Audio Amp gain

Hi Bill,
I have seen very few amplifiers with an input impedance as low as 600 ohms. Maybe for the preamp of a dynamic microphone. Line level inputs are usually 10k ohms.

Computer speakers already have a power amplifier built-in. Their input is line-level which has maximum RMS levels to about 500mV. Extra gain is used so that lower than normal signals play loud enough when the volume control is turned up. The amplifier might have a sensitivity of 100mV with the volume control at max.

An elecret or dynamic microphone has an output of about 5mV speaking at a normal conversation level about 10cm away. The gain from a preamp to boost it to line level is 500mV/5mV= 100 but extra gain is usually added for faint sounds to be heard. Then the preamp will have a gain of 200 to 300.

Most power amp ICs for cars have an output at clipping of 14W RMS at low distortion into a 4 ohm speaker with a 13.2V supply. The output swing is 7.5V RMS. They have voltage gains of from 30 to 100.

 

That 600 ohm figure seems to be the one for telco equipment line transformers.

With audio amps, the idea is for a high input impedance (47K used to be the common value) and a very low output impedance.

 

preamplifiers have the widest gain requirements, to output a "line level" signal no matter the input, such as phono cartridge or blu-ray player. The preamp handles the impedance matching and the volume control.

Power amplifiers have a lower gain with less range, but a very low output impedance. Typically, they can only take a "line level" input from the preamp.

Various systems do swap the volume and tone controls between preamp and power amp, however, especially in "all-in-one" systems such as a portable stereo/boom-box.

--ETA: For 600 Ohms, I've only seen that in telco things, otherwise 300 or so for dynamic mics. Older Tube TV's and such did have an abundance of matching transformers, unsure why 600 was chosen.

 

600 is the common impedance of Low-Z stage microphones which operate over a shielded balance line.

 

Here's the history of 600 ohm audio impedances, according to Bill Whitlock (see post #6):

See post #8 also.

 

Impedance can be any value so to speak. It solely depends on the type of amp.

But for maximum power transfer, Impedance shud be matched or else power will be reflected back in to the source.

Gain is also defined by the design and type of amp.
An amp can have different stages of different gains.

A preamp is mostly for high Voltage gain. A power output stage is designed for Maximum Power gain. To drive a Power output stage to it's rated output it should be driven by Voltage driver stage.

So for an amp. High gain voltage is for preamp. High Current gain is handled by the output stage