Greetings, all. This is my first post here ... please be gentle!
I recently acquired a cute little bass amplifier. It's a Silvertone BA Sx amp, which draws 26 watts and allegedly puts out 20 watts (10 watts RMS) through a 6-1/2"inch speaker. I've plugged it in and it seems to work as advertised.
A friend of mine is a bassist who wants to play her electric bass in camping venues where there aren't any electric outlets around. She's rejected the Pignose Hog 30 as too much for her needs ... too big, too powerful, too heavy, and too expensive. She only needs to accompany one or two acoustic guitars, and maybe a mandolin, all un-amplified.
It has occurred to me that the BA Sx might be the perfect amp for her if I could convert it to DC power. I've already played it powered from a car battery with a little Plug-in 100 W inverter without dramatic signs of failure, but that's a standard car battery.
I guess my questions to the group are:
1. Would it work using perhaps the sort of 12-volt lead-acid gel battery you see in home alarm systems or back-up systems, which typically push out 7 amps or so? How about two of them in parallel?
2. It seems it would be more efficient if I just bypassed the power transformer and rectifiers and tapped into the circuit that just fed it 12 volts (or close to it) directly from the battery, thereby eliminating any power loss from the transformer, along with the need for an inverter. Is this reasonable, and if so, what's the best way to find that point in the circuit? (Unfortunately, I have no schematic for the thing.)
3. Or would it make more sense to scrap everything but the speaker and the cabinet, and build a battery-powered amp from scratch, or from a kit that can put 10 watts RMS into the speaker? If so, can you give me recommendations for kits or plans that would let me do this?
In closing, I should add that my knowledge of electronics is fairly rudimentary, although I've built many kits and a few projects from scratch, using schematics that were provided to me. And I do know how to use a VOM and a soldering iron.
Hi JLT, welcome.
The short answer is to get an commercial mains voltage inverter and battery. The simplest, most direct, but possibly heavy and inefficient way.
2) is a reasonable idea, more power efficient, but how do you know the internals run on 12 volts? It's pretty unlikely, more like two lots of split rails.
Now automotive audio runs off 12 volts, and there are quite a few moderately powered amps kicking around (often cunningly disguised as small graphic EQ), so why not start afresh?
Put that amp to one side for home, and consider building something that is purpose-built, for her in her situation.
I'd start hunting second hand shops, garage sales and swap meets looking for a modern-ish car radio (cass/CD/MP3?) that has "Aux" in's. One channel of this should be enough watts for a campfire jam.
I can see this built inside a large plastic jerry can, 12-inch speaker, radio mounted near the top, batteries secured in the bottom, sundry methods of recharging, plugpack, car adapt, solar, hand-cranked gen, windmill, thermopile for the campfire...
(http://cdn.trendhunterstatic.com/thumbs/jerrycan-speakers.jpeg)
(Needs some speaker protection)
Add a couple of ply doors to protect the speaker in transit, but open to provide a rough horn in use.
As to your question about those gel batteries, are they 7AH ?
The ampere hours will determine how long you can run for.
26W input power would be approx. 2.15A at 12V, but that 26W may be idling.
But even if we double that figure, that makes 4A. So a 4AH battery should run it for an hour. This assumes the inverter is 100% efficient, probably not the case but should be close.
Or or or...
Have the bassist consider getting an acoustic bass. There are straight acoustic bass guitars, as well as acoustic-electrics. depending upon the bassists usual gigs, one might be appropriate.
Quote from: EnzoHave the bassist consider getting an acoustic bass.
Funny you should say that, was thinking similar because I knew a guy who had an acoustic bass guitar who brought it to campfire jams. He wasn't a very effective player, but it was certainly audible (and no risk of speaker "frapping").
Random pic to illustrate;
(http://www.carlradle.com/images/photos/photos=041305/Carl%20with%20Big%20Guitar.jpg)
Big mutha, but sure beats the battery problem.
{Somebody asked me to do some designs for portable sound systems for very small forest gatherings. It resolved to two 8" J-horns and an equipment pack with mainly car audio gear. And of course batteries. The idea was a sound system that didn't require the back-packing in of a noisy, smelly generator and fuel. Interesting set of design constraints.}
Quote from: Enzo on March 20, 2015, 12:17:29 AM
Or or or...
Have the bassist consider getting an acoustic bass. There are straight acoustic bass guitars, as well as acoustic-electrics. depending upon the bassists usual gigs, one might be appropriate.
