Starting to build some cadet modules and running into some issues with Mouser being sold out. Wanted to get a sanity check on these before making orders across multiple vendors (or buying a wrong sub)
Various SPDT On-On switches (Cadet VII Processor, IX VCO):
For the switch Iād just use a normal panel mount ON-ON mini (not sub-mini) toggle and use some old clipped resistor leads to connect it to the pcb. Iām pretty sure I used the substitution you mentioned for my first one or two Cadets, but itās not a perfect fitā¦ The legs are a tiny bit too short, so you have to put the switch in at a not exactly perpendicular angle to the front panel. Itās not a huge deal, and Iām not going to change mine, but on all the other Cadets and Castles Iāve built so far, Iāve just been using panel mount switches and I feel much better about it.
Example I saved from someoneās facebook post a while ago of how to do it with a panel mount switch (sorry forgot whose post!):
The capacitor substitutions sound fine to me. The things to match when it comes to capacitors is value (obviously), general type (electrolytic vs MLCC), voltage, tolerance, lead spacing, and you might as well match the dielectric too. Looks like you got that right in all those cases.
And if those restrictions are too tight to find a substitution, the voltage, tolerance, lead spacing, and dielectric donāt always have to exactly match the specād part, they just need to be good enough, but knowing when they are often requires a bit of understanding the circuit:
Most eurorack circuits will never deal with a voltage swing bigger than +12V to -12V (so 24V total) and often the way the capacitors are used in the circuits theyāre not seeing that max swing, so often 25V or even 16V caps can be used, but not always.
Tolerance of caps is like tolerance of resistorsā¦ higher tolerance is nicer but not always necessary depending on what itās purpose is (though in video accurate caps matter a bit more than audio).
Lead spacing wonāt affect the way the circuit works at all (unless the spacing causes you to have really long leads), but itās nice to not have to bend some 5mm-spaced leads to fit into 2.5mm-spaced holes and vice versa.
And the dielectric doesnāt always matter either, though sometimes you donāt have much choice for a desired capacitance and tolerance and voltage combo.
always try to buy as much as possible from the same supplier as possible - and from as few suppliers as possible - to reduce postage - so Iād either buy from mouser or digikey unless you can help it
substitutes for the capacitors at mouser - there seems to be an improved susbtitute finder at mouser - if you scroll down a bit you can tick what you need from the present spec and these just showed up first in the list and look ok to me
mouser switches are quite expensive - I got some from tayda, Iāve not used them yet, but had a bit of a play with them and they feel the same as the mouser ones - but about 1/5 the price or something
I always get my jack sockets from Thonk in batches of 50 - theyāre nearly half the price - but Thonk shipping to NL is not cheap, so I always try to add a lot of things I canāt get elsewhere in too
thonk also have reasonably priced switches - well, cheaper than mouser at least!!
potentiometers are pretty pricey - must have a look at ebay - as I will need some (a lot) more soon
looking at tayda for some resistors - they have a page where you can pick and mix 1/4W metal film resistors for $0.012 each - pretty handy
shrouded headers are also cheap from tayda - $0.18 each both 16 and 14 pin
they also have a coupon code at the moment - search for the Tayda Days thread on Muffs - itās updated as and when
pity they donāt do 0603 smd parts I need an extra 100nF capacitor
will check arrow theyāve been good for single parts with free shipping in the past!!! and they also have a sale at the moment!!
This feels like a silly question but can I use 3.9K resistors in place of the 3.92Ks in the sync gen? Also, for the 0r resistor - cool to just place a lowly old link in there? Thanks!
Great, thanks. Thatās what I was thinking. Iām more likely to find some that are maybe a little on the higher end of the spec than I am finding any 20r resistors to bridge them in my stash
Similar question about the 2.05k resistors on the Cadet VCO. Would 2k be okay? I measured all the 2k ones Iāve got (with a pretty cheap multimeter) and theyāre all within 1.98k to 2.00k.
Judging from a quick glance at the schematics, Iād guess this affects the triangle wave symmetry.
I canāt tell what a 2% change will do without crunching some numbers, but they are specified at 1% (so 2% is probably not a total failure).
Iām guessing you risk a visible asymmetry, but iād be surprised if it breaks everything
I have a few questions about the matched transistor pairs in the VCO BOM:
1: what do these transistor pair do? (as I gather, something to do with exponential CV input tracking? / drift)
2: Why is this important for a video oscillator? (Is it that you could control several VCOās in the same way without too much difference?)
3: What are the effects of non-matched pairs in this scenario?
In my own build I used non matched transistors (but from the same reel)
Iām building a system for somebody else now, so I want to be clear on these questions.
Q2A and Q2B are part of the exponential converter circuit.
Typically one of them does the exponentiation, and the other is used to compensate for temperature drift in the first one, so the degree of matching and the thermal contact between these will affect your tracking and offset over temperature.
What does a video oscillator gain from having an exponential response?
It lets a linear step in CV lead to a doubling in line density, which generally feels more ānaturalā.
It also means that if you have two oscās set to different frequencies you can
modulate both together with the same signal while keeping the ratio between them static.
iirc, the castle clock osc does not have an expco, so if you have that you can compare.
They are absolutely less critical than in an audio module, since the ear is very good at picking up on small errors.
For what itās worth, Iāve been thinking about trying to mod in a secondary linear input to one of my IXās.
What happens if theyāre not matched?
The tracking (i.e. how well a certain step in voltage correctly becomes a certain factor change in frequency) might change as the room or synth warms and cools. If your patches generally requires specific controlled frequencies or ratios, this will lead to you tuning a lot, while a properly compensated VCO is more a set and forget - type of deal.
Qās 4A, 4B, 5A, 5B, are all part of the linear triangle VCO core, combining the output of the exponential stage with comparator output and driving the integrator cap.
Iām not entirely sure what bad matching here will do, but iām guessing slight drifts and some amount of wave asymmetry.
i bought about 2-3 times as many transistors as i needed, did a rough matching by forward voltage across the base-emitter, and then pushed them together on the board after soldering - i.e. i assumed that i would never use this as a main audio osc and was willing to live with any consequences.
I have yet to regret that decision, but i do still have an entire lifetime in front of me full of opportunities to do so.
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