Wednesday, September 2, 2009

x0xb0x SS#01:Tweaking

One of the things I noticed with my x0x was that it wasn't *quite* a 100% match to the 303 sound I knew. The resonance wasn't quite there and it had a slightly harsher sound to it. The forums cover this in great detail so I won't repeat it all here but I'll touch on a few points that affected my x0x.

The biggest, most immediate change I noticed was swapping Q11 from a 2SC536F to a 2SC945. This noticeably muffled the sound but warmed it up. The harshness everyone describes can be eliminated right away via this transistor. The muffling effect it produced could also be adjusted via TM3, the VCF trimpot, so the range could match an original 303 with the 945s in them. I trimmed this pot after the swap and was able to match it to the real 303.

Secondly, the resonance level needs to match. This is such a signature to the 303 that's it's a must have feature. If the betas are too low in Q8, Q9, and Q10 it doesn't "scream" enough. I have to admit that I found it lacking in mine too. I started by swapping out my 2SA733Ps to 2SA733APs. The APs are found in all original 303s. Unfortunately it didn't have any audible impact for my x0x. As such I dropped the Ps back in and left them. What *did* work was adjusting the resistance in r97. The stock version calls for 10K. I found the sweet spot on this unit at 9.2K. This produced a perfect match with resonance at about 90% and left 10% more for me that exceeded what my original 303 had here. Some people add an extra pot to make this variable but I don't see the need. This produced what I wanted and still kept my 733Ps in use without using "fancy" APs.

During this tweaking I ran A / B comparisons the whole time with matching patterns and I can say that it is SO close to a match now that it's scary. For most ears it's an inaudible difference and in real world use it won't be noticeable. I can hear very, very small differences but it doesn't affect the overall performance and cloning ability of the x0x. This unit is without a doubt a near perfect match to the real TB-303 on my desk here.

So, to round up, it's at least three key steps:
  • Q11: swap the transistor from a 2SC536F to a 2SC945
  • R97: Reduce the resistor value from 10K to 9.2K (though subsequent units, or your unit, may be a different value)
  • Adjust TM3 to match the cutoff setting to your preference. In my case my preference was the TB-303 I had here.

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