The x0xi0 Standard kit is a mod package for the x0xb0x. New features are too numerous to summarize. The kit comes with everything needed to complete the mod including printed circuit boards, panels, electronic components, and hardware.
Gate On Mode Switch
Extended Bass response
Extended VCF Range
Extended Env Mod Range
The second VCO is the same design as the original TB-303 VCO. However it has an independent tune range of -1 to +4 octaves relative to VCO1. There is a coarse and fine tune control for VCO2, indicated as OCTAVE and CENTS. The original Tune control is the MASTER TUNE of both VCO’s. Each VCO has a waveform select switch to choose between saw or square. Each VCO has a Level control labeled VCO1 LEVEL and VCO2 LEVEL. When the VCF OD switch is in the off position and the Level controls are at max the signal level is equal to the stock x0xb0x. When the VCF OD switch is in the on position the VCF can be overdriven by each VCO by increasing the Level controls past 30%. VCO2 can be fed a separate CV from VCO1 by inputting a CV to the tip of the Headphone jack (optional). When a CV is input here it bypasses the internal or external CV that goes to VCO1, so VCO2 has independent CV control. This is most effective when the separate CV input is synced to the normal CV input. When no cable is plugged in VCO2 it gets the same CV as VCO1. The Tune and Width trimpots for both VCO1 and VCO2 are easily accessible from the front panel without having to open the enclosure. There are four small holes by the Tune controls where the trimpots can be easily adjusted.
VCO2 can be hard-synced to VCO1. The waveform of VCO2 will be reset for each period of VCO1. Sync is activated when the SYNC switch is in the right position. Since the tune of VCO2 can be adjusted by several octaves independent of VCO1 you can do sync sweeps. Also, since the new ENV2 envelope generator can also be sent to modulate VCO frequency there can be rhythmic control over sync sweeps as the envelope generator is triggered by accents, non-accents, slides, gate, etc.
The XMOD control determines how much one VCO modulates the frequency of the other. The switch above the Xmod control determines if VCO1 acts on VCO2 or vice versa. At the minimum setting there is no frequency modulation. When is the switch in the up position designated 1-2 VCO 1 modulates VCO2. When is the switch in the down position designated 2-1 VCO2 modulates VCO1. The pitch modulation will track the keyboard scale.
There is an analog noise source that is mixed with both VCO’s. The noise is pseudo-pink noise, actually somewhere between white noise and pink noise that I liked best. When the NOISE control is at minimum there is no noise added.
ENV2 (Envelope 2):
There is a new envelope generator with extensive routing. The mode can be selected as Attack/Decay or Attack/Release. In AD mode insert symbol the envelope decays as soon as the attack phase ends, independent of when the keyboard or sequencer gate is released. In AR mode insert symbol the release phase does not begin until the gate is released. If the attack time is reached before the gate is released the envelope will stay at maximum until the gate/key is released. The ATTACK time is variable between 1ms and 8 seconds. The DECAY/RLS time is variable between 1ms and 12 seconds. The SHAPE switch selects the envelope shape as normal or inverted as indicated by the graphic next to the switch. The Env Trg switch underneath the ENV2 attack control determines if the envelope is triggered by all notes or accented notes only, as indicated by the GATE and ACC text next to the switch. Also, when the TRG ALT toggle switch is in the up position the operation of the Env Trg switch is changed and now selects the trigger source as either all non-accented notes or slides as indicated by the alternate text next to the switch! The DEPTH control determines how much ENV2 affects the VCA (volume) or VCO (Freq Mod) depending on the position of the VCO/VCA switch. If the VCO is selected as the destination of the Depth control by default VCO1 is affected. If the ENV2 to VCO2 switch on the rear panel is in the on position then ENV2 will modulate the frequency of both VCO1 and VCO2. The VCF DEPTH control determines how much ENV2 affects the filter cutoff frequency. The LED indicator between the attack and decay controls shows the Envelope waveform.
