Converting a BA Sx to battery power -- is it worth it?

[JLT]

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.

[Roly]

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...


(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.

[g1]

  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.

[Enzo]

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.

[Roly]

Have 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;


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.}

[JLT]

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/

[J M Fahey]

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.

[Roly]

...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.   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.}

[J M Fahey]

WOW   

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.

[Roly]

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?


(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:

[JLT]

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).

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?

[Roly]

found 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.

[jay4cars]

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.

[JLT]

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.

[Roly]

It'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: