FETZILLA - The Builders Place
Fetzilla started when I got this idea of a simple Power Amplifier.
AKSA Hugh thought it was intesting and started to help us with the circuit.
Since then quite a number of people have joined in and started to build. I can even see group buy arranged.
To give you an idea what amplifier I think is Fetzilla, I will post a diagram.
- Input stage is a single transistor. In my case one JFET.
- VAS stage is often one MOSFET and bootstrap is used.
- The output is standard Push-Pull. MOSFETs can be used. But not necessarily.
I hope you guys who tweak your Fetzilla will join in here.
Here we can discuss the practical thing about this nice amplifier.
What's distortion like on this sort of topology?
At 13W//8R 1KHz, we have -72dB for H2, and 2-4dB reductions with rising harmonics.
It was not intended for it, but it is possible.
The biggest change would be in the output stage.
Say 2 pairs of IRFP240/9240.
What I want to know is what version of FETZILLA you have now.
Your amplfier has great sound you say.
If you give me a schematic here of your present Fetzilla
we could start from there.
AKSA Hugh Dean probably can answer this question, too.
My version is the one from groupsbuy 2.
Isn't there also a problem regarding the Jfet 2SK170?
I dont know if there is a problem with 2SK170.
Maybe it is hard to find to buy.
I will setup your circuit next two days.
I will make a suggestion how to make more power.
Have you got already EXICON transistors?
Otherwise we can use IRFP240/9240 maybe 3 pair of output devices.
I tought the SK170 is restricted due the operating voltage of the trannie.No, I don't have the Exicon trannies.It would be great if we could keep the positive things of the original FetZilla.Does the sound alter due the use of the IRF fets,I'm wondering?
Lineup, why have you posted a version of the circuit that has not been built and tested, and omits the lateral mosfets so pivotal in the original design?
I have built perhaps 9 versions of this amp topology with different device types (BJTs, etc) in the input, vas and output. I can tell you now that the JFET input, MOSFET vas, and lateral MOSFET output all play a key and synergistic role. Not only this, but the exact part used was decided on through extensive R&D. Substituting any one of them often causes problems. These problems can be solved but require circuit modifications. For more power we need higher voltage rails. Lets look at the problems this causes starting from the input.
1) The input jfet has a very limited dissipation capability. To run higher voltage rails means pushing it quite hard or switching to another input device. The other option is to run a cascode or buffer of some sort, both of which require additional circuitry and have their own sonic signature.
2) More power means more dissipation through the VAS due to the higher voltage rails and increased current required to run a larger output stage. The device selected by Hugh and I was chosen over numerous alternatives based on parameters like input capacitance, power dissipation and transconductance. It is already being run at about as much dissipation as we are comfortable with.
You can of course switch to a larger device such as the IRF610, or perhaps cascode the VAS. However, having tried both I can tell you that the sound quality suffers. Mikelm will also attest to just how much better the zetex mosfets sound in this amplifer. That said, the 610 might sound OK if you were to up the VAS current to 20mA or so.
Of course you can drop the current and run a buffer before the output devices, but that kills the elegance of the beautifully simple circuit.
3) The output. More power means we need either vertical mosfet outputs or a paralleled lateral mosfet output stage. Both of these options mean we need more current through the VAS or a buffer to drive the output devices. I can guarantee you that verticals sound terrible if simply dropped into the circuit as is. The increase in harshness is obvious. I don't know whether this is due to greater cross over distortion, poor drive capability of the vas, or the increased transconductance, but they really just don't sound as good IMO. Also, we need to consider thermal stability with verticals which means adding a Vgs multiplier or something.
All of the problems above are fairly easily solved but require a new circuit. Just this week I think I have come up with an alternative, though quite different to Fetzilla. I will post the results in coming weeks.
You will find a circuit which nearly meets your requirements here:
However, if you wait a few weeks I will be posting a 70W quasi BJT version which I think will be the best sounding...
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