The BA-3 as preamp build guide

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#722
kpsthakur said:
Humm...cost of pristine sound...well said
Click to expand...

Also quote from Papa's article;

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 R8 and R9 (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.
 
#724
Biasing starts...phase 1

6L6 said:
Bias and setting of P3

Bias -


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What's important - Iq (measured as the voltage across source resistors; the Mosfet bias) must be very low , offset is irrelevant in this moment .

Now turn one pot one turn ( assuming that you have multiturn pots) then turn other pot one turn. Continue, one turn at a time on each pot until something happens.

Observe voltage across resistors and output DC offset.

Proceed one then second pot , again just one turn

Observe Iq and offset

Again one turn + one turn

Now you are probably in range when you can see which pot is pulling offset in right direction - to 0 . It will feel like one of the pots is controlling the bias on both sides, and the other is controlling the DC offset.

It’s best to increase the bias a bit, and then zero the offset. As you zero the offset you will decrease some of the bias, so it will be two steps forward and one step back. That action is normal.

As you increase the bias and zero the offset, remember to always keep the offset near zero. If you run out of turn on the pots, determine your max bias, with zero offset. (It’s useful for troubleshooting)

Proceed iteratively with pots , while you set - say - 75% of desired bias, with zero offset. Remember, full bias is 1V across R10 and R11, with zero offset BEFORE the capacator. If you measure after the cap there should always be no DC.

Now - put lid on box and let it cook for a while - until you get thermal equilibrium on heatsinks

It's best to use wire/clips to leave those voltmeters in place ;

Open the lid , up bias to - say - 90% of desired one ,while maintaining offset



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Click to expand...




Now you are probably in range when you can see which pot is pulling offset in right direction - to 0 . It will feel like one of the pots is controlling the bias on both sides, and the other is controlling the DC offset.

For me the two pots work completely independent...none is pulling the bias together...and both the pots have similar impact on the DC offset...
 
#726
Forgot to check with you all...there is a tiny C3 Cap placement available on the FE plate..looks like a electrolytic cap could be fixed there...and looks polar too...I have yet not read anything...forum pages are exhaustive...is this some sort of by-pass cap...if yes, why?? and what value/rating (guess more than 25V for a 24V supply) to be used...
 
#731
IMG_0116.JPG
 
#734
Nice job kpsthakur. Mine drifts 100mv or so, depending on temp. But I could never get it down below 20mv for any length of time. As long as you are under 100mv or so you should be good.

if you want to see the offset drift blow on one heatsink...this circuit most definitely needs the cap on output.
 
#737
You now see why there are coupling caps. I have external leads on mine, so I can test with the cover mostly on. You don't want to know what happens when you put the cover on.

But don't sorry, that's why there is a cap. Price you pay for no feedback.
 
#740
Dear wise guys, I already started to gather the compontents for this preamp. I'm running my F6 very happily, and I'm wandering what is the benefit of this preamp in terms of audio quality. I mean, I have decent diy speakers from Troels Gravesen, my F6 is marvelous... so, what will I benefit from this preamp? Of course the gain and the inputs are important, but... musically speaking? will my speakers produce better sound at all? Can anybody describe what "can" get better?
 
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