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 Forum index » DIY Hardware and Software
Portable battery amp project
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SineHacker



Joined: Mar 09, 2010
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PostPosted: Sun Sep 30, 2012 2:07 am    Post subject: Portable battery amp project Reply with quote  Mark this post and the followings unread

As a result of hitting some festivals this Summer, I have found myself wanting to build a battery powered portable amp so I can join in with jams and the likes without the restraint needing a power point. Most of my gear is battery powered so I'm half way there.

The main things I am looking for are decent volume and battery life. Sound quality is also up there, but I don't mind if the final build has some kind of "characteristic" sound as most of my instruments are home made and I think that will just extend their uniqueness and might make for some interesting recordings by mic positioning etc. (if that makes sense)

I have a pair of 5ohm speakers extracted from a 25 year old Yamaha keyboard which I think would be a good place to start.

I would like to get some advice on amplifier IC's to drive these, I have used LM386 for a number of projects with varying results. I built a mini battery practice amp using the LM386 a short while ago, but the sound is always distorted, a friend pointed out that the input level is probably too loud which would cause this, so any advice on regulating this would be good.

So I'm thinking of using a pair of LM386 - unless anyone could recommend something better?
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JingleJoe



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PostPosted: Sun Sep 30, 2012 3:03 am    Post subject: Reply with quote  Mark this post and the followings unread

You can add both a gain control and an input level control on the lm386. For the input level control add a pot wired up as a voltage divider. This will allow you to "tune it in" to the maximum possible level without distortion.
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SineHacker



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PostPosted: Sun Sep 30, 2012 3:16 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for a fast reply as always JingleJoe!

I think I did use a voltage divider for my last battery amp - although perhaps I just used a variable resistor at input in line without tying one pin to GND, will have to look to check it out because it would be nice to have a quick fix for that!

Is the LM386 only suitable for 8ohm speakers though? how would a pair of 5ohm's play out? I was thinking about using a pair of LM386 (or a single IC designed for stereo) so I could switch for stereo input if I wanted
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JingleJoe



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PostPosted: Sun Sep 30, 2012 5:17 am    Post subject: Reply with quote  Mark this post and the followings unread

put something like a 1 watt 3.9ohm resistor in series with the 5 ohm speaker and it should be fine, I have done that before and the result was fine.
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SineHacker



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PostPosted: Sun Sep 30, 2012 5:20 am    Post subject: Reply with quote  Mark this post and the followings unread

cheers, I will post back with results
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richardc64



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PostPosted: Sun Sep 30, 2012 10:51 am    Post subject: Reply with quote  Mark this post and the followings unread

The LM386 datasheet has three graphs for Power Dissipation -vs - Output Power for 4, 8, and 16ohm loads. A 3.9ohm resistor will just waste a percentage of the chip's puny output power as heat.
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JingleJoe



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PostPosted: Sun Sep 30, 2012 11:39 am    Post subject: Reply with quote  Mark this post and the followings unread

Have you got a better IC to suggest? The LM386 has been plenty loud enough for my purposes in the past. Also I thought the minimum it could handle was 8 ohms, I suppose it'll just need a heatsink with a lower resistance load?
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SineHacker



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PostPosted: Sun Sep 30, 2012 11:43 am    Post subject: Reply with quote  Mark this post and the followings unread

Ah ok thanks for pointing that out, there are a couple of interesting graphs there that I haven't paid enough attention to before. So would I be right in thinking the graph for the 4ohm load shows less dissipation, and that would be a good thing? I'm guessing in terms of less power wastage (and better battery life?) which is something I definitely want.

I just had a thought, if I am running a pair of speakers, would the two 5ohm speakers have an overall load of 7.5ohms as opposed to 10ohms (I have faint memories of electronics lectures in college that I didn't pay enough attention to! Sad )

I guess I really just need to start testing

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richardc64



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PostPosted: Sun Sep 30, 2012 12:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

JingleJoe wrote:
Have you got a better IC to suggest? The LM386 has been plenty loud enough for my purposes in the past. Also I thought the minimum it could handle was 8 ohms, I suppose it'll just need a heatsink with a lower resistance load?


I like the LM380. The '386 is loud enough if you're right on top of it, I suppose. I don't think it would need a heatsink until supply V approaches the maximum.

