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Old 21st November 2006, 10:28 PM   #1
Fuling is offline Fuling  Sweden
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Default Cascoded ZV7

Good evening!

After years of junkyard digging Ive finally gathered all the stuff I need to build the world weirdest amp, the dreaded Son of Zen!!
The X:ed, current sourced version to be more precise.
To make a long story short:
I have two kinds of mosfets in stock to choose between; IRFP260N and 75645P. Both are high power, high current devices with seriously high Ciss (around 4000pF).
My question is: Would it be a good idea to cascode the whole thing and use smaller mosfets (IRF530 or something) in the lower position?
Im a bit paranoid when it comes to input capacitance and Im really fond of the idea of cascoding even though Id loose some output swing.

I should also mention that this will be a downsized version, there is no way my heatsinks can dissipate 375W or whatever the original circuit runs at.
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Old 21st November 2006, 10:29 PM   #2
Fuling is offline Fuling  Sweden
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This heatsink (1 of 2) is for the resistors only, the Fets will get separate heatsinks:
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Old 23rd November 2006, 11:57 PM   #3
Fuling is offline Fuling  Sweden
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Well, since nobody had an opinion I ditched the cascode idea and ordered some IRF640s to build the amp as drawn in Nelsons article. IRF640 seems lite a good compromise between capacitance and power handling.

Ill use 470R/4,7k inte the feedback "loop" and 120uF low impedance input caps bypassed with 1,5uF polyprops.
The PSU consists of 2 x 66 500uF and 1 x 1,1mH 10A choke per channel, the transformer will be either 2x18V 600VA or 2x24V 750VA depending on how much the heatsinks can handle, this will be subject for some experimentation later on.
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Old 24th November 2006, 04:35 AM   #4
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darn it fuling,

I should have caught your post earlier. Sorry. I'm very interested in your cascode ZV7. Can you post a schematic?

I like the idea of using small fet's in order to reduce distortion. Less wattage is no problem with midrange/hi frequency horns.

If you are still feeling inclined, I'd love to see what you were up to.

John
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Old 24th November 2006, 09:11 AM   #5
Fuling is offline Fuling  Sweden
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I can post a schematic later today.
The reason I gave up on the cascodes had more to do with lack of space for the extra heatsinks than lack of replies.
Its not impossible that Ill change the circuitry later but Ill follow the basic conscept to begin with.

My future BZLS will definitely be cascoded though!
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Old 24th November 2006, 02:30 PM   #6
Magura is offline Magura  Denmark
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How about throwing in some of those power jfets for cascoding?
They don't add that much to the heat issue or the space issue for that kind of matter.

Magura
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Old 24th November 2006, 02:47 PM   #7
Fuling is offline Fuling  Sweden
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Power Jfets would be sweet but I dont have any.

As I mentioned Ill finish this amp as in the ZV7 article at first, later on I might experiment with cascoding.
It pretty much depends on how much room there will be left inside the chassis, how much work it will involve to tear it apart to make the changes and how much power the heatsinks can handle.
To make the cascodes work I would have to raise the rail voltage = more heat to deal with.

With the transformers I have at hand I can choose between 24 and 32V rails, the lower figure will probably be ok without cascodes and the higher would lead to a significantly higher temperature... Im expecting about 4A total bias per channel, so only a few extra volts means big changes in dissipated power.

Though I still find the idea of cascoding very attractive and if I dont do it in this amp Ill definitely try it out in the future in a more efficient circuit.
I have a bunch of 112mH 2A iron core chokes waiting to be put to use in some inductor loaded amp soon...
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Old 24th November 2006, 03:55 PM   #8
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I switched to air-core chokes in the ZV7 and think they sound better than iron. My 28.5 volt supply dropped to 23 volts after the chokes. As a result, the amp runs pretty cool, though the chokes are rather warm. The sound is stunning.

btw: I cross the amp at 500hz, and have downsized the choke in order to take advantage of the limited frequency range. This saves money and offers less coil resistance.

According to Nelson's distortion graph, the choke loaded ZV7 only has 0.15 percent harmonic distortion at three watts output. This makes for one fine sounding amplifier!

John
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Old 24th November 2006, 05:03 PM   #9
Fuling is offline Fuling  Sweden
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Ive been looking at the ZV7-T for a while but I dont think my chokes are suitable for balanced operation, centertapped ones would be better.
I have another circuit including one P-channel Mosfet and one N-channel Jfet that has been tested with my chokes and it worked very good, maybe Ill build a 4x12W SE amp this way for my biamped TB speakers. Just for fun as a side project.

Well well, all I can do right now is waiting for the components to the Xsoz to arrive.
Meanwhile, I can always study some preamp circuits to find something to match the beast
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Old 30th November 2006, 07:41 PM   #10
Fuling is offline Fuling  Sweden
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The amp is now almost complete, all thats left to assemble is transformer, rectifiers and front & top plates.
Im currently running some tests with 24V bench PSUs and everything seems fine, the heatsink for the mosfets gets quite hot but not dangerously so. The main heatsinks only gets comfortably warm, which is good since the PSU caps are mounted very close to the power resistors.
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