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Old 30th March 2003, 01:34 AM   #1
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Join Date: Feb 2001
Location: Mars
Default IRFP240 Questions

Digging thru the archives revealed
that the Holton AV800, 1KW version
uses +110v/-110v rails, but under load,
the realistic values were +96v/-96v,
I think I read.

The VDSS rating is 200v. How accurate
do you think they ratings are ?
can we go let say 10% higher than
rated ? The reason I ask is... I
don't know if transistor manufactuers
under-engineer the parts slightly?

Randy Slone book of amplifier
construction said that a safety
factor of 2x would be good on
chosing transistors, ie, an
amplifier with +40v/-40v rails
would reguire a 160v transistor
(40v + 40v ) * 2 = 160v

Does anyone fear running the IRFP240
near 200v with +100v/-100v rails ?
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Old 30th March 2003, 02:01 AM   #2
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Location: Queensland, Australia
It's generally not a good idea and Vdss must also be derated for temperature and then you must work out max safe Idss from the SOA charts and derate that allowing for the reactance of a loudspeaker + x/over. I wouldn't build this design without making some changes.
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Old 30th March 2003, 03:58 AM   #3
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My latest thinking is....

1. PCB layout 4" x 10" to accomodate
14 IRFP240 and 14 IRFP9240 per
channel, double what the AV800
design uses. Then, perhaps using
only +90v/-90v rails and not
+110v/-110v as shown in the
Holton 1kw amp (modified AV800).

2. Referencing this schematic, bottom of page;
http://www.aussieamplifiers.com/downloads/AV800.pdf
Someone suggested to reduce R28,
330 ohms, to perhaps increase the
drive capability of the VAS to
accomodate the extra paralleled
output stage transistors, but I'm
not an amplifier designer so I don't
know what needs to be done on
this particular design.


The power transistors array will
solder on the edge of the board,
14 across a 10" span of PCB,
one rail per side.

silly pic;

t = transistor

tttttttttttttt <-- IRFP240
|----------|
|---pcb---|
|----------|
tttttttttttttt <-- IRFP9240

This allows me to put 2 modules
on the one slab of heatsink
which will make the amplifier chassis
about 22" depth, perhaps 8" min.
height, and 17" width, a 4 channel
configuration, with option to bridge
to make it a dual mono.


What do you think ?

I like monster amps

If this works, half of my brain
wants to layout another PCB
that that is 4" x 16" to accomdate
40 transistors per channel, perhaps
to make it 1 ohm stable using
much lower rails.

Need help from the amp gurus
here just to get me in the right
direction.....

Right now I have access to professional PCB sofware and
I want to take advance of my situation and do some free layout......

Gonna build the AV800 prototype
next month, all my surplus electronic
stores here from my childhood has
gone under so I need to get parts
online from somewhere.
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Old 30th March 2003, 05:01 AM   #4
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Default Re: IRFP240 Questions

Quote:
Originally posted by thylantyr
The VDSS rating is 200v. How accurate
do you think they ratings are ?
can we go let say 10% higher than
rated ?

Does anyone fear running the IRFP240
near 200v with +100v/-100v rails ?
The X1000 runs the IRF240 and 9240's at 150V peak, and have never seen a failure due to voltage.

I suggest that if you want to get to or exceed 200V you individually check each part for leakage. Fortunately in this regard Mosfets are far superior to bipolars, so you can probably use room temperature measurements.

Do we fear this? No, we are fearless amplifier builders: The best kind.

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Old 30th March 2003, 05:12 AM   #5
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The X1000 runs the IRF240 and 9240's at 150V peak, and have never seen a failure due to voltage.


Sweet!

Now the squirel cage in my head
is turning

Is Randy Slone correct in his
assessment that VDSS correlates
to BOTH rails or do you only consider
the single rail when trying to figure
out SOA of VDSS

Last question, should the source
resistor be increased in value
if you raise rails to Uberman
high voltage levels ?

ie, from lets say 0.22 ohms to
some higher value ?
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Old 30th March 2003, 10:06 AM   #6
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Both rails because when the output swings all the way to one rail, the device sees almost the full rail to rail voltage across it. Nelson states a peak of 150V across the device not 150V rail.
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Old 30th March 2003, 02:21 PM   #7
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Ok...thylanter...heres what I was told by Anthony. The av800/1kw amp can handle 10 device pairs (20 devices) per channel. He didn't mention 40. wow. Now, 40 sounds good to me, but I don't know how to do that. Once again, I think you should make the driver and output boards seperate. For instance, if you wanted to do monoblock amps w/ that design, and wanted to put the irfp240's on one heatsink and the 9240's on the other side, you'd need to make seperate output boards. Besides, I think it'd be cheaper...maybe..and monoblocks look cool. Again, if you can make more than one pair of boards, let me know and I'm in :-D email me at mattyo5@aol.com

Later!

