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Old 28th November 2011, 12:54 AM   #491
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Default Biasing

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Originally Posted by JimM View Post
Ok got it, my simulation has Idss of 10ma, I was biasing 2sj and 2sk way to high so good lesson to ensure P1 and P2 start at zero ohms and turn to get the 1.0volt drop across R10 and R11, unless you want a burning amp. In simulation with IDSS of 10ma pots were set at around 250 ohms.
I'm biasing mine at 1.1V across R10 and R11 (higher quiescent current helps with a slightly too-high power supply voltage). The heatsinks get faintly warm.

I had to add 330 ohm resistors on the back of the board to get past 0.85V on one side as well.

I also added 10uF capacitors across the power supply in the BA-3 box as per Nelson's latest schematics. Result of these changes is a smoother sound that appears a touch rolled off in the treble until I play something like harpsichord or percussion, when it becomes obvious that there's plenty of treble when needed.
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Old 28th November 2011, 01:32 AM   #492
Variac is offline Variac  United States
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EDITOR'S NOTE:
ALL info on the BA-3b, the Balanced BA-3, are moved here:
Burning Amp BA-3b (Balanced)
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Old 28th November 2011, 01:36 PM   #493
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What DC offset is acceptable for the BA-3 anyone? I have 4mV at present.
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Old 28th November 2011, 01:44 PM   #494
Zen Mod is offline Zen Mod  Serbia
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where ?

on output of FE or on output stage itself ?

in both cases - more than good

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Old 28th November 2011, 01:48 PM   #495
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Quote:
Originally Posted by Zen Mod View Post
where ?

on output of FE or on output stage itself ?

in both cases - more than good

4mV across the 330ohm resistor of the output of the BA-3.
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Old 28th November 2011, 02:30 PM   #496
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good .
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Old 28th November 2011, 03:22 PM   #497
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congratulation....... ! your J-Fets now seem to work......
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Old 29th November 2011, 09:44 AM   #498
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congratulation....... ! your J-Fets now seem to work......
Thanks to you.

I let the amp run for about 45 minutes, during which the DC offset swung between +60mV and -100mV until with a few adjustments it settled between -6mV and +6mV. R10 and R11 settled in at about 1.07V and R3 and R4 were both 0.058V with P3 set dead centre.

I plugged this unit (this is the left channel monoblock) into the system last night and I'm running it again this morning. Quite distinct from the BA-2 on the right channel but comparisons are not easy because the left BA-3 is powered by the Salas voltage regulator and the right one is not (not yet anyway). On first listening the slight chestiness to voices on the BA2 has been replaced by a more even and open sound. Percussion on cymbals is more distinct in the sense I can hear a clearer sense of impact and decay and a clearer sense of position. Instruments are easier to separate from the mix. However, some of this will be the upgraded voltage reg I am sure. I am certainly getting more detail from the amp at lower output levels than I was whilst there is no sense of hardness to the sound (not that there was any on the BA2).
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Old 29th November 2011, 11:19 AM   #499
generg is offline generg  Germany
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I could write very similar words, thanks for impression......

"more open, more distinct, sense of position, easier to separate, more detail on lower output level, no sense of hardness"


what do we want more......

till the next project.....
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Old 29th November 2011, 11:53 AM   #500
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I think we need to let those who are building for the first time that the BA2 is an outstanding amp in its own right. I hadn't heard anything as good as the BA2 even after years spent auditioning some very expensive amps and there will be those who prefer the BA2 sound. For those building for the first time I would go for the BA2 because I found the front end easier to build and set up. I have had a lot more problems with the BA3 and I think I might have given up had it been my first build.
Of course I wouldn't have given up; you guys would have sorted it out.
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