Frequently Asked Questions
1. Do I have to use the Power/Regulation PCB?
A. Yes, for the VCO, VCA, VCF, Triple EG, Sequencer, and Trigger Converters need to have this PCB in order to properly connect, filter, and condition the power from your mains. I did this to conserve space behind the module to accommodate shallow Euro rack users.
2. Will these modules run on +/-12 volts?
A. Yes. Nyle has been testing each module on +/- 12 volts and so far they all work fine. You still need the Power/Regulation PCB for the VCO, VCA, VCF, Sequencer, and Trigger Converters. Some of these modules, eg: VCA, VCF, etc., have additional filtering on the power for parts of the circuit and these components are on the Power/Regulation PCB.
3. Do I have to order the Power/Regulation PCB separately?
A. No. When you order a module, you get all the PCBs you need to complete the module. For example, the VCO includes the VCO PCB, two trimmer converter PCBs, and the Power/Regulation PCB. For the Sequencer, you get the two sequencer PCBs, Power/Regulation PCB, and a Trigger Converter PCB. For the Triple EG, you get two ENVDL PCBs, one ENVDT, PCB, and the Power/Regulation PCB, etc.
4. Why S-Triggers and this conversion stuff?
A. I wanted to keep these modules original. Not a modern interpretation or tribute modules. To that end, I'm making precious few modifications to the original schematics. Nyle has made some modifications, but they're his designs, aren't they? Nyle decided on S-Triggers, and I like them better than voltage triggers as well. However, I understand most if not all modern stuff uses voltage triggers. I decided for me, I would just build a separate module and keep the other Synthasystem designs un-modified. When Nyle gave me permission to share this work, I realized not everyone would want to have to patch to a separate module. For the sequencer, due to board space, I decided to make a separate PCB to connect between the sequencer PCB and panel jacks. For other modules with one or two trigger inputs/outputs, I put the circuitry on the PCB with instructions on how to bypass it for those (like me) who want to use S-Triggers.
5. What value pot and capacitor should I use in the envelope generator?
A. Since 2 Meg pots are very hard to find, I recommend you use a 1 Meg audio/log taper pot for the Attack, Decay, and Duration potentiometers. Now, what value capacitor? The basic theory is if you cut the resistance in half, you should double the capacitance. However, a 7.8 or 2.4 electrolytic capacitor really doesn't exist. A good option is to go with a tantalum since you can get an 8.2u and a 2.7u. The drawback is the 8.2u costs close to $3.00 at Mouser. The 2.7 is about $0.71, a bit more reasonable.
Let's think about this circuit. If you use the original capacitance with a 1 meg pot, your minimum times for rise and fall will be comparable since both a 1 Meg and 2 Meg pot go to 0 ohms or close to 0 ohms. No difference. On the other end, the maximum rise and fall times will be about half with 1 Meg than with a 2 Meg. That may not be bad.
If you double or nearly double the capacitance, you'll get the same response in theory. However, since any given pot may not always go to a true 0 ohms, on the CCW side, you may see longer minimum times with a doubled capacitance than if you use the specified capacitor, 3.9u or (1.2u for the Duration on the ENVDT).
What to do? Well, I used the tantalum option with 1 Meg pots. I'm considering going with a smaller capacitor to get faster minimum times, but love it as it stands.
Really it's a matter of taste and use. If you want some really long maximum time constants, then go with doubling the capacitance and use the tantalums.
If you would rather keep faster minimum time constants, then use an electrolytic that's the next size smaller than the doubled value.
Last, you can get two electrolytics which add up to the exact doubled value and twist the leads together so they are in parallel and solder the combination in.
There is a caveat. EG3, the ENVDT circuit, has trouble with short triggers if you double the timing capacitor to make up for the lower value pot.
The "fix" is simple, you change the value of two resistors and you can double the capacitor and halve the pot. See the Triple EG page for details.
6. The high frequencies are flat. What can I do?
Due to how the saw core operates, as you increase the input voltage, the frequency will become increasingly flat. Nyle did not have a high frequency compensation circuit in the Synthasystem, but he did add one to the Microcon.
I approached Nyle about this and he offered a couple of ways to compensate for this. I played with both, and I've put together a small PCB which can be added to any of the VCO PCBs to correct this high frequency error. Of the two ways, this seemed to be the simplest to implement and it works well. See the HF Correction page for details.
7. What is a UJT and which one should I use?
A UJT is a unijunction transistor. It was used a lot to make simple oscillators before the 555 timing chip was created. They are archaic and a little hard to find.
The original circuits used 2N4891 UJT. I think I found a source for some at Nikko (Dalbani). I ordered some and will make sure they are real and work.
There are some options. I've tried several different UJTs and found the one that works best is a 2N4871 or NTE 6409. the 4871 seems to work best in the Sequencer. The 4871 works in the Sample and Hold as does the 6409.
8. What's up with all the SSM2210s? They are expensive!
Yes, they are. Bridechamber (www.bridechamber.com) has them for a good price.
When I was laying out the PCBs, I noticed Nyle used transistor pairs quite often, exponential converter, current mirrors, etc. The exponential pair clearly should be a matched Vbe pair of transistors. It doesn't have to be a monolithic pair, but it is so much easier. The other pairs, like in the VCF, VCA, waveshapers in the VCO, etc. are not nearly as critical. You can certainly just use some generic small signal NPNs/PNPs in these locations. Using hand matched or monolithic pairs for some of them like in the VCF and VCA, will help the overall performance. Will you hear it? I don't know. I've built them both ways and they sound great either way.
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