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iBump Active Crossover FAQ
 
Q0: What is the iBump?
The iBump is an active crossover to enable you to add a subwoofer to an existing stereo system. It was featured in Make Magazine issue 08.

Q1: Why would anyone want a crossover?
To reproduce lower frequencies, speakers need to do more work. They have to travel farther. Half the frequency requires the speaker to travel four times as far! So a 200Hz tone that requires a speaker to move back and forth 1/4" would require the same speaker to move 1" back and forth to get the same sound level at 100Hz.

Large speakers are needed to move the large amount of air needed to make loud low frequency noises. But these large speakers are unable to produce high frequencies, because they cannot move quickly enough.

The accepted solution to the problem is to use a large speakers for the low frequencies, and a smaller speakers for the high frequencies. It is important to keep the high frequencies out of the large speaker and the low frequencies out of the small speaker. This is not just because failure to do so sounds terrible, it can also damage the speaker. The crossover network seperates the frequencies, and sends them to the correct speakers.

Q2: Don't the speakers in my old stereo already have a built-in crossover network?
Yes, they should have a built-in passive crossover. But to add a subwoofer, you'll need another crossover.

Q3: I already have a stereo, and I want to add a subwoofer to it. What are my choices?
The answer is 'it depends'. What have you got?
What you have:
#1. (Common in old systems) Amplifier has no subwoofer outputs Choose from solutions B, C, or D.
#2. (Common in new systems) Amplifier already has subwoofer outputs to drive a powered subwoofer You can use alternatives A, B, C, or D.
#3. (Rare) Amplifier already has a crossover network and an amplifier for an unpowered subwoofer Wow! Choose solution E.
Solutions:
A. Buy a powered subwoofer
B. Buy a powered subwoofer with both low-pass and high-pass filters
C. Buy a powered subwoofer and an iBump
D. Use a bass amp, speaker, and an iBump
E. Buy an unpowered subwoofer

Q4: How do bass amps compare with powered subwoofers?
Musical instrument amplifiers and stereos are really very different. Stereo gear assumes that your music will only be producing full volume about 10% or 20% of the time. If you push it beyond that, if you are lucky the stereo will have protection circuitry that will prevent meltdown or other damage. If you are not lucky, the amp, the speaker, or both will fail (permanently). Musical instrument amplifiers are designed to continuously produce their full power output.
A: Stereo Subwoofer: Typical: 1 10" speaker, $1/Watt to $2/Watt, built in low-pass filter
B: Bass amp with speaker: Typical 1 15" speaker or 2 10" speakers. $1/Watt to $10/Watt. No useful filter for stereo listening. The bass amp will withstand 10 times the abuse and still not blow up!

Q5: Really, you mean stereos melt, break, or shut down if you play them continuously at high volumes? And musical instrument amplifiers don't?
Yes, this is easy to prove. Use an adapter to plug a guitar into a stereo, and turn it up to get the distorted rock-and-roll sound. Play for a few minutes, and poof! You will have a dead stereo. Try the same thing with a guitar amplifier. It will keep working, because it is designed to handle full-power distorted signals.

Q6: Wait a minute, the stereo has power specifications! Are you saying that it doesn't meet its specifications?
There may be some words in the specification that reveal the truth about the effect. See if the specifications 'crest factor' and sometimes 'music power' are used as hints. There is no industry standard way to describe this problem. See the JBL reference below to read more about the effect.

Q7: But I listen to distorted guitar all the time on my stereo, at very loud volumes. How could a real guitar damage my stereo?
Very easily. I have verified this experimentally on several occasions. Try it yourself - but don't say I didn't warn you - it really will break your stereo. You can also read the JBL article linked below. The JBL article is about derating speakers for use in musical instrument amplifiers. The amp will also need derating in a similar manner to the speakers.

Q8: What is the design of the iBump?
The schematic is at the Make Magazine site.

There is an omission in this schematic. The power supply pins of the opamps U1 and U2 are not shown. I will upload a new schematic soon, but in the meantime add these connections:
U1 pin 4 connect to VN9
U1 pin 7 connect to VP9
U2 pin 4 connect to VN9
U2 pin 7 connect to VP9

Q9: What is the frequency and phase response of the iBump?
The response is called LR-2 and there is also an inverter to compensate for the 180 degrees of phase shift between the highpass and lowpass filters.

