Using Batteries For Odd Voltages

A guy I used to know had one of these portable amps, that he would carry around and practice with. They guy was a phenomenal guitarist, and used lots of electronic effects, but it was all portable. I don't know what sort of battery it used.

Since you are only driving a headphone or tiny speaker, 9V battery would probably drive it for a good long time. A simple amplifier circuit using an LM386 would be easy to build. No regulator required. If you search for the LM386 datasheet, there is a schematic for a 200 gain amplifier that would probably work just fine at 9V, using a guitar input. I think guitar inputs generally swing about 10mV, so a gain of 200 will give you a voltage swing of 2V. That into a 32 ohm headset will give you 125mW. I suspect that's louder than you need. The input pot allows you to turn it down. Lower output will mean longer battery life too.

These are just ideas, so you can then experiment. LM386 chips are widely available from surplus electronic mailorder places. Also, you can get them from more traditional outlets like digikey, mouser, arrow, or futurlec. You may even be able to get them at radio shack.

Here is plans for one:

This one uses a preamp, so perhaps an LM386 isn't going to drive a speaker from a guitar signal by itself.

Since no one seems to have answered your actual question yet, yes, you can stack batteries all day to get whatever voltage you want.

But I say, just use a 9V. They used to last pretty long in portable radios.

I wish you'd posted the schematic - but probably, if it's just an ordinary audio amp, it should work at 9V. At worst, you'll have to change a couple of resistor values. You haven't built the circuit yet, right? Here's a quick google:

The first couple I checked used 9V.

And yes, a mic preamp and guitar preamp are the same circuit since a dynamic mic and a guitar pickup are indistinguishable at the input of the amp.

Have Fun! Rich

Do you have a link to the schem? Maybe it could run on 9v. As far as battery types, if I'm picturing the type of unit you want to build correctly, I think that you'll have to suck it up and go with a 9v, even though it may not last that long. You'll certainly lose any portability if you need 8 AA cells to power the thing.

Thanks to everyone who's responded. Yes I should have posted a URL to the schematic:

Or maybe some more info like what OpAmp it uses, wattage and such. I'm sorry i didn't include that before. I'm sure that it will work on 9V, and probably would be ok with 18V. I just thought that 12V is somewhat of an odd requirement for something portable. Of course, the circuit is a pretty old design (iirc 1970s) and i was too young to remember what battery choices we had then :-D It's probably even closely based on the datasheet for the LM383

Roger- I don't do street performances at this time. The tube amp plugs into the wall directly. I was going to use this small practice amp as a "disposable amp" project for testing homemade effects pedals and such. I would hate to mis-wire something in a stompbox and damage my Reverend :-( It would also be useful to have a small portable amp like this- something i could even keep in the guitar case itself. I also intend to make a second amp on breadboard or pegboard with IC sockets just for experimenting with.

Robert- That amp design looks interesting too! I actually just ordered a bunch of TDA2002 (replacement for the LM383) and some LM386s from Jameco. Futurlec is awesome but they take forever to ship. The LM386 is typically 1/2W, and the LM383 amp is 8W. My tube amp is switchable between 5W and 15W.

Rich- I haven't built it yet, no. But I intend to. ;-) Thanks for the tip on similarities between mic pre's and guitar pre's. I had a feeling that an amp is an amp is an amp (more or less). I figured i could stack batteries like that, but i've never seen 9V batteries stacked, or dissimilar types or sizes stacked like that before in manufactured equipment. I assumed there's gotta be a reason for it ;)

You should go with batteries that have a high mAH figure, that will give you the longest run time at the required voltage. On that basis, forget the 9 volt cells.

What power source do you use for the tube amp? Do you do street performances?

Batteries have a certain current delivery capacity, for example a common battery can be marked with 3000mAh.

That means it can deliver 3000mA for one hour, or 1000mA for 3 hours, or

You need to find out this number for the different battery alternatives.

You did not say anything about the power rating of the amp, or what number of watts you need for your performance. You know how many hours you want to play before you must change battery (-ies).

So you can see what kind of batteries which can be used. You should avoid using batteries in parallell, but it is possible if necessary.

You say that you need 12V for this unit. Have you checked if it can be run from other voltages? Many 12V amp circuits can be used with 18V or other voltages.

Using a voltage regulator to get 18V down to 12V is not a good idea. Most power amps can use a range of voltages and the closer you get to the upper limit the more power you can get out of the amp.

If you use a compressor/limiter unit, or an effects box including some soft clipping, you don't use up the batteries so fast. Such a unit cuts down on the really high current peaks in the signal, and it still sounds loud. It is a good way to get a maximum of power from the batteries.

