Archive for the ‘diy’ Category

hey linux networking peeps

I’m finally getting back to working on a new gateway/router server and I’m basically setting up this old-school sort of DMZ, with the rest of our servers hanging off one card, and our internal LAN/DHCP/NAT side hanging off the other. (Using ISC, which Debian seems to like.) And all of that seems to be right from the new server’s perspective, which is yay!

Except there’s no packet forwarding from the DHCP side even though it’s enabled and I’m sure I enabled it and yes the kernel thinks its enabled but it isn’t happening.

Any ideas where to start?

show this thursday!

I’m playing with Leannan Sidhe and we’re doing some of my material this Thursday at Shoreline Community College. We’re the 1pm band at SunFest 2017, at the Rock River outdoor theatre, on campus, near Building 1600.

While this is largely a campus event, there is bus service (331 from North King County, the 5 from the south, also… something like the 342 from Northgate?), and also visitor parking on the south side of campus, which it’s not expensive, and some street parking, which is free if you can get it.

Hope to see some of you there!

it’s the latest issue!

I’ve updated the Kitting Out Cheap guide to building your home recording environment on a low budget. New additions include short commentary on kit microphones, updated interface information, and pointing out that hey, guess what – USB chipsets matter!

So, hiya, Home Recording panel attendees – here y’go!

Project Kohaku A-B-C

I made another thing! A magnetic (electric) Irish bouzouki pickup, the kind used on electric guitars, but with some extra features. I retconned a project name for it (Kohaku A-B-C) because it has three primary modes – “single coil,” “humbucker,” and “electric/acoustic mix,” which to be honest is kind of two modes depending upon the single coil vs. humbucker setting, but Kohaku A-B-C-D is too long and didn’t sound enough like a JDF anti-Godzilla plan.


Originally I was just going to do a basic wire-up with a level knob, but then I found out that humbucker pickups gave you separate access to both coils, and so can be used as a single-coil pickup as well as in humbucker mode. So that meant adding a switch. Which means it’s probably worth doing a tone knob while I’m at it. And while I was thinking about that I realised there was no reason I couldn’t also build in a tap to the extant acoustic pickup, which means I should also add a little mixer to control relative levels of the two pickups.

So I did! That was a good idea.

The two red knobs are the mini-mixer for the electric (left) and acoustic (right) pickups. Infinite relative levels! Or close enough. The two big black knobs are combined output level (left) and tone (right), the little switch is tone disable switch, the bigger switch changes the pickup mode – humbucker or single-coil.

I’ve had a bunch of those black rocker switches for a while, but I rarely use them because you need to cut a fairly precise rectangle for them to snap in correctly – there’s no screws or anything. Hearing that [CLICK] when you do it right and it pops into place and is straight and not loose at all or anything is so satisfying.

The pickup is held in place in the soundhole basically by foam and friction. Here’s the first test sitting, after I’d added the high-density foam to the pickup itself:

That was almost enough by itself, but not quite. I ended up adding a backplane and some spacer elements, as it had a tendency to rotate in place without those. The hot glue is mostly there to be cable stress relief:

The project box is an old Radio Shack room monitor transmitter – basically, a baby monitor. I don’t remember how I got it or anything. Regardless, I tested it a few years ago and it’d gone bad, so I gutted it and kept the cases for projects. I think it cleaned up well!

I can’t find a picture of a truly unmodified example, but I did find a decent picture with this mod where that modder kept the words “REALISTIC” and “FM WIRELESS ROOM MONITOR” on the box. I’d wanted to keep the REALISTIC logo, too, but I couldn’t – the knobs wouldn’t stay in place flat with the word still there, so I scraped it off with an xacto knife.

I did keep the big TRANSMITTER label though. That seemed important.

Here’s that little rectangular switch. [CLICK] into place. So satisfying. Looks factory. ♥

Here are the large-block components in place, without the wiring, and also without the tone knob bypass switch which was actually a later addition. (One that turned out unnecessary – 800K resistance and infinite resistance are pretty much the same as far as this tone circuit goes.)

The circuit itself is very simple. The larger switch controls which lead from the pickup (humbucker or single-coil) is fed into the electric pickup side of the mixer. The two mixer knobs are just straight-up variable resistance pots, as is the combined-output level control. The tone knob is based on the 1950s-1960s Les Paul-style electric guitar tone knob, only with a smaller capacitor which goes pretty well with the zouk. And the tone-knob enable/disable switch is just a ground lift on the far side of the capacitor and variable resistance pot.

