I'm going to start a project

[Boarder]

Browsing YouTube I found this video. I found it interesting that I'm going to build a boom box with two 8" speakers. When I ordered the stereo unit from Walmart it was $12 and change. Still not bad. It'll be fun. 

 

It'll be in May 2 (give or take)

 

 

[Tamesh16]

Great initiative !, could have gone under Hobby electronics, or Audio 

[Unimportant]

On 4/12/2019 at 9:27 AM, Boarder said:

<snip>

I'd attach extra heat sinking on the back of the unit, near where that TDA7389 integrated amplifier is sitting.

Metal is more costly then electronics these days, so one of the reasons it's so cheap is because they skimped on the casing. That flimsy piece of metal surely isn't enough to keep that TDA7389 cool. It's a linear amp IC, not class D, so it gets hot. Such heat on that sort of package tends to lead to broken solder joints pretty quickly.

 

 

[Boarder]

Good idea. I'll see to that.

 

After a lot of headache I finally figured it out. I was going overboard by making it big. No, I want it to be portable so it'll be 12" x 8" x 6" and use two 5 1/4" speakers. 

 

It can be all made from a 24" by 24" panel with wastage between the pieces. That will allow precise cuts around the pieces. The pieces will all have 45 degree cuts.

 

ETA: I was wrong. More headaches. I found a program online where I could create a pattern and work with it. I saw my mistake from it. By changing to 12x8x5 I got it.

 

[Boarder]

Looks like the project is determined by what's available. I got the 24" x 48" MDF  and some square dowels plus some hardware.

 

[Boarder]

Looked at the speakers on Amazon and found a pair I like. They're 5 1/4" set but the grills measures 6" so the specs above flew out the windows. Back to the drawing board but since I have the 24" x 48" panel I can build a bigger box and put the unit on face with the speakers and not on the top.

 

16" x 11" x 8"

 

[Boarder]

Today I got some tools to work with the sheet from HFT:

 

 

All I have is a jigsaw so these cut pieces are just rough cuts. Later I can sand them for custom fitting.

 

[Boarder]

This project is a bust. My "stereo" (notice the quote?) came in.

 

it's a roll of HDMI cable.

 

 

[Boarder]

I want to finish the project so I'll just buy another unit. It'll cost $30 but it's more powerful (60 watt x 4) and the speakers I plan to get will handle 200 watts total.

 

What I got so far:

 

Lots of sanding for custom fitting

 

 

Partly built box

 

[Bitter]

If you do some baffling internally with some fiber fill you can get much better performance out of the speakers than just shoving them into a big empty box, there's a big wide world of audio design out there to explore. With the right kind of porting that box could sound amazing.

[Boarder]

I could do that but it'll smother the PSU in it. It'll produce 20A so it needs to air. At first I'm going to try it open ended just as the box is now. I've prepped the dowels to seat the side plates on if desired.

 

I've noticed I had a shift in one corner so it's corrected. I'll need to get some spackle to cover the screw holes. When I took the screws out the panel popped over to its correct position. LOL

 

By the way, the stereo is 240 watts so it'll be really a boom box. When I bridge the wires into two channels it'll be 120 watts per channel. 

 

The advantage of such power is that not only it'll be loud but at listening level it will be high definition with virtually no THD. 

[Unimportant]

53 minutes ago, Boarder said:

By the way, the stereo is 240 watts so it'll be really a boom box. When I bridge the wires into two channels it'll be 120 watts a channel. 

Yes, but no. Marketing talk is just that, talk.

A car head unit is made to run on 12V car voltage, and a typical car speaker is 4 Ohm.

A speaker requires a AC signal so the 12V needs to be split in a positive and a negative part, +6V and -6V.

Theoretical RMS power, which is the figure one needs to focus on would be ((6V / √2) / 4 Ohm) * (6V / √2) = 4.5W. In reality you won't even get that because a real amp can't go rail to rail.

 

If you bridge 2 channels it allows driving the speaker with +12V/-12V. RMS power becomes 18W.

 

Other figures you see are peak power, which is nonsense that can be massaged any way they see fit. Higher wattage requires a voltage booster to boost the car's 12V to a higher figure first. I've never ever seen a head unit with a voltage booster so they're all quoting nonsense figures.

 

[Boarder]

Maybe so but by bridging I added the wattage together for more output. 

 

By the way, the car electrical system is 14V. Maybe I should get a bigger PSU.

 

Finally made the ends. Not perfect but I'm done with it.

 

 

[Boarder]

Unimportant, do you know for sure that the speaker output will be AC instead of DC? Is it because of a high output?

[Unimportant]

14 minutes ago, Boarder said:

Unimportant, do you know for sure that the speaker output will be AC instead of DC? Is it because of a high output?

A speaker output is always AC. In fact if there were to be a DC component on it it would fry the speaker.

[Boarder]

Never too old to learn. Thanks. 

 

A mockup of the components. Gonna be tight.

 

 

The rectangle cutout for the stereo unit are most likely from the dimensions of the box so it may not be that big.

[Boarder]

Apparently the paper cutout of the unit was the box it comes in but I found the correct dimensions. I redid the measurements and finally have the components in place.

