Conjunction junction

Well, after getting an arbor for a joint jigger, learning how to use a hole saw, and wheedling some welding this ship is finally RIDEABLE! I used two 3/8″ steel rods to brace the seatpost against the rear dropouts. Check it:

A close-up of the seatpost:

And another of the axle, with temporary bolts installed; those have since been replaced with stronger steel bolts:

The next challenge is to figure out how to mount the radiobox parts. There’s still about 8′ of 3/8″ steel rod left, so I’m planning on bending it into a rear rack that I can hang the amplifier from. I’m not sure where to put the subwoofer and the enclosure yet, but it might fit underneath the front part of the frame. We’ll see.

In the meantime, I’ve set up and tested the radiobox:

Radiobox Testing

It’s a relatively simple device. An iPeed with a split output feeds the amp:

And I’ve rigged up a computer power supply to provide +12VDC:

which runs through a multimeter so that I can monitor the load and size the batteries properly. The radiobox parts all work, which is great, and it draws around 4 amps at a significant volume, so I’ll be getting a pretty hefty battery to keep this thing making noise for 4-6 hours at a stretch.

30 Comments so far

  1. chris clark on February 16th, 2010

    I didn’t think they still made amps without an internal bridge.

  2. chris clark on February 16th, 2010

    So, it looks like you’ve got the Ipod split into R L channels and then parallelled out into 2 sets of signal. Then out to 2 speakers. Why the doubled signal? The Ipod puts out at line level or phone level? I think they only do phone level, if that’s true, you could probably drop your total power draw and distortion with a small pre-amp.

  3. chris clark on February 16th, 2010

    Oh, and your rear tire seems to be mounted backwards.

  4. h4ckw0r7h on February 16th, 2010

    There are non-bridgeable marine amps out there; Pyle makes one, model PLMRA400. I found that there was a lot of exaggeration in the amplifier market, mostly surrounding peak vs RMS wattage, so I spent a lot of time searching for and reading the manufacturer’s product manuals looking for detailed specifications. It was surprising what corners were cut on the lower end of the amp market.
    The iPod’s split into two inputs because the amp uses one high level input for channels 1/2 and a second high level input for channels 3/4. Thanks for the tip on the pre-amp, that sounds like something I can add later. I’m also considering a class D amp for just the subwoofer, but that increases the cost and weight.
    I don’t think the tread on the rear tire has any sort of V pattern, so the orientation doesn’t matter, although the bright red arrow on the sidewall is pretty noticeable. When the tire does have a V pattern, I mount it facing forwards on the rear tire and backwards on the front tire (thrust vs. brake), although on this bicycle I’d probably mount the V facing backwards on the rear tire for additional support during braking.

  5. chris clark on February 16th, 2010

    Yeah, adding a pre-amp later is just PnP. So whenever, if it’s even worthwhile, but my thought is that if it does really affect battery life (an it should)you might want to do that before they’re bought. So, you are using the 3/4 channels? Sub on a combined R/L mono signal? I may have a little crossover network I’m not using here that would handle bridging and band-pass for the sub if you’d like I’ll take a look. Specialized is weird with their tires, they seem to specify a direction for everything, I’ve got a set of pretty pure and sparse knobbies and they still specify a direction for that. On auto/motorcycle tires that matters in terms of casing life because it has to do with pry-wrap direction but I can’t imagine that’s a real issue with bike tires. I’ve found that with tightly spaced knobbies in the rear, backwards mounting tends to force water into the contact patch and traction really suffers, mainly in the winter. But yeah amp marketers are a pretty dishonest bunch always marketing by peak power when RMS is often less than half that and no/vague reference to distortion at rated power peak or RMS, some of those curves get really steep. It’s irritating and intentionally frustrating.

  6. h4ckw0r7h on February 16th, 2010

    Enh, I already bought the batteries :). 35Ah, should be enough and then some to power the system for 4-5 hours. I’m using the 3/4 channels bridged to drive the sub. There’s a band-pass filter built in to the amp, too.

    Honestly the knobbies are just for looks. On my commuter bike I ride a relatively smooth tread tire when it’s not icy out and notice a big change in the stickiness when handling the bicycle. If these tires don’t handle well in rain I’ll replace them with something smoother.

    I just did some subwoofer (PLMR W10) enclosure calculations and they show that I should use a bigger enclosure than the manufacturer recommends — 2.18 ft^3 vs 0.6 to 1.0 ft^3. I just plugged in the manufacturer’s T/s parameters, so I wonder if Pyle is intentionally recommending a smaller enclosure and if so what their reasons are. Any ideas?

