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Old 11th January 2006, 05:01 PM   #1
Darksog is offline Darksog  Finland
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Question Audio amplifier

Hi all
i have searched audio amplifier about 600-700w 4ohm
a-b class
constructiong details i have searched what about you think about this? http://www.ptt.yu/korisnici/s/r/srmarkovic/poweramp.htm

Post here more sites 600-700w amplifiers diagrams/constructions....
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Old 12th January 2006, 12:27 AM   #2
quasi is offline quasi  Australia
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Hi Darksog

At best this amplifier will deliver about 200 watts RMS into 8 ohms. I would be worried about driving a 4 ohm load with the output stage shown, it might be ok for hi-fi use but for PA the output transistors will die. This does depend on the power supply used but it sounds like you're after some power.

There are other parts of the cct that could be improved also.

Cheers
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Old 12th January 2006, 07:41 AM   #3
Darksog is offline Darksog  Finland
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Yeap i cant find good amp for my subfoofer
do you know amp what could deliver 500wrsm 4ohm load?
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Old 12th January 2006, 08:02 AM   #4
quasi is offline quasi  Australia
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You are welcome to use this design. Some have been built already by very satisfied DIY'ers. This link will take you to the midpoint of the thread. Later in the thread you will see a layout version that can handle more than 500 watts RMS into 4 ohms.

http://www.diyaudio.com/forums/showt...021#post601021

Cheers
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Old 12th January 2006, 10:16 AM   #5
Darksog is offline Darksog  Finland
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Thank you
do you mean this amp? http://www.diyaudio.com/forums/attac...amp=1111237713

What transformer VA you can faforite?
how much did amplifier parts cost?
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Old 12th January 2006, 11:36 AM   #6
quasi is offline quasi  Australia
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Yes thats it, but you will need to use the 10 FET board if you use IRFP250/350/450 or similar. If you find FETS rated at 250 watts or more then you can use the 6 FET board.

For 500 watts into 4 ohms you will need an 800va centre tapped transformer with AC voltages of 56v 0v 56v. This will give you rails of around +/- 78 volts.

I built my amp almost entirely out of second hand parts and it cost me only $80 US.

It is difficult to put a number on how much it will cost you, it depends on what standard you build it to. Decisions you make about the power supply can vary just that cost by a hundred dollars or so.

If you decide to build this amp but are unsure what a good complete setup would look like, take the time to look at the pictures I posted thoughout the thread and drop me a line (or join that thread) describing what you want to achieve.

Or if you aren't in a hurry you can track / join this thread
http://www.diyaudio.com/forums/showt...082#post759082
where a new (and more powerful) amplifier is being developed, again with the contributions of other members.


Cheers
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Old 15th January 2006, 08:35 PM   #7
Darksog is offline Darksog  Finland
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hi quasi
i think i use IRFP460 N-FET 500V 20A 280W
and add 2 fets more then it will be 8 fets in amp and 2 ohm load stabil? 75/2/4=9.375A per fet =)
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Old 16th January 2006, 09:45 AM   #8
quasi is offline quasi  Australia
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Quote:
Originally posted by Darksog
hi quasi
i think i use IRFP460 N-FET 500V 20A 280W
and add 2 fets more then it will be 8 fets in amp and 2 ohm load stabil? 75/2/4=9.375A per fet =)

Hi Darksog,

The IRFP460 can be used in the amp, and 8 FETs might be enough but only just. Let me explain;

Whilst a speaker load is rated at say "2 ohms" impedance it's actual impedance could dip under 1 ohm. This is because speaker loads are not resistive but reactive and in reactive loads the current is not in phase with the voltage. This causes real problems (and device failures).

The graph attached shows the possible (worst case) current draw for 8, 4 and 2 ohm speaker loads with 75 volt amp rails, as well as the SOAR of 4 IRFP460s at DC and at 10mS non-repetitive pulse. The graph calculations assume a heatsink temperature of 50 degC.

As you can see a "2 ohm load" can draw 44 amps peak or 11 amps per FET. The DC SOAR curve (blue) is well inside this curve and the 10mS SOAR curve is just outside.

Without going into too much detail this chart tells me that for 2 ohm loads normal music signals that drive the amp into full power only intermittently, 4 x IRFP460s per rail will survive. Extended low frequency signals at continuous full power will destroy the output stage.

Remember also (a lot I know) that the FETs will not share the current equally. So while the average peak current is 11 amps per FET, a few may be passing more.

For me there is not enough room for error with 8 FETs so if I were you, I would use 10 FETs per board as a minimum if the amp was to be driven into 2 ohms.

The IRFP460 also has a much higher gate capacitance (4.2 nF). This means that the driver transistors will have to work harder to cope with the extra charge & discharge current. Please make sure that the driver transistors heatsinking is adequate.

Cheers
Attached Images
File Type: jpg soar 4 x irfp460.jpg (68.3 KB, 679 views)
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Old 16th January 2006, 10:47 AM   #9
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Hi Quasi,
For 2 ohms stable operation into reactive or mismatched Loads for sustained periods with low frequency content.. upto16 IRFP460 FETs or 8 Pairs of FETs are required.....when driven from +- 100Vrails....

For +- 75V rails upto 6 pairs or 12 FETs are required..for safe 2 ohm operation..

Gate capacitance doesnot plays an important role in gate drive current requirements...The Foremost important parameter is Total Gate Charge and highest frequency of operation...

Calculate Ig from...this equation below...
Gate Drive Current per FET = Total Gate Charge Qg in nC X Highest Frequency of operation....in Hz

gate resistors must be lowered in value and current drain through gate to source bleeder resistor should be at least 5 times of total gate current of FETs


K a n w a r
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Old 16th January 2006, 11:25 AM   #10
quasi is offline quasi  Australia
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Thanks Kanwar,

So there you go. With Kanwar's experience in demanding PA applications his view is 12. I say 10 he says 12, in any case you get the picture.

But why the discrepancy??

It gets down to the amplifiers application and cost. If I was installing amplifiers into a concert venue, where the conditions are demanding and loud then Kanwar’s view is valid.

In a home theatre or hi-fi setup where full volume is out of place but the dynamic range is still needed then 8 might be enough.

The power supply capability holds some of the answer. A power supply sized for home use will collapse sufficiently to offer some self-protection. A power supply in a PA amp is usually stronger and likely to hold voltage better, good for the sound but bad for the FETs. In my calculations I factor in a modest 5% power supply droop. Most power supplies (even good ones) would droop more at full power.

As a perspective, most domestic amplifiers I've seen to date would fail the SOAR calculations I have used here.


Cheers
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