Bontempi B50 (beginners keyboard with gritty digital lo-fi sounds)

This initially extremely cruel sounding Bontempi sqeakbox would everybody else likely hated so much that he rather would throw it away or smash it with a sledge hammer than ever to dare to make a serious instrument of this ear tormenting tablehooter. But after modification it makes a great tekkno instrument because it as many freaky howling synth sounds with gritty stepping zipper noise envelopes.

main features:



The thing can now make incredible jungle/ tribal style rhythm patterns by letting the normal rhythm run and switching the drum chip either into its own drumrolling mode or pitching its clock so low that it starts to miss- recognize some of the beat pulses sent by the CPU. Drums and cymbals sound very bassy and pressureful and have a wild click/ knock at their start and end, which makes a perfect tekkno machine of this tablehooter. The rubber drum pad keys can even be locked by pushing them under the rear control panel rim to keep the drumroll mode running. Also the rough FM sounds of the main CPU are great for tekkno.


According to the package box picture (which differs much from my B50) there may 2 different versions of this keyboard exist those have different sounds and a different drum kit chip; the drumpads on the box correspond to the predecessor Bontempi B30. An interesting detail is also that the B50 employs on the control panel a narrower character font and a different terminology than the (older?) B30; e.g. instead of "Voices" there stands "SoundBank", instead of "Rhythms" "RhythmsBank" and the cipher buttons are labelled "BankSelect" instead of "Sounds/ Rhythm Control". All functions beside volume, tuning, tempo, rhythm start/ stop, demo, "plus play" and sustain, reverb are selected by entering numbers. Unlike the B30, here the buttons don't make a click but only successfully entered (3 digit) numbers are acknowledged by a snare sound and every rhythm and demo melody starts with a 4 step metronome bar. Unlike the B30, the B50 keeps no previous settings during power off, but without drum sequencer this wouldn't make sense anyway. (The B50 always resets by pressing the "on" button.)

The main voice sounds resemble coarse 2 operator FM timbres with complex envelopes those have audible zipper noise. All sounds contain a short sustain, which makes it impossible to play very short notes. Sounds with short attack phase start with a percussive click. Most wind instruments contain a delayed vibrato (often less extreme than with Bontempi B30). The "leslie organ" is a rough Hammond organ sound with strong, slow vibrato; the attack phase resembles more a steel drum. Also "brass" has a strong and the "crazy trumpet" an extreme vibrato. The "horn" fades duller. The "xilophone" is rather a vibraphone with strong vibrato, while the "vibes" has none (likely the manufacturer confused both names) and fades duller. The "vibraphone" is a duller version of "xilophone". The "jazz organ" is a plain dull Hammond timbre. The "piano" is rather an electric Rhodes one and fades duller. The "bandoneon" is a reedy tone that fades brighter. The "telephone" is a disharmonic digital phone with a little sustain. The "synth drum" howls down and up again an a unique way; this one has a shorter envelope than on Bontempi B30 and resembles a cat meow. The "ghost" is harsh hollow, halfway voice- like tone with very strong, howling vibrato and long sustain phase. The "fantasy 1" is a medium fast howling siren with a similar timbre, while "fantasy 2" is the same in faster and with sustain. (Unlike the B30, here siren sounds don't repeat their 2nd in the lowest keyboard octave.) With the "sustain" and "reverb" buttons the release phase of most timbres can be extended. The "reverb" adds an additional longer sustain with lower volume than the normal sustain to the envelope.

The digital volume control steals bit resolution from the main voice, which truncates the decay phase of the timbres at low volume and thus can be rather regarded as a sound effect. It anyway only affects the main voice and not the rest, which is not what a volume control is supposed to do.

Annoying is that the accompaniment of the rhythms plays always in a fixed key and can not be turned off, which limits their use very much. The accompaniment seems to be made from rough squarewave organ chords and a monophonic e-bass sound. Unlike the Bontempi B30, this one has no programmable drum pattern but the "sustain", "reverb" and "plus play" buttons instead of drum sequencer buttons. Also annoying is that selecting a new rhythm always switches the main voice sound to a default one corresponding to that rhythm. (The B30 makes no such nonsense.)

circuit bending details

To open the case, a piece of paper should be inserted under the black keys; otherwise you endanger it to crack the keys off. (There are no screws under the front panel sticker, though don't attempt to peel it off.) 

To add a simple sound output jack to the instrument, it is crucial to wire from the CPU sound output a resistor to the jack and from there a capacitor against GND that does the D/A conversion, because this instrument employs a crude digital power amplifier that sends amplified square bit stream signals (about 11 kHz?) directly to the speaker (halfway smoothed by an electrolytic cap), thus connecting here a normal output jack would toast your tweeter or even destroy the sound input of attached external devices by the high signal amplitude (5V?). 

