BigDad
Active member
Hi there,
I am designing a Direct Box inspired by the RCA BA-31B. To be more accurate, it uses the output stage of the BA-31B, while the input follows the topology from Rod Elliott's website (Figure 7.5):
https://sound-au.com/articles/jfet-design.htm
Here is the schematic of the full DI:
I have built a prototype, and it seems to work fine at first glance, but there are some aspects that disturb me.
Basically, the DI sounds really good, but there's a kind of upper-frequency fry or fuzzy sound that superimposes on the signal when I play loud with a guitar or a bass. It’s really subtle, but it makes the sound not totally clean. I'm not sure if this is coming from the input or the output stage. I plan to connect the input stage with an output capacitor directly to the transformer to be sure. But I want to know if BA-31 users have experienced this with the original design?
Also, this class A output design is clearly not the most efficient, and I don’t really understand how to set the R7 value. I’ve tried adjusting it by using a trimmer, but it doesn't seem to change anything except increasing or decreasing the power dissipated by Q3 and Q4.
I'm also unsure about the best way to calculate the values for R4 and R5. I left them at 2.7k because it seems to work with the 2N4416, and Q2 seems to auto-bias the whole thing, but maybe there's a better tuning?
I will post harmonic and frequency response analysis results, but I didn't see anything wrong during the first tests.
I would be honored to share build files with the community if I can figure out how to fix this problem.
I am designing a Direct Box inspired by the RCA BA-31B. To be more accurate, it uses the output stage of the BA-31B, while the input follows the topology from Rod Elliott's website (Figure 7.5):
https://sound-au.com/articles/jfet-design.htm
Here is the schematic of the full DI:
I have built a prototype, and it seems to work fine at first glance, but there are some aspects that disturb me.
Basically, the DI sounds really good, but there's a kind of upper-frequency fry or fuzzy sound that superimposes on the signal when I play loud with a guitar or a bass. It’s really subtle, but it makes the sound not totally clean. I'm not sure if this is coming from the input or the output stage. I plan to connect the input stage with an output capacitor directly to the transformer to be sure. But I want to know if BA-31 users have experienced this with the original design?
Also, this class A output design is clearly not the most efficient, and I don’t really understand how to set the R7 value. I’ve tried adjusting it by using a trimmer, but it doesn't seem to change anything except increasing or decreasing the power dissipated by Q3 and Q4.
I'm also unsure about the best way to calculate the values for R4 and R5. I left them at 2.7k because it seems to work with the 2N4416, and Q2 seems to auto-bias the whole thing, but maybe there's a better tuning?
I will post harmonic and frequency response analysis results, but I didn't see anything wrong during the first tests.
I would be honored to share build files with the community if I can figure out how to fix this problem.
