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Old 17th June 2003, 12:10 AM   #1
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Question 400 watt Holton amp upgrade

Ok, just a quick question. I'm building the 400 watt symmetrical amp and I'm wondering what the easiest way is to get the amp to be 2 ohm stable....or at least close. I have a 1kva toroid PER BOARD and lots of caps etc. which will give me about 75vdc on the rails. I hope. Anyway, a while ago Anthony said the amp can't handle very many more output transistor pairs...board has space for 4 pairs...I was thinking of doing 6 pairs (is this ok?). Also, which driver's do I need to heatsink? would this help drive more outputs if I heatsinked the Q8and Q9 drivers?

ok, to summarize

1) is 6 pairs of outputs too much? too little? (more is better :-D)
2) would heatsinking OR substituting some driver transistors help drive more output pairs
3) if I can sub a driver...(I don't mind using more expensive parts...like other output mosfets) what should I use?

thanks ya'll!

-Matthew K. Olson


www.aussieamplifiers.com/sym-sch.htm (I think thats the right link)


oh, and one other thing, building the av800 is a great idea, but I already spent $40 per board on these 400 watt boards...so I'd like to work with what I have for now...thanks guys
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Old 17th June 2003, 03:04 AM   #2
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I built an amplifier similar to that design
a decade ago except it had;

*65V rails
*same transistors (can type), 8 per rail
*Four 500VA transformers
* fans

1/4" thick alumimum base plate
for transistors, then I mounted
heatsinks on the base plate.

I used two 110 VAC inputs
and did test tone load tests
at 0.5 ohms for quite a while
with no problems.

*****************************

more food for thought.

An amplifier typically can handle
less "ohms" when not doing
"tone testing" and just playing
music.

An amplifier can typically handle
less "ohms" if you limit the bandwidth,
such as bi-amp or tri-amp. Case in point, my Adcom 555II bridged
is rated for 600w @ 8 ohms, I
can operate a 4 ohm woofer but
it gets hot -- need fans..
But, I can drive eight parallel 8 ohm
tweeters for a 1 ohm load with no
problems... tweeter don't draw much.

I think you can easily drive 2 ohms
with that desgn if you have "can type"
transistors on massive heatsinks
playing music.. If you "tone test" the
amplifier, it will run hotter.

The orginal design used 4 transistor
per rail, I double the outputs with
no circuit mods.. it seems to work
well... I bet you can add a couple
more transistors per rail with no
big side effects.
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Old 17th June 2003, 03:07 AM   #3
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Well...I'm using the plastic irfp240 and 9240 transistors...BUT...I have quite bit heatsinks. If I use 6 pairs...the 240's will be on one sink and the 9240's will be on the other. Each sink will dissipate 75 watts of heat easily. And, since this is monoblock design, thats 300 watts of heat total w/ the amps just coasting no fan...but I can put a fan if need be. Think that'll be enough for 2 ohms music? Should I sink the drivers? would that help drive more outputs?

-Matthew K. Olson
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Old 17th June 2003, 07:29 AM   #4
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I say you should try out your idea
and see how it goes..... I would
heatsink the drivers, the amp
I did, the drivers got hotter than
normal - hehe I mounted those
on the main heatsinks, works good.
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Old 17th June 2003, 07:54 AM   #5
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Remember to carefully match outut devices, otherwise one or two will do the most of the job and will probably fail if driving low impedance loads.

Cheers

Andrea
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Old 17th June 2003, 08:30 AM   #6
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- and it will sound bloody awful........

Hugh
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Old 17th June 2003, 12:24 PM   #7
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What will sound aweful? If I don't match the outputs? Or I don't sink the drivers? I was planning on matching the outputs anyway...so no big deal there. Sinking the drivers, thats a more hairy animal but I can do it.... Ok, so basically what everyone is telling me is just go for it and find out. k. later

-Matthew K. Olson
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Old 17th June 2003, 12:42 PM   #8
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Quote:
Originally posted by Andypairo
Remember to carefully match outut devices, otherwise one or two will do the most of the job and will probably fail if driving low impedance loads.

Cheers

Andrea
Won't the source resistors be sufficient to balance the load.
That's most certainly the reason (or one of several) why
they are there already and if works for four devices it should
work equally well for any number of devices, I think.
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Old 17th June 2003, 02:25 PM   #9
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Quote:
Originally posted by Christer


Won't the source resistors be sufficient to balance the load.
That's most certainly the reason (or one of several) why
they are there already and if works for four devices it should
work equally well for any number of devices, I think.

I tried it myself yesterday, with 0.22R source resistors you have only a slight balancing effect. Even 50mV between Vgs of two mosfets brings to a dramatic difference in bias current.

I had accidentally blown a mos and tried to substitute it without matching it (I thought it wasn't worthwhile to desolder the others to measure them..): it carried almost twice the current compared to the others. So I took it out and it gave a reading of 3.78V @ 150mA.
Then I measured the others taking them out of the circuit and they were between 3.83 and 3.84.

So I kept on measuring ( I have about 35 pairs of them from the same batch) and I got Vgs values between 3.40 and 3.61V for the IRFP240s and between 3.65 and 3.95 for the 9240s.

With such Vgs voltage spread and a bias current of about 100mA per pair the degeneration voltage ( 0.1 * 0.22 = 22 mV) over the resistors can do little if you don't match the devices within a 10 mV range.

Cheers

Andrea
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Old 17th June 2003, 02:31 PM   #10
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Quote:
Originally posted by Mattyo5
What will sound aweful? If I don't match the outputs? Or I don't sink the drivers? I was planning on matching the outputs anyway...so no big deal there. Sinking the drivers, thats a more hairy animal but I can do it.... Ok, so basically what everyone is telling me is just go for it and find out. k. later

-Matthew K. Olson
If you use the maximum admitted voltage you probably will have to put a small sink on the drivers, I run the amp at about 55V per rail so I don't use them. If you have doubts just put your finger on it: if it starts burning you definitely need a heatsink

Cheers

Andrea
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