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Messages - darwindeathcat

Yeah, I suppose it sounded like a way cooler conversion than it actually was!  ;) One thing I DID do, however, was to add a rotary switch to change which pin of the OT connects to the negative feedback loop. At the expense of some volume I can get it squeeky clean (i.e., as it was supposed to be), or if I want I can get it to have some pretty nice breakup (i.e., so it can be used more like a regular guitar amp).

Believe it or not, I found that MPA20 head in the Goodwill on sale for $8. The only thing wrong with it was a blown fuse. Gotta love those kinds of thrift-store finds!
It's a Realistic, MPA 20. That's the RadioShack brand. But it's one of the older ones (maybe from the 70's?) with discreet power transistors rather than a chipamp. It sounds really good, actually!
Amplifier Discussion / Battery power for street amp....
January 07, 2012, 08:11:02 PM
About a month or two ago, there was some discussion ( about building 12 volt battery powered amps with more power than the typical 1-5 watts you get with the typical 9volt designs. The discussion mainly centered around battery technology, and which one would give you the most output for the buck, as well as the one that would be the lightest (important for a battery amp). Well, I found the former (best bang for your buck), and bought one for a little combo amp I built for street busking. It's a 12v 7aH Gel-Cell marine battery meant for fish finder:

The battery weighs in at about 6 lbs, so it ain't light! But, for a little less than $30, you get the battery and a charger (not a smart charger, but not a totally dumb charger either. An "average intellegence" charger, if you will). The amp is 20 watts, class AB, discreet transistors (I didn't build the amp, it's a PA head I integrated into a cab). I've played it on battery power at a few busking sessions now, the longest was about 2 hours, with no sign of power loss. My math tells me I ought to get about 4 hours playing on full, but since I don't turn it up past 4, I'm not sure how long could actually go for (maybe 5-6 hours?).

Here's my review that I posted on Amazon:

QuoteThere are a few other 12v 7AH SLA batteries in this price range that also come with chargers. It's hard to tell the difference between them from the info posted here on Amazon, but this one has the most reviews, all positive, even though it's a couple of dollars more expensive than the other ones. So why buy this one instead of those others? Well, partly because this one is sold directly from That's a big plus because if you live in an Amazon hub city (like Phoenix, where I live), you can get your package very quickly even with the free super saver option. In fact, I ordered this battery at about 4pm on a thursday, and the package was delivered to me at 9am the next day! This is with the free super-saver shipping! Secondly, this one comes with a slightly nicer charger than the others do (at least judging from the pictures of the other chargers). The charger that comes with this one has a nice LED status light that changes color when the battery is charged (from yellow to green), and also has cross polarity protection that will keep the battery from shorting (and light a red warning light) if you hook it up backwards (VERY nice thing to have).

I am using this little 6lb battery to power a small 20watt amplifier I built for busking on the street. I get several hours of play time from it, and it holds a charge very well. Like all SLA batteries, you have to maintain it properly, and be sure that it never drops below 11.5 volts, or sulfation will occur, and the battery will be permanently damaged. But if you keep it charged up, and don't store it for too long, it should last for many years!

In sum, this is a great deal on a very good battery.
Schematics and Layouts / Re: TDA2003 Suggestions
November 01, 2011, 06:40:19 PM
If the Muff Boost doesn't give you enough volume on it's own, I've developed a second stage using an MPF102 (I had some lying around) to go after it. It pumps the volume up even more, but the sound still remains clean. I powered it at 9v, so you may have to adjust some of the bias resistor values if you are running it at 12v. I'll attach the schemo to this post.
Schematics and Layouts / Re: TDA2003 Suggestions
November 01, 2011, 05:42:32 PM
The AMZ "Muff Boost" (halfway down the page here: uses 2n5088's, and should give you enough boost to at least get the volume up on the tda2003. You can make it clean by omitting a couple of components (as directed in the build instructions). I've built a clean one for similar purposes, and had good success. Good luck!
Well, since I was down at the local Fry's Electronics shopping for an unrelated item, I decided to price out batteries and charging systems so I could get an idea of what I'd pay retail here in Phoenix Arizona. First off, I looked at the SLA 12v batteries. The cheapest was $22 and was only 1.2AH. I'd say it weighed about 5 lbs. The other 12v SLA they had was 4AH, and went for $36, and probably weighed more than 10 lbs. Both batteries were "Tenergy" brand, which I've heard of. SLA "smart' chargers went for about $15, and an adjustable voltage universal wall wart that could be used as a "dumb" charger (12v, 500ma) went for $8. Best deal on NiMh AA's was again with the Tenergy brand-name, and was $11.99 for 4 AA's AND the charger. Remembering that we'd need TEN AA's to equal ONE 12v, that means we could buy two of those deals, and get a separate pack of 4 AA's for $7, or we could just splurge and get THREE of those deals so we could charge all in one go. These AA's were 2500ma. For a weight comparison I held a ten pack of regular alkalines. It was about half the weight of the small SLA, so I'd say about 2.5 lbs. If we go the AA route, we need some battery holders. The best arrangement sold in the store was to get one holder for 8 AA's ($2.49) and one holder for 2 AA's ($0.79), and wire them in series.

