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BA-3 Amplifier illustrated build guide
BA-3 Amplifier illustrated build guide
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Old 30th June 2020, 10:27 AM   #321
Dennis Hui is online now Dennis Hui  Canada
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Location: Toronto, Canada
BA-3 Amplifier illustrated build guide
Increasing the resistance generally will work.
Some questions:
1) Are the voltages you quoted (+/36.5V with 120VAC, +/-32V with 111VAC) under load?
2) If so, how many of the 0.47 ohm resistors do you have parallel on each rail, and what is the voltage drop across them?
3) Are you trying to get to +/-32V under load with 120VAC?
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Old 1st July 2020, 01:54 AM   #322
Deebs is offline Deebs  United States
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Hi Dennis,

So, the voltages I gave you were not under load. I didn't apply load for fear of destroying the JFETs. However, upon slowly ramping up under load, I find that at 120.0vac, which is exactly what I have as line voltage at this location in my home, I get +/-32.65vdc on the rails. Is that acceptable? Or are the JFETs doomed? What if I go to someone else's house and connect to a wall socket at 123v, will that be their final farewell?

I'm using QTY 4 of the 0.47ohm 3watt resistors in parallel on each rail

Voltage across resistors (at 120vac primary) is 202mV on B- and 212mV on B-

Also, I noticed that when starting up I see a voltage of about 35vdc on the rail for about a second. I am using the soft start board from diyaudiostore to limit inrush current--not sure if this plays a role in that. Also not sure if I should be worried about it.

My goal is just to have a safe and long-lasting build. I followed the build guide here as closely as possible, hence where I got the target of 32v rails. I'm not opposed to having a lower voltage, as I'm not trying to push any limits. Just want it to sound great and last a while!

As always, thanks for the help.
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Old 2nd July 2020, 10:45 AM   #323
Dennis Hui is online now Dennis Hui  Canada
Hoopy Frood
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Join Date: Aug 2002
Location: Toronto, Canada
BA-3 Amplifier illustrated build guide
Thanks for the clarifications.

Page 6 of the BA3 article ( http://www.firstwatt.com/pdf/art_ba_3.pdf ) contains this relevant passage:


"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."

The rating here refers to the drain to source voltage. You can measure that on the two
outer legs of the jfets, though you may have easier access by probing at the connections
coming off the legs to the resistors. (See attached.) Given you are using Toshiba
mosfets I expect you are in the 30-volt range.

Let's say you want to lower that a little by dropping a volt from your rail. You currently
have ~0.47/4 ohm resistance in your CRC power supply and you are drawing about 1.8A.
If you replace each set of four 0.47R resistors, with a set of four 2.7R resistors, you will get
an equivalent resistance ~0.68R and ~1.2V (0.68 x 1.8) voltage drop. I suggest the power
rating of the resistors should be at least 3W.

Here's a sample part:

MOSX3CT631R2R7J KOA Speer | Mouser Canada
Attached Images
File Type: jpg BA3_jfet_vds.JPG (146.6 KB, 61 views)
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Old 4th July 2020, 02:58 PM   #324
Russellc is offline Russellc  United States
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Join Date: Mar 2003
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BA-3 Amplifier illustrated build guide
I built my BA3 exactly like the 6l6 did in his build guide. This included the same soft start board, and same 24 + 24 voltage transformer, (same voltage, not same unit) which in my case made for 32 volt rails.

While this has run a few years having no problems, I always thought cascoding the front end might be best option, could allow more rail voltage if one cared to.

The only thing keeping me from it is knowing how the passive parts are calculated. Or, is it not picky, and just poach the circuit used to cascode F5 turbo?

I figure the circuit is made for certain voltage rails and for certain voltage presented to the front end Jfets?

Thanks for any clarity on cascoding front end Jfets in BA3.

Russellc
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