In my instrument I rewired the digital single transistor amp as an analogue one by placing serial resistor and capacitor to GND in the line before its base pin, which improves the sound a lot. But to prevent transistor overheat, the speaker needed a serial resistor bridged by an electrolytic capacitor. (My circuit is quite complex, sorry for the vague description at the moment.)

aliasing filter with timbre controls:

I developed an active RC- filter circuit to remove the unbearable high whistling bit stream frequency from the main voice without muffling the trebles too much. I designed the thing by trial and error and a tube oscilloscope, but I think it works basically like an analogue notch filter. The timbre can be adjusted through 2 potentiometers. Because it is quite complex and not easy to describe without schematics, I now have included a photo of the crude hand drawn stuff, although it is likely incomplete and possibly even wrong. It also shows the modified amplifier and the standby circuit.
The standby circuit was designed to activate +Vs for the beep filter only when the CPU is active (and thus outputs keyboard matrix pulses). This way the thing can be still used with batteries using the given electronic power button. Below seems to be an old version of it.

Here are some other old schematics I found about this instrument; they might be outdated versions:

This is the power amplifier.
These are outdated drafts of the ant- aliasing filter.
An easier method to filter out the ear tormenting high beep may be to re-sample the sound on a PC using the same sample rate like the beep. On PC exist also various "lo-fi" distortion effect programs those can re-sample a sound in realtime with freely adjustable re-sampling rate, and also a normal sound filter effect program may work here.

rhythm section:

The rhythm IC "HT3010A" can not only play single percussion samples triggered by an external CPU, but it can play entire rhythm patterns/ drumrolls by itself, likely for stand- alone use in various drum sound toys.

The IC has an 18 pin DIL package with the following pin assignment:
pin function comment
1 IN base plays a sound
2 IN tom plays a sound
3 IN snare plays a sound
4 IN rhythm 4 beat plays built-in rhythm pattern (for a fixed duration)
5 clock connected through 82 kOhm with pin 6
6 clock  
7 OUT sound sound output
8 GND  
9 IN rhythm stop/ pause  
10 IN fill-in  
11 OUT1 connected to pin 12, pin 13
12 OUT2  
13 OUT3  
14 ? connected to pin 15 and through 47nF to GND
15 +Vs connected through 100 Ohm protective resistor to +6V
16 IN mode connected to pin 4, 17, controls input behaviours
17 IN rhythm on/ off  
18 IN hihat plays a sound

To control rhythm pitch, replace the 82 kOhm resistor between pin 5 and 6. Instead connect a potentiometer (470 kOhm?) with right end to pin 5, left end through 120 kOhm to GND and wiper through 470 Ohm to pin 6.

OUT1, OUT2, OUT3 draw pulses against GND (open collector: OUT1 = long, OUT2 = medium, OUT3 = short pulses - to connect LEDs against +Vs?).

All "IN" input lines react on connections to GND. Solder to pin 9 a button switch for "fill-in" and to pin 4 one to enable the internal rhythm mode.

The IC can now produce drumrolls by itself, but they would always stop after a while when there is no continuous GND signal at the rhythm input lines. Therefore disconnect the drumpads from the main CPU and solder them instead through diodes directly to the pins {1, 2, 3, 18} of the rhythm IC. For safety solder also a diode into each main CPU output line to these pins to avoid overload. The drumpads can now be used to make a continuous signal by pushing their rubber buttons under their rear case rim. 

After pressing the "internal rhythm" button, the IC will now produce 4 different drumrolls by pressing the drumpads. When the instrument tries to play its own rhythms in this mode, the result are quite wild tribal rhythms because the IC outputs instead of individual drums now a drumroll sequence for every single pulse. When the rhythm pitch pot is turned way down, the IC will skip some of the CPU pulses and thus makes freaky jungle grooves.

Attention: I have only very incomplete draft schematics of my modification, thus this description might be partly inaccurate or even wrong since it is based on them.

A predecessor of this instrument was the Bontempi B30, which has even a programmable drum pattern and different sounds, but misses the (anyway quite useless) accompaniment. The similar Bontempi B40 (stereo with accompaniment) and B20 (mono, only 15 sounds and 7 rhythms) have no individual buttons but select everything through keyboard keys (and a single "select" button instead of the white "power" button). The top of the line model of this series was apparently the very mysterious Bontempi BS3000 mini synth.

 removal of these screws voids warranty...    
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