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| Deletions are marked like this. | Additions are marked like this. |
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| ==== Design notes ==== | ==== Design notes / mods ==== |
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| * VCF gain - Since the VCF does not have a volume control, and the output also varies depending upon cutoff frequency and resonance, its gain was set so that its loudest signal would not clip. This means that average signals will be lower (usually half the amplitude). The gain of the VCF amplifier is set by R26. If you want to increase the gain, you can reduce this value. | * VCF gain - Since the VCF does not have a volume control, and the output also varies depending upon cutoff frequency and resonance, its gain was set so that its loudest signal would not clip. This means that average signals will be lower (usually half the amplitude). The gain of the VCF amplifier is set by R26 with a default of 12k. You can drop this value to 10k (and increase the gain) without much worry of distortion. |
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| * VCF mix - Another possible solution to the VCF amplitude problem is to adjust the VCF input mix. As the resonance is turned up, the amplitude of the VCF output drops. By mixing some of the signal from the resonance pot wiper, you can get an increase in signal as the resonance is turned up. If you only use the wiper output, you will only get signal when there is resonance, and if you only use the main output, you will get a signal that varies greatly with resonance. To accomodate for this, a mix of 2:1 is used on the VCF output (same as is used internally in the 303 to feed the VCA). These are set with R30 and R34. You can try different values to see if you can get a better mix, or perhaps put a pot in place of both resistors so you can vary the mix (and volume) as needed. | * VCF mix - Another possible solution to the VCF amplitude problem is to adjust the VCF input mix. As the resonance is turned up, the amplitude of the VCF output drops. By mixing some of the signal from the resonance pot wiper, you can get an increase in signal as the resonance is turned up. A mix of 2:1 is used on the VCF output (same as is used internally in the 303 to feed the VCA). These are set with R30 and R34. Dropping R30 to 100k gives a more consistent amplitude over the resonance range. |
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8. Solder in the jacks, switches and pots. 9. Run jumper wires for the remaining headers which do not directly mate to the x0x-heart. |
8. Place the jacks, switches and pots, and attach the faceplate. Then solder the jacks and such in. This will ensure that they are perfectly aligned. 9. Run jumper wires for the remaining headers which do not directly mate to the x0x-heart. The header pins are labelled with J(N), and match the x0x-heart board. The pin numbers on the pacemaker match the pins they go to on the x0x-heart. |
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Since the components on the x0x-heart are facing the pacemaker, it is impossible to tune the x0x-heart when assembled. So, to tune, disconnect the board, but leave jumper wires in place. Next, run a jumper wire from the TUNE pot to the TUNE header on the x0x-heart. Now you can tune it as described in the x0x-heart manual. You will need to probe at J6 - pin1 for the VCO output. |
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| * [[attachment:pacemaker_bom.pdf|BOM .pdf]][[attachment:pacemaker_bom.ods|BOM .ods]] |
x0x-heart pacemaker (Eurorack adapter board)
About the pacemaker
The pacemaker is the shortest path to getting your x0x-heart beating in your modular setup. It pins out all of the knobs, and allows for CV control of frequency, gate, accent, slide, env-mod, decay, and cut-off. There are also switches for waveform select, external oscillators, and both the VCA and VCF outputs are available. The front panel was designed by Grayscale.
Design notes / mods
A few tradeoffs were made in the design of the pacemaker. Most of these were done in hopes of making it easy and inexpensive to build, at the cost of some component fiddling. So, as you build, you can modify your pacemaker to suit your needs.
- CV to PWM converter - This is probably the most finicky part of the circuit. A microcontroller would have been a better solution, except that would have required things that most people do not have on hand. So, instead it is based on a 555 timer. But, it is being run at a frequency in its upper limits, so it does not behave the same across all 555 timers. If you are picking up some new 555's, be sure to get an LM555 from Fairchild, as these are what the pacemaker was designed around. But don't worry if you don't have that exact part, it will still work. Just be sure to follow the test procedure in the Assembly tips below.
- VCA gain - The output of the x0x-heart varies greatly depending upon how it is being run. If the resonance is down, or accents are triggered, the output can be 10x that of other conditions. To accomodate for this a large gain was put on the VCA output stage, with the volume knob attenuating the signal before the gain stage. The gain is set by R29, for which the default (2.2k) ensures that even the smallest signal can be boosted to +/-5V. But, this means that the noise floor is also boosted, so for high level signals, where the volume pot will need to be turned down, there will be more noise than otherwise. So, if you want to increase the SNR at the expense of not getting full volume for 100% of the case, you can increase R29. A value of 10k would probably be fine 90% of the time.
- VCF gain - Since the VCF does not have a volume control, and the output also varies depending upon cutoff frequency and resonance, its gain was set so that its loudest signal would not clip. This means that average signals will be lower (usually half the amplitude). The gain of the VCF amplifier is set by R26 with a default of 12k. You can drop this value to 10k (and increase the gain) without much worry of distortion.
- VCF mix - Another possible solution to the VCF amplitude problem is to adjust the VCF input mix. As the resonance is turned up, the amplitude of the VCF output drops. By mixing some of the signal from the resonance pot wiper, you can get an increase in signal as the resonance is turned up. A mix of 2:1 is used on the VCF output (same as is used internally in the 303 to feed the VCA). These are set with R30 and R34. Dropping R30 to 100k gives a more consistent amplitude over the resonance range.
Assembly tips
You may want to build your pacemaker in the following order, to make it easier to solder.
- Solder female 0.1" SIP headers on the x0x-heart.
- Place male 0.1" SIP headers into the female headers, and then place the pacemaker onto the other side of the male headers. Now solder them in place. This will ensure accurate alignment between the boards.
- Solder remaining components to the bottom of the pacemaker (caps, regulator, and 0.1" DIP header for Eurorack power).
- Solder in the LM555 and LM339 (CV to PWM), and associated components. If you have the option, use an LM555 from Fairchild, as the circuit was designed around this. If you do not have one handy, don't worry, any imperfections can be accounted for in the next step. Do NOT solder in the positive input resistor chains at this time.
- Solder in 1 channel of input resistors, and verify that the CV to PWM circuit works. It should start making high pulses at 0.5V, and go completely high at 4.5V. If you do not have an oscilloscope to check this with, you can put a high efficiency LED (and 1k resistor) at the output of the LM339. The LED will have no light if there are no pulses. You can reverse the direction of the LED to check for pulses in the other direction. Other methods like using diode peak detectors, or just measuring with a multimeter will probably also work. If your circuit doesn't perform as you would like, adjust the 51k/36k resistor combos to shift the level into the right range.
- Solder in the remaining channels once you have verified that the values you have chosen work well. In most cases the default values will be just fine.
- Solder in the remaining ICs and passive components.
- Place the jacks, switches and pots, and attach the faceplate. Then solder the jacks and such in. This will ensure that they are perfectly aligned.
- Run jumper wires for the remaining headers which do not directly mate to the x0x-heart. The header pins are labelled with J(N), and match the x0x-heart board. The pin numbers on the pacemaker match the pins they go to on the x0x-heart.
- Test to see if everything works. Rinse and repeat.
- Wash the flux off the board.
Tuning the x0x-heart after installation
Since the components on the x0x-heart are facing the pacemaker, it is impossible to tune the x0x-heart when assembled. So, to tune, disconnect the board, but leave jumper wires in place. Next, run a jumper wire from the TUNE pot to the TUNE header on the x0x-heart. Now you can tune it as described in the x0x-heart manual. You will need to probe at J6 - pin1 for the VCO output.