-Matthew K. Olson
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Old 30th March 2003, 06:20 PM   #8
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Quote:
Originally posted by Mattyo5
Ok...thylanter...heres what I was told by Anthony. The av800/1kw amp can handle 10 device pairs (20 devices) per channel. He didn't mention 40. wow. Now, 40 sounds good to me, but I don't know how to do that. Once again, I think you should make the driver and output boards seperate. For instance, if you wanted to do monoblock amps w/ that design, and wanted to put the irfp240's on one heatsink and the 9240's on the other side, you'd need to make seperate output boards. Besides, I think it'd be cheaper...maybe..and monoblocks look cool. Again, if you can make more than one pair of boards, let me know and I'm in :-D email me at mattyo5@aol.com

Later!

-Matthew K. Olson

My definition of mono block is
one PCB per side, channel 1
PCB left side heatsink, channel
2 PCB right side heatsink,
then bridge the two modules
for uberman power.
I think I read somewhere in the
archives, ? Holton? suggested
+70v/-70v rails for bridged.

Why settle for 90v swing single channel mono block when you can
get 140v swing with two channels,
more punishment for the speaker

... think bigger .....

more fireworks when something
goes wrong

It's all good, the transistors
cost $1.25 for n type. $1.75 for p type. Cheap firecrackers.......
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Old 30th March 2003, 06:25 PM   #9
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Ok...so you are gonna use 4 "channels" then....2 channels bridged for 1 channel. I see. Ok, so use 10 pairs of devices per board, total 40 devices, bridge it...use lower voltage...maybe even +-50v ...shoot...I'm sure that'd be killer power and it'd work into 2 ohms then! (hopefully :-D ) that'd be like...600 watts into 8 ohms. you'd need a nice toroid to do that...sweet. Let everyone know when you finalize boards. Maybe others would be interested ...especially b/c Anthony is out of stock

-Matthew K. Olson
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Old 30th March 2003, 07:19 PM   #10
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Quote:
Originally posted by Mattyo5
Ok...so you are gonna use 4 &quot;channels&quot; then....2 channels bridged for 1 channel. I see. Ok, so use 10 pairs of devices per board, total 40 devices, bridge it...use lower voltage...maybe even +-50v ...shoot...I'm sure that'd be killer power and it'd work into 2 ohms then! (hopefully :-D ) that'd be like...600 watts into 8 ohms. you'd need a nice toroid to do that...sweet. Let everyone know when you finalize boards. Maybe others would be interested ...especially b/c Anthony is out of stock

-Matthew K. Olson

Actually the plan is to <try> to
make one pcb with 28 output transistors per channel, 14 per rail, 2x more than AV800, 4 more transistors
per rail than AV-1KW.

Later, design another PCB which is
the same but longer, to allow 20
output transistor per rail, this would
be an experimental PCB... the
fireworks board - hehe

I want a modular channel design
to minimize the wiring rats nets
and keep all the parts close together..

The pcb will most likely mount to
a metal interface plate, either aluminum or copper.. The output
transistors would be directly mounted
on the interface plate with no
electrical isolator on the tab to
get best heat transfer from device
to hunk'o metal..... Then electrically
isolate/thermal grease the interface
plate to desired heatsink. So, both
pcb and interface plate will have
random mounting holes to make
mating easier to any heatsink.

So.... if you have a modular design,
then insert as many modules
as you want in any particular
chassis. If you want a 8 channel
amp, line up 4 modules per side,
that would be a chassis size of
17"W x 14"H x 22"D. This is
worse case scenario, I wouldn't
do this, I'd rather have 4 modules
in a small chassis, but you can
do it......

Realistically, the 4 channel chassis
would be the same size except the
height would be 7" - 8", more manageable.

Also, you don't have to solder
all 28 transistors to the pcb
for lower powered applications.
Suppose you want to build a 2 channel amplifier for the tweeters
only, well, you can populate just
a few transistors ... spread out ..
on the pcb and get a chassis with
12" depth.

Since I like fully active stereo systems
where each speaker gets it's own
amplifier channel, the modular design
allows you to build each amplifier
slightly different.

If you want to use the new high
xmax monster woofers that have
1 ohm - 2 ohm voice coils, then
build the bridged mono with all
transistors, scale down rails
and place both modules in a larger
chassis with more heatsink due to
1 - 2 ohm load
heheheh.......

The problem is.. I need to build
the AV800 next month to get
familiar with the design, I'm seeking
parts right now to make two, I
have two Holton designed AV800 pcb's...... won't fit 28 output transistors, but maybe I can build
an interface PCB later after I get
the generic one operational.. One
step at a time..

I have a bench test PS with +75v/-75v rails *and* +150v/-150v
rails...

My email is lordpk@pacbell.net
if you want to keep in touch
since you will have some experience
with AV800.. perhaps you can find
some kewl mods.........

The Randy Slone book has
some 'generic' protection circuits,
plus I have other schematics....

I've read archives here and some
people with rail fuses have had
big problems, one rail fuse blew,
the other did not, this caused big
DC offset --> blew up speakers...
For such a crazy project, I think I
need a rock solid protection circuit
designed into the PCB with beefy
output relay.. Speakers are very
expensive, would not be sweet
to have them go up in smoke
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