[sdiy] Roland PB-300 kick: is it an 808?

[Benjamin Tremblay]

Yes, what I see is somewhat special about the TR-808 kick drum is primarily that transistor in the T network where one might put a trim pot to have coarse control of the pitch.
The problem with most T filter drums is they usually have a highly resonant sweet spot and RC components are chosen to produce the desired pitch resonating in that sweet spot.
This means if you do adjust the coarse pitch it will self-oscillate anywhere near the sweet spot. 
By adding positive feedback to filter, you can adjust the RC components and amplifier gain to be in the right range, not self-oscillating, and then add back oscillation as desired. 
This lets the 808 have a pitch-sweeping pulse that only starts to ring when you tell it do. 

The only other distinct feature of the 808 kick is a simple treble knob to manage how much click passes through. 

Other Roland drum circuits have used a germanium diode there in the T circuit to automatically sweep the pitch down as the pulse decays. This creates an automatic sweep like in the TR-606 toms. The 808 and the PB-300 are the only Roland kick drum circuits I know of with a transistor-controlled pitch sweep. So you can literally control the sweep speed with an envelope circuit like you would use to control the decay of a cymbal sound generator. 

I borrowed this idea and applied it to the 1-transistor Monotron kick drum circuit: It’s pretty awesome for a simple pitch-swept kick drum but it lacks the positive feedback op-amp to give you more deterministic oscillation.


Benjamin Tremblay
btremblay at me.com
330 Fiske Street
Carlisle, MA 01741




> On Nov 24, 2022, at 9:36 PM, brianw <brianw at audiobanshee.com> wrote:
> 
> I'm assuming that the op-amp with twin-T network in the feedback loop is the specific oscillator that makes the TR-808 kick (and toms, right?). Most other parts of the circuit would not be as important (like how the trigger is created). More tweakability is usually better, but sometimes having less control gives better chances of success.
> 
> i.e. If an analog kick drum has the twin-T feedback network, then it's in the family.
> 
> Brian
> 
> 
> On Nov 24, 2022, at 5:12 PM, Adam wrote:
>> Ermagawd, dontcha just love Roland stuff from this period?
>> 
>> I found a SM circuit description for this thing.
>> 
>> Typically it uses their “swing type VCA”  and twin T networks for the drum sounds, but then it also features:
>> 
>> 
>> “BASS
>> PB-300 utilizes the CPU’s internal TIMER as an oscillator to generate BASS Notes. The signals from the oscillator for BASS Note are taken from CPU via PC7 (Pin 24).
>> When PB-300 is played with BASS switch on, it progresses the development of BASS line in accordance with a combination of keys pressed, which is received via MIDI IN. The varying data of BASS Notes are sent from CPU via PC7 to Q4 of Voicing board, where bass tone is gated when TRIG from PB7 (Pin 16) is applied to Q2 at a certain rhythm tempo. Having colored while passing through IC1a and Q1 (LPF), bass tone is routed to BASS VOL CONT VR2.”
>> 
>> There’s also some wonderfully wacky stuff explaining how it will generate arpeggios from incoming midi chords, and there’s a dedicated VCA for “Nuance” (elsewhere known as Accent) from incoming midi note velocity!
>> 
>> Those Roland engineers! There must’ve been something in the water…
>> 
>> On 25 Nov 2022, at 8:44 am, Adam wrote:
>>> Interesting!
>>> Good basic analogue sounds - ripe for tweaking. 
>>> 
>>> Is there no clap? Most complex voice circuit of this era.
>>> 
>>> I wonder what the controls marked “Hold, Arpeggio,etc” are doing on a rhythm machine?
>>> 
>>> And don’t knock the DR-55!! There is no muddy live PA ever existed that could muffle that rim shot!!
>>> 
>>> On 25 Nov 2022, at 7:43 am, Benjamin Tremblay wrote:
>>>> It lacks any knobs, but the transistor-controlled pitch sweep, trigger circuitry, and positive feedback decay look similar to me. Am I reading this right?
>>>> I’m looking to add some knobs to mine.
>>>> The cymbals etc remind me of the CR-5000 but the kick looks like is just needs a little love.
>>>> Huh I will try to dig up the schematic.
>>>> Ben 
> 
> 
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