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Old 5th December 2012, 10:27 PM   #51
GregH2 is offline GregH2  Australia
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BTW, R5 is a fixed value resistor and will need to be determined empirically. I once tested four devices and made a chart which should provide a guide (see attachment).

The Vgs multiplier is used for biasing because 40mA is probably too much for a trimmer, though you could use one if you like. Probably worth a go actually, just be sure to put a zener across it in case it goes open circuit. If you go with the Vgs multiplier it needs no heatsink.

The input also needs zener protection and a DC offset voltage reference. I would make this with a zener and a trimmer - like fetzilla but with the LED replaced by a zener.

I will try to get a final circuit posted this weekend.
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Old 8th December 2012, 10:41 PM   #52
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Hi GregH2,

please excuse my lack of electronics Knowledge, is the feedback via R2/R1 and via C1? how does this effect phase?

Please excuse me for my lack of knowledge in the 1st place. Im also Looking at something that goes down stable at 1.8ohms and I am hoping that this will do the trick. I currently have an old 250w Hitachi base jfet/mosfet amp that is deployed but looking at different approach that can give me better microdymanics and better separation of instruments
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Old 10th December 2012, 01:03 AM   #53
AKSA is offline AKSA  Australia
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Greg,

Sorry for late reply; a bit slow off the update.....

Yes, use a Baxandall diode, along with a 100R and a 10nF cap, all in parallel, supporting the emitter of the phase transistor. This works as well as anything and some amps that sound wonderful use this approach, as in deed JLH used then for years too. You cannot expect that clip is identical however, so don't expect perfection at the highest levels.

Cheers,

Hugh
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Old 10th December 2012, 09:48 PM   #54
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Great thread - I'll be following this with interest. I really appreciate the knowledge here, I've been lucky enough to have recently built three amps, an LM3875 chip amp with regulated 28V rails and a 6DJ8 based tube buffer, a Hypex NC400 Ncore set of monos, and the highlight of course was the Fetzilla. I would be hard pressed to sell the Fetzilla, it is such a good amp. The thing I like about it most is the lack of listening fatigue, it's got all the attributes I like, punchy bass, detail etc, but the gem is in the relaxed presentation. When comparing it against the Ncore for example, the Ncores can sound more dynamic, more detail, however ultimately the Fetzilla always snuck back and replaced them after a while. I really thought 50w (I used 120va) would not be enough to drive my 87db 8ohm speakers, especially as the trafo's were a little smaller than I would have liked and always intended to replace them with 160's at some point, but the specs don't always tell the whole story, the Fetzilla seemed to punch above its weight, that's how I would characterize it. I never tried driving it to party levels though.
I'm not smart enough to contribute along with others here on achieving higher power output, but all the same I will be interested in how it comes together and how the sound will compare with the original.
AJ
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Old 11th December 2012, 05:55 AM   #55
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Quote:
Originally Posted by ajst2duk View Post
Great thread - I'll be following this with interest. I really appreciate the knowledge here, I've been lucky enough to have recently built three amps, an LM3875 chip amp with regulated 28V rails and a 6DJ8 based tube buffer, a Hypex NC400 Ncore set of monos, and the highlight of course was the Fetzilla. I would be hard pressed to sell the Fetzilla, it is such a good amp. The thing I like about it most is the lack of listening fatigue, it's got all the attributes I like, punchy bass, detail etc, but the gem is in the relaxed presentation. When comparing it against the Ncore for example, the Ncores can sound more dynamic, more detail, however ultimately the Fetzilla always snuck back and replaced them after a while. I really thought 50w (I used 120va) would not be enough to drive my 87db 8ohm speakers, especially as the trafo's were a little smaller than I would have liked and always intended to replace them with 160's at some point, but the specs don't always tell the whole story, the Fetzilla seemed to punch above its weight, that's how I would characterize it. I never tried driving it to party levels though.
I'm not smart enough to contribute along with others here on achieving higher power output, but all the same I will be interested in how it comes together and how the sound will compare with the original.
AJ
AJ,
Nice review,anyone that's heard the FetZilla says the same.This is realy a good sounding amp people must build it they will not be disappointed .
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Old 11th December 2012, 07:11 AM   #56
GregH2 is offline GregH2  Australia
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Oops,

I didn't see that there had been a reply.

Music lover, yes, you have the feedback concept correct. As to your questions about phase I will leave that for someone who knows more than me to answer.

Hugh, thanks for your response. Can I ask what the 10nF cap is for? I haven't seen it used anywhere.

Meanman, try this:
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Old 11th December 2012, 06:12 PM   #57
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Greg,
Is this design covering the whole frequency spectrum?
Looks very nice by the way
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Old 12th December 2012, 10:23 AM   #58
GregH2 is offline GregH2  Australia
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Quote:
Originally Posted by meanman1964 View Post
Greg,
Is this design covering the whole frequency spectrum?
Looks very nice by the way
Thanks,

Yes it should cover every frequency you are capable of hearing....

Do you think you'll give it a go?
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Old 12th December 2012, 11:31 AM   #59
AKSA is offline AKSA  Australia
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Nice GodZilla design, Greg! My compliments! With lower rails, say 42V each, it would be very suitable for ML's 1.8R speakers, kapitz?

A 10nF, in parallel with 100R and a diode, is used in the Baxandall circuit between the output and the phase inverted driver emitter on a quasi amp.

Cheers,

Hugh
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Old 12th December 2012, 05:53 PM   #60
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Originally Posted by GregH2 View Post
Thanks,

Yes it should cover every frequency you are capable of hearing....

Do you think you'll give it a go?
Greg,
Yes I'll give it a go,a friend of mine is planning to make the layout.
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