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Recessionist 19th December 2011 09:27 PM

Newbie boombox project questions
Hello all.

Before I begin, please be aware that I have never in my life worked with electronics, let alone an audio system. As a consequence, I am trying to keep it simple.

I want to make a simple boombox which can play music from an MP3 player and that (thus far) only has an on/off switch and a volume knob.

The main reason for my posting here (please excuse me if this didn't require a new thread or is in the wrong section) is to, firstly, confirm that all of the components that I am planning to use will work correctly together to deliver acceptable sound and, secondly, to receive any advice on how to proceed.

I was thinking of buying two sets of the fusion CS-CM60 component speakers primarily because of their aesthetic but also because of their relatively low price.

I was then thinking of buying this small amplifier which seemingly matches the speakers perfectly if they are wired in series.

I must admit I didn't quite understand the power requirements for the amp, but I ended up looking at toric transformers with essentially the same numbers on them, such as this one.

Would this set-up be able to play music from an mp3 player of acceptable quality? Also, what kind of a battery can I use to power it? (preferably rechargable)

firechief 19th December 2011 10:27 PM

Well you are going to need two transformets best I can tell. Here from the text

"Input voltage:AC24V-0-AC24V/

AC 9V 1A

Transformer Power need about:150W-200W

OutPower:80W+80W 8ohm"

Is running the finished Box off of batteries your goal? Not that it cant be done It just makes it a little complicated. If you will be in a position to run it on AC line power then I think you could get there with two transformers and some other parts like electroydic caps. I wonder what they are using the 9VAC for? It could be the filament supply for the tube, The +- 24 will be the supply for the chip amps. I always cringe with someone advertises on E-bay and offers a link to the schematic but it is not really usable link. If you want to go portable with this you should look around for something based on chip amps that will operate off of a single supply. They are out there and this would make it a whole lot easier to run off of batteries.

Recessionist 19th December 2011 10:38 PM

Thank you for your extremely prompt reply. I'll dig around for another suitable amp and come back if I have any other questions.

I would indeed like to run the box off of batteries eventually.

Recessionist 19th December 2011 11:34 PM

I think this one has a single DC supply. However, if I understand correctly, each channel only dispenses 60W @ 8 ohms, in which case I would be missing 20W per channel (if I want a perfect match). I've also found a 2x50W @ 8 ohms and another which is apparently 2x100 @ 4 and 8 ohms.

Could it be possible to simply use two 2x40W amplifiers? If it's at all possible, this one might do the trick.

riccoryder 21st December 2011 08:46 AM

Most of the chips are already running in bridge mode to try and push more power so you probably cannot use more amplifiers per driver. However if you bi-amp it takes less power to deliver the same undistorted voltage to the drivers.

For signals of random phase, like different instruments, the peak voltages add up. If you had 2 Volts r.m.s. of bass into 4 Ohms that's 1 Watt. And 2 Volts of high end into another 4 Ohm driver would also be 1 Watt. But if both were handled by one amplifier, the voltage would add up to 4 Volts, and with power being the square of the voltage divided by the load resistance it would take a 4 Watt capable amp to deliver the two signals that were 1 Watt along.

The advantages of biamp go beyond the above. If the low-end amp gets driven to clip, the harmonics generated don't go into your tweeter. So tweeter damage is less likely, and the harmonics that are above the working range of the low end driver will be less audible due to the response rolling off.

Additionally, bi-amping lets you target the most appropriate amps for the job. You can get very tight bass and very high efficiency with a Class-D (switching) amp for the low end. Although many of those amps would be okay for the high end, they're generally not as good as the specs suggest (at slightly lower power where the distortion is way below 10% seen with the inflated power figures). Due to low loop gain at high frequencies the distortion goes up, but some of the filtering to removing the switching components filters some of the harmonics beyond 20 KHz making the numbers look better. Not that you could hear those anyway, but other distortion products that aren't shown in the specs will be more audible from the slightly marginal high-end performance. If you had high level 19 and 20 KHz tones at once there'd be an intermodulation product at 1 kHz. The regular I.M. test with 60 Hz and 7 kHz tone would not be very revealing of something that's degraded higher up in frequency.
Tube equipment isn't viable for portable battery use. Even class AB solid-state amps can be a bit much of a power drain. But if you bi-amp and use a DMOS class AB amp for the high end, it'll be running at low enough average power that the lower efficiency won't be such a big deal, and a good DMOS amp is cleaner sounding for the high end.

Maybe you can get an amp that can work at 12 Volts for portable use (pretend the 12 is closer to 15, and try one of the amps that runs on 15 and up), then switch to a higher voltage external supply when AC is available and you'll get a bit more power. As long as you're making something fairly big/heavy something like a motorcycle battery would provide a fair amount of energy at low cost. Just make sure it doesn't leak acid. If you want 2x100 Watts a car or golf cart batteries would give more run time. The point is don't go nuts with the power ratings and watch efficiency. Be sure you know what you're doing if you attempt to make your own battery charger. Don't just hook a random power supply across the battery. Some batteries will give very poor life or even have risk of fire/explosion if improperly charged.

Since the bass takes most of the energy, a Class-D amp for that and an efficient speaker are the two biggest things you can do to be light on the battery drain for a given acoustic output. Although sealed-box speakers tend to have excellent response, their efficiency is very low and a box for the lowest frequencies is huge. An enclosure with a tuned port for the bass will give much higher efficiency. While they can sound awful with randomly thrown together drivers and boxes, performance can be good if you take the time to see that things are properly paired and tuned. You can always start with your full-range speakers and a 12 to 20 Volt or so class-D amp, then experiment with adding things to bi-amp, or alternate bass boxes later.

Most of the power figures shown for those amp kits are horribly inflated and at 10% distortion (usually at 1 kHz). Downloading the spec sheets for the chips used will tell you much more. But the actual power you need will likely be less than you expect so even if that 2x50W amp is really only 6 Watts clean bridged into 8 Ohms at 12 Volts (about double in 4 Ohms), it'll do more than you would expect.

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