In the near future there will be the graphs showing these frequency response, phase response, and group delay linked from here...

Q10: Why is 12dB/octave better than 24dB/octave?
The technical reasons are covered in the Linkwitz Lab web site, see below. The descriptive reason is that 24dB/octave doesn't sound as good.

Q11: Why is the iBump so expensive?
We used high-quality parts, such as the OPA2134, which provide sonic transparency. These parts cost more. The iBump also includes a line module and cables. Our manufacturing volumes are too small to allow the economies of scale found in mass-produced electronics, so they are built and tested by hand. We also provide email support for the design, in case you have questions or need help modifying the circuitry.

Q12: How does a crossover network make your system louder?
A crossover can have a significant impact on available volume because it lets your components do what they were designed to do and frees them from attempting to do what they weren't. You still need a power amplifier and speakers, though, the crossover output is line level, not high-power.

Q13: How can you claim 'The World's Loudest iPod?' It's not even an amplifier!
For the system in the picture in Make Magazine, the volume is limited by the house. Things start falling off of shelves, and we worry about loosening the nails in the house! If you have an enclosed steel structure (or a car) you could build something quite a bit louder.

The "World's Loudest" claim is meant to be silly, of course, but the unit helps you build a really loud system that sounds great - especially if you want to add a subwoofer to an old stereo, or if you have a bass amp or guitar amp handy. If you just want a better crossover, it is that, also.

Of course there is nothing about the unit that prevents you from achieving the "World's Loudest", but presumably there can be only one "World's Loudest" iPod, so if we sell more than one...

Q14: What is the equipment in the picture in Make Magazine?
  • JBL 18" PA subwoofer
  • 400W Sunn Bass Amp bought off eBay to drive the subwoofer
  • One pair of Klipsch Heresy
  • Cheap receiver/amp from Costco to drive the Klipsch speakers
You really need a sub that powerful to keep up with a pair of Klipsch Heresy speakers when you play Dark Side of the Moon, especially when the Heresy's only have to work at frequencies higher than 100Hz!

Q15: Why would someone want an iBump? Does it really do anything?
It amazes me how much better I like this crossover design than the one built into the receiver. I suspect this is because the rolloff in the receiver crossover is too fast. In the receiver, the transition between the bass and the midrange sounds like it has a gap.

I have listened to various 'home theater' systems and some good stereos, but I hadn't heard a home system with the true volume and vibration that I have heard at a real rock concert until Wendell and I built this system. I get to listen to clean studio recordings, also. I can clearly hear every word and feel every bass note.

The system also sounds great at low volumes. You can't even tell that the subwoofer is playing, because it blends in so naturally with the midrange. Your ears are fooled into thinking that the all the sound is coming from your regular stereo speakers.

Q16: What speaker positioning should I use for the subwoofers?
The best position is for the subwoofer to be in the middle between the speakers, either in front of you or behind you. If the subwoofer is to one side, it may sound like the bass player is moving, depending on what note he is playing. It's a very weird effect.

Q17: Isn't hearing loss a serious problem with such a loud system?
Yes, hearing loss is a real problem. I encourage everyone to take care of their ears! Mine are not in too good shape, so I do plan on designing myself a hearing aid at some point. From what I have seen so far, existing hearing aid designs can be greatly improved if you are willing to use a less unobtrusive microphone. I hope I can get used to wearing a baseball hat with a pair of shotgun mikes mounted on the bill!

These days I use ear protection when listening to loud music, and I give out fresh ear plugs to those around me. I buy big jars of them!

Q18: How do I order an iBump?
Order the iBump Active Crossover Online - Click here and scroll down

Q19: Can I order the iBump directly from Quaketronics instead of from O'Reilly?
No, please use the O'Reilly web site. They are handling all the shipment and payment details so that we are available to answer questions and design new projects!

Q20: What if I have a question that is not answered here?
Please send mail to Quaketronics info.

References:
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