SineHacker wrote:
I just had a thought, if I am running a pair of speakers, would the two 5ohm speakers have an overall load of 7.5ohms as opposed to 10ohms (I have faint memories of electronics lectures in college that I didn't pay enough attention to! Sad )


Ohm's Law can be applied to Impedances as well as resistance, so two 5s in series would be 10ohms, which isn't much more than 8ohms, which seems to be the optimal load.

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bubzy



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PostPosted: Sun Sep 30, 2012 1:02 pm    Post subject: Reply with quote  Mark this post and the followings unread

theres a N-4 version of the 386, this outputs a whole 1 watt!!! might help a bit.
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SineHacker



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PostPosted: Mon Oct 01, 2012 2:02 am    Post subject: Reply with quote  Mark this post and the followings unread

they are definitely something to look into, but the lm380 and lm386n-4 both push up the supply voltage requirements, the lm380 might just be manageable with 10v min but the lm386n-4 needs 24v which is going to be a bit of a pain with batteries.

I wanted to use AA's just because they will typically last a little longer than 9v PP3's so I would need about 8. I found this http://www.ebay.co.uk/itm/C1207-1-RC-Battery-Holder-Case-Box-Pack-8-x-AA-Futaba-/260846742670?pt=UK_ToysGames_RadioControlled_JN&hash=item3cbbad6c8e#ht_2069wt_952 so maybe that wouldn't be too annoying (The double sided holders would be a pain!)

I will order some lm380 ic's and give them a blast along with the lm386's - I saw on a website that the lm380 uses some of its pins as heat sinks, would it be safe to test on a breadboard or am I likely to experience meltage?

Richardc64, if you wouldn't mind just clearing up this bit of theory for me, would there be a difference in load if the speakers were connected in parallel then?

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PHOBoS



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PostPosted: Mon Oct 01, 2012 2:48 am    Post subject: Reply with quote  Mark this post and the followings unread

For the impedance you could use another speaker in series instead of a resistor, less heat more sound.
(Oh, I see that was allready sort of mentioned, still waking up here Wink)
Another thing you could look into (allthough not as DIY as making it yourself completely)
is taking the amplifier out of a set of computer speakers. Those usually work on a low voltage, are small, and can get pretty loud.

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LFLab



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PostPosted: Fri Oct 05, 2012 12:18 am    Post subject: Reply with quote  Mark this post and the followings unread

I would look elsewhere for the amplification, the LM386 is complete and utter shite (to my ears anyway). Maybe use one of the TDA car amplifier chips? A bit more (clean) power.
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JingleJoe



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PostPosted: Fri Oct 05, 2012 1:04 am    Post subject: Reply with quote  Mark this post and the followings unread

LFLab wrote:
I would look elsewhere for the amplification, the LM386 is complete and utter shite (to my ears anyway). Maybe use one of the TDA car amplifier chips? A bit more (clean) power.

I was thinking only this morning, anyone with a strong opinion is usually as wrong as they are adamant.
Sure the sound quality is not on par with a hi-fi system or something like a "professional" audio system, but a lot is down to the speakers used and components selected for the circuitry- people often use very low capacitances for the input parts, which act as high pass filters.
On top of all that, we aren't making some high fidelity audio system, look at the first post; the main thing Mr Hacker wants is volume and battery life Smile

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bubzy



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PostPosted: Fri Oct 05, 2012 3:06 am    Post subject: Reply with quote  Mark this post and the followings unread

http://www.youtube.com/watch?v=9SclQIWkOtk

this guy has used a neat little transistor preamp before the 386 amp, seems to get nice results in the way of volume from it.

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analog_backlash



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PostPosted: Fri Oct 05, 2012 3:31 am    Post subject: Reply with quote  Mark this post and the followings unread

I agree with JingleJoe about the crappiness or otherwise of the LM386. I've seen this over and over again on music electronics forums e.g. never use the 555 timer/LM358/LM324 etc. etc. The original circuit requirements are often not considered, just people's personal prejudices against a particular component. I use the LM386 all the time (especially in headphone amps) and it is perfectly adequate for the job. If you're trying to build a hi-fi amp, then that's a completely different matter.

Gary

P.S. he's right about the highpass filtering as well, as he pointed out as a problem on one of my projects.
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SineHacker



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PostPosted: Sat Oct 27, 2012 3:43 am    Post subject: Reply with quote  Mark this post and the followings unread

sorry for the late reply, I have been working on this and not letting the project stagnate!

Bubzy, excellent vid thanks!

I think I am going to work with the LM386, should have some stuff to show soon - finishing off a new drone box at the mo Smile

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