The VCA envelope is a dedicated envelope for the VCA. This is the bottom row of controls to the left of the Volume control, indicated by VCA between the decay and sustain controls. This envelope generator is based around the stock x0xb0x/TB-303 circuit. Attack, Decay, Sustain and Release controls have been added. When the controls are in the default positions the VCA envelope is equivalent to the stock x0xb0x. The ATTACK time is variable between 5ms and 1s. DECAY is variable between 5ms and 6 seconds. SUSTAIN is variable between 0 % and 100%. RELEASE is variable between 10mS and 5s.
Note: There is some interaction between the attack and release controls. Because of the way the attack/decay portion is triggered you will notice an interaction between release time and attack time. When release time is set to higher values you will find that the attack time is limited. This is unavoidable without compromising the original design of the TB-303 VCA envelope.
This control allows for variable Env Decay times on Accented notes. Usually the stock Filter Envelope has a decay time of 200ms on accented notes. With this control the Decay time can be varied. There are two options for Accent Decay depending on the option chosen when the kit is installed. There are LED indicators for both normal decay and accent decay time above the Decay and Accent controls.
Option 1: Semi-independent accent decay. This is the typical setup used on every other TB-303 accent decay mod that I have seen, including Devilfish, Oakley TB-3030 etc…In this configuration the filter envelope has an independent accent decay control whose decay time can be less than or equal to the parallel combination of the normal decay control (VR6) and the new accent decay control. In other words, the decay time on accents set by the accent decay control can never be greater than the current setting of the normal decay control. This is because the new accent decay control and the normal decay control (VR6) are effectively in parallel during accented notes.
Option 2: Independent accent decay. In this case accent decay can have a greater decay time than normal decay. This also works well as it can do everything option 1 can plus more. It must be noted that the filter envelope during accented notes is truncated as soon as the accent gate signal has ended. The duration of the accent gate signal is the same as the duration of the sequencer gate-on signal, about ½ of a step. For longer settings of accent decay the tail of the envelope is truncated when the accent gate goes low. For faster tempos you may not get a smooth envelope decay with higher decay settings because it will be truncated. This happens with option 1 as well.
This TRACK control determines how much the filter cutoff tracks the keyboard control voltage. At the minimum setting there is no filter tracking. At max setting there is several times over-tracking. The x0xb0x/TB-303 filter control input has a resting voltage of 3.25v. This means any control voltage below 3.25v will decrease cutoff frequency and any voltage over 3.25v will increase it. Filter tracking is scaled so that this transition happens at note E1 (2.33v), so as the filter tracking control is increased the cutoff freq will decrease for notes below E1 and increase for notes above it.
VCO FM (filter mod):
The VCO FM control determines how much the waveform from VCO2 affects the cutoff frequency of the filter. At the minimum setting there is no VCO to VCF modulation. At max setting the modulation is extreme. The filter will be modulated by whatever VCO2 waveform is selected with the VCO2 waveform switch.
This switch increases the max filter resonance. When the HI RES switch is in the up position the filter can self oscillate at higher settings of resonance. There is an LED indicator above the resonance control to indicate when high resonance is turned on.
The GATE ON switch activates a gate on mode when in the up position. There are two modes for Gate On as determined by the MODE switch below the slide control. Since the controls are far apart they have been related by the grayed out box around the title for each controls. When the gate on switch is activated and the mode switch is in Mode 1 the keyboard gate is held high and the ENV2 gate is held high. In this case both the stock filter envelope and ENV2 are held high and will not reset to re-trigger the envelopes unless the ENV TRG button is pressed to manually retrigger both envelopes. Also when in Mode 1 the VCA can be held open by setting the VCA Sustain control to max. This is useful for playing legato lines. Also retriggering the envelopes manually can be used to great effect. When the gate on switch is activated and the mode switch is in Mode 2 only the ENV2 gate is held high. I this case the stock filter envelope is retriggered as normal for each note but ENV2 is not. In this case it can only be retriggered manually using the Env Trg button. This is good for many things including using long envelope times that can last up to 15 seconds or rmore without being reset, such as long filter sweeps or pitch sweeps.