Now I just need to find some way to make the controller stay attached to the face of the zouk (or any part of the zouk really) without making any change to the zouk itself. Maybe a suction cup?

I wasn’t even really thinking about that part when I started this project, past “keep it small so maybe if I want to think about it later, I’ll have the option.”

It almost looks like some kind of foot pedal, doesn’t it? The reason the controls are so large is just so they’re easier to grab without looking. I’m already controlling this thing by feel – if I can come up with some way to attach it, I think it’ll be legitimately viable as a stage device. It’s certainly no fussier than that piezo pickup I made for the octave mandolin, and I’m using that.

So – yeah! Kohaku A-B-C: much more fun than I expected. If you want to make one, Here’s where you can get the pickup bar, the only thing I bought new. The pots are all 1M variable-resistance linear pots, the single coil/humbucker switch is type SPDT and should be pretty clean since the signal goes through it, the capacitor is… 14nF film cap, because I had it and it was handy, and the tone bypass switch isn’t worth doing but if you want to regardless just use anything that’ll let you connect a wire to ground – it’s not in the signal path so you don’t have to worry about quality. The knobs were from the Radio Shack bankruptcy sale at 90% off, and since they’re going through another round of that now, maybe you can get some 90% off too. Or just use the metal posts for an industrial look, it’s not like you need the knobs.

As always, higher-resolution pictures are on Flickr. Have fun, and if you build it, let me know! ^_^

a user interface question

In an audio device with a tone knob, and an on/off switch by that tone knob, which “ON” makes more UI sense?

[poll id=”29″]

This is an honest question. I mean, I’m going to wire it up so that “ON” means “tone knob on,” because that’s how I roll, but I see this go the other way a lot.

(Also in this case “tone knob” means “low pass filter” which means “removes high frequencies” because AND THERE YOU GO. See? SEE?)

“kohaku” electric test harness nr. 1

Literally the first signals off the electric test harness for Kokahu.

Recording is from a couple of days ago. There have been meaningful additional developments since this, but this is what I’m posting now.

How many electric Irish bouzoukis are there in North America? One more than before last week.

More to come.

a late addition to an old report

Remember a long time ago – like, a little over three years ago – I built a ribbon microphone? I had all kinds of problems chasing radio interference ghosts and stuff, it was strange and messy but came out with a neat sound in the end.

Except… even after fixing the RF problems, it was kind of noisy. Not unusably so, not for direct-miking, which is how I’ve used it, but still… kinda noisy. Noisier than it should’ve been.

I rediscovered this when trying to use it in a “mid-side” type mic setup with the new RK-47, which I was doing just to see how that would work. (Tony of Vixy & Tony has been after me to try that for a while.) And because it involves playing towards a figure-eight microphone from the side – the point of least sensitivity – it required enough extra gain that the noisiness became a problem.

Since the special preamp (also a kit) was the entry point for the RF noise, and since said amp works with dynamic microphones, I tested that for noise, using an SM-58 as input. Dead silent, cranked all the way up. Result: it wasn’t the microphone preamp’s fault.

Then I remembered how the RK-47/990B build manual talked so much about making damn sure you had no solder rosin or finger oils at the high-impedence connection points in the circuit, and to just scrub those connections with isopropyl alcohol. So I took apart the ribbon microphone, redid those solder connections while I was in there, and then scrubbed the hell out of them. A downright confusing amount of old solder rosin came up when I did so.

Result? Problem sorted. Huge drop in noise. There’s still a little at the high end at probably more gain than I even need here – this may be an “only elves can hear this” moment, at least in part – but a little -3db cut starting at 14-15kHz sorts it. It might not even be true noise, it could be something like air movement – ribbons will pick that up in ways nothing else will, and I didn’t turn off the HVAC, etc.

So, yeah! Turns out that finicky bit about solder rosin and flux is real important, kind of generically, at high impedance. Good to know. (And is why I’m posting this, and why I’ll link to it from the old microphone buildout writeups.)

I still have not the vaguest idea why so much rosin ended up on those connection points. Seriously, it’s weird. That was my old stock of Radio Shack silver solder, which I’d had since I Don’t Even Know When, and not the BenzOMatic solder that gave me so many problems. I never noticed it doing that before – but then again, I wasn’t really looking. ¯\(ツ)/¯

Anyway, have a test recording I made at 1:30 yesterday morning trying out that configuration, with the Micparts RK-47/990B kit mic being used as the “mid” and the above-mentioned Austin OTA-1 ribbon microphone as the “side.” It’s intermixed with a recording made simultaneously using a pair of M-Audio Novas in a spread X-Y configuration. Both versions are mixed directly to mono, rather than spread-stereo, which is not what you usually do, but does allow maximum left-right placement in a mix.

