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Old 1st August 2007, 08:47 PM   #1
featherspeed is offline featherspeed  South Africa
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Default M250

Hallo i am a first year electritian aprentice and i am interested in building te 250W amp i ave tride a few books and learned that the amp is a push pull class b amp but i still dont realy understand the amp i only found amp's coupled with a transformer becuase of the better gain that it offered round about 30db i think it was.

my question is can any body please examen the scheamatic and explain what the verios sections do ens.

the info on the amp can be found from this link
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Old 2nd August 2007, 01:43 AM   #2
djk is offline djk
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A nice tutorial on amplifier design:

Candidates for the Darwin Award should not read this author.
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Old 2nd August 2007, 03:06 AM   #3
jaycee is offline jaycee  United Kingdom
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I suspect the transformer coupled amps that you found may have been Valve designs, and not solid state. The main reason that a transformer is use there is to isolate the speaker from the high voltage needed to drive the valves.

A brief tour of the M250:

T1 and T2 are configured as what's known as a "long tailed pair". Think of the bases of T1 and T2 as the + and - inputs on an op-amp. T3 is coupled up through LED1 to act as a current source.

The audio signal enters the base of T1 through some filter components (C1, R1 and C2). The waveform causes the currents in the collectors of T1 and T2 to change accordingly. This is then voltage amplified by T5 (with T4 functioning as a current source for this stage of the amplifier).

From there, we basically have a "current gain" section.. ie, to provide the same signal as through T5, but with more current behind it. The output transistors are T10-T13 for the positive half of the wave, and T14-T17 for the negative half. T8 and T9 function as "drivers" to run the transistors and provide them with the base current they need.

T18 and associated parts form what is known as a "VBE multiplier". This serves to regulate the "idle current" flowing through the output stage.

T6-7 and their associated parts function as a safety circuit called a "VI limiter". The bases are connected so that they sense the current flowing through T10 and T14. If this becomes too high, then T6/7 (as appropriate) turn on and "steal" the base current from the driver transistor T8/9. This in turn, turns off the output stage to prevent damage.

http://sound.westhost.com/amp-basics.htm This is a very good article, especially relevant here is part 2, where Rod explains how an amplifier like the M250 works.
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Old 3rd August 2007, 01:57 PM   #4
featherspeed is offline featherspeed  South Africa
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Thank you that was very help full i will go and check the pages that you recomended aswell
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