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Old 18th August 2014, 03:26 AM   #671
K-amps is offline K-amps  United States
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Quote:
Originally Posted by lawbadman View Post
Thanks for the answer AndrewT.

So based on the formula (4 volts drop) you sent me 4 ohms works out to be 685W.
So now how many pairs of output devices should I use? I approximate 100w per pair, so 6 pairs should be fine, correct?
In real life (mid-fi audio like Adcom/Forte etc) you will be lucky to get over 240 watts into 8 ohms and 400 into 4. This assumes a 700va transformer.

Bump it up to 1kva and you can go over 300 watts into 8 ohms. These are real life numbers what you may end up getting as opposed to ideal world calculations. figure 2.5x rated transistors for given wattage you need. so if 800 watt at 2 ohm, built an output stage worth 2000 watts of trannies.
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Old 18th August 2014, 09:22 AM   #672
AndrewT is offline AndrewT  Scotland
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Quote:
figure 2.5x rated transistors for given wattage you need. so if 800 watt at 2 ohm, built an output stage worth 2000 watts of trannies.
I have arrived at 5 to 6times for BJTs and 4 times for FETs.
Cordell has a similar multiplier, but arrives at it using a different method of analysis.

Big amplifiers generally don't use ClassAB and have gone to ClassG, or H, or D to save expense on output devices.
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Old 18th August 2014, 09:28 AM   #673
K-amps is offline K-amps  United States
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Quote:
Originally Posted by AndrewT View Post
I have arrived at 5 to 6times for BJTs and 4 times for FETs.
Cordell has a similar multiplier, but arrives at it using a different method of analysis.

Big amplifiers generally don't use ClassAB and have gone to ClassG, or H, or D to save expense on output devices.
5-6 times over the 8 ohm output or 2 ohm output?
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Old 18th August 2014, 11:30 AM   #674
AndrewT is offline AndrewT  Scotland
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if you want 100W amplifier with a BJT output stage then expect the output devices to have around Pmax total ~ 500W to 600W.
If mosFETs then expect around 400W of total Pmax.

The load impedance does not matter.

But there is a secondary effect, due to secondary breakdown.
As power goes up and/or impedance goes up the Vce goes up, a lot.
The increase of Vce brings in the temperature de-rated SOAR, very much sooner, when driving a reactive load.

A 100W into 4ohms amplifier will have a very low Vce and thus very little of the secondary breakdown comes in to limit the peak output current.
Here just 400W of output devices will probably suffice even for an amplifier used for drunken partying duty (probably worse than PA duty).

But 200W into 8ohms will have a very much higher Vce. Thus 6 times may be a more appropriate factor. This would require 1200W of total Pmax.

Compare the terrible 1943/5200 SOA to mj15003/4 SOA (slow), or to MJL4281/4302 SOA (fast).
You will probably need 5pair or 6pair of 1943/5200 to match the reliability of 3pairMJL4281/4302 or 2pair of mj15003/4 in a +-70Vdc amplifier.
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Last edited by AndrewT; 18th August 2014 at 11:33 AM.
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Old 18th August 2014, 11:36 AM   #675
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Based on your example a 200 watter needs a 1200w o.p stage.

A commercial 200 watt amp will give about 325 @ 4ohm or about 480w at 2 ohms.

Looks like 480 multiplied by 2.5 is 1200, which is what I suggested anyway.....
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Old 18th August 2014, 11:40 AM   #676
AndrewT is offline AndrewT  Scotland
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if the amplifier is rated for 2ohms speakers and it's rated output into that reactive load is 480W then the guidance for the power capability of the output stage would be ~4times for mosFET or 5to6times for BJT.
I.e. 480W into 2ohms would require a ClassAB BJT amplifier to have approximately 2400W to 2880W of total Pmax.

ClassG, H, D would have very much less.
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Old 25th August 2014, 11:16 AM   #677
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hi apex,
can i add this protect sch to ba1200?
any idea to put protect led in the protection board?
Attached Images
File Type: jpg Snap403.jpg (61.4 KB, 603 views)
File Type: jpg BA1200 PROTECT NE555 Ver1.0 Blue.JPG (893.8 KB, 598 views)
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Old 26th August 2014, 03:19 PM   #678
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Quote:
Originally Posted by AndrewT View Post
if the amplifier is rated for 2ohms speakers and it's rated output into that reactive load is 480W then the guidance for the power capability of the output stage would be ~4times for mosFET or 5to6times for BJT.
I.e. 480W into 2ohms would require a ClassAB BJT amplifier to have approximately 2400W to 2880W of total Pmax.

ClassG, H, D would have very much less.
2400W Pmax=1200W Prms.
so efficiency is (480/1200)*100=40%....hey it is very bad class AB amp...
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Old 27th August 2014, 05:11 PM   #679
AndrewT is offline AndrewT  Scotland
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nothing to do with efficiency.

And

Wrms does not exist.
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Old 27th August 2014, 05:40 PM   #680
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Hello guys ,Apex on this protection i added parallel resistor 47K on selected capacitor,DC protection when involved again not turn on OK now
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File Type: jpg BA1200 PROTECT NE555 Ver1.0 Blue.JPG (499.7 KB, 430 views)
File Type: jpg Protect LEDs.JPG (44.0 KB, 370 views)
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