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Old 10th June 2013, 09:40 PM   #1
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Question recomendations for a +-12v portable amp?

Hi there, im asking to you the most experienced people,what would you advice.
Iwant to build a portable sistem with some 8 or even smaller speakers,
to be used by a singer, in places where there is no ac supply.
Sometimes it has to be used on the move.
My plan is to use a pair of motorcycle batteries, small size so it is not so heavy.
The cuestion is about what chip will suit the needs so as to have
decent quality audio and not deplete the batteries so fast.
Aiming for like 20w x2. Ive built several different chipamps, but none to be
used this way; any comments? all be apreciated.
Thanks in advance for your replays.
Martin martinez
uruapan michoacan
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Old 10th June 2013, 10:27 PM   #2
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I did a bunch of research on that because I wanted to build a small portable Hi-Fi for camping. I eventually decided I didn't have time for such a project, but I did conclude that the Texas Instrument switchmode amplifier chips may be the best, and you want to use switchmode (class D or whatever) both for efficiency (86% instead of about 40% with analog circuits), and to get more than a few watts out of a 12 volt battery. Using a single 12 volt lithium battery means you could recharge the battery from your car if necessary. But switchmode amplifiers have a lot of very high frequency noise to deal with, so board layout and part selection would be tricky.

Another perhaps more sane approach is to buy a car stereo that plays CD's and has AUX inputs, SD chip and/or USB chip slots, and even bluetooth, which should be able to pick up the 16 bit stereo wireless signal from my android smart phone, which has about 10Gbytes f music on its 32 G micro SD chip. All current car stereos have switchmode poweramps in them. Some reasonably priced 12 volt batteries are available on the web. They are often used in the CCTV industry. I'd go for at least a 10Ahr battery. Hope this helps.
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Old 11th June 2013, 01:02 AM   #3
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Thanks, also tought about class D, but analog chips are easier to get than class D modules around here, also heard some little class d amps in portable devices and were awful sounding with recorded music, then i dont trust those for good sounding live vocals. Also saw the car amp option, but then my diy personality suffers.
and i think ican get better results than buying caramp. I plan t make mic pre thats why ill use +-12V, for the pre opamps then run the amp at either 24V se or split supply 12-12, but really want it to sound at least decent. Thanks anyway your comments are really apreciated.
martin martinez
uruapan michoacan
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Old 11th June 2013, 01:04 AM   #4
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It really is not a matter of just more watts, but of quality sound output and maybe alittle less current consumption. It will be used for one or two hours, but need those 2 hours to sound good, or better!

Last edited by martin martinez; 11th June 2013 at 01:12 AM. Reason: add
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Old 11th June 2013, 03:07 AM   #5
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If you use a class AB analog poweramp (push-pull output stage) with a +/- 12V supply, the output might swing up to +/-10 volts. Power = E (voltage) squared over R (load resistance). For RMS wattage you have to multiply 0.707 onto that peak sinewave voltage reading. So it's (10V x .707) squared / 4 ohm (speaker) = 12.5 watts rms. If the speaker is 8 ohm, max power will be 6.25watts. If used in a large room or especially outdoors, that may not be nearly enough. The speaker drivers would need to be VERY efficient, which is often a trade off with fidelity. I agree that many of the switchmode amps have been called shi*ty sounding, which is why I would be willing to consider buying a name brand car stereo that might give more power that sounds decent (maybe Pioneer). A big company like Pioneer or perhaps Kenwood? Sony? you would think they would take the time to get the circuit to sound real good (?)

Another idea that I would probably do is build two poweramps driving two speakers (probably in the same cabinet - ideally with angled panels for more separation), and either drive both inputs with a mono source or get a digital reverb guitar pedal that creates a stereo reverb output ( I think they all do that these days). I use such a device on my guitar along with a stereo guitar amp I built, and it sounds fantastic.
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Old 11th June 2013, 05:18 AM   #6
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yes ilike your idea maybe avery subtle chorus efx, i used to do this for live vocals reinforcement, with a dedicated pre and compressor and then an aux send to the chorus set at barely noticeable level and slow modulating like .3 to .7 sec it gives alot of depth.
And another question what about bridged mode? i was thinking about a pair of chips per 8 ohm speaker, bridged to give alittle power push.
Ive been using an old chip amplifier made for car PA, had to mod the power res caps, and many of the signal path caps, also changed some values and types on caps and res in the input circuit to give it a better sound of the microfone when singing.
It does its job but id like to make a better sounding and a little more powerful
one so as to have more dynamic range and less distortion.
By the way you play guitar, goood!
Thanks again 4 taking your time.
martin martinez
uruapan michoacan
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Old 11th June 2013, 06:11 PM   #7
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I forgot about bridging. Great idea. With twice the output swing you'll approach 4X the power. Probably 3X in reality due to sag of the power supply. Linkwitz does that with the 3886 chip in his Pluto system. I'm not sure if you can run that chip on that low a voltage though. Check the spec sheet on it.

One thing that is often forgotten in commercial amps is a passive Rf filter at the input. I always include these. All it takes is a R in series (maybe 1Kohm) & then C to ground (maybe 10n, which would put you down 3dB at 16kHZ with a zero ohm source Z). Reducing the cap to 5600 pF would put the 3dB down point at 28.4kHZ. In a guitar amp or PA amp I'd gladly give up the energy above 16kHZ in order to have better filtering of Rf. Most opamps are great if you don't ask them to do something they can't do well, such as giving them significant energy above about 50kHZ. With proper layout and good local power supply bypass caps within an inch of each opamp, many opamps can go much higher in freq., but feedback is rolling off up there, so distortion correction too, and the opamp can start demodulating the Rf noise as it becomes non-linear, splattering difference frequency noise down into the audio spectrum, while getting tangled up in the sum frequency energies (intermodulation distortion results). This Rf filter will form a shelf since there probably needs to be an R to ground at the input of the chip. Since the R at the chip is much bigger than the 1K, the filter is still substantially effective, and may reduce distortion and noise significantly depending on proximity to Rf noise sources. This is especially important when you are near light dimmers or any other Rf noise sources. Since mics are low Z and often deliver a differential signal through an XLR connector, these problems are greatly reduced if the mic preamp has a differentail receiver ("balanced input"), but an Rf filter never hurts anything. If the source Z of the mic is 1kohm, that must be added to the 1K R when calculating the 3dB down point. You should know what the mic source Z is, so you can take that into account. It's not likely to be zero ohms. Any such distortion produced in the front end of a PA amp will get amplified by about 1000 times by the time you hear it coming out of the speaker, so I consider this a good idea.

Last edited by Bob Richards; 11th June 2013 at 06:17 PM.
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Old 18th June 2013, 01:16 AM   #8
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well its all over now did my searching and foun plenty of options for this project. seems i{ll be using several small chipamps,running at +-12V with 2 series batteries, and one for eahc 8 ohm speaker. like 3 or 4 .
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Old 18th June 2013, 05:22 AM   #9
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I noticed the 3886 chips are good down to +/- 10volts power supply. Which chips did you end up going with?
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Old 18th June 2013, 02:36 PM   #10
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TDA 1562Q a nice choice
http://www.reber.si/TDA/TDA1562Q/e002044.pdf
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