(This may be called “T” rather than “mid-side” since mid-side includes a kind of subtractive mixing not used here? I dunno. But this is, again, just a straight mix to mono.)

The recording starts with the RK47/990B plus OTA 1 pair, then switches back and forth between that and the Novas. Remember: all of this is mono.

Kinda neat, eh?

let’s listen to an rk-47 microphone!

Build reports are nice enough. (I wrote up a little errata post yesterday, by the by.) But the real question, of course, is how does the RK-47/990B kit mic sound?

Early impressions are surprisingly good. Even with only the single microphone, there’s a sense of presence and space – even with a purely mono signal path – that I normally have to dual-mic to attain. Also, it has tremendous precision – this is a mic capable of great subtlety. And the amount of gain built into the microphone itself is crazy – this is one spicy meatball of a microphone. That’s something you won’t hear in recordings, but it results in a lower noise floor, which is always good.

Let’s start with some unsubtle differences, ones that’ll show up on laptop speakers. Because while I’ve never liked the MXL-990, they sell a zillion of ’em, and we should make a couple of direct comparisons.

Here’s a snippet of chords from “Lukey,” alternating between the MXL-990 (unaltered factory) and the RK-47/990B. It starts with the MXL-990, then transitions in-song to the RK-47, then back and forth. It ends with the RK-47. It’s a pure mono signal path until prepped for uploading to Soundcloud.

And here’s a short melody, on zouk – again, starting on MXL-990 (factory stock), then RK-47, then back to MXL-990. The last phrase is repeated to allow us to end on the RK-47; also, I wanted that ending bit to be presented on both microphones. The glissando really highlights some of the differences.

But that’s shooting fish in a barrel, as it were. The MXL-990, while popular, is not a good microphone. We should do comparisons to microphones I actually like – let’s say, the M-Audio Nova. At about twice the price of the MXL-990, it’s still a cheap microphone, just one I consider entry-level competent. But it has issues – not the least of which being it’s kind of a noisy beast as these categories of microphones go.

So let’s take the easy swing – here’s a sharply boosted noise level comparison of the RK-47 to the M-Audio Nova, at equivalent gain levels. This is not the noise you’d actually hear; I recorded a silent room at gain appropriate on each microphone for instrument recording, then cranked that recording up 32db for easy noise levels comparison.

Unfortunately, this really requires headphones, because it’s RK-47 on left, Nova on right:

NOT SUBTLE. But also, an easy shot. The Nova is noisy and everybody knows it. There are some mods out there to improve that, but they change the sound a bit in ways I don’t like, so I work with it.

comparison of waveformsSo let’s dig down a bit. Pictured here is a snippet of waveform from a bit of music played, in mono, over my studio monitors, into identically positioned microphones relative to those speakers. These two recordings were made simultaneously.

You’ll note in this highly-zoomed-in render how the RK47 waveform remains clear and unmuddled in these extremely rapid changes, while the Nova’s blurs into a bit of a mush. That’s the sort of thing I’m talking about, and also, the sort of thing you can hear in these very short snippets of horns from a jazz track. This comparison requires headphones, possibly good ones:

They’re short because they must be uncompressed for best comparison – sorry about that – but listen to them a few times and compare. Note how the edges of detail – bits which add flavour – are blurred in the Nova, but retained in the RK-47. Neat, eh?

That out of the way, let’s step up a level in comparator microphones. Oktava 012s are considered very good affordable microphones, particularly strong in their price ranges, and street for a new 012 and one pickup is comparable to the cost of this kit. With a second head (to add a second pickup pattern, as this mic has), it’s a bit more. They’re small-can capacitor instrument mics, rather than large-can, but we’re doing instrument recording, so that’s fair. The components inside – particularly older ones picked up used – can be a bit dodgy, but the design is great and the pickups are great, and you can upgrade the iffy capacitors and the suspect transistor if necessary. I have, of course, done this with mine.

Here’s the intro to “King of Elfland’s Daughter,” on the Oktava 012 (upgraded components) and the RK-47/990B kit. This recording repeats phrases, with the Oktava 012 first, then the RK-47/990B. Pure mono signal path, identical recording setup made within a few minutes of each other, but not simultaneously, as you can’t put two microphones in exactly the same place and I wanted the most equal comparison I could, modulo performance limitations. This probably also requires headphones, as the 012 is a pretty darned precise microphone itself:

44.1khz/16-bit uncompressed WAV file version here.

