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Old 12th May 2008, 10:44 PM   #11
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Update on output/driver configuration................

First of all, thanks for the replies! It helped me rethink and replan the setup, and now it's even better.

I re-cut some new Aluminum clamps, and moved the transistors around so they all fit on the heatsink now. I decided to use ThermalTrak transistors for Drivers anyway, I just used the lower power 180W ones so I had all my 200W ones for other projects. This saves me from mounting temp co transistors and worrying about driver and output temp. I plan to use oversized VAS stage transistors so they don't get hot at 50mA or so, and have plenty of drive.


Transistors for two BTL sub amps:
12 Outputs: 6 MJL4281 6 MJL4302
4 Drivers 2 NJL0281 2 NJL0302
2 VAS MJW0302
2 CCS MJW0281
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Old 12th May 2008, 11:57 PM   #12
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Default PICS!

Here's the Subwoofer Amplifier Sink and outputs.
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Old 12th May 2008, 11:59 PM   #13
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Here's the Main 4 Channel Amplifier ThermalTrak 4281/4302 outputs on the same type heatsink.
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Old 13th May 2008, 12:03 AM   #14
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This shows the thick base of the heatsink, and the large dense fins.
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Old 13th May 2008, 12:05 AM   #15
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When the amp is finished, they will sit side-by-side on the back to look like one really big heatsink.
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Old 13th May 2008, 09:01 AM   #16
gain is offline gain  United States
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Quote:
Originally posted by AndrewT
OK, you've satisfied yourself.

But, I disagree.
can you tell us where the error(s) are in EWorkshop's analysis? i think it would be beneficial if we could learn the proper way to figure the current requirements of the VAS and drivers for the reactive loads when designing amps.
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Old 14th May 2008, 11:19 PM   #17
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Think of a resistor in series with a capacitor - essentially what the speaker looks like above resonance. Now let the cap charge to positive rail. The amp then swings full negative. This gives a peak current that is V(clip)*2/R(e). It might even be worse due to resonance buildup. Then again, this is a very strange signal too. It's probably not very common for this to happen.
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Old 15th May 2008, 12:09 AM   #18
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Quote:
Originally posted by megajocke
Think of a resistor in series with a capacitor - essentially what the speaker looks like above resonance. Now let the cap charge to positive rail. The amp then swings full negative. This gives a peak current that is V(clip)*2/R(e). It might even be worse due to resonance buildup. Then again, this is a very strange signal too. It's probably not very common for this to happen.
Hoping that these are some of the best BJT around, I surely would hope they can take extreme loads. I'll clamp them with diodes like in my other amp so any high voltage on the output drains into the rails.

Have any of you here tryed the Thermaltraks as drivers for regular BJT before?

I've looked around, and I haven't seen any here that do that, so this may be a first. It just makes it so much easier to hook up than a seperate transistor vbe multiplier.

peace
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Old 15th May 2008, 04:13 AM   #19
wg_ski is offline wg_ski  United States
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Quote:
Originally posted by megajocke
Think of a resistor in series with a capacitor - essentially what the speaker looks like above resonance. Now let the cap charge to positive rail. The amp then swings full negative. This gives a peak current that is V(clip)*2/R(e). It might even be worse due to resonance buildup. Then again, this is a very strange signal too. It's probably not very common for this to happen.
Driving either a pure capacitor or pure inductor puts full rail at peak current on the transistor. But loudspeakers aren't that bad a load - at the impedance minimum, it's always pure real. When the impedance goes reactive, it's ALWAYS above the minimum. Resonance buildup is *high* impedance so you don't have to worry much about that.
Design for an ohm and a half resistive and maybe 3 ohms full reactive within whatever comfort zone on SOA you're going to follow (DC, 100ms, or whatever).
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Old 15th May 2008, 08:12 PM   #20
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I were not talking about pure capacitors or inductors. You are right about the dissipation.

It's just the peak currents that might be that bad, what I'm talking about has no significant effect on transistor dissipation. It's resonant buildup and then a transient in exactly the right (wrong?) place that will demand huge peak currents. It might not be too important, loudspeaker distortion will probably mask any clipping due to current starvation on the transient, and it will also be a very rare occurence.
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