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Old 29th November 2006, 10:14 PM   #1
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Default Help ZEN's cousin to come to life!

Hi guys,
I am currently building PLH and mini Aleph and lack of the final fets that my supplier will send me soon, so I jumped on something else to have my time occupied by something cool...so I jumped on Nelsons schematics for the ZV8 but had no J fets here, so I took a N channel mosfet I had lying here and a IRFP 240 and built this(picture) yes it works ...sound is not bat at all, but on sinewaves seems that the upper part of the sinewawe is cut straight, would anyone be able to have a look at this schematic ant tell me what I did wrong? i'd really like to fix it as the amp sounds really good til 5watts then it starts ti clip on the upper side of the sinewave..
Vince
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Old 29th November 2006, 10:40 PM   #2
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I'm no genius, but it seems to me that you're cascode fet should be idling at half your supply voltage, not 24 volts.

I'd replace the 22k fixed resistor with a pot for adjustment purposes.

Sorry if I'm wrong...

John
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Old 29th November 2006, 10:47 PM   #3
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Quote:
Originally posted by carpenter
....................

Sorry if I'm wrong...

John

for my toob brain,you aren't
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Old 29th November 2006, 10:51 PM   #4
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I was finally able to offer advise, hurray!

John

Thanks for the confirmation, Choky.
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Old 29th November 2006, 10:54 PM   #5
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Hi Guys!
thanks for comments, I actually tried to trim the 22k (that is varianle resistor) and managed to get 9V at the gate of the IRFP 240 ...
but this adgusment does not help to solve tho whole problem...
any other idea? why i can't get the right rail value?
Thanks mates!
Vince
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Old 29th November 2006, 11:06 PM   #6
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Quote:
Originally posted by bobsinclar
Hi Guys!
thanks for comments, I actually tried to trim the 22k (that is varianle resistor) and managed to get 9V at the gate of the IRFP 240 ...
but this adgusment does not help to solve tho whole problem...
any other idea? why i can't get the right rail value?
Thanks mates!
Vince
you need to readjust biasing (consequently-Iq,too) and Rload (in this case 14E) to achieve symmetrical clipping;

when you achieve that ,with output node at ~ Ub/2 ,that's it .
in other words- tweak divider for cascode biasing,Rload and even 0R47 to ground
I can't tell you more without looking at precise graphs.......my toob brain works just in that way
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Old 29th November 2006, 11:23 PM   #7
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HI, I love the looks of that thing because it's simple, and I have seen that such circuits can work pretty well.



The bias on the lower fet : looks like a 300K resistor from drain to gate, and no resistance from gate to ground - meaning, I think, that the driving source has a big fat gate-to-source capacitance to deal with - which is not so good.

I believe that if you put a voltage divider pot on the lower fet, of a value between 10K and 25K, you can tweak it to maximum output.

You have a 14 ohm resistor dropping 13 volts, so the Idle current
would be 926 mA. For 5 watts into 8 ohms(?), we need 18 volts pk-pk, 9 volts pk, so 9/8 = 1.12 Amps Idle current minimum is needed. So there must be some modulation of the current source going on there, I think. I am not familiar with the operation of that current source you are using. I have a very simple one I like, but it can't be modulated. It uses the gate-to-source voltage to regulate itself. In my experiments, I seem to recall it provided better regulation than the Aleph current source, using dummy loads (although whether that is so important, I don't know).

If you use resistors in the value around or below 10K on the lower mosfet, and make it a standard voltage-divider bias, I think you will have more success.
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Old 29th November 2006, 11:44 PM   #8
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Hey thanks guys!
here is the improvment:
carpenter and zen higlighted the fact that the supply value should be divided by 2 at the drain of the 240 by trimming it's gate resistor, actually I just had to use another value of resistor and we got it the amp clips symetrically, but only when trimming gives me 16v at the 240 drain when I have 18v at the supply (I don't know why, but when I put half the supply at his drain clipping isn't symmetric...)
Now the amp sounds wonderful very punchy bass and sweet meds...
peak to peak I get about 12v that is to say (if i am correct 6Vrms) on a dummy 8ohm load
I'll try to work on the cascode small fet now just like JCM proposed to...
but this seem a very good start , maybe i'll had to apply some feedback? any comment is welcome!
Vince
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Old 29th November 2006, 11:50 PM   #9
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Oh just forgot,
what could I do to get more power out of it without replacing the supply? I'd like to get 10wattsrms is that possible?actually I didn't choose to clip at about 4 watts it's just the way it ended but i'd prefer 10!
Vince
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Old 29th November 2006, 11:51 PM   #10
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Quote:
Originally posted by bobsinclar
...................
but this seem a very good start , maybe i'll had to apply some feedback? any comment is welcome!
Vince
agree with everything ,especially with glorifying Carpenter and me ........

feedback-you already have that ,but if you mean on regular loop job,than is time for standard resistor in line with lower transistor gate and one from ldsp node to gate........in this case you have cap induced mumbojumbo included in feedback
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