That's what she was using the last time I saw her. I had my own acoustic bass, and we had the same problem: neither bass could be heard when other instruments were playing. Both had pickups for amplification, and it was obvious to us that outside of practice, the pickups were essential for playing in groups.
(And, yes, I've seen the sort of acoustic bass guitar that Roly's friend had. A tad unwieldy, but it probably doesn't need amplification.
Thanks for your suggestions, all. I've got a lot of experimentation ahead of me, I see. I'll keep you informed.
And BTW, I've written up something that may amuse you all. It's an essay called "Incense and Radio Shack" and can be found at http://jayeltee.blogspot.com/
I've built and sold tons of battery powered amps, and most bang for the buck was always the lead acid gel/alarm type, either 12V 7AH or the 12V 4AH , two of which in this case is what I suggest: your current amp most certainly has dual rails, probably around +/- 19V or so.
It can work quite well with +/- 12.6V rails.
You'll need to add the proper heavy duty switch to switch on/off both rails at the same time (you'll need at least a 6A switch) or you will damage it.
...and fuses (or circuit breakers) too, one for each rail, say 10 amp, real close to the battery terminals. A short circuit can light up a cable in a second.
{1AM, gig over, vehicles packed we set off home in convoy. Only a couple of minutes up the road and the car in front suddenly pulls off the road and the driver door opens and the driver bales out before the car has even stopped. Now he appears to be break-dancing in the middle of the road :duh - quite impressive for a tired drummer pushing sixty.
Now he appears to be stripping, ripping his jeans down and trying to get them off over his boots. We watch in awe. Now there is the smoke drifting up from his jeans. :o Impressive. Pity he didn't pull this sudden outburst of creativity mid-evening when it would have wowed them at the pub.
Seems he was helping the guitarist with a 9 volt stomp battery problem before the show, and in the swaps one when into his jeans pocket.
This all went well until a coin also in the pocket shorted the fresh battery on the trip home.}
WOW :o
Glad it was a 9V battery.
If it had been a Lithium cell phone battery the one he shorted, no tiome to strip, he'd have a HOLE in his body where tha battery was.
FWIW one of the official (no kidding) theories about the Malaysian plane which mysteriously disappeared with no trace 1 year ago was that a cargo of Lithium batteries in the cargo bay caught fire :'(
And many airlines let you carry a notebook on board, but not a spare battery for it.
In the Bush you tend to see the odd battery catastrophe; and yeah, Lithium's are a worry. There are lots of Utoob vids of people stomping and otherwise abusing them into reaction, but there is also CCTV of a girl on a bus in China poking at her phone when it suddenly bursts into flames and she rapidly leaps off the bus with it burning like a traffic flare.
pprune forum for all the guff on MA370. I'm keeping a very open mind (you know I've got a bit of a history in accident investigation, eh?) and I don't have any scenario strongly in mind, but I have tried to fit what little we do know. There have been two incidents of oxygen fires on these aircraft, and looking at the state of the cockpit controls after they put it out, the computers are flying the plane, but how can you fly the computers?
(http://static1.nydailynews.com/polopoly_fs/1.1738032.1396033366!/img/httpImage/image.jpg_gen/derivatives/index_635_390/new-theory-blowtorch-fire-cockpit-similar-2011-incident-eyed-malaysi.jpg)
(transponder controls: centre console, bottom-right)
If I was sitting in the right hand seat and an oxygen fire started right next to me I would be up and outta there like a flash (standing on the transponder control panel on the lower right of the central console) in the panic. If you have never seen an oxygen fire, then just think "petrol". "Intense" doesn't cover it. The pilot on the left might have some more time to try and reverse course, but does he put on his smoke hood? Is it even working now? How hot is it getting? Pretty soon it would be abandon the cockpit or die, but the opinion has been that at that altitude, once it had burned through the hull it would have been self-extinguishing due to decompression effects. So maybe they can go back into the cockpit, but what's left?
It has been demonstrated many times that an aircraft can still be flown with badly impaired controls, but this is the world of Fly By Wire. How much control would you have over your computer if your keyboard and mouse had melted into a puddle? The computers themselves are all still snug down in the avionics bay, chugging away.
I think the FDR will confirm what we know about what it did, but won't answer the question "why"? And the CVR will only have the last 30 mins before ditching, so that's unlikely to cast any light on anything.