The ENV TRG button retriggers the stock Filter Envelope and ENV2 whenever it is pressed.
The SLIDE TIME control varies the slide time between 70ms and 500ms. There is compensation so that VCO tuning remains constant regardless of the setting of the control.
Note: This control has been implemented backwards! The shortest slide time occurs when the control is full clockwise. This is equivalent to the stock x0xb0x. The longest slide time occur when the control is full counter-clockwise.
Gate On Indicator:
There is a white LED above the volume knob to indicate when the keyboard gate is on.
Control Inputs & Outputs
Gate In. Positive input. +4.0V to +12V for gate on. Gate In is usually used along with CV In to play the x0xb0x from an external device. When a patch cable is plugged in both the internal sequencer and the external input can turn on the gate. -15V to 15V input will not damage the unit but extreme voltages should be avoided.
CV In. The CV input allows external control of the pitch of the notes being played. The x0xb0x uses 1V/Oct where +1V is equal to note C1 - corresponding to 65.4Hz. This is a switched jack that is normally-closed. When a patch cable is inserted into the jack the internal CV is bypassed. There is CV protection against negative voltage and over-voltage.
Acc In. Positive input. +2.5V to +12V for Accent on. Negative voltage up to -15V will not damage the x0xb0x but will not trigger an accent and should be avoided. Accents will be triggered by both the internal sequencer and the external input. Typically this input would be controlled by a CV from other analog gear or from the velocity output from a MIDI to CV converter.
Sld In. Positive input. +2.5V to +12V for Slide on. With the x0xb0x/TB-303 Slide function the gate is also open when a slide is held. Anything over +4V will initiate a slide and hold the gate open for normal 303 operation. Input between +2.5V and +4v will only trigger a slide and not open the gate. Negative voltage up to -15V will not damage the x0xb0x but will not trigger a slide and should be avoided. Slides will be triggered by both the internal sequencer and the external input. Typically this input would be controlled by a CV from other analog gear or from an aux output from a MIDI to CV converter.
Filter CV In. This is the filter modulation input. It is 1V/Oct. Input between 0V and +6V is more than enough range to cover several octaves. Because of the biasing initiated by components Q9, R64, & R65 the resting voltage of filter cutoff is +3.25V. Any CV input lower than this will decrease cutoff frequency. Anything higher than this will increase cutoff frequency. For example, a CV input of 4.25V will increase the filter cutoff about one octave. The input will take anything from 0 to +10V. Anything above +10V or below 0V should be avoided. The filter modulation source can be any CV or audio input, perhaps an envelope generator or low frequency oscillator from other analog gear or an audio input from the x0xb0x or external gear.
Swp Out. This jack gives a direct copy of the accent sweep signal normalized to 0V. This is the short burst (wow) envelope that affects filter cutoff on accents.
Acc Out. The Accent Out feature is optional. When wired up there is +5V on the tip of the headphone jack. It can be used to trigger accents on a second x0xb0x/TB-303 or to trigger velocity or other gate functions on other hardware. For example, it could be used to control the gate in on another piece of gear so that notes only play on accents.
Audio Inputs & Outputs
VCO Out. This is the output of the VCO, the sum off VCO1, VCO2 and the noise source. This is a switched jack that is normally-closed. When a patch cable is inserted into this jack the circuit is opened between the VCO output and the VCF input. The signal level of this output is in the range of 1 to 3 Vrms. It can be used for several purposes. One use would be to process the x0xb0x waveform with external hardware such as an effects processor, delay, modular gear, etc… before possibly returning it the VCF or VCA input.
VCF In. This is a direct audio input to the VCF. It is ac-coupled and is suitable for most line level signals – preferably in the range of 500mV to 2V. This is where you would input external audio signals to be processed by the x0xb0x filter (and VCA, output mixer & overdrive if desired), or where you could return the x0b0x waveform that was taken from the VCO Out jack.
VCF Out. This is the output of the VCF. The signal level varies depending on VCO level and cutoff & resonance settings but is typically 500 mVrms. This is a switched jack that is normally-closed. When a patch cable is inserted the circuit is opened between the VCF output and VCA input. This is done so that you don’t get the original VCF signal mixed into the VCA if you are taking the VCF Out signal and returning it to the VCA already. If you’re using the VCF Out signal and also want to send it to the VCA simply route it back in to the VCA Input jack.
VCA Input. This jack is a direct audio input to the VCA. It is ac-coupled and is suitable for most line level signals – preferably in the range of 500mV to 2V. If you’re using the VCA input and don’t want to hear the output from the VCF simply plug a patch cable into the VCF Out jack. The external signal also passes through the Overdrive as well since it is post-VCA.
On/Off. When in the off position the overdrive circuit is bypassed. When in the on position the x0xb0x output is sent to the overdrive circuit. This is post-volume control so the volume control is used to drive the input of the overdrive.
Type. This switch selects between overdrive and distortion. When in the up position overdrive is selected. When in the down position distortion is selected.
Amount. Controls the amount of overdrive or distortion.
Tone. Controls the mid & high frequency tone of the overdrive or distortion.
Low Freq. This switch selects between three different low frequency contours. The up position is the moderate setting. The down position gives more low frequency. The middle position gives the least low frequency. In the case of the Distortion circuit it affects the color of the mid and low-mid frequencies more-so than the lower frequencies.
Bass Response. The low frequency response is increased approximately one octave to 35 Hz.
VCF Range. Both the low range and high range of the filter cutoff frequency is increased. The lowest cutoff frequency is lowered from 400 Hz to 80 Hz. The highest cutoff frequency is increased from 1.2 kHz to 3.5 kHz. In addition, activating the Filter Hi Range switch increases it further to 14 kHz (especially useful when using the VCF input with a full-range signal).
Env Mod Range. The range of Env Mod is changed to give no modulation when the Env Mod control is at the minimum setting.
VCO FM. This is a frequency modulation (FM) input to the VCO. It is ac-coupled and is used for audio rate frequency modulation. It is expecting a line level signal in the range of 100mV to 1V.
Filter Hi Range. When this button is on the maximum cutoff frequency is increased to 14 kHz.
ENV2 to VCO2 Switch. This is only used with the full x0xi0 mod. See the full manual for details. With the I/O-only mod this SPDT slide switch is free to be used for any additional mod.
Details & Circuit Description
Control Inputs & Outputs
Gate In. The signal from the tip of the Gate In jack goes to the base of Q37 through R326. The turn-on threshold is +4.0V. Both the internal sequencer and the external input can turn on the gate. -15V to 15V input will not damage the unit but extreme voltages should be avoided.
CV In. The CV input allows external control of the pitch of the notes being played. The x0xb0x uses 1V/Oct where 1V is equal to note C1 - corresponding to 65.4Hz. This is a switched jack that is normally-closed. When a patch cable is inserted into the jack the internal CV is bypassed. More specifically, the internal CV input coming from the node of R88/R89/R90 goes to the switched jack so as when there’s no patch cable inserted the signal continues to the tip of the jack. The tip of the jack then goes to pin 5 of IC11, the slide buffer. In the case of the full x0xi0 mod the tip of the CV In jack goes to the slide time circuit, which then goes to pin 5 of IC11. In either case the trace on the x0xb0x mainboard between R88/R89/R90 and IC11 Pin 5 is cut. CV In is protected from negative voltage by D307 – which shunts any negative voltage to ground. D308 and R328 protect it from over-voltage. D308 is a 6.2V zener diode. If the CV In exceeds the zener voltage the diode will conduct and limit the input voltage. R328 current limits from constant or extreme over-voltage so the zener diode doesn’t burn out.
