed around the THAT 1512 integrated circuit.
The focus will be on the component function regarding; Input level, Phase, Pad, Gain adjust and Balanced output, referring
too the circuit diagram and component values. This will also outline the role
the 1512 chip. Further there will be a section dedicated to the testing of the
final build, along with analysis of the data collected. Following this an
evaluation, then a conclusion.
The aim of this report is to understand
the fundamental operation of a preamplifier and recognise why it is crucial as
part of the signal chain. Additionally, the results from the test will give
data on the success of the preamplifier. Allowing areas to be highlighted for
improvement or better soldering ect.
Detail component operation
Review component necessity
Consider THAT 1512 operation
Test preamplifier values
Assess circuit success.
Highlight areas to improve
1.4 Rationale ?
2.1 Preamplifier Circuit Description
An effective preamplifier is design to
amplify low level electrical signal captured using a condenser or dynamic
microphone to line level. This is achieved by adding “A voltage gain of 1000 (60dB).” (Ballou, 2015) However in doing
this the circuit also have to exhibit a wide range of qualities in order to
capture clear audio these include;
high a stage gain as is practical have a wide AC bandwidth and a low noise
level and are capable of a adequacy output voltage swing.” (Hood, 2004)
Preamplifiers also require; Adjustable
gain up to 40dB, however some applications would require up to 70dB, 48V
Phantom Power with an On/Off switch, standardised by IEC EN 61938?and a Gain Pad. With the optional extra
of a phase reversal switch and a Line In.
2.1.1 Input level?
the preamplifier to work a input bias current is required this switches the
mains AC signal to DC through a 1K2 resistor connected to input pins and the
ground. (Fig 1.1) The values of the 1K2 and 6k81 (used for Phantom power supply)
This phase reversal latch allows the
input signal to have it polarity switch, so that the preamplifier can be used
to record with microphones techniques that require to signals of opposite
The 20dB input attenuation pad is a
U-pad made up of two 1k1 and a 267 ohm resistor. In order to pad the signal a
SPDT latched switch is used to switch in the U-pad, however adding this into
the signal path will add noise to the signal. When the latch is switched out
the signal passes directly through the input capacitors.
2.1.4 Gain adjust
The gain of the input signal is
controlled using a variable log taper audio type potentiometer with a 5 ohm
resistor and a 6800uF capacitor is series with it. This resistor acts to limit
the available gain of the pot to approximately 60dB. Using the potentiometer to
change the gain of the system can create DC offset changes at the output. The Capacitor
is used to overcome this, setting the DC gain to fixed value.
2.1.5 Balanced output
Balanced lines are fundamentally better
at reducing noise generated within a circuit as well as maintaining the
integrity of the integrated circuit. This part of the circuit is made up of for
1N4004 diodes. These reduce the
“likelihood that accidental
electrostatic discharge (ESD) or electrical over stress (EOS)”
The two 1N4148 diodes are used to
prevent reverse biasing on the above components. Which would lower the noise
performance of the preamplifier.
2.1.6 Radio Frequency Interference.
In order to reduce noise interference further
RFI protection is required within the circuit. This protection is created by
the 100pF capacitors at the input to remove common mode and differential Radio
Frequency Interference. Furthermore a 470pF capacitor is used across the input
of the THAT 1512 to reduce differential high frequencies from both internal and
2.2 Power supply description
2.2.1 Main supply
2.2.2 Phantom supply
2.3 Integrated Chip
3.1 Gain Test
A preamplifiers primary function is to
amplify low level signals effectively to a maximum of 60dB effectively. In
order to test the gain of the preamplifier, it was connected to a; Power
Supply, Signal Generator and an Oscilloscope. The signal generator was set to
produce a sine way with the properties of:
The gain of the signal was incremented
using the potentiometer 20 times to gather a large data series. The oscilloscope
was then used to measure the Peak to peak voltage of the sine wave after each
increment. (Figs Below)
Gain Positon (approx)
VPP of Preamplifer (V)
Gain Positon (approx)
a short conclusion identifying the key issues drawn from your discussion.