Was at a friends place while he was replacing his crawler battery (big) that had been on charge. Accidentally flashed the terminals to the machine frame and the battery violently exploded while he was holding it, enveloped in a large cloud of fog (O+H=water), shattering the plastic case and sending shards several metres. He was very very lucky that he was only hit in the face by small stuff, and it all missed his eyes. He had an outside shower nearby so I frog marched him under it, and no great harm was done.
My travels have taken me to many "new settler" homes and properties because if my experience with off-grid systems. On one property I came across a sorry sight, several solar panels that where all blackened internally, and some 100A/Hr house lighting batteries that had ... well ... melted.
"It's okay, I know what I'm doing". This young fesbian lemonist was building a dwelling, and wasn't gonna ask for no advice from no man. I can only guess that she reasoned that the batteries in a radio go positive-to-negative, so she wired the batteries like that. So far so good. 12 volts. Cool. Now all I have to do is connect the solar panels the same way, positive to negative.
Well what she got in very short order was a whumping great silicon diode forward biased across a 12 volt 100A/Hr battery set - and a lot of heat everywhere in a real hurry. In fact I suspect that it went pear-shaped so fast she couldn't safely disconnect anything, it all would have been alight with burning melting insulation very fast. :(
These are some of the reasons I want to see a final backstop fuse/interrupter close to each battery terminal. :dbtu:
Quote from: Roly on March 19, 2015, 10:56:17 PM
Hi JLT,
I'd start hunting second hand shops, garage sales and swap meets looking for a modern-ish car radio (cass/CD/MP3?) that has "Aux" in's. One channel of this should be enough watts for a campfire jam.
Done. I found an old Radio Shack car radio. Don't know the wattage, but it was advertised as "28 Watt" measured god knows how. Four speakers, so perhaps 7 watts peak per channel or 3 watts rms. Not much. But since I got it for free, I'm sure I can spring for a 12 volt amp that will give me 10 watts RMS and feed it from the "AUX OUT" jack (originally intended for a power amp).
Quote
I can see this built inside a large plastic jerry can, 12-inch speaker, radio mounted near the top, batteries secured in the bottom, sundry methods of recharging, plugpack, car adapt, solar, hand-cranked gen, windmill, thermopile for the campfire...
That's where this project seems to be going. Any idea where I can get a low-wattage (10W RMS) bass speaker without paying a lot? Would any small woofer do?
Quote from: JLTfound an old Radio Shack car radio. Don't know the wattage, but it was advertised as "28 Watt" measured god knows how
On the one hand you a quite right to be sceptical about "X watts" (I've got some computer speakers somewhere that say "400 watts" on the front - and take four AA batteries on the back. :duh "400 Watt" brand I guess. ::) )
On the other hand all car audio has to compete with high noise levels (and crap speakers in crap enclosures, i.e. car doors) so it has to
actually produce some sort of power, a few real watts at least. The most reliable way to know is flip the lid and see if you can read the type numbers on the chip amp(s), then we know for sure. From a battery point of view starting with an amp at the lower end of the power scale is a good thing.
Taking on this path means that you are going to have to use a fair bit of cunning and ingenuity to get your result - there aren't too many trails blazed in the forest in this direction.
Generally speaking small speakers, car speakers, and Hi-Fi speakers are not noted for their efficiency (dB/Watt), and in battery operated gear this is a particular interest because doubling the conversion efficiency (+3dB/W) will double the operating time at a given playing level.
For bass I'd be inclined to go with a 12-inch, but given the low power level you might be able to scrounge something suitable (e.g. I have a few light duty 12's I've recovered from console organs).
A possible alternative if it's a four channel "quad" radio is to actually use four 8-inch speakers (but I'd be a bit dubious about bass).
Remember, an awful lot of different stuff fits in a standard dashboard radio cutout, same for a 12-inch speaker, so there is a lot of latitude to rethink based on experience; in other words it may need a couple of cycles of refinement to get it right.
While the
Aux input is about the right signal level (~half a volt) it is typically fairly low impedance (10-47k) so might at least require a FET buffer or the like so the guitar is happy; not difficult, just a small detail.
Wow its been some year ago but i converted my marshall vs-10 to dc using radio shack d-cell batteries holders (2x6 d-cell batteries) and diode and adjustable trim pots. We could play for about 10 hours at low to medium volume.
