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BA-3 Amplifier illustrated build guide
BA-3 Amplifier illustrated build guide
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Old 26th July 2014, 01:35 PM   #11
bcmbob is offline bcmbob  United States
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Quote:
Originally Posted by 6L6 View Post
The Front-end has enough gain to drive it to full output from less than 2V in, so no preamp is strictly necessary.
So a B1 or similar will be suficient in most cases. I sill need a little help (using JC-2 currently) with my speakers and the SE outs. Still might have to build something more efficient just to stay "pure" Discovered this video this morning where NP has some interesting comments on the (speakers) subject.

I really like the way you stacked the bias boards - very efficient.

Why the thermistor from PS to chassis if you are using the soft start?


(P.S. Maybe a note on this over on the pic thread where traffic is higher. I was a month late finding this thread - others may be looking.)
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Last edited by bcmbob; 26th July 2014 at 01:52 PM.
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Old 26th July 2014, 01:36 PM   #12
AndrewT is offline AndrewT  Scotland
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R5 sets the gain. (R6+R7)/R5
So it's 2times?
Or is it?

So the article answered one of the four questions.
What about the other three questions?
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Old 26th July 2014, 01:38 PM   #13
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BA-3 Amplifier illustrated build guide
article sez everything

read edited Pa's text , in one of my replies
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Old 26th July 2014, 01:40 PM   #14
bcmbob is offline bcmbob  United States
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Andrew, Watch the video Jim posted. He answered all my questions on P3 adjustments there. Nice job Jim
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Old 26th July 2014, 01:43 PM   #15
AndrewT is offline AndrewT  Scotland
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I have watched the video when he put it up.
Seeing the distortion change was informative.
That is not what I am asking.
GAIN !
Quote:
The Front-end has enough gain to drive it to full output from less than 2V in, so no preamp is strictly necessary.
6L6 claims that a unity gain buffer output and a claimed unity gain BA3 gives "enough gain" to drive it to full output.
Quote:
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.
How you we/he arrive at that?
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Old 26th July 2014, 01:44 PM   #16
AndrewT is offline AndrewT  Scotland
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Originally Posted by Zen Mod View Post
article sez everything

read edited Pa's text , in one of my replies
which Thread?
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Old 26th July 2014, 01:45 PM   #17
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BA-3 Amplifier illustrated build guide
Default BA3 - explanation from article , proper parts nomenclature by Mighty ZM

from here

Quote:
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. "
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Last edited by Zen Mod; 26th July 2014 at 01:53 PM.
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Old 26th July 2014, 01:59 PM   #18
AndrewT is offline AndrewT  Scotland
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Links in post10 and post17 point to the same article.
Is that intended?
Or is there a Thread that has more info?
Quote:
read edited Pa's text , in one of my replies
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Old 26th July 2014, 02:05 PM   #19
Zen Mod is offline Zen Mod  Serbia
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BA-3 Amplifier illustrated build guide
considering that 6L6 is usually aware of what he's writing ....... and , at least this time , same case is with me - that's most probably intended

regarding dedicated thread , there are few with BA3 in title

though , everything you need to know is pretty much visible from schematic itself
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Old 26th July 2014, 02:07 PM   #20
AndrewT is offline AndrewT  Scotland
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So If I read again and more carefully
we have:
first stage gain = (R6+R7)/R5
second stage gain <R13/R10 and then double that giving < R13/(R10||R11)
The total gain would be first stage gain times second stage gain:
BA3 gain < (R6+R7)/R5*R13/(R10||R11) < (100+100)/100 * 332/(22||22) < 2*30.2 <60.4

So back to my questions:
What is the BA3 gain?
Does the P3 being set off mid resistance change the gain?
Can the gain be reduced or increased to suit other requirements?

or is unity gain (+0dB) to somewhere around 60.4Times (+35.6dB) close enough for every one else?
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Last edited by AndrewT; 26th July 2014 at 02:09 PM.
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