Acc In. The threshold to trigger an accent is +2.5V. The signal from the tip of the Acc In jack goes to R329 and then Q302. The emitter of Q302 goes to the base of Q35 so that both the external input and internal sequencer can trigger accents. The signal at Pin 12 of IC13 and the emitter of Q302 is 0V on accents and 5V when no accent. Q35 and related circuitry inverts the polarity of the accent input so that there will be ~10V at the collector of Q35 on accents. This 10V goes to a control input on the 4066 analog switch (IC12) which effectively allows the filter envelope signal to pass through. This copy of the filter envelope then goes to the accent pot. The accent pot controls the depth of the filter envelope which is the sent to the VCA and to the accent sweep circuit.
Slide In. The threshold to trigger a slide is +2.5V. The signal from the tip of the Slide In jack goes through R331 to the base of Q29. Both the internal sequencer and external input can trigger a slide. The external slide input also goes to R330 to the base of Q37 to open the gate during slides for standard TB-303 operation. The value of R330 is set to open the gate with an input voltage over 4V. So anything between +2.5V and +4V will trigger the Slide In circuit only and not open the gate. Anything over +4V will trigger the slide circuit and open the gate.
Filter CV In. The tip of this jack goes through R332 to the filter CV input. The input is 1V/Oct. The usable range is typically 0V to +8V. This is a switched jack that is normally-closed. When no patch cable is inserted the tip is connected to the ac ground of the filter control input to eliminate the possibility of noise coming in through the floating tip of the unused connection. The final filter cutoff frequency is determined by several factors. First is the position of the cutoff pot. Second is the settings of all the other sources for filter modulation, including Env Mod, the accent sweep, and the Filter CV In. Additional sources for filter modulation with the full x0xi0 kit include VCO to FM, Filter Tracking, and ENV2 to Filter Mod. One important thing to be aware of is that the Filter CV input has a resting voltage of +3.25V because of the bias voltage created by Q9, R64 & R65. Keeping this in mind you will find that filter frequency will decrease with an external CV input that is less then 3.25V and increase with anything greater than 3.25V (assuming all other sources of filter mod are inactive). Biasing the filter modulation input was implemented in the TB-303 to increase the effectiveness of envelope modulation (ENV MOD). Since the TB-303 is primarily a bass machine, Roland decided it would be more useful for the filter envelope to be able to decrease the cutoff frequency as the depth of the filter envelope is increased, rather than just increase it to higher frequencies with less interesting higher harmonics. If you were to scope the filter envelope at the emitter of Q40 you would see that it shoots up to an amplitude of about 9V and decays to a minimum of about 0.7V. This signal then goes to the Env Mod pot before going to the filter mod summing node. At lower settings of the Env Mod pot (VR5) this bias voltage has a greater influence on the filter envelope signal, as the ratio of the resistance between pin 2 and pin 3 of the pot versus pin 1 and pin 2 becomes greater. This effectively offsets it more towards 3.25V rather than 0.7V. As the Env Mod pot is increased the less influence the bias voltage has on the filter envelope.