Once again, I’m finding that the RK-47 has a real staging advantage. There’s a sense of in-the-room presence with the RK-47 that I can make happen by dual-miking with my other microphones and mixing down, but not directly in mono.

Now, I don’t want to leave the impression that it is BEST AT ALL THINGS, because it’s not. These aren’t the only recordings I made – they’re just ones that show differences best. The first example I found was mandolin – the Nova likes my mandolin better than the RK-47 does. The specific response behaviour and foibles of the Nova work in its favour; a single RK-47 may have more presence and precision than a single Nova, but the Nova recording sounded more musical just the same. I’m sure there will be other examples as well.

In the end, I think this will probably become a heavy-use microphone in my kit. It may even become my go-to mic on the zouk – I need to do some stereo and multi-distance comparisons before I will know that for sure, but it’s looking very good. I also like what it does with piccolo and flute. I haven’t done any playing around with fiddle or drums, and one thing I want to play with is a two-mic setup with the ribbon kit mic I built, to see how those behave together – it’s a mic placement technique I’ve wanted to try for a while, but have never got round to testing. Now is probably the time.

I kind of wish I’d ordered the matched-pair version of this microphone kit. But it would’ve cost twice as much and I couldn’t know in advance I’d like it this much, so.

a bit of errata for the rk-47 build report

A bit of errata on the micparts rk-47/990B kit microphone build report yesterday:

First, I didn’t mention that the board kit includes extra capacitors, to be used either for different capsules than I chose, or to tailor the response curve of the microphone. I went for flattest response, but I could’ve had more or less high-end responsiveness by choosing which included tone capacitors to use.

Second, I said that the “omni” switch setting probably wasn’t really going to be actually “omni,” but was more likely to be figure-eight or the like. This was in response to both the capsule design, grille design, and older pictures of the circuit board which appeared to indicate that. In testing, I discovered that I was mistaken – as you can see here, levels as seen in waveforms made by holding a tone generator at the four cardinal directions remain constant. But read on, after the graphs, because there’s a catch:

rk-47 set to omni, against tone generator at 30cm, front, right, rear, left positions

And here’s the same test, with the mic set to cardioid:

rk-47 set to cardioid, against tone generator at 30cm, front, right, rear, left positions

And here’s an M-Audio Nova, same conditions, for comparison:

nova, cardioid, against tone generator at 30cm, front, right, rear, left positions

The total pickup is certainly omnidirectional. But this is not a true basket head on this thing, there are support brackets on the left and right sides, and that does affect tone! High-end harmonics definitely fall off on the left and right sides.

So while it does qualify as an omnidirectional microphone in “omni” mode, I’d have to call it flawed in that mode, and use something else – like an Oktava 012 with the omnidirectional head, which is built for true omnidirectional pickup and does not exhibit this behaviour.

Alternately, I suppose it might be usable in that mode if you’re recording someone who is a tad screechy – aim the wrong side at the performer and presto, fewer high harmonics. EVERYBODY WINS XD

let’s build an rk-47 kit microphone!

After a few suggestions that I do so, I finally went ahead and splurged for another kit microphone – the micparts RK-47 capsule and amplifier circuit board, both sized to use the dirt-common MXL-990 microphone case.

The MXL-990 is kind of a go-to microphone for people just starting at home recording. I don’t recommend it, even in that role; I think it’s a nasty piece of kit and sounds terrible. Even at $50 – the used price, roughly – you’re better off spending another $40 and getting a low-end AKG or Nova. But it is about the cheapest condenser microphone with XLR out that you could pretend to treat like a real mic, and that has meant a thriving mod community.

Here are the basic parts included with the kit. I didn’t have the capsule out – the actual thing that vibrates to sound – because it needed to stay in its protective box until installation.

This is technically an MXL-990 mod, one wherein nothing is kept but the case and socket. It’s also a nontrivial assembly, so I’m going to talk about building in this one, and have some sound samples in the next.

You can see the instruction booklet over on the right. It’s quite clear, and has some important commentary, about extra care being needed not to touch certain components any more than you have to, and which parts are particularly sensitive to heat. It does not say up front that about 25 pages in it will suggest that you use a couple of chemical products if you have them, so I ended up having to order some purple loctite and conformance coating.

If I had it to do again, I probably wouldn’t order the conformance coating, because the MDA sheet on that is not attractive. But now that I have it, I might as well use it. If anyone local needs some, let me know.

As you can see, the board is clearly labelled:

For the most part, this is one of those insert-and-solder projects. The tolerances are reasonable, though they were emphatic about working at a meaningfully lower iron temperature than I prefer. If you use 60/40 lead solder, their temperature would be fine.

