Cryptic words about TB-303... ?

[antto]

Thank you, so the Accent directly affects the Cutoff frequency too ?

It's commonly stated that the Accent controls exclusively the filter Resonance and the VCA...

What's the TM3 ?

as mystran said, the resonance "parameter" has a 2nd role (which has nothing to do with the actual filter), where at "minimum" setting, the accent modulation is passed thru (so it's a sharp decay-down, a spike basically), while at "max" setting, that spike is completely smoothed into the so-called "accent-wow" .. so it first goes up, then down, smoothly.. and due to this filtering effect, it can now carry accumulation (so several accents close together make each "wow" charge higher, tending to some max level)

the accent also adds to the VCA, the VCA here takes two control voltages - the usual Volume envelope and this accented signal

here's how the accented signal looks like at first:

this signal is created by taking the actual main envelope signal, and "gating" it during accented notes (so the result is 0 during normal notes)
this signal is added to the VCA
it's also sent thru a diode (which waveshapes it) and into a peak-follower (filter) where the resonance parameter acts as a balance between the dry version of the signal, and the slew-rate-limited version (where it looks like a wow and has ability to accumulate) the result of all that is then called "Vaccent" in the formula
"TM3" is the trimpot for tuning the 303 filter cutoff

you might find this video interesting

read the description for details, but it basically shows the two "303s" on a spectrograph where you can see the resonant frequency and its envelope (in linear frequency scale), you can see the "wow", the accumulation, and then there's a hack which allows you to see how "Vaccent" looks like when the slew-rate limiter is not in effect (injecting the Vaccent signal from one unit into the other which keeps being at max resonance, so you can see the modulation)

[xhunaudio]

Thank you antto for this ultra-accurate post, much appreciated...

Just one thing is not 100% clear :

is the gimmick circuit output sent to BOTH the vca and the filter cutoff, or just to the cutoff ?

I mean :

both vca and filter :
filter_eg * accent_step_gate ---> gimmick_circuit ---> BOTH filter_cutoff AND vca

or

filter (separate):
filter_eg * accent_step_gate ---> gimmick_circuit ---> filter_cutoff (= gimmick circuit involved for filter)

vca (separate):
filter_eg * accent_step_gate ---> vca (= NO gimmick circuit involved for vca)

?

[mystran]

is the gimmick circuit output sent to BOTH the vca and the filter cutoff, or just to the cutoff ?

Ignoring the current drawn by the gimmick circuit (which I'd imagine is mostly negligible), the envelope (after the knob scaling) is sent to the VCA "as-is" and to the VCF through the gimmick circuit.

[xhunaudio]

Thank you !

[Dasheesh]

I forget the actual specifics because it was a long time ago, but someone that opened up a 303 said they found an extra circuit that he didn’t expect to find, like an extra capacitor.

[xhunaudio]

Hi,

on the TB-303, from the infos and measurements I gained, the frequency range of the "cutoff" KNOB should be from about 350 hz up to 2500 hz.

And, the "env mod" KNOB - when fully anti-clockwise (min value) - should enable the filter envelope to about 20%.

Could any 303-expert confirm this data/measurements ?

EDIT : ok, it doesn't matter, I found it. I made 2 wrong calculations.