The building your own portable soundsystem thread

[fractionMan]

I've been inspired by the boominator and I'm thinking about putting a T-Amp based system into a pram. Unfortunately I'm completely new to this kind of thing so I've got a bunch of questions.

We'll start with Impedance.
Right, so I've got the basic idea here. if you stick two 8Ω speakers on the same channel then the channel can have 4Ω or 8Ω impedance depending on how you wire it, right? Your amp is only rated to drive a certain range of impedance and it's best to be at the bottom of that range.

To make it 4Ω you go amp->speaker->speaker
To make it 8Ω you go amp->speaker and amp->speaker

Is this correct?

If so, how do I go wiring in piezzo tweeters. Apparently they don't add to the impedance or something? The don't need a crossover?

It boils down to getting the most out of the small t-amp It's suggested you run two (very sensitive) woofers and two tweeters on each channel. How do I wire this up? Would it be easier to run two full range drivers on each channel and skip the tweeters?

 

[beesonthewhatnow]

Errr, quite a lot wrong there

Two 8ohm speakers on one channel will either be 16ohm or 4ohm. 16 if you wire them in series (R=R1+R2), or 4 if in parallel (1/R = 1/R1 + 1/R2). And you want to wire them in parallel.

As for the tweeters, throw them in the bin and just use two full range speakers.

 

[beesonthewhatnow]

Best bet for speakers if you're doing a compact mobile thing is ones designed for a car btw...

 

[fractionMan]

I think in the first instance I'm going to buy a bunch of cheap electrovision 8" drivers and experiment. If you pick the right ones their quoted sensitivity is pretty good (94dB) for such cheaparse drivers and the fact they don't handle much power (10w/15w) doesn't matter when I'm only running 20w a channel anyway. If they're total crap it's only 13-20 quid down the drain.

http://www.bluearan.co.uk/index.php?id=EVNL030AF&browsemode=manufacturer (94dB - 10w - 8" - 3quid) or
http://www.dhwelectronics.com/Speak...ck_204_mm_15_W_Full_Range_Round_Speaker_8_Ohm (96dB - 15w - 8" - 6quid)

 

[fractionMan]

Best bet for speakers if you're doing a compact mobile thing is ones designed for a car btw...

Really? The folks on diy audio were pretty scathing. A few people recomended the JBL GTO series but they're all 2ohm

 

[fractionMan]

Errr, quite a lot wrong there

Two 8ohm speakers on one channel will either be 16ohm or 4ohm. 16 if you wire them in series (R=R1+R2), or 4 if in parallel (1/R = 1/R1 + 1/R2). And you want to wire them in parallel.

As for the tweeters, throw them in the bin and just use two full range speakers.

So you can either double it or halve it depending? So if you run four speakers on a channel you could do both and end up with the original impedence, right?

 

[fractionMan]

And you want to wire them in parallel.

This always confuses me. Parallel is amp->speaker->speaker right? Even though it looks like a series (as in they're all in a row)

 

[beesonthewhatnow]

Really? The folks on diy audio were pretty scathing. A few people recomended the JBL GTO series but they're all 2ohm

Ignore people on forums

Think realistically what you're aiming for - are you after hifi sound, or something to provide tunes in the park? The advantage of car speakers IMO is that they are designed for environments prone to vibration, come with built in mounting hardware and more often than not are voiced bass heavy to satisfy the max power brigade.

 

[beesonthewhatnow]

This always confuses me. Parallel is amp->speaker->speaker right? Even though it looks like a series (as in they're all in a row)

No, if it looks like a series, it's, errr, series

To wire two speakers in parallel is easy - wire one up as normal, then wire another one as normal to the same amp outputs.

 

[beesonthewhatnow]

So you can either double it or halve it depending? So if you run four speakers on a channel you could do both and end up with the original impedence, right?

You could, but bear in mind how much power each speaker would then be getting. You can't magically get more out the amp.

For a pram based system just stick with 2 speakers, one on each channel.

