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.

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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.

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.

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.

This circuit 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.

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: 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 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!

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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).

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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.

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: 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: 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: 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).

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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: 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/

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.