(I ran at 350°C, which is a little hotter than they recommend, but I use silver solder, and it seemed to work out. In these cases, I solder only one leg from each component, then cycle back to the start, and do the second leg, then cycle around again for the third if necessary. This lets the heat that is building up in the component itself dissipate, reducing peak temperature inside the component. This is most important with semiconductors, usually – particularly transistors.)

They also have you cleaning the board with isopropyl alcohol regularly, and suggest cleaning your iron between each solder point. I don’t know if they just meant wiping the tip or actually using the sal ammoniac, so I kind of compromised. (Though for the most sensitive components, I did do a complete sal ammoniac cleaning between each point.)

This is the board essentially complete:

The process is a bit fiddly, I have to say. They’re quite strict in the manual about flux cleanup and so on, so you’ll end up using tweezers and foam swabs and lint-free cloth and and and. Still, with kits this expensive, it’s best not to take chances.

You might note that the connections on the switch are not made yet; that’s because you’re wiring two of the leads from the pickup to the circuit at these points directly. Those connections are where they’re most persnickety about how things are to be done – but I followed their directions precisely and it worked out.

Here’s the new RK-47 pickup canister in its mount:

I’m liking this canister already, at this point. I know, I know, “good feel” is silly, but this capacitor pickup has a good materials feel to it. The mounting system you see in use is also partly a replacement – the original was a ring mount system; this is a saddle, larger, and is also where you need the threadlocker. I’m not convinced MXL used a threadlocker on that post, but I see how the larger, heavier pickup does kind of call for it.

Once the board is mounted back into the bracket and grille, you end up with this:

And once you’ve wired the three leads from the pickup and the three from the XLR connector, you’re pretty much ready to test.

I did have an initially-worrying test result: voltage at the primary test point – the one you conduct before any audio checks – was running high. The manual had a lot to say about what you’ve probably done wrong if it runs low, and further tests to isolate your mistake, and spends a couple of pages going on making sure you’ve got a bare minimum of 45V going to the exciter, with a preferred minimum of 50v, and an “operating range” of 50-62v, where 60v is optimal.

The range on mine ran from a low of 62.45v to a high of around 68v, so I was concerned. But it turns out the actual operating range is 50v-70v – they just don’t expect anybody to be able to get to the upper end. Support even said that if I was mostly recording quiet material I could safely crank it up to 65v full-time for an even hotter pickup, but, heh, that’s not what I generally do, so I’ll leave it where it is. XD

As for the sticker – I put “MP-SVS” over the “MXL” logo because at this point, it’s not an MXL microphone in any way, so why would I keep that? It’s just silly. MP-SVS is MicParts-Supervillain Studios, naturally. I also changed the model number to RK-47, after the canister number.

I did not mention before that this is a switch-selectable dual-pickup-pattern microphone:

That will let me try an instrument miking style I’ve wanted to try for a while anyway. The only downside of using an existing microphone housing is that the switch has to live on the inside, and you have to unscrew the bottom of the housing to get to it. But it’s just twist-open, so not really an issue.

Regardless, as you can see, the switch says “cart” (for cartioid) and “omni” (for omnidirectional), but I think I’m pretty sure it’s really more figure-eight, given the construction of everything and what the switch does. And pictures in the manual actually agree with that – the printing on the board is different in the photos.

An optional modification to the case involves coating the lower half with silicate caulk (or a couple of other listed materials). I, of course, did that too:

The idea is that this reduces case resonance. Does it? I dunno, I just went ahead and did it because silicon caulk is dirt cheap – the local hardware store down the hill had a small tube for three dollars. The lack of silicon at the top is necessary – the upper part of the case scrapes into the lower half quite tightly, and any added material would be scraped off right away. As you can probably see, it’s even scraping off its own coating.

This is what it looks like all put together, and put in the standard MXL-990 shock mount:

As always, larger versions of the photos are at my Flickr account.

All in all, I’d call it a straightforward well-documented build. It’s far from the most difficult I’ve done. Not a good starter project, but if you’re reasonably good with a soldering iron, you’re going to be fine.

And this is already a long post, so I’ve decided to make it Part 1 of 2, and will talk about the important part – what the result sounds like – next time. Spoiler: a whole hell of a lot better than an MXL-990. But it also has audibly (and visibly, in the waveform) finer and more precise pickup than, say, my Novas.

I’m also going to see if I can’t do some comparisons to my Oktava 319 and possibly the (already-improved via mods) Oktava 012s as well before next time. That, too, might be interesting.

Have a good weekend, everybody!

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