Swp Out. Accent Sweep was a term coined by Robin Whittle to name the filter sweep that is derived from the filter envelope during accents. This is the circuit based around D24, R46, C13, and the second gang of the resonance pot (VR4). When there’s an accent the control lines of the 4066 analog switch (pins 5 & 13) go high and the filter envelope goes to the accent pot. The accent pot determines the depth of the filter envelope that is sent to the VCA and the accent sweep circuit. It is sent to the VCA through R119, C36, and R22. The signal is attenuated and smoothed by R119, R120 & C36 and sent to the VCA control input. This is what makes accented notes louder. It is sent to the sweep circuit consisting of C13, R47, D24, and VR4. From looking at these components on the schematic you can see that at lower settings of resonance the sweep signal at pin 2 of the resonance pot is pretty much a direct copy of the filter envelope since all the signal goes from the accent pot to D24/R46 and straight to R72 to the summing node of the filter CV input. As resonance is increased C13 will be charged and discharged by the arrangement of D24, R46 and the voltage divider formed by the resonance pot between signal going straight to filter cutoff and that going to C13. The charging and discharging of C13 gives the rounding off of the attack and decay of the filter envelope signal and becomes the sweep signal. The signal at the Swp Out jack is a direct copy of this accent sweep normalized to 0V. The arrangement around IC304b simply creates a DC offset for the accent sweep signal. The resting voltage for this output is ~10mV (when calibrated with TM301). This was done to make it more compatible with external hardware. Normally the resting voltage of the filter sweep signal is ~3.4V. Since the sweep signal normally varies between 3.4V and 6V, taking a direct copy of it would eliminate most of the useful range when used as a control voltage. Below shows the normal accent sweep signal and the offset version that is available on the Swp Out jack :
The amplitude of the sweep signal is typically between 0v and 3v depending on the position of the accent and resonance controls. This output can be used just like any other CV to control filter modulation, frequency modulation, velocity, etc…
Acc Out. The accent signal from the collector of Q35 goes to the junction of R346 & R347. The other side of R347 goes to the new +12V supply. When there is no accent the collector of Q35 is 0V so the node between R346, R347, and D309 is also 0V. When the collector of Q35 is ~10V the voltage at this node goes to ~6V based on the values chosen for R346 and R347.
Audio Inputs & Outputs
VCO Out. This is the output the VCO (or VCO1 & VCO2 if you installed the full x0xi0 mod). This is a switched jack that is normally-closed. When a patch cable is inserted into this jack the circuit is opened between the VCO Output and the VCF Input. The signal level of the VCO output is generally in the range of 1 to 3 Vrms. The output impedance is 22k. C17 is an ac-coupling cap. The value of R342 is set to maintain the same signal level to the filter as the stock x0xb0x when no patch cable is plugged into the jack.
VCF In. This is an external audio input to the VCF. Connections Q and R are placeholders for the full x0xi0 mod as there is a level control for the VCF Input on the top panel. This input uses a switched jack that is normally-closed. When no patch is inserted the Noise signal from the main mod (when installed) is sent through connection Q to the level control. When a patch cable is inserted the white noise signal is disabled and the external input is sent to the VCF. When installing the x0xi0 I/O mod only there is jumper placed between connections Q and R since there is no level control. The input is ac-coupled by C323. The input impedance is 22k and is compatible with signals in the range of 500mV to 2V.
VCF Out. This is a switched jack that is normally-closed. When no patch cable is plugged in the VCF Out signal is sent to the VCA for normal operation. The value of R334 is chosen to give the correct signal level to the VCA when no patch cable is inserted. When a patch cable is inserted the VCF output is available on the tip of the jack and the circuit is open between the VCF and VCA. The output impedance is 47k. The VCF output is typically around 500mV.
VCA In. This is a switched jack that is normally-closed. The VCA input is very sensitive since the VCA is high gain, so when there is no patch cable inserted the VCA input is grounded. C327 is an ac-coupling cap. R345 is chosen so that the input to the VCA is compatible with signal levels in the range of 100mV to 1V.
The goal of the overdrive was to provide a lot of adjustability to achieve a broad range of sounds without the need to reach for other overdrive or distortion boxes. It would not have made much sense to integrate a simple overdrive without the additional controls as you would most likely still find the need to use external gear. In addition to controls for Amount, Tone, and Low Frequency contour, the overdrive also integrates two completely independent circuits selectable with the Type switch. It has an overdrive and a distortion. Double pole switches and dual gang pots were used for the Amount, Tone, and Low Freq controls so they work double duty for both circuits. In this way there is a big range of sonic character. The overdrive and distortion circuits are completely independent so there is no overlap in the resulting sound. The overdrive circuit will be described first.
