What's the difference in transformer coupled vs Direct coupled power amplifier ?

Hi !

I was wondering, transformer coupled amplifiers were used a lot in old sound systems, but the role has been taken by direct coupled amplifiers.

Besides the very little power loss due to coil resistance etc, what would the reasons be ?

Inshort, what's the difference in efficiency of circuit when using Transformer coupled amplifiers vs direct coupled amplifiers ?

A direct coupled amplifier needs to be able to supply the +ve and -ve parts of the audio waveforms into the speaker resistance. Modern amplifiers often have PNP and NPN transistors, or P-MOSFETs and N-MOSFETs, so symmetrical circuits can be made that are reasonably efficient.

If symmetrical active devices are not available, or they don't work at voltages and currents that match speakers, then a transformer will often increase efficiency by allowing the active devices to work in one direction only or to work with a different impedance load than the speaker resistance, or both.

Valve amplifiers usually need output transformers. The valves only work in one direction, and they need a large voltage to work. With those two constraints, a transformer will usually simply the design.

A modern direct-coupled amplifier costs less, is smaller with less weight and produces lower distortion and a wider bandwidth than a transformer-coupled amplifier.

Valve amplifiers needed the transformers because of impedance matching. Tube operated at much higher voltages and lower currents.
They could not cheaply put amps of current into a 8 ohm load.

Some speakers in the valve era were 40 ohm or so. Hifi had taps for 8 or 16 ohms.

There is and was a technique called line driving from a 70 V line. Each speaker had a line to voice coil transformer and an L-pad set the volume. This allowed many speakers to be driven from a single output. You might find these in a department store.

The transformer could be expensive because of the frequency response.

Capacitive coupling is also used with solid state amplifiers.

Class AB and class D are common for higher power audio amplifiers. Class A is also used.

Bridging is also used so that a single supply like 12V, allows an effective +-12 V supply. The speaker impeadance is lowered to at least 4 ohms to get the power up.

What bridging does is, it drives two amplifiers, one 180 degrees out of phase. The speaker output is taken from the two + outputs relative to the other + output.

If the transformer is on the input of the power amp or the inputs and outputs of low-level electronics it is to have a balanced line (google that). This reduces hum and noise on the system. E

canadaelk said:

If the transformer is on the input of the power amp or the inputs and outputs of low-level electronics it is to have a balanced line (google that). This reduces hum and noise on the system. E

Click to expand...

It could be, but that is more often done electronically (eg. a differential amp).

A transformer could be impedance matching, ground isolation, balanced drive to a push-pull output stage, "phantom" power or various other reasons.