Quote from: jay4cars on March 31, 2015, 11:01:42 PM
Wow its been some year ago but i converted my marshall vs-10 to dc using radio shack d-cell batteries holders (2x6 d-cell batteries) and diode and adjustable trim pots. We could play for about 10 hours at low to medium volume.
I don't know anything about the Marshall VS-10. What sort of re-wiring was required to convert it?
Update on my own project:
The Radio Shack unit turned out to be wonky, so I ended up ordering two kits from Alltronics: a preamp kit and an 18 watt amplifier kit. Got them yesterday, but won't be able to put them together until next week. Found what was described as a "sub-woofer" in a thrift shop for $2.50, and I'll see if that works as a speaker.
It's funny how these projects seem to get just a little more complicated (and expensive) with each plot twist.
Quote from: JLTIt's funny how these projects seem to get just a little more complicated (and expensive) with each plot twist.
Yeah, it can be a bit like stepping onto a running exercise treadmill - but you really can't complain about a mystery speaker for $2.50.
You have plunged off the beaten track here a bit, so there will be some experimenting, you'll have try what seems a good idea and see if it really is.
Planning is good - "long think, short do". :dbtu:
Quote from: JLT on April 05, 2015, 04:20:39 PM
Update on my own project:
The Radio Shack unit turned out to be wonky, so I ended up ordering two kits from Alltronics: a preamp kit and an 18 watt amplifier kit. Got them yesterday, but won't be able to put them together until next week. Found what was described as a "sub-woofer" in a thrift shop for $2.50, and I'll see if that works as a speaker.
Update to my update: I built the Alltronics kits and hooked them up to the "woofer" and found that the woofer just couldn't handle the power. I hooked up a speaker from a compact stereo and that worked better, but started distorting with the volume control on "2" out of "10." Do I need to drop another resistor between the amp and pre-amp to lower the gain? Or should I just get a speaker rated for 60 watts rms and see how that works?
And speaking of speakers, would any sub-woofer or woofer do, or should I get one specifically for a bass guitar amp? I do have an EQ box I can run the signal through to optimize the sound. The speakers I'm looking at are:
http://www.ebay.com/itm/Single-8-inch-8-ohm-Home-or-Studio-Woofer-Replacement-Speaker-Custom-Cabinet-/201028325758?pt=LH_DefaultDomain_0&hash=item2ece38d17e
http://www.ebay.com/itm/8-Woofer-with-Poly-Cone-and-Rubber-Surround-70W-RMS-at-8ohm-DJ-PA/261566982131?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D30251%26meid%3D3710a194a5454fe9aabf1a8d2dcbfc77%26pid%3D100005%26rk%3D2%26rkt%3D6%26sd%3D201028325758&rt=nc
and FWIW, the schematic for the preamp is at
http://www.alltronics.com/mas_assets/acrobat/KIT_98.pdf
and the power amp:
http://www.alltronics.com/mas_assets/acrobat/KIT_105.pdf
I currently have a 20K pot installed between them for a volume control. That's not specified in the on-line schematic for the power amp, but is included in the one I got for the kit.
Quote from: JLTcouldn't handle the power
Which is likely to be the case with just about any consumer/domestic speaker these days that you pump a genuine 15Wrms into.
The preamp has a problem, very low input impedance, only about 60k without the optional 10k killer resistor. Passive guitars don't like. You need at least a buffer of some sort (e.g. FET) to get it up to at least 1Meg.
It also could have more gain than you need and you can moderate it by increasing the value of R6/100 ohms to say 220 ohms.
HTH
Quote from: Roly on May 01, 2015, 12:10:09 AM
Quote from: JLTcouldn't handle the power
Which is likely to be the case with just about any consumer/domestic speaker these days that you pump a genuine 15Wrms into.
The preamp has a problem, very low input impedance, only about 60k without the optional 10k killer resistor. Passive guitars don't like. You need at least a buffer of some sort (e.g. FET) to get it up to at least 1Meg.
Thanks for your response. Can you give me a little more information on this ... what kind of FET, and how to wire it in, and what other components might be needed to support it? If you could just direct me to a schematic, I'd be grateful.
I omitted the "killer resistor" as specified in the kit instructions, since I wasn't going to using an electret condenser mike. Instead I'm using two piezo pickups, as furnished by JJB electronics.
