class ab amp

i have found this ckt of class ab amp..
i jst need some one to please confirm this ckt
one problem is that i have seen almost all the amps that one input must b ground,,but here its not like that ,,,why?
the link for this ckt is:
http://mrcshobbies.blogspot.com/
i have designed the one with 21w.(.according to my requirement..)
please help

This is an AC amplifier and the input is capacitively coupled. That mans the DC level is set on the other side of the cap. I don't know why they return the input to the positive rail, but since it is capacitively connected to ground, it should work. I would return it to ground.

The upper rail becomes an alternative ground when doing an AC analysis. You can use it, but I prefer using ground instead.

ok...what about this one...if i use this amp instead of the last one,,then it will be good or not?
http://www.redcircuits.com/Page1.htm
this is the link..please help me to choose..

and in the link,,it also says that:
"D1 must be in thermal contact with Q1."
what dose that mean???

Hello,

Thermal contact means that the body of D1 is touching transistor Q1.
It is mounted body to body.
You can use some thermal paste to have a good thermal bound.

Bertus

bertus said:

Hello,

Thermal contact means that the body of D1 is touching transistor Q1.
It is mounted body to body.
You can use some thermal paste to have a good thermal bound.

Bertus

Click to expand...

so can u suggest me ,,that which circuit should i use for an amp?i mean which circuit u think is more accurate..?

Thermally bonding D1 to Q1 improves the accuracy and response as both semiconductor devices have similar properties. If you only need a basic amp then this is unnecessary.

Hello,

On this page you can find the definition of an class AB amplifier:
http://www.wordiq.com/definition/Electronic_amplifier

Bertus