just built a lm3886 amp

[carbonzit]

sorry but i must ask this as it is important.. WhaT is class-D,A,AB Guru?

Like the guy said, you could look it up.

I'll get you started, though:

• Class A: "Single-ended" amplifier; a single amplifying device (transistor, vacuum tube, MOSFET) conducts throughout the AC cycle. Can be highly linear (= low distortion) but inefficient (wastes power) because current flows throughout the cycle.
• Class B: Two amplifying devices operating in "push-pull" mode, where each device only conducts 180° of each cycle (one device amplifies the positive half, the other device the negative half of each cycle). Eliminates distortion due to even-order harmonics and is much more efficient than class A.
• Class AB: A class B amp where a small amount of overlap is allowed between the push-pull devices (conduction is slightly more than 180°) to reduce "crossover" distortion.
• Class C: A highly efficient mode of operation used in RF (radio frequency) only, where the amplifier is tuned to a specific frequency using a resonant tank circuit and produces a series of pulses that are converted into a sine wave at the output. Not usable for audio.
• Class D: An amplifier that uses PWM (pulse-width modulation) or "chopping" to achieve very high efficiency (90% or more) by driving the output stage with high-frequency pulses (significantly higher than the highest frequency to be amplified) of varying widths. The PWM frequency is removed at the output, leaving only the amplified audio wave. Capable not only of high efficiency but surprisingly low distortion. Used extensively in automobile audio amps.

 

[Jackdaniel]

thanx carbo, and what is "crossover distortion"

 

[audioguru]

what is "crossover distortion"

Look at its description in Google.

It sounds like a buzz attached to speech or music. It is caused by a class-B output stage which does not have any idle current so there is a dead zone around 0V and the signal needs to be +0.7V or more to turn on the NPN emitter-follower and needs to be -0.7V or more to turn on the PNP emitter-follower at the output of an audio amplifier. Crossover distortion is eliminated by biasing the output transistors a little with a small amount of idle current (then they operate in class-AB) so there is always a transistor conducting with no dead zone.

 

[carbonzit]

Hey, Audio McDuck: very good explanation of crossover distortion there (the illustration). Where did you get that graphic and text?

 

[audioguru]

Hey, Audio McDuck: very good explanation of crossover distortion there (the illustration). Where did you get that graphic and text?

I entered Crossover Distortion into Google and this site was the 5th link:
**broken link removed**

 

[Jackdaniel]

okey..What kinda meter do u use to measure output of an amp?

 

[audioguru]

okey..What kinda meter do u use to measure output of an amp?

Do you want to measure the AC output level of an amplifier or measure the DC offset voltage?

Any voltmeter can measure the DC offset voltage.
An analog or digital multimeter is made to accurately measure only the low frequency (50Hz or 60Hz) of the mains electricity and are inaccurate at higher frequencies.
An oscilloscope can be used to see the output level plus it can show any distortion.

A distortion analyser can accurately show the output level plus it can measure the distortion.