Have you already acquired the transistors and boards for the front end of the BA3?
The complete BA3 is a 3-stage amplifier while the F4 is a 2-stage amplifier.
In addition, the BA3 has 6 pairs of output devices per channel while the F4 has 3 pairs of output devices per channel.
Do you just want an amplifier or do you really want exactly the BA3?
Answer the two questions above and it will be more clear what help you actually require.
No, BA3 has same output stage as F4, 3 pairs...would have to be mono blocks for 6 pairs.
Scroll down to pic of output boards:
https://www.diyaudio.com/forums/pas...fier-illustrated-build-guide.html#post3973243
You are correct about the front end. You probably got the idea of six pairs from various schematics of BA3, which do show six pairs. ( schematic with 6 pairs appears in same link above, just before pic of output stage.
I don't know if the complementary bias cards are the same for BA3 as F4, somehow the spare BA3 complementary output boards I bought some time ago for BBA3 monoblocks don't have bias boards with them....
Russellc
The standard BA-3 output stage is shown here with the bias board. The schematic shows 6 pairs, the PCB has 3 pairs, and the ability to daisy-chain additional PCB if you happen to have a large enclosure. 
As to the question of can a F4 pcb be used in lieu of the BA-3 output stage, (with the F4’s input buffer removed) the answer is yes, with very little modification. Being a simple mosfet follower stage with a clever bias circuit, the amp circuit is the same in theory and almost identical in details.
As to the question of can a F4 pcb be used in lieu of the BA-3 output stage, (with the F4’s input buffer removed) the answer is yes, with very little modification. Being a simple mosfet follower stage with a clever bias circuit, the amp circuit is the same in theory and almost identical in details.
The standard BA-3 output stage is shown here with the bias board. The schematic shows 6 pairs, the PCB has 3 pairs, and the ability to daisy-chain additional PCB if you happen to have a large enclosure.
As to the question of can a F4 pcb be used in lieu of the BA-3 output stage, (with the F4’s input buffer removed) the answer is yes, with very little modification. Being a simple mosfet follower stage with a clever bias circuit, the amp circuit is the same in theory and almost identical in details.
Just occurred to me the F4 is all in one so to speak, without separate bias cards like the BA3. I have complementary output boards I got for future BBA3 build, but when organizing things the other day, I don't have the bias cards. I guess When again available, I can rebuy them for the BBA3, and daisy chain these extra boards for a behemoth 12 output pair BBA3. Now to find 2 behemoth cases with requisite heatsinks.
Russellc
What chassis have you considered for such dissipation?
I still have to power up my balanced BA3 which accounts for the same 12 pairs and contrary to what i calculated i fear grose overheating in Dissipantes 500mm deep
Source Resistor Question
I am in the middle of my BA-3 build, and I am basically just duplicating the build in 6L6's build guide (thank you for writing this). There are a couple small differences, I have a 4U 400mm chasis, and 22,000 uF 50V power supply filter caps, and I am planning on using 1 ohm source resistors, but when I read the following from Papa's BA-2 write up, I wonder if I should be using a lower value:
"For fewer output pairs, consider using proportionally lower values of Source resistors on the power Mosfets. For example, with 2 pairs, 0.33 ohm might be a better choice than 1 ohm."
If I am following this correctly, I have 3 output pairs per channel, so I would use .5 ohm source resistors.
Any thoughts or suggestions? I already purchased 1 ohm, 1%, low ppm resistors, but I would certainly purchase new resistors if that is the right thing to do.
I am in the middle of my BA-3 build, and I am basically just duplicating the build in 6L6's build guide (thank you for writing this). There are a couple small differences, I have a 4U 400mm chasis, and 22,000 uF 50V power supply filter caps, and I am planning on using 1 ohm source resistors, but when I read the following from Papa's BA-2 write up, I wonder if I should be using a lower value:
"For fewer output pairs, consider using proportionally lower values of Source resistors on the power Mosfets. For example, with 2 pairs, 0.33 ohm might be a better choice than 1 ohm."
If I am following this correctly, I have 3 output pairs per channel, so I would use .5 ohm source resistors.
Any thoughts or suggestions? I already purchased 1 ohm, 1%, low ppm resistors, but I would certainly purchase new resistors if that is the right thing to do.
It really depends on how well matched your mosfets are.
You could try .5R and se if there is any "bias hogging" (imbalance of current among mosfets). This can be done by measuring across all the mosfet source resistors and comparing current draw.
1R is a safer choice and provides more of a buffer if the balance is not perfect between the mosfets.
And yes, a lower value will sound better. But the lower the value the more likely bias hogging and thermal runaway will occur.
You could try .5R and se if there is any "bias hogging" (imbalance of current among mosfets). This can be done by measuring across all the mosfet source resistors and comparing current draw.
1R is a safer choice and provides more of a buffer if the balance is not perfect between the mosfets.
And yes, a lower value will sound better. But the lower the value the more likely bias hogging and thermal runaway will occur.
It’s my understanding that the 1ohm were specified because the original test chassis(s) way back when had not particularly matched transistors, and the big source resistor helps makeup for that.
If you have really tight matches you could go as low as 0.22ohm. 0.47ohm is very safe if you know that the mosfet are nicely matched.
If you have really tight matches you could go as low as 0.22ohm. 0.47ohm is very safe if you know that the mosfet are nicely matched.
Thanks for the informative replies. I really like the term "bias hogging" it helped me understand the purpose of the source resistors. I purchased my mosfets as a matched set from the diyAudio store, so I am guessing they are pretty well matched. However, I would like to check the matching so I know that I am good to go with a lower source resistor. Would checking with a low current level as shown in the thread: "Nelson Pass' Easy Peasy MOSFET Vgs Measurement" be sufficient to verify mosfets that are already believed to be matched, or is matching at the suggested 500mA still required?
Nelson Pass' Easy Peasy MOSFET Vgs Measurement
Nelson Pass' Easy Peasy MOSFET Vgs Measurement
Store mosfets are matched to at least .01 Vgs. I'd use 0.47ohm resistors without worrying about it at all, and use 0.22 if you wanted to, but at that point you'd probably want to match your source resistors since that will give more variation than the mosfets. And it's likely you don't have a meter that can reliably measure 1/5th of an ohm.
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Thanks - do you think .22ohm 2% rated resistors would be matched close enough? The resistance range would be .2156 thru .2244 ohms, and you are right this is definitely not something my multimeter could measure.
If it makes a difference, the resistors I am looking at are Vishay/Dale CPF3R22000GNB14.
CPF3R22000GNB14 Vishay / Dale | Mouser
Are there plans to restock the complementary output boards+bias cards in the store? Somehow I managed to get the output boards some time ago, but more recently discovered I don't have the bias cards to go with them.
I suppose there is even a smaller chance that some extra of those bias cards alone would be left over at store?
Russellc
I suppose there is even a smaller chance that some extra of those bias cards alone would be left over at store?
Russellc