I am looking for a small hifi quality amp that can be driven off 12v (motorcycle battery etc)
I have googled up a few but they all seem to be aimed at public speaking/p.a type applications.
Anybody know any?
Might be able to help if you give a few more details. small ? .25W, 1W, 10W? Amp to power what? from what source?
Hi TwoJ
Thanks for your reply.
I am looking for the loudest portable sytem that is practical subject to current drain on a small (4.5ah) 12v battery.
The source is a MP3 player and the speakers are some amazing cardboard speakers using a NXT exciter. These speakers take up to 50W at 8ohms. They also collapse and can be packed flat. Check them out at www.soundpax.com.
I have bought the 12v amp shown on this site. It is rated at 15W per channel at 4ohms. It sounds great with the speakers but is not really loud enough.
Paul
What about a dedicated car stereo amplifier ? These can often match hifi standalones (but require a lot of amperes, too) ...
Max. output power of a class A/B amp for 12 Volt power supply is typically 5 Watts into a 4 Ohm speaker. This is simply because of the fact that you have to be able to make a symmetric sine wave from your 12 Volts, i.e. +/- 6 Volts pp, equaling about 4 Volt RMS, resulting in U²/R = 4 - 5 Watts. Advantage : these amps are very low power consumption.
A next step is a so called bridge amp, where you have basically 2 amps driving one speaker, while the speaker pins are both connected to one of the amps output pins and amp1 is giving out the same signal as amp2, but phase shifted by 180° . With this trick you can almost double the output voltage from a normal 12 VCD power supply, resulting in about 16 Watts into a 4 OHM speaker. These amps are widely used in normal car stereo units these days, and as many of them have even 4 output channels ( 2 front / 2 rear ) they are sometimes working with 8 amps all togehter ( 4 x 2 ), of course all coming in form of fully integrated ICs today. These amps are available as external units also, all major car stereo producers will have those in their program, and they should be rather cheap also. Their efficiency is still pretty good, and they should be your first choice.
Higher powers than 16 Watts / 4 Ohm can only be achievedby using internal DC/DC converters ( high frequency switching power supplys, similar to PC PSUs ), they will take the 12 VDC on their input and transform them to +/- 30 - 80 Volts, so that a normal class A/B amp can be used then. These amps can be boosted to almost any output power, only the sky is the limit, 1000 Watts / channel are achievable . Now, dont try to use such an amp for your purpose !! They are monster energy consumers, one hour of listening joy and a veritable car battery may be empty, even if you dont use a lot of the available power, as the DC/DC converters alone have a poor efficiency even in standby.
I'd go for the cheapest external 12 VDC amp in the Kenwood/Alpine/SONY/Pioneer program, it will be a integrated circuit based bridge amp with about 16 Watts output power / channel into a normal 4 Ohms speaker, cost about 40 - 50,- € and should do a fine job ....
Great post ChristianHJW - I'm happy you typed that out instead of me.
but - 12VCD?, 12Vdc?, 12Vcc? - minor point but i was curious - maybe thats how it is expressed there (current direct)?
Anyhow - Christian is quite correct that your only real choice is a bridge amp because of your requirements, you should be able to get a car amp such as this Pioneer (http://www.pioneerelectronics.com/Pioneer/CDA/CarProducts/CarProductDetails/0,1429,92459,00.html) which should be an easy hook-up to your motorcycle battery, get a 1/8 plug to RCA and attach from your mp3 player to the amp. You also might want to get a battery protector - a device that will cut the power to any accessories before you drain the battery completly and can't start the bike. That pioneer is rated 240W: 2X 50W@4ohm, since you say your speakers are rated 50W@8ohm - which is usually based on p-p values and not RMS, so you are probably ok at underdriving them a bit instead of getting a 400W model and popping some voice coils or whatever they have.
I would be a bit worried about the current drain (12V*4.5ah = 54W/h) and at full volume you'll be draining that battery at maybe 100W - I would test the setup before trying it out in the woods for the first time
In my family's camping car we use a power converter (dunno the english name for it) which is able to transform 12V DC to 220V AC @ 250W. There you can easily use your home amplifier and blow your speaker! One inconvinient is that your battery will very soon be empty. In order to prevent this, even if we do not use 250W amplifiers, we have two 55W solar panels which keep the battery loaded.
Sounds cool fragtal. But solar panels are expensive aren't they?
We call you power converter a transformer.
On the other side of the pond it is also called an "inverter" - although I must admit that transformer might be a better discription.
But adding one of these would not be a good idea since you need another unit so it will not be as small as it could be, as well the modern inverters use a push-pull MOSFET high-frequency design - similar to what computer PSU use. These produce a lot of strong harmonics within the line voltage going to the amp. Most amps are just design to reject the 50-60Hz line voltage noise - but not many I would think filter out 10KHz+ harmonics. I know when I hooked up my stereo to the inverter I had a lot of noise from the inverter coming through the stereo.
Although these are great things to have in the car you are far better off with the car amp mentioned above.
Great post ChristianHJW - seconded
Also many thanks to TwoJ - the Pioneer amp looks like just what I need.
This quest was triggered by a desire to avoid dragging round a heavy, battery munching CD player, boxes of CD's and plenty of arguments with SWMBO about who forgot the one CD that I just have to listen to next. Now with a HD MP3 player no more CDs found lying in the sand/camp fire the morning after plus no more forgotten CD's.
I am now getting resigned to carrying more than one lead acid battery but at least I wont have to look at the horrible macho styling of the old CD player.
Once again - thanks all
Dont you just love web forums
Paul
Sounds cool fragtal. But solar panels are expensive aren't they?
We call you power converter a transformer.
Yep, they are. The equipment cost 500€. It consists of two 55W panels, one electronic device which manages to reload the battery and prevents "overloading" and some cool copper cable with a huge diametre.