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Class D Switching Power Amplifiers and Power D/A conversion

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Old 24th April 2012, 05:03 AM   #1
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Join Date: Apr 2012
Default Party Tricycle: UC Davis!

Hello everyone!

Roughly one month ago I got the itch to build a giant tricycle to pedal around my university campus (University of California Davis). At around the same time, I decided I wanted to try and build a high power sound system for the contraption in order to really push it to the next level of “swag,” as us young folks call it.

The tricycle part was easy as metalworking and bycles are things I am quite familiar with. I quickly realized that the sound system part would be much more difficult. And so started my journey into the world of sound engineering.

Right of the bat a read a great article on speaker design that said something like “Everyone wants to make their own speakers in such and so way with such and so drivers and imagines it will work out great. It never does.” I thought to myself “Hmmmm, very good advice! I should make sure to not do that.” Now, one month later, I seem to have fallen into just that trap!

My system is not “bad,” as it provided a great deal of joy for 12 straight hours at “Picnic Day,” with plenty of juice left in the tank. But from a pro audio stand point, it is certainly not “good.”

This is what I have rigged up currently:

(2) 110ah deep cycle AGM batteries from a VW bug conversion (1) 6 channel car audio head unit
(1) type A/B car audio amp, roughly 600 watts total, 4ch (my brother gave this to me, I am using two of the channels as two seem to be dead)
(2) very inexpensive MCM 12” woofers (they say 97dB...) Part number 55-2952 MCM Audio Select 12" Woofer with Paper Cone and Cloth Surround - 175W RMS at 8ohm | 55-2952 (552952) | MCM Audio Select
(2) 3-way pioneer car audio 6-9's (horribly inefficient, 89dB)

I also purchased a type D car audio mono amp of 1000 watts that I planed on running the woofers on. When I realized that the amp only dealt with 300hz and bellow, I decided to run everything off of the inefficient A/B amp I have in order to get a bit more use out of the 12s. Everything other than the batteries are mounted in a pyramidal enclosure of roughly 6 cubic feet. Roughly 1.75 cubic feet of this space is partitioned off for the 6-9's. There are two ports, 3 inches in diameter and 4 inches long. The box is (I believe!) tuned to roughly 50hz. As I am sure you can see, this set up is much less than ideal. Luckily, it is still quite a bit of fun, and thus sets a pretty satisfying lower bound on the project.

And so the question remains: how *should* I have done it?! Here is my current plan for party trike V2, as well as a few questions.

System overview:

(2) PA style loudspeakers (mounted high)
(1) Sub (mounted in the belly of the beast)

System details:

PA style loudspeakers:
-Two way
-10” woofer (perhaps Selenium 10PW3-SLF 10" Driver 264-332)
-1” compression driver (perhaps Eminence ASD1001 1" HF Titanium Horn Driver 1-3/8"-18 TPI 290-525)
-Type D amp (perhaps from 41hz. I am not sure which one would be best)

Sub
-Large ported or sealed enclosure (size is not an issue)
-Something in the 500-1000 watt peak range.
-Type D mono block car amp (best to run at 2-4 ohms).

Now a few questions:
  1. In reading boombox threads here (Boominator), I have heard of giving up on frequencies bellow ~80 hz. I will have between 1 and 2 kwh of energy in my batteries, significantly more than a boom box. Given my power levels, perhaps it is worth stretching down low?
2) I will be playing mostly electronic music (house, popmixes, what have you) as well as hip hop, or "slaps," as they say. I have heard that a lot of the electronic music doesn’t go much bellow 80hz. Is that really true?! I feel like my current system really struggles with the bass hits from the likes of daft punk.

3) Any advice on drivers, especially for the sub? Also, what's the deal with the 12” MCM drivers I purchased? For $25, they cant possibly be 97db SPL... I would guess that I am putting 80 watts into them peak. Any higher and they seem to become unhappy. Perhaps their low power justifies the cost?

Thank you so very much for your time! Any advice would be greatly appreciated. This is already turning into a very rewarding and addicting hobby : )

Regards,
Michael Norcia

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Old 24th April 2012, 04:23 PM   #2
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Join Date: Dec 2003
Location: Munich
If you want to go down below 80Hz or not depends on your personal taste.

80Hz are deep enough that you can feel it in the stomach and high enough that the required electrical power and required volume displacement of the drivers is not going unsaint.
If you allow lower frequencies the sound will be richer, but system requirements are going up massively overproportional.

You can start to experiment on this with your existing system, of which you state that it struggles with the low frequencies.
I guess you do not use a subsonic filter (just a high pass that cuts the lower end)? Furtheron I guess basically you are happy with the sound in the lower power range, but when going really loud.... it does not become really impressive before starting to distort...?
For experimenting you could put a simple 2nd order high pass between the output of the head unit and the input of the power amp.
Reasonable range for experimenting with the cut off frequeny 50Hz...80Hz.

When looking for any class D type amp, which is not intended for car hifi, you have to provide the higher supply voltage.
Option 1) Multiple car batteries in series to fit the required voltage
Option 2) Car SMPS (typical push pull with a control IC like the SG3525..)

Low cost drivers:
For sure they cannot compete with 300USD JBL drivers.
On the other hand, most likely the difference is by far less than the factor in price makes you guess.
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Old 2nd May 2012, 11:16 PM   #3
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Join Date: Aug 2008
Hey Norcia,

I just wanted to say that your boom-box trike is bad-***! Well done! If you guys can pull a little beer cooler behind you, you will go down in campus history!
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