QuoteIt also could have more gain than you need and you can moderate it by increasing the value of R6/100 ohms to say 220 ohms.
At this point, I'm not sure how much gain I'll need, since that will depend on the speaker. I'm assuming that a speaker with a 70-100 watt (RMS) rating will need a true 15 watt output to drive it, at the very least.
Which prompts me to ask that question again: are either of the two speakers I cited earlier going to be good "fits" for this project? If not, where should I be looking.
Again, let me thank you for your response. I didn't expect to need such hand-holding, and I'm grateful for your help. I get questions all the time like this in my own areas of expertise (lutherie, etc) and I know how exasperating such questions can be.
Quote from: JLTInstead I'm using two piezo pickups
In which case the input impedance will be
way too low, 2.7Megs being more typical. Will sound very thin and trebly.
(http://www.electronicproducts.com/images/e736f65ae3af0ca914d479cf09c9962b.gif)
N-channel FET such as an MPF102 or 2N3819 (o.n.o.). Drain to V+, Source to ground via "RS", that 10k resistor (see, it came in handy for something!) and to the input. 2.7Meg resistor from Gate to ground, "RG".
Shouldn't need C1 and I think your amp already has a cap at C2 (input).
I'm talking
voltage gain which should not be confused with power output. 15 watts into
any speaker is going to make quite a racket. Whatever maximum power a speaker is rated for it will still produced quite a noise even with only one watt.
I'll leave speaker recommendations to
JMF.
Quote from: Roly on May 02, 2015, 11:47:59 PM
Quote from: JLTInstead I'm using two piezo pickups
In which case the input impedance will be way too low, 2.7Megs being more typical. Will sound very thin and trebly.
Definitely not what I want for a bass amp!
Quote
(http://www.electronicproducts.com/images/e736f65ae3af0ca914d479cf09c9962b.gif)
N-channel FET such as an MPF102 or 2N3819 (o.n.o.). Drain to V+, Source to ground via "RS", that 10k resistor (see, it came in handy for something!) and to the input. 2.7Meg resistor from Gate to ground, "RG".
Shouldn't need C1 and I think your amp already has a cap at C2 (input).
Exactly what I needed. I'll have to see if I have that 2.7 Meg resistor in my big bag o' parts. And if the cap at C2 is the same as the 100 nF C3 on the Alltronics schematic, you're right, and I can dispense with that. As for the FET, it looks like Radio Shack doesn't stock it, so I'll see if one's available at our real electronics store here in Sacramento (Metro Electronics).
Quote
I'm talking voltage gain which should not be confused with power output. 15 watts into any speaker is going to make quite a racket. Whatever maximum power a speaker is rated for it will still produced quite a noise even with only one watt.
I'll leave speaker recommendations to JMF.
I'll await his input before I order.
Thanks for the trust vote :)
Both speakers will work properly, pick one.
As of "it doesn't handle the power" it's not exactly that but simply power amp clipping.
1 W makes a big noise if using a guitar, 15W is not *that* much for a Bass, because:
* human ear is much less sensitive. Big time.
Ever wondered why alarms, beepers, sirens, etc. *all* operate at high mid frequencies?
That's because we are genetically tunes to them.
Low frequencies? ... not that much.
Only animals I know who prefer them are whales.
* speakers lose tons of sensitivity for every octave they go down, as in 8:1 per octave
So we have reduced ear sensitivity coupled to reduced speaker efficiency ... bad combination.
That said, your amp will be among the best *portable* 15W amps found in the Market.
That distortion you hear can easily be tamed by setting volume slightly lower, I suggest you use an A/Log/Audio volume control instead of the one which you are using now, which I guess is B/linear.
Commercial amps use an automatic limiter/compressor to do so all the time and minimize user tweaking, but you won't find EBay kits with that feature , the only option is to fully clone a commercial MI Bass amp, which *is* doable, but leave it for a future project, because it involves designing your own PCB and Layout from the ground up, better go step by step.
Just to show that it's not for the faint of heart, have a look at this Fender BXR25 schematic.
http://support.fender.com/schematics/bass_amplifiers/Bassman_25_schematic.pdf
The limiter/compressor is based around variable gain UC7 , a CA3080 ; the distortion detection circuit which drives it is made out of everything below and to the right of it; for reference including Q2, U8a (TL072) , half a dozen diodes and capacitors, a dozen resistors .
