Does Voltage Step Down in Audio Frequency Transformer?

[tahir4awan]

We know that audio frequency transformer are used for Impedance matching in Amplifiers to convert high impedance to low impedance. Therefore its input and output coils turns are not same. Does it mean it also steps down the input voltage?

Secondly, does impedance of audio frequency transformer consist of purely dc resistance of coil or both dc resistance of coil and inductive reactance?

 

[rohitkhanna]

Who uses AF transformers these days ?? Curious to know.

Having *said* that, the idea of the transformer is of course to manage impedance. A typical magnetic speaker's coil is anywhere between 2 - 16 ohms usually. 4 & 8 ohms being the most common (at least at the time that i used to dabble in 'em some *years* ago).

However it's not the *voltage* that the transformer is managing - rather its the *power* - which stays the same, and is transferred to the load most efficiently when the impedances are suitably set.

Next - the word "impedance" implies a combination of resistive and reactive elements. Since the audio spectrum has such a large dynamic range, usually this impedance is specified for some standard fixed frequency. Probably 1KHz - but i'm guessing here.

The mechanics of a speaker add complications to the actual impedance. For a moving coil type of speaker, there are many many variables which affect this impedance over the audio frequency spectrum - hence its not just a simple "resistor-in-series-with-inductor" model.

 

[FvM]

Who uses AF transformers these days ?? Curious to know.

100 V audio distribution systems are still used in buildings as far as I'm aware of. They involve voltage step up at the amplifier (at least if it'a a transistor amplifier) and step down at each speaker. https://en.wikipedia.org/wiki/Constant_voltage_speaker_system

Microphone and line level audio transformers have been mostly replaced by active electronic circuits, but are still an option. The very low impedance of a ribbon microphone can be still best matched by a transformer. https://en.wikipedia.org/wiki/Ribbon_microphone
Besides historical ribbon microphones, there are active products like Shure KSM 313.

 

[godfreyl]

We know that audio frequency transformer are used for Impedance matching in Amplifiers to convert high impedance to low impedance. Therefore its input and output coils turns are not same. Does it mean it also steps down the input voltage?

Yes. let's say we have an output transformer from a valve amplifier, and the transformer is specified as 5KΩ to 8Ω. The turns ratio of the coils will be N = sqrt(5000/8) = 25 to 1.Then (ideally) output voltage = input voltage / 25 and output current = input current * 25.

Secondly, does impedance of audio frequency transformer consist of purely dc resistance of coil or both dc resistance of coil and inductive reactance?

The resistance of the coils is kept as small as possible, but may be significant.

The primary inductance is important as it affects low frequency performance. The output valve(s) driving the transformer will see a load equal to 5K in parallel with the primary inductance of the transformer.

At high frequencies, performance is limited by the unwanted capacitance between the windings in the transformer and also the leakage inductance caused by imperfect coupling between the input and output windings.

Who uses AF transformers these days ?? Curious to know.

There's still a few uses I can think of:

• Valve amplifiers are still used. They normally have an output transformer, and some have interstage transformers as well.
• Step-up transformers are sometimes used with microphones and moving-coil phono cartridges.
• Isolation transformers are used quite often these days in car sound systems. In systems where a head unit (CD player or whatever) in the dashboard is connected to a power amplifier in the boot/trunk at the back of the car with a long cable, there can be problems with interference/noise due to the "ground" voltages at the two ends not being the same. Putting a transformer in line isolates the grounds and reduces the noise.