SW301 selects whether the signal is sent to the overdrive or bypassed. When in the off position the overdrive circuit is completely bypassed and the signal from the x0xb0x mixer stage is sent directly to the mixout jack. When the on/off switch is turned on the signal is sent to SW302, which selects between the overdrive and distortion circuits. IC303 is a rail splitter that takes the output from the new 12V supply and creates a bias voltage of 6V. C330 is a bypass cap for the bias voltage to reduce any noise in the bias voltage. The circuit based around IC301a is the clipping stage. It is a non-inverting op amp with a gain control and clipping elements in the feedback loop. The overdrive Amount control (VR301) varies the gain between 1 and 200. D301, D302 & D303 are the clipping elements. The input signal is clipped as it is amplified and exceeds the forward voltage of the diodes. The diodes are placed in both directions so they clip on both positive and negative halves of the waveform. This circuit is generally referred to as an overdrive since the diodes are in the feedback path of the gain stage. Placing the diodes in the feedback loop results in what is generally referred to as soft-clipping since the non-inverting op amp always has at least unity-gain regardless of how much the diodes conduct, since the output of the non-inverting amp adds the input signal to the clipped signal. The overdrive circuit is placed post-VCA so the volume control is used to drive the overdrive – just like if you were using an external stomp box. The output of the x0xb0x can be anywhere from a few mV to 3V peak to peak or more. This is important to note since the behavior of the overdrive will vary depending on the signal level at the input to the overdrive. With normal setting and the volume control at the 3 o’clock position the output of the x0xb0x is usually around 2V peak to peak. Since the turn-on threshold of the diodes is around 0.5V to 1V it is easy to imagine that the subtlety of the soft-clipping will transition to more of a harder clipping as the overdrive amount is turned up, especially as the op amp itself starts to hit the supply rails. R302, C304, C305 & C306 create a high-pass filter in the gain stage. SW303 allows for selection between three different low frequency contours. The frequency response measurement for each position of the switch is shown below:
After the clipping stage there is a tone control stage to manage the mid to high frequency response. R304 and C308 create a first-order low pass filter at 1 kHz. IC301b, VR302 and related circuitry is an active tone control. The values for the tone control (VR302) and surrounding components have been chosen to give a wide range of tone. The frequency response for min, mid, & max settings is shown below:
The output of IC301b goes to an ac coupling cap C321 and then to the SW302. The second pole of SW302 then goes to SW301. The second pole of SW301 goes to the output of the x0xb0x.
The distortion circuit is selected when SW302 is in the down position. Q301 and related circuitry is a discrete gain stage. IC302b is another gain stage with a gain determined by the setting of the Amount control (VR301). C313 rolls off some high frequencies. Since it is placed in the feedback loop the cutoff frequency decreases with increased gain (as the ratio of resistance on each side of the amount pot is varied as gain is adjusted). After the gain stage are the clipping diodes D305 and D306. Contrary to overdrive designs, distortion circuits generally place the clipping diodes after the gain stage connected to ground, as opposed to the feedback loop. Placing diodes to ground after the gain stage results in what is generally referred to as hard-clipping since the entire signal is clipping after the gain stage. VR301, R319 and C314 create a first order high-pass filter. The cutoff frequency is dependant on the position of the VR301.The higher the setting of VR301 the higher the cutoff frequency, although the cutoff frequency is set low enough in this circuit that the low frequency response doesn’t change much for different settings of the Amount control. The low frequency characteristic is determined mostly by the C310 and C312. With higher settings of the Amount control the op amp can run out of headroom and will clip at the supply rails, this is part o the design. After the gain stage R320 and C316 create a first order low-pass filter. This takes some more of the edge off the higher harmonics of the overdriven signal. VR302 and the surrounding components create a passive tone control. R323 & C320 form a low-pass filter and C317, C318, C319, R321, R322 form a high-pass filter. The high-pass and low-pass sections are weighted by the setting of Tone control. Below is the frequency response with the Tone control set at the min, mid, and max settings:
The Low Freq switch changes the contour of the mid and low-mid frequencies by changing the characteristic of the high-pass filter section of the tone control. The up position retains a slightly scooped response by adding component C318 in parallel with C317. The middle position increases the scooped response and raises the center frequency by raising the cutoff frequency of the high-pass filter by de-selecting C318 & C319. The down position gives more of a flat frequency response by changing the high-pass filter by adding component C319 in parallel with C317. Below is the frequency response for the three settings of the Low Freq switch:
An effort was made to keep noise down as much as possible. Overdrive and distortion circuits can get quite noisy because of the high amount of gain, as the dynamic range of the signal is compressed and the noise is amplified during the gain stages. I’ve made a point of using low noise components and managing the gain structure to work well with the x0xb0x. However, I have still allowed for extreme amounts of gain for those who want to get a really filthy sound and there will still be audible noise at the max settings (this is especially true for the distortion circuit). Varying the settings for the volume, tone, and amount controls will greatly change the character of the distorted signal.
