Donner B1 tech info/ modifications

Please reference me when reposting images from this page. Thx

This is all very preliminary and a tad frantic as I’m pretty busy otherwise, but more concise info will follow soon. In brief: partly super close to TD3 in terms of IC choice and VCF/VCA setup. VCO tuning, however, seems to be done per MCU scheme (trimmers not populated!), and generally, this has a very tidy layout and partly more selective choice of parts. 4580 is the dominant op amp (with LM358 for v/oct and TL072 for distortion being exceptions). The general sound engine character is closer to the TD3 than to the TT303MK2 or the TB-EFA module I am familiar with. Especially the resonance character is pretty TD3 style (that’s the matched transistor pairs in the filter for you). As for the VCA, once more the OML discrete Ba662 clown has been honoured by being used, as far as I can tell. While I can’t say how this reflects on Donner, I think this speaks for guest’s accomplishment and the quality of what they did there!

VCF and VCA section…

What else? CPU: ARM GD32F, distortion is your usual Boss clone (better than on td3, need to trace that first though), delay, yasss! uses a PT2399 chip – these are fun to mod!


Disassembly of the unit is super easy and in places some modifications are practically almost already implemented – the VCF audio input and the VCA control input for instance, sports a bridged jumper, which you can hijack for using potentiometers or input jack or other stuff – nice!

simply unscrew these six and pull knobs of the pots on the front, the gently pry open
voila, the jack PCB and the sound PCB and control PCB

The only thing with which you need to be a tad careful is the ziff connection between the smaller sound PCBon top and the larger PCB underneath. Best lift the smaller one up and turn it slowly and carefully. After unclipping the larger connector leading to the jack board, you can then focus on the little ziff.

pry black ziff brace up first, then gently pull awm ribbon


Sound engine PCB

You find more detailed pictures below, but this should suit for a general overview. PSU I need to look at in more detail, but there’s the voltage generation bit per op-amp and we can do a PSU starve like on the TD3 by changing one resistor. Trimmers for OSC scale and offset are not populated, so I guess the software takes care of that. The mini IC left of the waveform switch is a headphones amp and the rest (transistor selection, use of caps, etc.) largely as per Behringer clone. Accent is switched per 4066IC, and, if I’m not tripping, slide is done per software here. Thankfully, the Donner has manual accent trigger and manual slide trigger as features and you can also set gate length, which goes a long way in terms of finding some grove. Sooner or later I will of course mess with the circuits…

More pics at the bottom of this page.


Most of these I hastily tested for an hour yesterday, so treat this more as a shopping list and preliminary inventory. Completely untested stuff is in italics. Moe pics and mostly also sound soon!

Unmodded Donner BS1 for initial impressions

like the factory made it
and one more ex factory clip


Pulse wave adjust: R136 (22k) – confirmed, no demo though


Filter trimmer up/down – not really a mod though, innit?

VCF trimmer

Pre-filter boost: R69 (200k) replaced by 250 k pot

VCO overdrive

Resonance boost: Reduce R140 (10k) a bit.


Min decay mod: R189 (68k)

Accent decay mod: 250k/500k pot between cathode D2 and PCB pad.


Bass boost: augment C143 and C 145 by 100nf

Bass Mod engaged

VCA crunch: remove R200 (220k)

crunch in, crunch out (or so)


Decay mod: R178 (1,5m) replaced by 1m pot

Gate ON forever: anode D17 -.- well, you have a hold option on the interface, so not really needed?

VCA ON forever/VCA CV IN: jumper at VCA via 100k or higher

Rubber Mod: that’s what made the maffez famous for randomly dropping caps, lol: 560nf across C131 (that cap is not populated — funny, since a similar setup on the TD3 leads to clicks, here it does not…)

560nf at control pin of VCA (slight clicks when I switch the mod in and out)
2.2uf cap at C131 – just for fun as this is somewhat too much – synth starts dragging its feet


delay mayhem (fb and time modded)

Feedback galore: reduce R113 (10k) to ca 6K8 (optionally per switch)

Time: R48 (2k7) higher or lower –> the time pot and R48 in series set resistance between PIN6 of the delay chip and GND. Read up details here on electrosmash (page is safe, just outdated certificates) –


Change R161 (10ohm) to 100ohm (I like) or, if you find this waaaay to much/not magical enough, to 47ohm.

Detail pics