Help me understand the "pre" stage levels.

Let me know if I have this right.

As far as I understand, pre-power stages (from mic/pickup to preamp; from preamp to power amp) are all about voltage. In other words the audio signal is transported in an electrical form using the voltage amplitude as an analog to the sound regardless of the current (amperes) level. In other words, since those stages are not driving any load (like a speaker) the wattage (voltage/current) is not important as long as the voltage levels are the right ones.
Example:
The pickup has a 500 mV signal that is increased to 2000 mV using a preamplifier. <-- Here the ammount of amperes (hence watts) is without importance.


On the other hand, power stages are all about, yes, power (watts) because they need that power to move the speakers to create sound.
Example:
The power amplifier takes that 2000mV output from the preamplifier and adds the necessary current (amperes) to create 50 W of power.


Is that right? Im I missing something?

Thank y'all.
 

benb

Member
2010-04-24 1:52 am
Well, yes and no.

For "line level" signals, for the the inputs of tape recorders and power amps, what you say is exactly true. Input impedances of such inputs is usually around 10k, though it can go up into megohms or maybe down to 1k or 600 ohms in special cases. And yes, outputs that drive these basically generate a voltage that these inputs respond to. Such outputs have impedances in the hundred ohm or less range, so there's virtually no voltage drop when connected to a line-level input. In fact, usually one line-level output can drive several line-level inputs with no problem.

When you get into electromechanical sources such as microphones and phono cartridges, impedance becomes very important for several reasons, none really having anything to do with power. Standard moving-magnet phono cartridges have a specified load (usually 47k ohms resistance, and a few hundred pF capacitance) they need to be plugged into to have their stated frequency response.

Dynamic microphones (basically a diaphram placed on a voice coil, the exact opposite of the standard dynamic loudspeaker driver) also have a frequency response that's sensitive to loading. Most mics used in PA and studio recording work are low-impedance (and balanced, but that's not important in this conversation) and work well into an impedance of 2k to 3k ohms.

Ribbon microphones are VERY low impedance, like 1 ohm, comparable to ribbon speaker drivers. These mics usually have a transformer to bring the voltage and impedance up to something a preamp can use.

Condenser mics, both the small electret type used in cassette recorders and as "computer mics" as well as the larger ones used in studios, have small "impedance converters" built in, essentially buffers that convert the super-high 20 megohm impedance of the capacitive membrane to a few hundred ohms. Since the buffer isolates things, it doesn't matter what impedance it drives, as long as it's not too low.

Also, the impedance of the preamp is important for low noise performance. Moving-magnet phono cartridges have about 5mV output, which is pretty high as far as these things go. Moving coil cartridges have maybe 1/10th that, and a much lower impedance. Microphones can have even lower output depending on what they're "listening" to. The noise of resistors becomes significant at these levels, so careful design must take this into account.
 
Voltage, current and power are always related. In your example, the 2V output by the preamp results in very little power because the load provided by most power amplifiers is thousands of ohms and P=V^2/R. If however you tried to swing 2V into a 0r1 load, you would be delivering 40W.

With some exceptions, all stages in the audio chain are voltage stages that must be capable of delivering the current determined by the voltage delivered into a specific load impedance. Until you get to the last stages of a power amp, the load impedance is typically high in relation to the voltage so the power delivered is insignificant. Most power amplifiers are nearly ideal voltage sources - the input signal is multiplied by the gain and the output voltage is constant over a wide range of load impedances.

There are some power amplifiers that are more like current sources, meaning that the output current has a direct relationship to the input voltage and the output voltage will vary with load impedance.