So you'd better be your own "human limiter" , set the amp just to clipping (if you want to maximize power, that is) and play louder/softer by yourself picking *just* what's needed.
And in the middle of a hot jam, letting it growl a little won't hurt, being surrounded by loud instruments is not the same as playing alone.
In a nutshell, your battery amp will be *very* usable, just not a wall breaker.
Quote from: JLTDefinitely not what I want for a bass amp!
Oh, I had forgotten that detail. IMO piezo pickups are totally unsuited to bass guitar by their inherently bad low frequency characteristic, whatever the input impedance, even infinity (which we can do for you sure :) ). Are you trying to pick up a signal from some sort of
acoustic bass?
1Meg is typical for a magnetic pickup, so that will do for the time being (or series 3x 1Meg).
There has to be a DC blocking cap between the buffer output and main amp input, but you only need the one.
Get whatever N-channel small signal FET's they
do stock, a source follower is pretty non-critical.
The
maximum power handling (W
RMS) and
sensitivity (dB/Watt @ 1m) of a speaker are not really related. A speaker may have a high power rating yet also have a high sensitivity, or any other combination.
But
JMF is the real expert here - he actually
builds guitar speakers. :dbtu:
Quote from: J M Fahey on May 04, 2015, 05:02:06 AM
Thanks for the trust vote :)
Both speakers will work properly, pick one.
<snip>
In a nutshell, your battery amp will be *very* usable, just not a wall breaker.
That's what I want. I have another amp for large venues.
Quote from: Roly on May 04, 2015, 05:46:05 AM
Quote from: JLTDefinitely not what I want for a bass amp!
Oh, I had forgotten that detail. IMO piezo pickups are totally unsuited to bass guitar by their inherently bad low frequency characteristic, whatever the input impedance, even infinity (which we can do for you sure :) ). Are you trying to pick up a signal from some sort of acoustic bass?
I am indeed. The bass is a Dreadnaught I converted into a (very) short-scale acoustic bass. I've played it through other bass guitar amplifiers with good results. Not as pure a signal as one I'd get with magnetic pickups, but a warmer, more resonant sound.
The peizo pickups were the ones recommended for the application by JJB, and I've had good luck with their other products, as well.
Here's the link to that product:
http://www.jjb-electronics.com/prestige-220.html
Quote from: JLTHere's the link to that product:
...and not a word about the need for high input impedance with piezos, all the more so with smallish ones like this.
I'd be thinking of a high impedance FET buffer as close to the piezo elements as possible, built in, beltpack, or FET lead (http://www.till.com/articles/PreampCable/).
This circuit (http://www.ozvalveamps.org/techsite/hydromic/hydromic.htm) would also work.
Quote from: Roly on May 06, 2015, 06:14:11 AM
Quote from: JLTHere's the link to that product:
...and not a word about the need for high input impedance with piezos, all the more so with smallish ones like this.
I'd be thinking of a high impedance FET buffer as close to the piezo elements as possible, built in, beltpack, or FET lead (http://www.till.com/articles/PreampCable/).
This circuit (http://www.ozvalveamps.org/techsite/hydromic/hydromic.htm) would also work.
I'll keep that in mind, but I'll start with the circuit that Roly suggested. The cable FET may not be practical, since phantom power may not be available, and it may not be necessary. I'll find out by and by.
I ordered the FET today from Jameco, as our last real electronics store was out of stock, and they won't restock because they're going out of business. I am bummed. There'll still be Radio Shack and Fry's Electronics, but they aren't the sources they used to be.
Believe it or not, this project is still live, although I haven't done much in the last month due to other projects, sick family, and life in general.
But the speaker has been acquired and mounted in its cabinet (a re-purposed 1950s-era record player), the electronics are ready to mount, and a lead-acid 12-volt battery has been used to power it, along with an external power supply (120 vAC in, 12 vDC out).
One final question for those who have been so kind as to put up with me until now: I'd like to arrange it so that, when the external power supply is used, it also re-charges the battery. Something tells me that simply connecting the hot leads of the power supply and the battery together isn't going to work for that, and that some sort of isolating circuitry or voltage regulating circuit would be required (as it is in cars). The question is: what sort of circuitry would be required for this, or is it even possible? As usual, schematics would be a big help.
It is possible but the power supply voltage, under load, must be higher than 12.6V , is that so?
In the simplest way, you run the amp straight from the battery, and the supply has, say, 15V output.
