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Old 19th November 2012, 06:55 AM   #21
lineup is offline lineup  Sweden
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Quote:
Originally Posted by Bigun View Post
Hi Lineup,

I like this general topology, the way you draw it makes it look very simple.

Reminds me of the JLH Class AB published in 1970, [http://sound.au.com/tcaas/jlhab2.htm]which of course was all BJT.


I do like the use of Laterals in the output for simplifying the thermal stability of the bias. I suppose this is the only output device that we can use without adding some temperature compensation ?
Bigun.
If you mean circuit in my first post,
this was the original Idea.
The idea that got AKSA Hugh started.
He suggested several changes including bootstrap.
If it wasnt for Hugh it wouldnt have been built.
Hugh is a man liked and trusted.

Yes, it isa nice circuit.
It is good to see that there are other ways to think and build good amplifiers.

Regards, LIneup
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Old 19th November 2012, 06:57 AM   #22
lineup is offline lineup  Sweden
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Originally Posted by meanman1964 View Post
The FetZilla I've build is realy a special amp with nice mids good staging and detailed hights and very strong tight bass.If we need power it is only for the bass region.Is it possible to design a kind of a Fetzilla topology that goes to max 200Hz wouldn't this make things easier to design?
meanman
It is true what you say.
If we are to build only for BASS region .. upto say 200 Hz
then it simplifies the design very much.

Is it a BASS amplifier you are after?
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Old 19th November 2012, 06:59 AM   #23
lineup is offline lineup  Sweden
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Quote:
Originally Posted by Bigun View Post
meanman, how would you compare your FetZilla with your DX blame ?
Blame is a conventional and a bit boring amplifier.
Fetzilla has another topology and should be way faster.
I think a compare would be in favour of Fetzilla.
But lets hear what destroyer X has to say
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Old 19th November 2012, 07:13 AM   #24
lineup is offline lineup  Sweden
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meanman.
I had in mind 2x40 VAC transformer.
This would give like +/-55 VDC supply.
The max power would be like 150Watt into 8 Ohm and more into 4 Ohm.
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Old 19th November 2012, 09:19 AM   #25
lineup is offline lineup  Sweden
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Originally Posted by GregH2 View Post
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?

Meanman,

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.
If you have built 9 versions of this amplifier, Greg
then you are a guy to listen to.

I hope you will assist meanman with his high power version he wants for his BASS loudspeaker.

Great to have the knowledge bank of someone who actually built this thing.

Regards, Lineup
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Old 19th November 2012, 09:20 AM   #26
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You could need three pairs of laterals for 150W, and I believe that increasing the VAS current around 30% and inserting a cascode on the input jfet you would be in business.

However, a complete rejig of the pcb would be needed, of course.

Cheers,

Hugh
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Old 19th November 2012, 09:23 AM   #27
lineup is offline lineup  Sweden
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Just what I have in mind, Hugh.
3 pairs of EXICON.
Have rather plenty bias for the VAS.
And a cascode for the 2SK170.

Rail voltage I suggest is +/-55 VDC from 2x40 VAC transformer.
This will give something like 150 Watt into 8 Ohm.
And more into the BASS if the impedance will go down towards 4 Ohm.
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Old 19th November 2012, 10:22 AM   #28
GregH2 is offline GregH2  Australia
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Quote:
Originally Posted by lineup View Post
If you have built 9 versions of this amplifier, Greg
then you are a guy to listen to.

I hope you will assist meanman with his high power version he wants for his BASS loudspeaker.

Great to have the knowledge bank of someone who actually built this thing.

Regards, Lineup

Thanks Lineup. The other person to talk to is Mikelm. He has his own very special direct coupled version...

One version I did like was a small signal mosfet for the front end and a BJT VAS. It sounded quite nice and if anyone is willing to experiment I think you might be pleasantly surprised.
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Old 19th November 2012, 03:20 PM   #29
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Quote:
Originally Posted by lineup View Post
meanman
It is true what you say.
If we are to build only for BASS region .. upto say 200 Hz
then it simplifies the design very much.

Is it a BASS amplifier you are after?
Well lineup,I was thinking the most power gets into the bass right?My first choice would be a fullrange amp like the FetZilla but with higher output power.
If it's possible you may design a fullrange amp.But don't forget I'm spoiled with the FetZilla it has to be as good.On the other hand my speakers that I've to build will be all active with two 20cm bass drivers each side and for this a real bass amp would be a great idea,what do you think?
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Old 19th November 2012, 03:23 PM   #30
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Quote:
Originally Posted by lineup View Post
meanman.
I had in mind 2x40 VAC transformer.
This would give like +/-55 VDC supply.
The max power would be like 150Watt into 8 Ohm and more into 4 Ohm.
That would be great
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