  7. chris clark on February 16th, 2010

    So far as for the enclosure size, two offhand guesses, they’re using the enclosure to optimize a different part of the frequency range. Or just to sell more speakers. There’s marketing advantage to claiming you can achieve similar sound from a smaller box since it’ll be easier to fit into the cramped quarters of a vehicle. That is a pretty big difference though, but a .6 ft^3 box is really tiny, that seems silly to start with. But are you guys apples to apples? Is there rec. and your calculation based on the same kind of box? You’re going to have different size recs. for ported and sealed boxes.

    Yeah I run slicks on all my bikes intended for anything remotely hard. Ice aside, Ice is just different. Knobbies are better for sand, mud, snow, stuff like that but on hard Surfaces it’s all about compound and siping. But I do kind of like the buzz big knobbies make on smooth concrete.

  8. h4ckw0r7h on February 16th, 2010

    The recommended box size for a sealed enclosure is 0.6 – 1.0 ft^3, for ported is 0.8 – 1.25 ft^3. But this is Pyle, which is known for marketing hyperbole, so I suspect that my own calculations are more accurate. They’re based on a sealed enclosure. Assuming my calculations are right, and I pack the enclosure with polyfill or something similar, I’d need a cylindrical enclosure (concrete voids) measuring 10.375″ ID x 33.42″ high. I think I’ll need to build an isobaric enclosure, though, to fit it on the bicycle, since there’s no way to mount a 33″ long tube. The isobaric enclosure would only be 15.32″ high, with one speaker extending 4.5″ out from the enclosure. I can mount something that’s 19.82″ long x 11″ O.D. That creates another challenge, though — I’d need another amp to drive both subs.

    The studded tires sound like popcorn on concrete. 🙂

  9. chris clark on February 16th, 2010

    I don’t see any reason you’d need the second amp. You’ve already got a 4-channel amp, right? combine the second R L channels into one and split it out to channels 3 and 4. How long are forks? You’ve got ape hangers and long forks don’t you? Seems like that must come out close to your 33″. Mount the speaker at the bottom and a Simon at the top of the tube angled and styled to look like some sort of control panel you’ll be stylin’.

    And studded car tires sound like the road’s wet.

  10. h4ckw0r7h on February 16th, 2010

    Right, 3 and 4 are bridged into one channel for the subwoofer. But in order to attach two 4 ohm subwoofers to the bridged channel, the resistance decreases to 2 ohms, and this 4-channel amp needs 4 ohms of load on the bridged channel otherwise it will overheat. I can’t seem to find reasonably priced 8 ohm marine subs.

    The other reason to get a monoblock amp is that Class D amps are *much* more energy efficient.

    The long fork creates a significant tendency for the front wheel to want to flip around backwards, so I have to actively resist that when steering. If the subwoofer enclosure is mounted on the FRONT part of the fork, which is the only part long enough to accomodate a 37.5″ tube (+4.5″ for the subwoofer back) that will increase the fork’s tendency to want to flip backwards, possibly to a level that is uncomfortably dangerous to me. However, if it’s mounted on the rear part of the fork, that will counter some of the imbalance that makes the front wheel want to flip backwards. Also, mounting it on the rear of the fork will cause less strain on the mounts, which will support a hanging weight on the rear instead of a weight above on the front.

    Thank you for all your feedback, Chris! This is great.

  11. chris clark on February 16th, 2010

    Why couldn’t you install the subs in series? Alternatively couldn’t you augment with a dummy load. An Isobaric enclosure should really be driven by identical feeds. Does this amp have contingency for 3V inputs?

  12. chris clark on February 16th, 2010

    Wait, no. You’re parallelling the inputs, not the outputs. You still have 2 discrete channels each driving a sub. They just have identical inputs. You shouldn’t be dropping output resistance. I don’t know, am I missing something? there’s nothing in my hands, so I’m not sure I’m getting it.

  13. h4ckw0r7h on February 16th, 2010

    I could be missing something, and probably am. This is new to me; I’ve never dealt with car audio before — just instrument audio.

    I could install the subs in series! Good idea, that was in the back of my head as I was riding home. But won’t that affect the available wattage to both subs? Instead of one at 70 watts, I’ll have two at 35 watts, yes? And if I have two speakers at 35 watts per channel and two subs at 35 watts…. hmm…. actually. That’s still 70 watts total of subwoofer power, despite the fact that they are two different subwoofers, so it should provide a comparable amount of sound energy between the two speakers and the two subs. My concern was that the bass from the subs would be quieter than the rest of the frequencies from the other two speakers.

    One sub would be out of phase with the other, driven by an identical input, and in series they’d have 8 ohms of load, which is fine.

