240W class AB amplifier

Thread Starter

Experimentonomen

Joined Feb 16, 2011
331
Hello!

Just thought i'd share my latest project, a class AB amplifier capable of pushing about 245 Watts into a 4 ohm load before clipping.

The circuit is a basic Douglas Self "Blameless" topology but with a single transistor VAS instead of the darlington "beta enhanced" VAS used in the blameless.

The rail voltages are about 60VDC per rail with a 800VA power transformer, so its no toy, and should NOT be attemted by someone that doesent have experience in electronics and such.

A few pictures, the little board is a speaker dc and thermal protection board:
http://i.imgur.com/Eybjs.jpg
http://i.imgur.com/Y8JVR.jpg
http://i.imgur.com/yVJNH.jpg
http://i.imgur.com/mJDmO.jpg

A video of the thing in operation:
http://www.youtube.com/watch?v=E5KR4TeSXYU

Schematic:
http://i.imgur.com/9UU4a.png
 
Last edited:

Thread Starter

Experimentonomen

Joined Feb 16, 2011
331
On the same heatsink i am also putting a stereo amplifier that does around 50-100 watts/ch into 4 ohms, using the same circuit, it will run off a separate 300VA transfomer with 2x35VDC rails.
 

Thread Starter

Experimentonomen

Joined Feb 16, 2011
331
More like a 2.1 channel amplifier, the 245 watt mono channel will also have a active lowpass filter with selectable xo frequencies, around 50, 90 and 120 Hz and a level control.


And beeing forced air cooled im calling it a PA style amplifier as it will be able to take more of a beating than a normal convection cooled amplifier would, like beeing able to sit at rated power all day long without overheating, and if it does get too hot, the thermal protection will just shut off the power supply.

Im willing to bet most commercial consumer amplifiers cannot survive outputting rated power for more than a few minutes before they overheat and break down.
 

R!f@@

Joined Apr 2, 2009
9,728
I am way ahead of you Mate.

I have a 3 way active Tri Amplifier I made. Been using it for like I dunno 3 years may be.
 

R!f@@

Joined Apr 2, 2009
9,728
Need to open it up. It was before I joined AAC.

Bloody thing weighs like a TON :eek:

But, it needs a bit of dusting now. And I am thinking of remaking it a bit stronger.

PIC's will come soon.

Let u know.. keep visiting.
 

Audioguru

Joined Dec 20, 2007
11,249
Maybe your amplifier produces more power than you measured.
Most DMMs can measure 50Hz and 60Hz voltages but are wrong (the measurement is too low) at 1kHz.
Your 'scope should show the output p-p then divide it by 2.828 to get the RMS value.

Your supply is 120V (but maybe it drops when the amplifier is blasting?). The amplifier has a loss of about 8V p-p. Then the output is 112V p-p which is 39.6V RMS. Then the power into 3.9 ohms (maybe its value is a little higher or lower?) is 402W.
 

Thread Starter

Experimentonomen

Joined Feb 16, 2011
331
The dmm may be very cheap but it actually shows the same voltage all way from 20Hz to 20kHz with minimum rolloff, which is suprising for a cheap dmm, especially for one thats not even true RMS.

I noticed this last time i measured an amps output and tried different frequencies.

At 400 watts output, the transistors would explode! According to the SOA diagram in the datasheet, im already exceeding SOA at 200 watts, i guess its rail sag that so far prevents the output transistors exploding, and the fact that i always look at the SOA curve for DC, i do this as to have a small safety margin.
 

thatoneguy

Joined Feb 19, 2009
6,359
Question, Q7/R17/D1 make a current sink.

What's Q6 doing as it is already inline with the current sink, and it has a 4.7uF cap across C-E?
 

R!f@@

Joined Apr 2, 2009
9,728
Egad !! ur work bench is worse than mine...

But I see you got toroid..mmm I luv to get me couple of those hefty ones.
 
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