Saturday, March 2, 2013

First Xanu Module EVAR: The Xanu Delay

Before I get started, I'd like to quote an earlier post:
"Q: If you plan to turn this into a company, why do you post all of your schematics and explain how they work?
A: Why not? I mean, I technically own all of my designs, therefore I should be able to use them however I'd like. If you think about it, I'm just saving you time. You can go online and see the PCBs of any module, and you can look at the connections and build one yourself. I'm just saving you some time."

Ok, now, onwards to the subject.

I recently was talking about a module idea I had, a digital delay. Well, after figuring it out a bit and submitting the idea to Muff Wiggler, I'm ready to post a little more about it. It seems like it could be a rather successful delay.

First, general circuit design. What I can post isn't much because the actual delay circuit is rather large and I don't feel like drawing it, but you can get a general idea:
Click to make it larger. Something you may notice is how strangely the delay time is controlled. I used a FET in place of the actual time resistor, which means several things: VC is easy, delay time can be INSANELY long (given the FET is pushed to its limits), and it's really the only way I could have made it VC given the delay line I'm using (PT2399's). The problem with using FETs is that, to use them as resistors, you need to apply a negative voltage to the Gate (hence the inverter). Positively biasing them for a long time may make them melt, and it won't act as a resistor (well, technically when it melts it'll be a resistor of nearly infinite resistance and thus really long delay times, but whatever).

Enough technical stuff. Let's move on to ALL THOSE FREAKING ATTENUATORS WTF. Yeah, so there might be a few attenuators, but it's for a good reason. It means you can mix various signals very precisely and get a ton of different delays. And, given the somewhat high-gain output amp (gain of 11), there can be no volume loss by the end of the chain. You'll also note the input attenuator (the fixed resistors). That's useful because modular signals are typically pretty hot (high-ish voltage) and could probably very easily burn out the delay chip as well as clip through the output amp.

You also have the delay time manual control. This is actually just an attenuator lowering the voltage of the main power source to the FET. The delay time circuit also has a switched jack, so you can use manual delay time or control it via an external voltage. And, as a final note, You'll note that there is a Range rheostat on the same line as the FET. This isn't exactly necessary and could easily be replaced with just a 1k resistor (it needs some resistance), but including the Range control allows finer control on the delay time, which is good for manually syncing it with some kind of clock source. If I figure out how to automatically sync it with a clock, I'll let you know.

Now that that's generally explained, here's the front panel:
It's pretty basic, like most of my designs. But I did label this one and add lines on the knobs. Makes it more awesomer n stuff. The two outputs are exactly identical, I guess just because most people use Delays at the end of their signal chain and it makes it easy to interface with outbound gear. I may make it just mono, though, I dunno. I also grouped things: time controls on top, audio controls on bottom. I may move the Time-CV input to the bottom, though.

Controls:
Input volume (input signal volume)
Output volume (total effect volume)
Feedback (literally feeds delay signal back through the delay)
Original (volume of input)
Delay (volume of delay, 0% bypasses delay circuit)
Time (delay time within range)
Range (sets max delay time)
CV amount (controls how much incoming CV controls delay Time)

Specs:
CMOS-based with HQ AD and DA converters
less than -90dB noise
less than .5% THD
44kHz sample rate
Max delay time: ~2 seconds (guessing, will probably be more via the FET)
12hp
~15mm deep, will have to check when it's done
currently unknown current draw, but I doubt it's much.

Now, the bad part: everything works perfectly... except the actual delay circuit. It does delay, which is kinda expected, but for whatever reason you can hear the internal VCO and it has constant feedback. So basically it's a constant rhythmic noise source. I blame cats. But, this is more likely due to not having EXACTLY the right parts (digital circuits are so picky), so when I get/find the right parts I'll remake it and try again. I mean, who seriously just has 3900pF, 590pF, and .082uF caps on hand, let alone random resistor values? Though I did just get my Futurlec shipment in (finally), so maybe they're in there.
Now, bear in mind I can't make it or sell it RIGHT NOW, because I still need a few materials (12hp faceplate, jacks, knobs, etc.), but it generally works. I just need to fine-tune it a bit.

Neat fact: I'm literally only using this delay chip because Korg used it in their monotron Delay. And, I mean, it's made by Princeton Tech and has very few external parts compared to other delay lines. You really can't beat that. And before you ask, no, I'm not going to use BBDs and make it analogue, although the general principles can apply. I've never seen them have very long delay times (usually less than 1 second) and they cost a LOT more -- these were like $8 for 20 chips. BBDs are $8 for ONE.

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