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Old 17th August 2011, 04:41 PM  
Nelson Pass is offline Nelson Pass  United States
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Default Burning Amplifier BA-3

Introduction


In the BA-1 and BA-2 projects we constructed two different amplifiers using very similar input and voltage gain stages (aka the “front ends”), but used them to drive two different Class A Mosfet follower output stages. Now we are going to flip this approach and begin...

Last edited by Jason; 14th September 2011 at 09:22 AM.
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18th September 2011
kees52
diyAudio Member
Looks very fine, but maybe a mosfet cascade with the Q1 and 2 the rail voltage can be higher and so more headroom.
18th September 2011
kees52
diyAudio Member
It looks good, I like to go play with it, I was afraid that the j-fets has trouble driving the mosfets capacity but I saw the 100 Khz square who did look very good, with my hybride 20 khz did this slewing already, but it was afcourse a 2 Ma valve driving a powertube and then the mosfets, a bigger tube and I had the same effect like you did, no feedback..

You have very nice designs.
18th September 2011
woody
diyAudio Member
Nelson, you mentioned Q3 & Q4 are 25W devices. Can I take it that we are using something other than the IRF610 and it's compliment?
18th September 2011
generg's Avatar
generg
diyAudio Member
Sounds really very very good! It is a sound world of it's own!

Unfortunately there are many faults in the nomenclature between the text and the scheme, that makes it difficult to understand Nelsons explanations completely.
and the First Wtt is......a really First Class PSS Idea
18th September 2011
Zen Mod's Avatar
Zen Mod
diyAudio Member
oooooh Greedy Lazy Boyz

Papa must do all hard work for ya .....

here it is - nomenclature by schmtc :

" This is familiar enough. Q1 and Q2 are JFETs which self-bias into resistors R3 and R4 at currents around 8 mA. R1 is chosen to avoid oscillatory interaction with whatever source impedance you might have, and R2 provides a DC reference to ground in the event that the source does not, and also establishes the nominal input impedance.

Q1 and Q2 are largely degenerated by R5, setting the amount of AC current which flows through them for a given input voltage. The voltage gain of this initial stage is approximately

the value of Drain load resistors R6 plus R7 divided by R5. In this case we have roughly unity gain – the Jfets are used as unity gain DC level shifters to Q3 and Q4.

Coming off the Drain of Q1 is the loading network of R6, C1 and P2, and there is a comparable network of R7, C2, and P1 attached to the Drain of Q2. R6 clearly sets the AC load for Q1, but the DC requirements to bias up Mosfet Q3 are higher than that, so P2 in parallel with C1 provides a higher resistance value below about 0.5 Hz, and gives the approximately 3 volt DC drop required to bias the Mosfets.

P1 and P2 are adjusted so as to set the DC bias of Q3 and Q4. You will want to set them at zero when you first fire up the circuit, and increase their resistance to achieve the correct bias voltages across R10 and R11 (about 1 volt) while also keeping the output DC offset voltage at a minimum. This circuit is capacitively coupled at the output, but low offset measured at the Drains of Q3 and Q4 will maximize your output voltage swing.

The voltage appearing at the Gate of Q3 is amplified by something less than the ratio of R13 divided by R10, and with the same happening at Q4(R11) and considering the transconductance of the Mosfets, comes out at about 15. Both of them added make a system voltage gain of about 30X, or 30 dB.

R10 and R11 help set the voltage gain, and they also help stabilize the bias of Q3 and Q4, else it would tend to drift upwards as the parts warm up. The bias current here is about 50 mA, and it will deliver peaks of approximately 100 mA. Q3 and Q4 require heat sinks.

Of course you can bias this circuit higher if you wish – 100 mA bias is perfectly OK as long as you properly heat sink Q3 and Q4, and if you are crazy (like me) you can experiment with higher bias, remembering that the parts are rated at 25 watts, and that it costs you voltage losses across R10 and R11. If you want to play with even higher bias, you can consider lowering the values of R10 and R11 and also R13, all in proportion.

The supply voltage is only critical with respect to the voltage rating of the input JFETs, which are nominally 25 volts. In actual testing, they break down around 40 volts. I wouldn't worry about running them as high as 30V. Hot-rodding this circuit would likely involve cascoding the input Jfets to allow higher voltages. "
my Papa is smarter than your Nelson !
clean thread; Cook Book;PSM LS Cook Book;Baby DiyA ;Mighty ZM's Bloggg;Papatreasure;Papa...© by Mighty ZM
19th September 2011
generg's Avatar
generg
diyAudio Member
I started to do it, but I got confused, especially with the gain of 30. I did not take into consideration R13. But it is clear now.....

Thank you Zen Mod now I understand completely.....
and the First Wtt is......a really First Class PSS Idea
19th September 2011
Zen Mod's Avatar
Zen Mod
diyAudio Member
lucky you ;

I still don't understand it



my Papa is smarter than your Nelson !
clean thread; Cook Book;PSM LS Cook Book;Baby DiyA ;Mighty ZM's Bloggg;Papatreasure;Papa...© by Mighty ZM
19th September 2011
generg's Avatar
generg
diyAudio Member
Oh, when I was four I also claimed to understand everything.....!
and the First Wtt is......a really First Class PSS Idea
19th September 2011
ross57
diyAudio Member
Hello everyone, this is my first note on the diya forum. I'm from Italy and I would like to recognize this forum as the best in the world concerning diy audio equipments. Sincerely. I'm really positive to build this front-end, but I would like to know which output stage would be worthful to couple with it. Thank you in advance for any suggestion.
19th September 2011
Zen Mod's Avatar
Zen Mod
diyAudio Member
ross57 - outputs are those second and third pics from top ;
feel free to ask questions in BA3 thread : http://www.diyaudio.com/forums/pass-...-new-post.html
my Papa is smarter than your Nelson !
clean thread; Cook Book;PSM LS Cook Book;Baby DiyA ;Mighty ZM's Bloggg;Papatreasure;Papa...© by Mighty ZM

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