Posts Tagged ‘WTPA v2.0’

WTPA2 4046-based V-to-F Circuit, take 2

Saturday, June 12th, 2010

Boom! Current sinks all the way:

Because I am late to a BBQ I will not bother here to kick on the corporate dog and grumble about how not a single Rat Shack in downtown Brooklyn sells solderless breadboards. I will however sing the praises of H&H (buy that book. I’m not kidding) and the value of the current sink in the previously discussed VCO clock circuit. I soldered this mess up on some perfboard and in short order had a VCO that happily went from DC to >> 1MHz! The linearity is TERRIBLE, but it proves that the range is there. I’m pretty sure I can fix it adequately for my needs with just fiddling with the passive components in this circuit. Hopefully. Here’s the schematic, captured in time honored analog fashion:

Those component values are way stupid, but I think this is the topology. I may yet get away without the op-amp. Live and learn. And happy BBQing.
xo
TMB

WTPA2, Current Sinks, and the 74HC4046

Wednesday, June 9th, 2010

So once again somebody on the forum came through and found this guy’s page about designing a switched capacitor filter, where he talks about using a CURRENT SOURCE (sink, actually) instead of a resistor on one of the 4046’s frequency setting pins. This was a real forehead smacker. Thanks Kyle (erschlagener) for the heads up, and thanks to Tim for putting that out there in the first place!

Requisite googling found an EDN note about this, too:

Which claims a 1000:1 frequency range out of the 4046’s VCO!
It also confirms what I’d found on the breadboard before, namely that the 4046’s frequency output is not particularly linear outside of about 1v-4.75v, and that inside there you can expect to get about a decade of range. So I felt less dumb.

This note also has the bright idea of putting the sink in the feedback loop of an opamp, which almost certainly helps linearize the sink’s behavior at low voltages by getting rid of base current related weirdness.

Enter a quick trip to findchips for some pricing data. Turns out that the 4046 and a transistor are certainly cheaper than the LM331, although they take up more real estate. Even throwing in an cheap LM358 still costs less than the LM331, although the savings start to get less significant. Now, if there was some good use for that second op-amp in a ‘358, that’d be something…

This recalls some quote I heard and want to attribute to Jim Williams, though I can’t recall exactly where it’s from, about how in modern circuits like 80% of the thing is digital and takes 20% of the time to design, whereas the 20% that’s analog takes the 80%. Sometimes it does seem true, although it might just be (for me anyway) that the analog parts are a lot of fun :-)

Back to the breadboard!
xo
TMB

WTPA2 Initial Hardware Release Announced! With a Pretty Case, too.

Monday, May 31st, 2010

First beta WTPA 2 release date got announced on the Narrat1ve Forum today. I promise to have pictures of a more-or-less-working printed circuit board HERE by July 1st. I’ve already got a rep at Future who I think can save me some dough in parts sourcing, which is good, and I’ve got initial hardware specs (RAM, rotary encoders, new MCU etc) done. The board is starting to come together too. Once all that happens the firmware will start to get changed and I’ll make sure the pretty case fits and looks nice.

WTPA2 Initial Clock Test

Friday, May 21st, 2010

Normally I hate solderless breadboards, but I was kindof inspired by doing recent work with Adafruit that they were OK for iterating through low frequency, proof-of-concepty kind of stuff.

And a good thing I was, too. WTPA2 was slated to have a VCO as a sample clock source, bringing that feature back from the murky WTPA v0.98 days. However, a standard 4046 clock had some troubles, at least as I breadboarded it.

Using a 5v supply, and a CV at the VCO input of 0-5v, and varying the values of the VCO resistors and caps I wasn’t able to get much better than a decade of frequency out of the part. I scribbled these numbers all down on various post-its that no longer make a lot of sense to me, but getting an FMax of 20KHz and an Fmin of <100Hz seemed pretty much impossible, both experimentally and once I bothered to do the math problems in the datasheet. Moreover (and I'd seen this before with the 4046 in video synth stuff) once the CV gets too close to 0v or too close to the positive rail, the oscillator tends to either stop or jump up in frequency. If you haven't already, it's worth referring to the 74HC4046 datasheet while reading this pontification, at the very least to convince yourself that I am not totally making this up.

This is not the most rigorous breakdown, I know, but it was enough to convince me that a standard 2 resistor, 1 cap, and and CV style 4046 VCO was not going to hack it without some kind of magic. I’d been curious about the LM331 V-to-F so I got its datasheet and did some more scratchpad math and convinced myself that it was a lot more likely to come close to what I wanted without serious massage. It’s quite a bit more expensive than the 4046, but at least it’s physically smaller.

WTPA2.0 Hardware Feature List Announced!

Monday, April 5th, 2010

It was with great pleasure that I posted the initial list of hardware features that will make it into the next hardware revision of WTPA on the Narrat1ve forum. Until you start seeing boards rolling out, there’s still time to change this stuff, so feel free to let me know what you think and what you’d like to see.

Here they are, as well as what they do for the kit and what they’ll do to the price:
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1.) WTPA 2 will have 8 controller buttons and not six (the very first one had 5 I think). This will allow us to avoid the dreaded 3-button combo when accessing functions. However, this means two more buttons, and one more shift register IC and socket. This costs more.

2.) There will be a through hole RAM chip. This means the whole kit will be through hole (no smt). This will probably cost about the same, but the board will need to get a little bigger.

3.) The analog sample clock will be based on a 4046 and not an op-amp relaxation oscillator. I can do this because I don’t need the extra op-amp section for overdubbing. This will cost a little less and will also allow CV input to the clock.

4.) The control pot will be replaced with an endless rotary encoder. This will help in all kinds of subtle ways. This may cost a hair more. Given enough pins, I may add a second encoder.

5.) The MCU will be either an Atmega644p or 324p, simply because I want the freedom to add new functions and we’re pretty much out of flash memory on the 164. This will also give us more RAM which may help some of the granularizing functions, and may be able to help with ISR speed if I’m smart about buffering. Those chips are still a 40-pin dip. They cost more.

6.) Some of the values in the analog section will change, which will result in slightly more headroom. This will probably cost about the same.

7.) The bus which is just hanging out doing nothing on the v1 WTPA will be used to access permanent memory. There will be an optional daughterboard which will contain a flash memory chip and will be able to store either 8, 16, or 32 samples depending on how some pricing and availability works out for me. I _might_ implement a sample loader via MIDI. Maybe. The daughterboard will come assembled (all that stuff is small smt biz) and will cost more than $20. Not sure how much more. So permanent storage will be optional.

8.) Instead of a hardware goodie bag, there will be another optional board which will have PCB mounted jacks for MIDI, Audio, and DC. Second to the RAM chip, most build problems in V1 were in the wiring. This should eliminate that problem. This board will mount under the main board. This will _probably_ be more expensive than the goodie bag, but may not be depending on some of the jack pricing.

9.) The routing from the input amp to the AVR will change A LOT. This should lower the noise floor a little.

10.) Because of all these changes, the board will be bigger. Probably not a lot bigger, though. My guess is the price will go up a little too.