Issues with the VRC6 Implementation of the Everdrive N8

I'm a huge fan of the Everdrive series. After having bought the sd2snes to get to play some of the extremly rare SNES games (Rendering Ranger, Manfred Trenz for the win^^) I decided to also let the Everdrive N8 join the fun.

The shop said it even features expansion audio so the japanese release of Castlevania III namely akumajou densetsu can be played with the sound of the VRC6 which the american NES was not designed for.
First of all I'm from germany, I bought the card and put the ROM on the card and played it with my PAL NES. Well it didn't run quite well as the game couldn't handle 50 Hz. So i had the decision. Buy the Famicom or a US NES with an expansion audio mod.
I didn't liked the idea of having my controllers directly soldered to the console so i went for an american NES.
The game ran well but the audio was still quite strange as the mod was not yet performed.

So i opened the NES and connected pin 3 and pin 9 of the expansion port.
I don't know if this also goes for Famicom to NES converters but this is what the Everdrive needs.

I started the game and something was wrong. The sound was not quite right. Missing notes?
With an NSF player on my PC i listened to the 'reference' I was used to listen to. I heard a strong baselines on 'Beginning' and 'Aquarius'.
On the Everdrive however the baseline which is played by the Sawtooth channel of the VRC6 was incredibly quite.

I registered on the forum of krikzz the developer of the Everdrive and complained about this. Sadly no one was able to help me as this is a known issue.

I then contacted krikzz directly and wow I love you man. He has supplied me with the source code for the FPGA. I investigated and compared the design with the infos I've found on nesdev and found an issue with the internal mixing circuit that was putting the two pulse waves and the sawtooth together. Also the pulse waves did have exactly twice the volume they normaly should have.

I synthesized the design with my changes and there it was. The baseline of Beginning coming out of the Everdrive. I've added a poti to allow changing volumes while playing until I'm satisfied.

You can download my modified version here.
Replace 024.RBF inside EDFC/MAPS on your SD card with this version. But make yourself a backup first. Please keep in mind that this mod is not yet finished as further testing is needed for perfect accuracy. Also the LO/HI volume setting is ignored. I can archive quite good results at the moment with 77kOhm between pin 3 and pin 9.

I've tested it for a few minutes but be sure you are doing this at your own risk. If this mod damages your Everdrive N8 or your NES I can't help you.

Happy whipping some vampires!