So, in a local retail setting the cheapest SLA set up would be about $30 for only 1.2 amp hours, and a dumb charging system, and the best SLA system would run you about $50 and you get 4 amp hours. You'd be adding 5-10 lbs of weight onto your amp. For a NiMh system, you'd spend between $40 and $45, and you get 2.5 amp hours.

Now, remember, this is buying retail. Better prices online could be found, but unless you can get free shipping (e.g., for an Amazon order above $25) or you can combine everything into one shipment for a reasonable rate, the price of shipping may be greater than the potential savings. For example, that 12" speaker listed on ebay sells for $12.99, but they want to charge you $28 in shipping!!!

Tubes and Hybrids / Re: hybrid mini 12au7 practice amp?
October 13, 2011, 05:18:03 PM
Thanks for the response. Yeah, the fetzer valve is a neat circuit. Easy to slap together and sounds so good! Interesting that it sounds better than a space-charged 12ax7... I do have some 12au7's sitting around, so I might give that a try one day, but I'd have to order some tube sockets, and I'm lazy!
If the frequency response really does go that low, this may be a good amp for harmonica players too. We like low wattage amps as well, as they tend to sound better for harp...

EDIT: Agree that sound samples and/or video is necessary. It *looks* good, but the proof is in the pudding (and by pudding, I mean audio)!
Well, I let it charge for about 8 hours. The battery voltage now reads 13.25, which is about 2.2v per cell, which I think is good. It seems that it all worked out! Thanks guys, for all your help!

If anyone is interested in hearing sound samples of this amp, let me know and I can make some...

Just a quick update: I've got the adapter hooked up to the input of the "smart charging" circuit using test leads (the plug wasn't exactly correct for the jack). Prior to connection, the battery was reading 12.2v (which is low, but not in the "danger zone" for SLA), and the adapter was reading 26v just hanging unconnected in mid-air. Connected to the circuit, the output of the adapter reads 18 volts, and the voltage at the battery terminals is 15v. All that seems pretty good to me. I can't get a DC current reading, however (but that may be a problem with my crappy multimeter). So far, I'm ten minutes in and don't have any heating or odd smells! Hopefully this works out!

EDIT: A couple hours have passed, and the wall wart is barely warm to the touch. No heating of the battery, and everything seems ok. Voltage at adapter output reads 24v now, and voltage at battery leads reads 14.2v. Seems to be going okay. I'll leave it for several more hours and then disconnect and then check the battery voltage.
Thanks again for the good common sense advice. I'll do just that and then keep you updated!

Well, I  just got back from the Goodwill, and the closest wall wart I found is 20v DC, but only rated for 500Ma. I bought it anyway (it was only 99 cents). Will I be able to use this adapter? If the only ramification is increased charging time, I can live with that (until I find a higher wattage adapter), but if there will be too much current draw for this transformer, I don't want to risk damage to the circuit.

And, yes, I know I'm being a bit lazy by not building one myself to spec, but the simple fact is that I AM lazy (;D), so I'd like to take the laziest route possible!  ;)

Thanks for all your help!!!!
Great! Thanks a lot for all the help! Looks I'm heading down to goodwill to sort through their pile of wall warts again! I should definitely be able to find one with those specs. Worst comes to worst, I have several raw transformers in the 16-18v range, as well as several diode bridges and even the correct value filter cap, so I could brew my own!


Tubes and Hybrids / Re: hybrid mini 12au7 practice amp?
October 06, 2011, 04:49:03 PM
Hey, did you ever get this wired up and working? I've had it in my head to do the same sort of thing for a while now... I've got all the parts sitting around, but wanted to see if someone else had made this work (and what it sounds like!) before i give it a go...
Thanks Joe Cool! I posted a schem I drew of the charging circuit in the other thread I started ( plus a gutshot, so you can take a look at it over there. Yeah, I've sent a couple of e-mails to them with no response. I do have a "dumb" charger, so I think I might have to go that way in the mean time, because the battery is getting dangerously low... JMFahey is giving me some help over in that other thread, so i'll wait to hear back what he says after looking at the schem I drew before taking that measure... Thanks for all the help!