  14. chris Clark on February 16th, 2010

    Wait, they’re 2 different subs? To do an isobaric sealed box like you were talking about. Wouldn’t that require that pretty much everything be the same, they have to be in phase, and move the same amount of air and should be driven by the same source and matching amps, not to say it couldn’t work, but it’s definitely sub-optimal and if the subs are very different and out of phase they will wreck themselves/each other? No? But, I still don’t buy that combining the R L inputs like that is cutting output circuit resistance. Bridged you’d get 70W on one, but putting one each on channels 3 4 will yield 2 @ 35 in phase which seems to me to be ideal for the box you want.

  15. h4ckw0r7h on February 16th, 2010

    Same subwoofer, model number, brand, etc. By ‘different’ I meant physically separate objects. That was probably misleading.

    If I wired them in parallel it would reduce the resistance.

    I’d want them out of phase exactly with each other, yes? And facing each other.

  16. chris on February 17th, 2010

    In an isobar box you can have them in pretty much whatever linear configuration you’d like, face-to-face, magnet-to-magnet these are antiphase options, you can also do face-to-magnet which is the most common way but requires they be in phase, and I think gives the best sound. But without additional amps will not give you more than the 35W of sound. And any additional amplification will have to be identical to the current amplification or the subs get altogether their own amp(s).

  17. h4ckw0r7h on February 17th, 2010

    If I put one sub on channel 3 and the other on channel 4, antiphase, I’ll get stereo separation, which won’t result in matched output to each sub. So, two subs in series, antiphase, facing each other in an isobaric enclosure should be fine. Or two subs in series, in phase, face-to-magnet, in a slightly more complicated enclosure. Each sub will be driven by 35W of power (70W output on the bridged channel), producing 70W of subwoofer sound total and 2x35W speaker power (channels 1 and 2). And if I find that I need a more efficient monoblock amp later, I can add that later and move the subwoofers over to that amp.


  18. chris on February 17th, 2010

    No you won’t have stereo separation if you bring R L together and split them back into two for 3 4. You won’t get 70W of sound out of an iso box in this format it’s the nature of how they work, you half the space required for one sub but it takes two subs to make it work. That’s exactly the reason they’ve fallen out of favor is they require twice as much amp and speaker for a given output. That’s what I’ve been getting at. So long as you are fine with 35Ws coming out of the sub and not insistent on the 70 it’s fine and so far as phasing all that matters is how you point the things. If you want the 70W of sub at rated F response with the stuff you’ve got you need the big box.

  19. h4ckw0r7h on February 17th, 2010

    I think what I’m not understanding is that if I bridge channels 3/4 and drive one sub, that will be driven at 70W. If I drive one sub from channel three and a second from channel four, and bring the R/L inputs together to make them both mono (hadn’t thought of that, thanks), I’ll have two subs with 35W power output. 2x35W = 70W total, yes? So the total power available for sound production is the same. But maybe this doesn’t mean that the VOLUME is the same… I’ll get to that in a minute.

    I don’t ultimately care how many watts are available to the subs, as long as the sound volume is roughly balanced with the speakers. Wimpy bass strong mid and high bands is not going to make for a decent sounding audio system. I know that I can turn up the woofer gain and turn down the speaker gain for some rough balancing but everything I’ve read so far implies that I should be driving the bass frequencies with significantly more power than the mids and highs.

    I think what I’m going to do is build an iso box out of MDF and sonotube simply because there isn’t enough space on the fork to mount a larger sealed enclosure. If there isn’t enough battery power to use the existing amp for the subs, I can add a class D amp later to power the two subs. The class D amp would be a lot more efficient power-wise, and some of those efficiencies would balance out the power cost of adding the second sub.

    One other consideration is that the woofer’s sensitivity is rated at 93 dB and the speaker’s sensitivity is rated at 91 dB. So, if the woofer is driven with the same wattage as a speaker, the woofer will sound louder than the speaker.

  20. Chris on February 17th, 2010

    If you bridge 3&4, you have to connect the subs in series. If you use an iso box, orientation will dictate phasing. Creating a mono signal to work from is easy enough, combine and split, or, many people just use Left. The iso box will pull 70W electrically but will deliver 35W acoustically. The gain control is useful to a point. The difference in sensitivity is very small, probably not noticeable outside really. Now I really know little about how you intend to use this thing, is it to entertain the troops en-route, or is it to be a mobile rig to be a sound system at the destination? I’d say most everything you’ve read on this topic was geared toward inside applications be it car or home. Most outdoor systems barring concert type situations where audience location is highly under control and large stationary equipment can be placed are mono. With it being bike-based the stereo separation is minimal in most directions unless like, you’re behind the audience like if this is for you to entertain a line of bikes, the stereo is worthwhile, but a mono system will be better as a destination sound system, and just generally simpler. But, I think what i’d do is make the mono signal, power them each with 35W, unbridged, in the iso box, antiphase, and point that thing as directly ahead as possible and just try it out. I think it’ll be fine this way. But it’s hard to build for open air and high sound quality. If you use sonotube and MDF, just make sure you apply a good sealer they don’t tolerate moisture.

