F4 power amplifier

[Skorpio]

If you have a source which can drive 5 Kohm and a couple hundred pF,
you can dispense with the JFETs altogether and simply drive the Gates
of the Mosfets.

A little more work, and you can use a single pair of Mosfets and raise
the impedances of the bias circuit, and then it would be about 22K and 70 pF.

I would like to try without the JFET buffer and connect my Tube preamp (cathode follower output 12AU7) directly to MOS-FET output stage.

#1: Would there be any issues with bias setting (drift, optimal values, etc.) with one set of MOS-FETs? I would like to make bias selectable (low and high), where low would be for muzak and around 100-300mA...

#2: If the two 220uF capacitors are lifted from ground and connected to output terminal instead, they would bootstrap the input and making the input impedance higher (and load lighter for the preamp), correct?

 

[Zen Mod]

if that's plain Jane CF - just forget . in that case - leave buffer stage in .

you can find one pair mosfets output iteration on few first pages of thread .... bufferless little later

 

[Nelson Pass]

Of course you could try it and see what you get.

 

[vitalstates]

I can't find any info. other than 1 volt .05 (pre-out). Maybe it does I don't know. They don't give enough info. in the manual to know.

Onkyo Europe - PR-SC5507

here are some figures I took some time back...I can't remember why I did them so You'll have to take them as they are....they might address your question

 

[Skorpio]

Of course you could try it and see what you get.

I have tried and found the following up till now:

Standard: input impedance= 5k, gain approx. 0.93gg
Bootstrapped: input impedance= 64k, gain approx. 0.93gg

I have added a switch to be able to select between the two, but with my tube prestage bootstrapping is best.
Next step will be lowering the two 220uF input capacitors to 10uF PS

I will also like a switch to change bias from approv 1A to 0.3A, will this could operate without major problems during operation?

 

[Nelson Pass]

No. But the DC offset is likely to change.

 

[generg]

Hi Zen Mod,

why didn´t you tell me how wonderfull F4 with Pumpkin sounds.........?

.... in the moment F5 lies in the corner.

 

[Nelson Pass]

Zen Mod is a man of great modesty.

 

[generg]

Zen Mod is a man of great modesty.

He will be happy to hear this from You!

 

[Audiofanatic]

Zen Mod, Choky !

He will be happy to hear this from You!

He already know this

Best regards,

Audiofanatic

 

[Zen Mod]

Hi Zen Mod,

why didn´t you tell me how wonderfull F4 with Pumpkin sounds.........?

.... in the moment F5 lies in the corner.

well - there is a small tweak available for you :

omit output caps on , omit everything in front of two electrolyt caps on F4 input ( tossing out jfet buffer and everything related) , bypassing two elcos with 10-68nF polycarbonats ........ then tell me what's your impression

Zen Mod is a man of great modesty.

yeah ....... but only when I'm forgetting fact that I'm spitting image of Modesty Blaze

Modesty Blaise - Wikipedia, the free encyclopedia

 

[ma_coule]

yeah ....... but only when I'm forgetting fact that I'm spitting image of Modesty Blaze

Splitting image?! No way!
I always knew that Zen Mod is Mr. Nelson undercover!

 

[generg]

O.K. Zen Mod!

What about the bootstrapping parts C3, C4, R24 and R25 in the version r0 4/17/07?
I suppose no jfet-buffer, no bootstrapping?

Oh I just remember Scorpio measured both versions, there was more input impedance putting the capacitors to output instead of ground.

 

[Zen Mod]

O.K. Zen Mod!

What about the bootstrapping parts C3, C4, R24 and R25 in the version r0 4/17/07?
I suppose no jfet-buffer, no bootstrapping?

Oh I just remember Scorpio measured both versions, there was more input impedance putting the capacitors to output instead of ground.

bootstraping parts - whatever ; I think that will hardly feel difference

try like this - and put bootstraping caps to output , if you like

 

[Luke]

My first real DIY audio project!

Thanks to cviller for help with the boards and jacco and steenoe for advice on the big RIFA’s.

And, of course, to Nelson – from one ’51 baby to another, thank you for providing the reason and means for an old dog to start learning some new tricks.


I've tried to finish the amp in the Aussie vernacular style – still looking for the right bit of weathered corrugated iron to put on top.

heres an aussie classic in corrugated iron:

HQ Holden - Collections Online - Museum of New Zealand Te Papa Tongarewa

 

[Audiofanatic]

F4

bootstraping parts - whatever ; I think that will hardly feel difference

try like this - and put bootstraping caps to output , if you like

Hi Choky,


Thanks for the schematic, this is how I use my F4, more or less.
It sounds fantastic! Thank you very much Nelson Pass for sharing !

As soon as I finish my F5 board's I'll let you know which one is my favorite one.
But so far, it's the F4 like in the schematic above.

Best regards,

Audiofanatic

 

[generg]

well - there is a small tweak available for you :

omit output caps on , omit everything in front of two electrolyt caps on F4 input ( tossing out jfet buffer and everything related) , bypassing two elcos with 10-68nF polycarbonats ........ then tell me what's your impression



Hi ZenMod,

thanks for the picture combined with Nelson, its a treasure.........!
It looks like "back to the future" and like a puzzle in same time!

And the comment of ma_coulee is perfect.

I´ll inform myself about Mister Blaze!

 

[Zen Mod]

......

I´ll inform myself about Mister Blaze!

you mean Miss Blaze ?

naah - that was just a joke , with her name as main trigger , combined with her certainly attractive look ; comic itself is just barely above average .

in fact - this picture is certainly more adequate

just search for pirsigbike.jpg ....... it's worth .

in fact - don't search - here it is :

zen and the art of motorcycle maintenance, robert m. pirsig

 

[jackinnj]

I think you mean Blaze Starr.

 

[Zen Mod]

I think you mean Blaze Starr.

nice .