In that case you use a resistor between supply and battery which is always charging it, whether you play or not, and lets pass some current, usually called "1/10C" , where C is the (C)apacity of the battery in Ampere Hours.
Suppose yours is 7AH so C is 7
Then charging current is 7/10=0.7A
Resistor is (15-12.6)V/0.7A=3.5 ohms (search for a reasonably close one, not critical at all) , and dissipation will be at least: (15-12.6)V*0.7A=1.68W .
In the real World your battery may be over discharged and have only 11V so resistor might dissipate (15-11)V*0.7A=2.8W
In practice, you will use a ceramic resistor between 3 and 4 ohms and capable to dissipate 5 to 10W`(they are inexpensive anyway).
These calculations were made with a 15V supply, repeat using the values of your actual one.
That is a safe current which will not stress your battery or the charger and is easy to calculate.
It has only 2 problems, both quite bearable and which of course can be expected from such a simple circuit.
1) it's a sort of slow charger , because since it's not controlled, you try to err on the safe side, so although it would charge your battery in 10 hours in practice it will take a couple extra ones.
2) it "never stops" so it can overcharge your battery if you forget to unplug it for a couple days.
There are intelligent chargers which supply higher current at the beginning (so they charge faster) and automatically turn off when the job is done, but of course they are somewhat more complex (not NASA complexity, of course)
An intermediate solution is to feed the battery regulated and current limited 13.8V which has been proven to be both faster and safe if you forget to turn it off.
Really there's many options and schematic on the Internet, but the simple solution posted first works quite well, it only requires an intelligent owner ;) who checks voltage every few hours and remembers to turn it off when done :)
Quote from: J M Fahey on July 01, 2015, 12:06:44 AM
It is possible but the power supply voltage, under load, must be higher than 12.6V , is that so?
Um, no. It says 12 v at 5 amps ... it's the kind of replacement power supply for laptops; the eBay URL is http://www.ebay.com/itm/121606465910
So I think I'd be better off another power supply with a little more voltage, as you recommended. Any suggestions as to sources, etc.? Remember, this is a "budget" project.
Quote
It has only 2 problems, both quite bearable and which of course can be expected from such a simple circuit.
1) it's a sort of slow charger , because since it's not controlled, you try to err on the safe side, so although it would charge your battery in 10 hours in practice it will take a couple extra ones.
2) it "never stops" so it can overcharge your battery if you forget to unplug it for a couple days.
There are intelligent chargers which supply higher current at the beginning (so they charge faster) and automatically turn off when the job is done, but of course they are somewhat more complex (not NASA complexity, of course)
An intermediate solution is to feed the battery regulated and current limited 13.8V which has been proven to be both faster and safe if you forget to turn it off.
Really there's many options and schematic on the Internet, but the simple solution posted first works quite well, it only requires an intelligent owner ;) who checks voltage every few hours and remembers to turn it off when done :)
You're asking a lot of me, my friend! But I'll try! And thanks again for your help.
Your power brick is probably regulated 12V, perfect to run your project ... but it won't charge the battery.
To charge it you need to have current flow into it, so you need a higher voltage (plus a limiting resistor) than what the battery has.
As a reference, a charged lead acid battery has 12.6V DC across its terminals , already higher than the 12V supply, but it's even worse: probably because some wicked sense of humour , the battery demands *even higher* voltage, while charging.
If you feed it:
* 13.8V : it will charge well, its own internal voltage rises beyond 12.6V but it practically stops "pulling" current from the charger when it's full, that's why it's considered a "safe" voltage.
At the same time, besides having regulated 13.8V it does not require much, it "self limits" so it's a very popular battery charging voltage.
* as large as possible current from alternators (in cars) to speed up the process, you don't want to run your engine for 12 or 14 hours just to charge the battery, so they feed it a lot.
Fine, but there is the possibility of bursting it easily, so car regulators are set to cutoff above hair rising 14.4V :o
That's why some car power amps, VHF transceivers, etc. are specified rated power at 14.4V , it's what's available in a car with the engine running.
* as you see, battery voltage rises when charging way above 12.6V , that's why my example used a 15V DC supply.
Try to search for some power brick similar to the one you found, but either in 13.8V (I bet you'll find a few) for simple self controlled charging or higher (up to 18/19V DC) to use with a limiting resistor.
Or maybe you find a ready made battery charger for about the same price, just plug and play :)