One important detail I have noticed after testing several x0xb0x’s is that there can be a lot of clicking if the VCA happens to have a lot of DC offset. BA662A’s generally good, BA662B’s are always bad, and BA6110’s vary from good to bad. This clicking is amplified during the overdrive or distortion stages and can get quite annoying in some cases. If you hear a lot of popping or clicking it is most likely the VCA.
There are a few additional things to note about the distortion circuit. When the x0xb0x level control is near max there will always be some distortion even if the Amount control is set to minimum. This is because the discrete gain stage is still clipping since it is before the op amp variable gain stage. Try adjusting the x0xb0x level control to something less than the maximum setting if you want a less harsh distortion. Also, it may go without saying, but instead of always maxing out the controls try adjusting the settings of the Amount control, the Tone control and the x0xb0x level control to greatly change the character of the distortion. One last thing to note is that the output level of the distortion circuit is less than the overdrive. I’ve purposely adjusted the levels so they are somewhat close but the output of the distortion will always be lower because there is no gain stage after the clipping diodes – only a passive tone control, so the once the signal is compressed by the behavior of the clipping diodes the signal level is lowered.
Bass Response. The low frequency response has been extended by one octave from 70Hz to 35Hz. If less bass is needed you can cut low frequency on the mixing board or within your digital audio workstation or wave editor. Leaving the bass response stock and trying to recover more low frequency later by using equalization would not be the ideal way to increase the low frequency response. The high-pass filter is before the VCA and there are distortions that occur in the VCA, output mixer, and overdrive, so it’s not equivalent to boost the low frequency at the final output of the x0xb0x. Below is the frequency response of a stock x0xb0x versus the new extended response:
VCF Range. The high range of the filter is increased from about 1.2 kHz to 3.5 kHz by simply setting TM3 to its lowest setting (full clockwise). The low range of the filter is also extended by replacing R47 with a lower resistance. This changes the minimum cutoff frequency from about 400 Hz to 80 Hz. Below is the frequency response for min and max cutoff for both the new settings and the stock settings:
Env Mod Range. R61 is replaced with a 0 Ohm resistor to give no envelope modulation when VR5 is set to minimum.
VCO FM. An audio signal can be input on the ring of the MIXIN jack to frequency modulate the VCO. C325 is a coupling cap. IC304a and R338, R339 and R340 form an inverting amplifier to amplify the input signal so that it can modulate the VCO to a greater amount. The signal is biased to the 6V supply to allow it to work on single supply. The output of the op amp goes to the frequency modulation input of the VCO through R341 and another coupling cap C326.
Filter Hi Range. This is a DPDT switch. One pole simply inserts R348 in parallel with R73 to increase the maximum cutoff frequency of the filter. The second pole is used to turn on an LED on the top panel when the switch is engaged. It is not used with the I/O only mod. When installing the full mod connection G goes to the top panel mod. When the switch is on the maximum filter cutoff frequency is increased to 14 KHz. The filter can start to become unstable with higher settings of resonance. The frequency response for both positions of the switch is shown below:
ENV2 to VCO2 Switch. This switch determines whether the ENV2 to VCO amount control on the full x0xi0 mod is sent to VCO2. By default it is hard-wired to VCO1 only. This switch is not used on the x0xi0 I/O only kit and can be used for any other mod.