Modified MAM MB33 Analog RETRO Bass Synthesizer w/ Circuitbenders Acid FM Mods

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Published on Mar 21, 2017 circuitbenders.co.uk

"MAM MB33 Retro with Acid FM mods: This video shows the filter FM mod modulating the filter cutoff at audio frequencies using the internal square and sawtooth VCO waveforms as FM sources. It also demonstrates the Filter Lock mod that holds the filter and VCA open with no envelope, the resonance boost, and the overdrive mod.

The MB33 retro seems to be designed to slightly distort the output stage at full volume, much like the TB303 did. The Drive mod boosts this distortion savagely for some grungy acid techno sounds."

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"The MAM MB33 Retro is the latest version of the MB33 / FAT FREEBASS clone of the infamous Roland TB-303. As standard it features all of the original TB-303 controls, and also has an external audio input through the filter and VCA. Overlapping midi notes gives you that essential 303 note slide, and any midi note with a velocity of 120 or more will trigger the note accent. Realistically if this synth was programmed correctly, you would be VERY hard pressed to distinguish between this unit and a real 303 in a mix. In a side by side comparison i'd say it sounds a little meatier than the Behringer TD3, with a little more girth and analogue weight to the sound.

We have modded this machine with our circuitbenders Acid FM mods to give a huge range of extra sounds, without diluting the essential TB-303 character.

MODS INCLUDE:

VCO On/Off Switch: Normally when you plug a jack into the external VCF input, the MB33's VCO is automatically removed from the mix. As there may be occasions where you don't want this to happen, or situations such as when you're using the filter lock mod (see below) when you want to be able to turn the VCO on and off yourself, we've added a switch to add manual control of this.

VCO Drive: The MB33 retro seems to be designed to slightly distort the output stage at full volume, much like the TB303 did. The Drive mod boosts this distortion savagely for some grungy acid techno sounds. This mod increases the maximum amount of distortion at maximum volume. The intensity of the distortion can be set using the normal volume knob.

Resonance Boost Switch: An annoying issue with these machines is that the resonance never reaches self oscillation (but then neither did the TB303) and also drops off considerably when the cutoff is over 3/4 of the way up. We've solved this problem by adding the resonance boost switch. With this switch activated the filter starts to squeal with the resonance about halfway up, and then just screams with self oscillation at any higher settings. The resonance boost is especially effective when used with the VCF FM mods.

VCF FM: Essentially VCF FM (Frequency Modulation) allows you to modulate the filter cutoff at audio frequencies to create all kinds of clanging cross modulation and bizarre alien splatters & sweeps. This effect works best with the resonance cranked up into self oscillation which is where the resonance boost mod comes into play. The FM controls allow you to select one of four sources for the FM signal. The first is the normal filter input socket round the back. This means you can run a sound through the filter section, but also use that sound to modulate the filter cutoff at audio frequencies at the same time. The second source is a new FM input jack on the right hand side of the case. Using this source allows you to run a sound through the filter from the VCF input or just use the internal VCO, and at the same time modulate the filter cutoff with an entirely independent sound source at the FM in jack. The actual audio from the FM in jack is not heard. The remaining two sources are the sawtooth and square wave signal from the main oscillator. These can be selected as sources completely independently of the setting on the VCO waveform knob The FM source select knob also has an 'off' setting. The amount of FM applied to the VCF is controlled using the VCF FM knob.

VCF CV input: This mods adds 3.5mm jack on the right side of the machine that will accept an external CV to control the filter cutoff. The CV input should range from 0-5v. Higher voltages, or negative voltages are NOT recommended. This mod also adds a switch on the front to toggle between internal and external cutoff control.

Filter Lock: Another annoying feature of these units is that if you want to use the VCF input to filter external audio signals you have to trigger the filter and VCA envelope using a midi note message, the filter then closes until it receives another midi trigger. This also means that by default the external signal will have the VCA and VCF envelope applied to it, so regardless of what kind of sound the external signal actually is, it will have a TB303 filter and amp envelope. While this can be useful, some of the time its just annoying so what this mod does is add a switch that locks the filter and VCA open at a constant level without having to use any midi input. The filter can then be adjusted in realtime using the cutoff and resonance knobs. Obviously if the VCO is turned on, you will hear a constant tone as soon as you activate the filter lock. You can turn the VCO off using the VCO on/off switch, but you should try playing some notes with the VCO and filter lock on, as it can generate some interesting organic sequences, especially when using the accent feature.

With none of the new mods activated the unit functions as normal"