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Old 19th September 2011, 12:03 PM   #111
Zen Mod is offline Zen Mod  Serbia
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Default just in case ..... schm and part of text , finally in peace



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" 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. "

rest of story - still here :

Burning Amplifier BA-3
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Old 19th September 2011, 12:13 PM   #112
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Old 19th September 2011, 12:27 PM   #113
Zen Mod is offline Zen Mod  Serbia
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naah ....... that was just a game "checkin' the checker checkin' the checker " ........

when Official Editor was lazy to play the same .......

OK - he's busy layin' da Red Carpet somewhere at SF Bay
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Old 19th September 2011, 12:36 PM   #114
generg is offline generg  Germany
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Yes laying out the Red Carpet for Nelson and his Court Jester, do not be shy turn around on the carpet and show yourself to the people, having mobile Papa besides you!
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Old 19th September 2011, 01:34 PM   #115
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Quote:
Originally Posted by Zen Mod View Post
he's busy layin' da Red Carpet somewhere at SF Bay
Quote:
Originally Posted by generg View Post
do not be shy, turn around on the carpet

Obviously.

(just think, you may become the 1st Red Carpathian in the Bay)
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Old 19th September 2011, 02:07 PM   #116
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Boards arrived this morning. Ordered last Thursday from London for UK delivery. And they say the world is falling apart.
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Old 19th September 2011, 02:17 PM   #117
CosmoA is offline CosmoA  United States
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Default Caps and biasing

For those who don't like electrolytics why not bias the mosfets with a couple of green leds with a 5 Kohm pot across them (one pair for each mosfet connected to each power supply rail). The wiper of each pot could be connected to the mosfet gate by a 100 Kohm resistor. The mosfet gates could then be AC coupled to the 100 ohm jfet drain resistors. 1uF or larger film caps should be used.

This would also leave the distortion trim pot free from influencing the Mosfet
DC bias pot setting and allows fredom from interaction of pot settings.

I have already ordered my PCB board and parts. When Mr. Pass throws something out there that is so different than what everyone else is doing
one must become very curious.

Fred Dieckmann


1st Red Carpathian in the Bay

Surely the most obscure pun of the week......

Last edited by CosmoA; 19th September 2011 at 02:21 PM. Reason: addition quote
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Old 20th September 2011, 10:08 PM   #118
Botte is offline Botte  United States
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Default Loweing the gain a bit?

So what do you guy’s think about cutting the gain down a bit, to about 12.5X or 22dB. We could sub R10 and R11 with 50ohms and increase P1 and P2 to 1K so we can still keep the bias about 50mA. The voltage drop across R10 and R11 would need to be 2.5 volts. We would be wasting output voltage but I was thinking about a regulated supply and running the front end about 5 volts over the output section.

Comments appreciated
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Old 20th September 2011, 10:13 PM   #119
Zen Mod is offline Zen Mod  Serbia
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why not ....
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Old 21st September 2011, 05:59 AM   #120
Tichu is offline Tichu  France
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Hello Gentlemen,

This is my first post here on the BA.
I ordered the boards, and I am trying to get all needed parts.

One thing that is not specified in the BOM is the mechanical parts: heatsinks and chassis.

I would be interested to know which heatsinks you use (size, K/W, ...) for the BA-1-2-3 power section, and where in Europe I could buy them.
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