  21. h4ckw0r7h on February 17th, 2010

    It’s meant to entertain en route; we usually ride in a more-or-less straight line. The stereo separation is nice from a speaker perspective, at least to the person riding the bike. When I combine the R/L channels for the subwoofer input I can just put diodes in-line to prevent that from muddying the signal to channels 1/2, which are heading out to R/L speakers. To get the outputs, I’ll need to Y-split from the source (an iPod).

    The sound quality isn’t going to be awesome, but I’d like to be good.

    What about setting up the subwoofers in-line? One midway through the sonotube, one at the end. That way there’s no exposed unprotected driver, and also the driver in the center of the sonotube will double as a brace, making the sonotube stronger.

    Yes, definately need to apply a good sealer. Got any recommendations?

  22. chris on February 18th, 2010

    Third time trying to respond and losing the form in a misplaced keystroke, Arrrgh. Anyway, key points: Valspar and Awl-Grip are well-regarded marine finishing products, used both, seem good, but haven’t seen the projects long-term. I’ve saturated MDF with Fiberglass resin and that will last virtually forever, however wet you get it. MDF soaks up unbelievable amounts of finish. Sonotube, I’ve never finished, I think it may get soft if you put on too much too fast, might be best to use a spray, I don’t know. I think making the tube face-to magnet with the face-out sub half way back might provide some useful directionality to the thing. Could also be a detriment I suppose, but I’d play with that first, if it’s too directional a cone can be added later. The back one will need a cap over the back as well. Make sure you seal however the cable enter the tube well. Double-shrink all your solder and RCA connections just across the widest part of the exterior if the shrink overlaps where it starts to narrow it channels water in and RTV all the t-strip connections if you think moisture may be a real issue. I think the diodes are unnecessary, I’ve done this a bunch and never had a problem but they don’t cost much and they’re easy enough to do so whatever.

  23. h4ckw0r7h on February 18th, 2010

    Cool. Thanks for the input.

    I took some measurements this morning and I have a LOT more room to play with than I’d anticipated — I can fit a 10″ diameter tube that’s 24″ long on the fork. Or, a 12″ diameter tube as long as 22″. I might *not* need isobaric loading if there’s enough volume. Gonna re-do some math now.

  24. chris on February 18th, 2010

    That’s awesome, this whole thing gets a bit easier and better if you can just make a straight bass tube. You’ll get the full 70W if you want. I still can’t quite picture how you’re gonna hang the tube under the fork.

  25. h4ckw0r7h on February 18th, 2010

    If I max out the enclosure size with 12″ sonotube that’s 22″ long, and use polyfill inside it, I still get a reasonably good frequency response curve. -3 dB at 50Hz, -6 at 40 Hz, and no significant peak above 0 dB before 100 Hz. That’s a big enclosure, but I can use a single driver, and it WILL fit on the bicycle (although just barely).

    If I use 10″ sonotube instead, 24″ long, with polyfill, I’m at -3.5 dB at 50 Hz with a slight rise of 0.5 dB between 80 and 100 Hz. Still pretty good, and it will be a better fit on the bicycle (more room to pedal).

    So it looks like there are reasonable options without isobaric loading.

    I’ll definitely seal it as tightly as possible.

  26. chris on February 18th, 2010

    Hey, you have a bunch of model rocketry stuff don’t you? The girls have been asking about rockets lately and I thought it’d be fun if we built a couple rockets, but I know nothing about it and don’t really want to buy a bunch of stuff, do you think you could give me a little tutorial and let me borrow the basics?

  27. h4ckw0r7h on February 18th, 2010

    I used to have a bunch of model rocket stuff back in high school, but it’s all been destroyed through abuse and the discovery that I could ignite other incendiary devices with the parachute ejection charge. I don’t even have the launch system anymore, that gave up the ghost after I used it to remotely set off one too many crude mortars.

    However, the launch systems are really cheap. You can buy a kit which includes two rockets and the launch pad for $30.

    If you want to build the launch system AND the rockets from scratch, that’s pretty easy, too, and I’d be happy to sketch out the details on Sunday. But I’d recommend buying the kit just because it illustrates the theory of operation and construction thoroughly enough to DIY more rockets.

    The one part you would need to purchase regularly are the motors. Making your own motors is a lot more work, plus, working with explosives can be dangerous.

  28. chris on February 18th, 2010

    I didn’t realize the startup was so cheap, thanks. C-ya Sunday.

  29. chris on February 22nd, 2010

    I mean really, how can you compete with prices like this? The market’s gotten wacky. Damned globalization.

  30. h4ckw0r7h on February 23rd, 2010

    That’s ridiculous…

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