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.

x0xb0x SS#01: VCF

Wow. What a frustrating stage this was. I hit a brick wall and the project languished as a result. No point in adding day numbers to my titles anymore!


I put the VCF together shortly after the last blog. I took my time just as I had with the other sections of the x0x and thought I has put it together successfully. Once I began testing it I couldn't get a signal to confirm it was working though. I didn't have scope so I tried to whip up the diy audio probe that's suggested on the x0x build page. I fudged my through it but never really got it working properly or to my satisfaction.

Because this section of the x0x has several rare parts, I started to wonder if one or many of them were faulty. So I spent tons of time checking resistor values, voltages, betas, and the like, all with mixed results. I was sure something was wrong. At the same time, I thought it was high time to get a scope but I couldn't justify hundreds or thousands of dollars on a new one, and frankly, I didn't want yet another 50 pound tool in my arsenal. My hope was to track down a proper soundcard probe and use free software. Between these two challenges it took 6 months. It was really discouraging.

The GOOD news is that I had success with both of these challenges. First things first: I found a probe at a great price by Virtins. $30 US, shipping included to Canada. Can't argue with that. Once I plugged it into my line input on one of my low end sound cards, I wired it all up to the x0x, got testing, and low and behold: the damn thing seems to be working just fine. I just couldn't measure it properly.

I can't begin to express how relieved and elated I am to have past this step. I feel like I can continue and know that I don't have to revisit a problem.

On a side note, the OTHER good news is that I managed to track down 2SA733-APs as well so I'm that much closer to making an authentic 303. The critical components are right in the VCF: Q8, Q9, & Q10. It turns out my hFE reader on my multimeter is also crap so the component log component log isn't accurate either. (I always thought they were unusually low.) I'm going to leave them in and revisit this later. I need time to find a better way to measure these too but this less important than starting to solder again!!

A real Sight for sore eyes:

The VCF test with the cutoff and resonance at 0%.



The VCF test with the cutoff at 100% and resonance at 0%.



The VCF test with the cutoff at 100% and resonance at 100%.