 

[fractionMan]

No, if it looks like a series, it's, errr, series

Dammit, I'm sure someone else said the other way around So I want to run separate wires from the amp to each speaker to halve the impedance, right?

It confuses me cos my 12v boat batteries are in a line yet they're wired in parallel

Cheers bees, this is really helpful

 

[fractionMan]

You could, but bear in mind how much power each speaker would then be getting. You can't magically get more out the amp.

For a pram based system just stick with 2 speakers, one on each channel.

Would you recommend coaxial car speakers or full range speakers?

 

[beesonthewhatnow]

It confuses me cos my 12v boat batteries are in a line yet they're wired in parallel

http://www.vonwentzel.net/Battery/00.Glossary/

Might make things clearer...

 

[beesonthewhatnow]

Would you recommend coaxial car speakers or full range speakers?

Coaxial, they have separate tweeters so should sound better.

 

[Crispy]

 

[fractionMan]

View attachment 13843

Awesome. So they wire up the same way as batteries then

 

[fractionMan]

Coaxial, they have separate tweeters so should sound better.

How do you wire them up then? Cos they're all rated for impedance yet have more than one driver

 

[Crispy]

They almost certainly have their own crossover. I have some coaxial speakers from the 70s and they have built-in crossovers.

 

[fractionMan]

They almost certainly have their own crossover. I have some coaxial speakers from the 70s and they have built-in crossovers.

Ah, ok. That makes sense.

 

[beesonthewhatnow]

How do you wire them up then? Cos they're all rated for impedance yet have more than one driver

They almost certainly have their own crossover. I have some coaxial speakers from the 70s and they have built-in crossovers.

Wot Crispy said. Their stated impedance will be for the whole speaker. This is why it's so much easier than dicking around with separate bits. Just wire them up and off you go

 

[fractionMan]

I also want to make a backpack sound system. And one in a plastic tube. I think I've got a bug or something.

 

[fractionMan]

Wot Crispy said. Their stated impedance will be for the whole speaker. This is why it's so much easier than dicking around with separate bits. Just wire them up and off you go

yeah, I did think it would be loads easier

 

[fractionMan]

So I'm going to start out with a tripath TA2020 based 12v amplifier (2x25w) going into two (maybe 4) full range speakers.

The plan is to eventually build a 24v AMP9 Basic (http://shop.41hz.com/shop/item.asp?catid=42&itemid=796) based system (4x50w) - possibly into 4 2ohm or 8 4ohm speakers

 

[beesonthewhatnow]

Remember that the more speakers you add, the less power they'll be getting each. More is not always better

 

[fractionMan]

batteries

I'm looking at CCTV li-ion rechargeables. They're dirt cheap from hongkong

http://cgi.ebay.co.uk/12V-DC-Rechar...UK_Gadgets&hash=item1c1701c187#ht_3202wt_1085

6800mAh for £15, with leads ( that will handlily fit into the t-amp! ) - could by lying about capacity though, it seems to good to be true. These 4800 are also 15 quid http://cgi.ebay.co.uk/4800mA-Rechar...tteries_SM&hash=item27b85cccb2#ht_3605wt_1085

Other options are SLA batteries (heavy) and 8 or 10 AA that may or may not be rechargeable

 

[Crispy]

Nice battery, esp as you can swap them out easily

 

[fractionMan]

Yeah, for some reason the CCTV ones are much cheaper than similar capacity batteries marketed for other uses (hobby/camera/etc)

 

[Crispy]

Gotta wonder what quality you'll get at that price, mind.

 

[fractionMan]

Ok, drivers. In the first instance I'm going cheap and simple. 2 full range high SPL speakers.

Are these not a totally terrible choice?
(50w 6.5" 94dB £7.43 8ohm Electrovision) http://www.dhwelectronics.com/Speak...ck_166_mm_50_W_Full_Range_Round_Speaker_8_Ohm

 

[beesonthewhatnow]

Well, ideally they need something like an 80W per channel amp.