Befaco A*B+C turned out to have a noise floor issue. Befaco issued a schematic diagram with the fix. I’ve done this modification and confirmed that the noise level is no longer a problem.
Since the schematic is a little hard to read, I’ve listed all the changes here. (If you’ve bought a full kit of parts, be sure to double check that you have received the latest parts values. Otherwise you’ll have to source the correct parts for yourself.)
It’s fairly easy to work on the module. Just remove the single screw holding the boards together and separate them. Most of the rework is on the main PCB.
If you are building it fresh, simply omit the following resistors from the Control PCB.
Cut off from Control PCB
R102, R103, R104, R105, R106, R107, R110, R111
Change to the following new values.
If you are changing these parts, fortunately the PC board markings that identify the parts are still visible after the parts are installed. The PC board is good quality and should not be damaged by rework. I used a good quality solder wick to help remove the parts. Then I cleaned flux from the board with isopropyl alcohol before soldering in the new parts.
R1, R37 -> 12K
R2, R36 -> 10K
R5, R33 -> 100K
R6, R32 -> 10K
R7, R20 -> 10K
R8, R30 -> 100K
R12, R26 -> 100K
R13, R19 -> 100K
R14, R24 -> 47K
R17, R31 -> 10K
R18, R25 -> 10K
C2, C13 -> 10 picofarad
Options
R11, R22 -> Either a bare wire or 1K. I used 1K so as not to have a jack connected directly to an input pin of the AD633. Using the 1K will reduce the signal by about 1%, which doesn’t really matter because the maximum gain is 2X.
R27 -> 4K7 or omit if you don’t want to mix the top section into the bottom one. I chose to omit.
Modifications
Use an ohmmeter to verify which end of R12 and R26 to lift.
R12 – Lift the end going to R14 and connect it directly to IC1 pin 13.
R26 – Lift the end going to R24 and connect it directly to IC4 pin 2.
I just did this modification at a Befaco workshop and confirm it’s correct (although I haven’t built a non-modified version and so can’t compare if one is less noisy).
One clarification — both R12 and R14 are connected to ATT_C2 on the schematic. This is one of the pin headers that connects the main board to the control board. Similarly, R24 & R26 are connected to ATT_C1, another connector.
So a better way to find the trace to cut would be to test for continuity between R12 & the pin headers, and then R24 and the pin header. I forget off the top of my head which one they are, but that is a more direct way for finding the trace.
Finally, one other tip is that one trace is on the bottom of the main board, and another is on the top.
I didn’t have to cut traces, because I simply lifted the ends of R12 and R26 out of the board and soldered flying wires to them for connection to the op amps.
Hey, thanks a bunch for posting this! It was a really helpful supplement to the Befaco-provided supplement.
Thanks for the detailed post. I can buy the version 1 of this kit at a good price (~$50). Just wondering if it’s worth the trouble and the additional cost of sourcing the needed replacement parts. Do you still consider that this module ” … has to go on the disappointing modules thread” (saw your comment on muffwiggler).
Is the noise eliminated?
Hi Robert. That’s a good price and the extra cost of a few resistors and capacitors is small. It’s also a lot easier to build the modified version, rather than remove over twenty parts to modify an original build (which I did for two A*B+C).
It was disappointing because of the noise floor problem in the original circuit. The noise problem is gone in the updated circuit. (Some people were happy with the original circuit because the signal to noise ratio was not too bad. The noise showed itself when patching an output with no input signal and turning up the volume on your amp.)
So I would recommend to get the needed parts and build it correctly the first time. The only tricky bit is soldering one end of a resistor over to an op amp leg instead of going into a hole. But if you DIY regularly, you’ll figure it out.
Appreciate your input. I’ve order the kit and will give it a go. Your Mouser part information was a good guide too. Thanks for sharing.
Thanks for this. how did you come up with this? Certainly no info on the Befaco page. I find their kits to post quite a lot of challenges to work (a pity),
Hi Eike,
There was a long discussion here.
https://www.muffwiggler.com/forum/viewtopic.php?t=198755&postdays=0&postorder=asc&start=0