 

[Bitter]

On 4/19/2019 at 11:05 AM, Boarder said:

I could do that but it'll smother the PSU in it. It'll produce 20A so it needs to air. At first I'm going to try it open ended just as the box is now. I've prepped the dowels to seat the side plates on if desired.

 

I've noticed I had a shift in one corner so it's corrected. I'll need to get some spackle to cover the screw holes. When I took the screws out the panel popped over to its correct position. LOL

 

By the way, the stereo is 240 watts so it'll be really a boom box. When I bridge the wires into two channels it'll be 120 watts per channel. 

 

The advantage of such power is that not only it'll be loud but at listening level it will be high definition with virtually no THD. 

Most aftermarket stereos are capable of about 50W peak per channel for a total of about 200W peak, and about 20W RMS per channel or about 80W RMS total. There's a few outliers that can do more and with class D amps getting better packing more power into smaller package is easier especially with head units omitting optical drives. The limiting factor starts to become the size of wire to the unit and to the speakers and the added resistance from that.

 

All car audio amplifiers are split into two sections, power supply and audio amplification. The power supply takes the 12V and runs it up to however many volts the audio amp needs, the audio amp takes the low level AC audio in and magnifies it, turning the boosted voltage from the power supply into high level output. Setting the gains on the amps in my car I was aiming for a specific voltage level which indicated a specific wattage output and then you run that and check for clipping from the speakers. From memory I was shooting for about 40V AC from my JL 300/2, so yea 12V DC goes in but WAY more than 12V AC comes back out!

[Boarder]

Interesting. So you have a power booster built in. You think this unit has it? I'm thinking it'll only put out a few volts.

[Bitter]

I think it'll probably do at least 70% of the rated RMS wattage, probably a little better than 70% but who knows what the RMS is because I doubt it's listed. It'll probably do like 50% of what they claim as peak though because cheap junk. A used $40 head unit from eBay from a reputable brand would be a better base for the project or include a small class D 2 channel amp to get some real power from it. Drives up the cost but the result will sound so much better and cleaner, driving speakers with crappy power gets crappy results and distortion is what kills speakers.

[Boarder]

At the beginning I was going to get this unit shown in the first post. Got screwed by Walmart so I'm ordering a better unit though it cost more. It's so I can finish the project. I think I can get HD at a higher RMS before THD.

 

I'm not looking to get loud though that would be fun. Just a nice little jamming box.

 

I just had a thought. For a long time I thought some stereos had DC output, high power would use AC output. Now for those with AC output why would the speaker wires be designated with + and - ?

[Bitter]

So what unit will you be using?

[Boarder]

I just added an afterthought before I saw your post so check previous post.

 

The one I'm interested in seems to be the best bet (so far, still browsing)

 

https://www.amazon.com/Aigoss-Bluetooth-Receiver-Speakerphone-Hand-Free/dp/B07J4JV391/ref=sr_1_8?keywords=car+stereo+receiver&qid=1555640237&s=gateway&sr=8-8

[Unimportant]

4 hours ago, Boarder said:

I just had a thought. For a long time I thought some stereos had DC output, high power would use AC output. Now for those with AC output why would the speaker wires be designated with + and - ?

Audio is always AC. The + and - are simply designations to make sure you connect all your speakers in phase. If you were to connect speakers in anti-phase 1 speaker would be "pushing" while the other is "pulling" and vice versa. Their audio waves would (partly) cancel each other out and that sounds bad.

[Unimportant]

On 4/21/2019 at 9:19 PM, Bitter said:

Most aftermarket stereos are capable of about 50W peak per channel for a total of about 200W peak, and about 20W RMS per channel or about 80W RMS total. There's a few outliers that can do more and with class D amps getting better packing more power into smaller package is easier especially with head units omitting optical drives. The limiting factor starts to become the size of wire to the unit and to the speakers and the added resistance from that.

 

All car audio amplifiers are split into two sections, power supply and audio amplification. The power supply takes the 12V and runs it up to however many volts the audio amp needs, the audio amp takes the low level AC audio in and magnifies it, turning the boosted voltage from the power supply into high level output. Setting the gains on the amps in my car I was aiming for a specific voltage level which indicated a specific wattage output and then you run that and check for clipping from the speakers. From memory I was shooting for about 40V AC from my JL 300/2, so yea 12V DC goes in but WAY more than 12V AC comes back out!

That JL300/2 is a standalone amplifier, most of those indeed have the necessary voltage booster on board to deliver the claimed wattages. However; we're talking about a plain head-unit here, and a cheap one at that. I have never *EVER* seen a headunit with a voltage booster on board for the amp in over 40 years, not even the expensive ones. 

Without it, the best you can muster is 4.5W RMS single channel or 18W RMS bridged on a 4 ohm speaker with a 12V supply, that's just math. Class D or not does nothing to help in this respect. So the numbers on all those headunits are meaningless peak values or just lies.

 

Now, that does not mean it won't be loud. The problem is audio amplifier output numbers have been exaggerated for so long (I remember tiny PC speaker sets which claimed 300W P.M.P.O - Where P.M.P.O is yet another bullshit metric - they were really 1W RMS) that most people can't imagine how loud a few real RMS watt's can be (very). But that is another issue.