MFB-522 / the heavy, heavy monster sound

Just scored a cheap MFB-522 and post a couple of PCB pics and descriptions here, which might come handy in case you need to repair yours. So far, I don’t plan on modding much (this thing sounds great!) but took the liberty of adding more snappy to the snare and more body to the clap.

Check also my Dimonion1 and my Synth Pro pages.

First of all, manuals:

https://medias.audiofanzine.com/files/manuel-simple-mfb-522-478076.pdf

https://archive.org/details/synthmanual-mfb-522-owners-manual

That frankensteinered capacitor is fat – and so is the sound of this little MFB (lol)…

Processed stereo sum and ABL3 bass line
cheapo dremel style case fitting…
three different types of screws – what’n not to love about that?

Quick inventory

Power: PSU 15V DC, at least 300ma (best use 1000/1200ma); 3x voltage regulators: L78L05 (+5V), L78L12 (+12V), L78L06 (+6V) — if I get this right, there is no negative rail in this machine. I tested op-amps for supply and all were +12/0V…

Digital Control: ATMEL AT89C4051-24PU Microcontroller; ATTINY13A Microchip; TDA8444T Octuple DAC; LC549C ADC; 24C64WP EPROM; CNY17-1 opto coupler

2x HC505C shift registers;  HEF4053 switch (switches the “B”s –> RS, Clave); 2x HEF40106 Hexinverter (one for the metal noise); Dual op-amps are TL062.

Transistors, diodes: BC487 NPN; BC587 PNP; BZX84C – ZENER DIODES (SOT23); single Schottky diodes too

Trimmers – adjusting sounds

Trimmer 1 (leftmost) – BD maximum length: Set decay to maximum and tune to minimum. Turning the trimmer anti-clockwise sets your max length. The latter can be near infinite.

Trimmer 2: Tom/Conga maximum length: turning this anti-clockwise increases the maximum decay of ALL toms/congas. Best play a midi/sequence loop going though all sounds and then adjust to taste while listening to that sequence.

Trimmer 3 – Hi Hat Oscillators Tune: While Cowbell D-tune and Cymbal Tune affect these potions of the metal noise that are prominent in CB and CY, this trimmer here tunes those metal noise oscillators more prominently featured in the hi hat sounds. The scope is not extreme, but you can nicely tune in some in/harmonic shimmer as per taste.

Trimmer 4 (rightmost) – CLAVE length. Tuning this trimmer anti-clockwise lengthens the clave sound.

Modifications

More snappy on the snare: the mfb snare is cool but I wanted some more snappy action, so I changed the snappy filter a little bit. On the picture you see a 3k3 resistor which I simply soldered over the 22k resistor that was originally there. No need to remove that 22k, just add your 3k3 on top.

sounds like this:

More clap body: the noise component of the clap is highpassed through a cap. By adding a cap on top (I used a 560nf cap) you can lower the fundament a bit. Can be done from the back of the pcb.

Sounds like this ( i tweak decay, filter and attack while playing)

Potential Trigger Inputs: Triggers (velocity sensitive!) are sent via the 8xDAC to individual sound components. A couple of these have diodes at the trigger entry point (I marked some of them yellow below). The cathodes of these diodes would be your pint for sending in external triggers (I’d stay below +5V and send these through diodes as well, so you have a nice AND-gate). Haven’t tested this though!


More when I have more…