What is a Transformer?
A transformer is a static electrical device that
transfers energy between two or more circuits through electromagnetic
induction. The transformer can Step up or Step down
signal voltage. The transformer has no direct connection between the
primary and secondary windings, the electrical energy transferred using
electromagnetic induction. Due to this isolated property between primary
and secondary, the transformer provides electrical isolations between
primary and secondary, which means from the input and output or vice
versa. We have covered detailed article on Transformers.
A Transformer receives a sinusoidal input signal
and converts it to an output signal. During this conversion process,
there are no physical connections between this two. This conversion
actually happens by the two or more insulated copper wire coils (which
are denoted as windings) wrapped around a magnetic iron core.
Audio Transformer uses this isolation property and
creates isolation between the output speakers or audio circuitry with
the transformer’s input side amplifier system. In such case, the primary and secondary winding turns ratio fixed to 1:1.
Due to this, the transformer does not alter the voltage or current
level. It does only create isolation between the Input amplifiers with
the output speaker system.
Other than the isolation transformer there is
another audio transformer too, which will change the output voltage
level depending on the input AC signal. The loudspeaker is a huge load
and needs to provide the required current and voltage across it to
produce proper sound vibration. An Audio transformer with Step-up feature will step up the voltage
or the current level to drive a load across it. Same happens for the
Stepdown transformer too. It converts the voltage from higher to lower
with the increased current output.
The audio transformer also provides impedance matching specifications.
When the output of one circuit or device is directly connected to the
input of another device, it is very important that the device output
impedance and device input impedance both are matched. An impedance
matching transformer provides this feature and converts higher impedance
output to lower impedance to drive a low impedance speaker or feeding
to another low impedance device.
Working of Audio Transformer and its Construction
Although an Audio transformer does not have a
physical connection between his primary and secondary coil, the
transformer provide bidirectional feature between this two windings. We
can also use the same primary side as secondary and secondary as
primary. In such case, the transformer provides signal loss in one
direction and signal gain in reverse direction or vice versa.
The audio transformer works at frequencies between
20 Hz to 20 kHz. So, the operation of an Audio transformer has much
wider frequency range.
As discussed above, the audio transformer uses Impedance balancing
technique. It is very useful for balancing amplifiers and loads
(Loudspeaker and other) that use a different input or output impedances
for maximum power transfer application.
In modern days, speakers impedances ranges from 4
to 16 ohms, typically 4 ohms, 8 ohms or 16 ohms speakers are available
whereas Transistor or Solid state amplifiers use 200 – 300 ohms output
impedance. If the amplifier is a retro design, such as old Valve or Tube
amplifier then the output voltage sometimes reach 300V with 3k
impedance. We need impedance matching transformer which will convert the
High impedance to low impedance and should convert the voltage and
current to a level which will directly drive a loudspeaker.
A Transformer can have multiple windings in the
primary and secondary side. The ratio between primary and secondary
windings, the number of coils turns in the primary side (Np) and a
number of coil turns in secondary (Ns) is called the turns ratio.
This turns ratio also defines the primary and secondary voltage ratio
as the voltage is directly proportional to the primary and secondary
winding turns.
So, NP/NS = VP/VS
Audio Transformer Impedance Ratio
Impedance is the most important
factor for impedance matching transformers. For impedance matching
transformer the impedance ratio between primary to secondary can be
calculated using the primary and secondary turn or the primary and
secondary output voltage.
To calculate the impedance ratio we need to square the transformer turns ratio or the transformer voltage ratio.
In the above equation, ZP is primary impedance and ZS is secondary impedance. NP/NS is the turns ratio of the transformer and VP/VS
is the voltage ratio of the transformer. Impedance ratio is the square
of turns ratio or voltage ratio. So, a transformer with 4:1 turns ratio
or voltage ratio could provide 16:1 impedance ratio.
Example
We can calculate some practical values depending on the formulas given above.
Suppose, a transformer with a 25:1 turns ratio is
used to balance the power amplifier output with a loudspeaker. The Power
amplifier provides 100 ohms output impedance, what would be the nominal
speaker impedance needed for maximum power transfer?
Solution:
So, using the 25:1 turn’s ratio transformer across
100Ω power amplifier we could efficiently drive 4Ω Loud Speaker with
maximum power transfer.
Types of Audio Transformer
As discussed in the above segment, the Audio transformer can be used in multiple applications. But generally, three types of Audio Transformers are mainly used for audio related purposes.
- Impedance matching Transformer
- Step up Audio Transformer with Wide frequency range which is within the audible frequency.
- Step down Audio Transformer with Wide frequency range which is within the audible frequency.
There is another specific Audio transformer
also available, which are useful for digital audio applications and
generally works in high frequency.
Transformers can also have multiple primary and
secondary taps, which provide flexibilities to the user to change the
output devices without changing the costly audio transformer. For
example, A transformer can have multiple secondary taps to connect
multiple loads with 4 ohms, 8 ohms or even 16 ohms impedance but only
one tap need to be connected to the load when working with it. Such
transformers are generally costly and can be found in retro musical
systems or amplifiers.
The transformer can have different bodies depending on where it would be used. A chassis mount transformer needs a supporting chassis to support the bulky weight. Also, there are PCB mounted audio transformers available in various shapes and sizes depending on their specifications and usages.
Microphone Transformer
A microphone transformer mainly used to balance
the impedance between Amplifier system and the microphone. It is
essential as there will be signal loss due to unbalanced impedance on
Amplifier input and microphone output.
A microphone transformer does not reduce Hum
noises. A microphone transformer needs a twisted pair with ground
shielding wires to connect. The wire consists of two conductors which
are tightly twisted together with surrounded by a conducting braid or
foil. This wire effectively reduces humming noises and external noise
interferences.
A transformer which have a single primary and
receives unbalanced input, and have a center tapped secondary which
provides a balanced output, is called as Balun Transformer. In such a configuration, the Amplifier gets a perfect balanced signal.
100V Line Audio Drive Transformer
There are such scenarios where multiple
loudspeakers are connected together in long-range public address systems
which are connected with a single amplifier system. The problem arrives
when long wires are used to connect the Amplifier output and
Loudspeaker input. The wire resistance creates trouble for signal quality and the signal loss happens with poor signal amplitude across the speakers.
Due to this, two special transformers are used, one is step up and another one is Step Down.
The step-up transformer increases the audio output signal voltage to
100V. Due to the Formula P(W) = V x A, when a voltage is increased the
current decreases for a given power. The resistance would not effective
for the low signal current. The signal will transmit perfectly.
On the other end, across each loudspeaker, a
step-down transformer with impedance matching facility, Steps down the
100V to the speaker voltage and increases the current. The Transformer
also matches the impedance for maximum power transfer.
This type of audio transformers is called a transmission line matching audio transformer.
They have multiple connections in both primary and secondary side. In
general, primary side taps are used for suitable power level thus the
amplification gain can be controlled by tap connections. And the
secondary side has multiple taps which are useful to connect different
impedance speakers to different impedance speaker as per choice and
availability.
Many modern Professional Amplifier line
transformers provide high power handling capabilities as well as
multiple configurations to connect parallel or series, loudspeakers
together.