With 2*9V batteries, the small common size, you could maybe play for half an hour if you play in a common living room and the audience is 5-10 people, if the amp is suitable for 12-18Volt, and with compressor.

The TDA2002 has a quiescent current draw (current draw when powered, but with the output unloaded) of typically 45mA. Add to that maybe say 20mA for the required current for producing your audio output and that yields a typical power consumption of say 65mA.

Standard 9V alkaline batteries have a rated capacity of around 600mAh, so expect battery life of something vaguely around ten hours. Although the TDA2002 expects a supply voltage of 18V (or two 9V batteries in series), it will probably operate acceptably with much lower voltage (since you don't need 8W output capability). Nevertheless if you use two 9V batteries, the operating cost could be quite significant. The TDA2002 is massive overkill for driving headphones. When operating from 18V it would probably be very easy to configure the TDA2002 to produce way too much power which will end up burning out your headphones and/or eardrums.

The LM386 is a much more suitable part for your application. It operates from lower voltage, so a single 9V battery would supply it just fine. The quiescent current draw is typically around 4mA, so expect dramatically better battery life (and you only need one to begin with). As it is the LM386 is still a fair amount more powerful than you actually need to drive a set of headphones, however it should do just fine. As an added bonus the LM386 is available in a DIP8 package, whereas those TDA2002 devices come in what appears to be a relatively cumbersome TO-220-5 package. Five legged TO-220 parts don't fit properly in a solderless breadboard, while the DIP8 fits nicely. I highly recommend playing with your circuits on a solderless breadboard.

Standard headphone rail to rail input and output amplifiers can operate from as low as 3V or less and still yield adequate volume for use with headphones. A more advanced design running from two AA or AAA batteries could be built from a part such as the TPA6100A2.

It's probably lifted verbatim. :-) The ranges on R3 and R4 tell me that. And yes, they just used 12V because it was common. (as in ordinary, not circuit ground. ;-) )

There is - there's no point to it. :-) Nine volts will be fine. I'e noted the schematic indicates ranges for R3 and R4 - so you'll have to play with them to get the gain you need for your ax and phones anyway, so just do it. :-)

It might not hurt to look at quiescent current - I don't know the figures on the LM383, but it _is_ a power amp, so you might want to look at low- power opamps or preamp chips (or discretes) if you're concerned about battery life.

For street performance, of course, you'd want a car amp and golf cart battery. ;-)

Cheers! Rich

He is right. It takes a long time for them to ship. Also, the packages have something that looks like arabic or farsi on them, which makes it easy to spot them amongst various deliveries and rip them open immediately.

I think you can speed up their shipping by paying more.

To protect the input of a power amp you can put two diodes in antiparallel over its input. That ensures that the input voltage can never get higher than plus minus 0.6Volt. Add a series resistor 1k too to protect against very low impedance sources which could blow up the diodes.

For headphones you can probably use common op-amps, like the TL071 series. A TL072 for stereo.

Or no amp at all. You probably use some guitar effects boxes before the amp, and most of them have output power enough to drive earphones directly. There are schematics for thousands of guitar effects on the web, and they are suitable as beginner projects in electronics. Check out geofex.com, for example.

That is interesting. Have you tried them or do you base that on some information?

pedals

damage

True that it would protect the input of the amp, it's also a standard waveform clipping setup- it may introduce distortion (which may or may not be desirable ;)) but even for gits and grins i'd like to build this small practice amp :-D

the

None of the commercial effects boxes will drive headphones...

out

Yep! Along with generalguitargadgets.com

Yes, my initial investment in parts (i bought one each of all their value packs) was from futurlec. It was just over 2.5 weeks between the time i placed my order and the parts shipped. Most everyone i talked to in #electronics on irc.freenode.net said their experience was similar. However, that's my ONLY complaint. Otherwise, they were very inexpensive, courteous, and all the parts i received were packed well, organized and marked well, and appeared to be brand new and good quality.

I would buy from them again, but sometimes i want stuff a tad sooner, so i go to Jameco ;-)

Anyways, you (and everyone else in this thread) have been very helpful. thanks!

-phaeton

weeks

Actually, I placed the order, and it didn't get packed and shipped for

It then took another week to travel halway around the world. The week for travel time is acceptable, sure. It's just that they sat on the order for a while after I placed it.

But hey, everything else about the transaction was excellent. For things that I don't need in a hurry (read: If i plan ahead a little better) I'll happily order from them.

-phaeton

If they sent the stuff halfway around the globe by ship I think 2.5 weeks total shipment time is very acceptable.

That is very irritating. I hate when that happens. The company should tell all customers about such delays at their web site, so the customers know about it before even considering placing an order.

On the other hand, most companies in the world never inform of that, so it is not like they are alone in that behavior.

Maybe they are not storing all the products they say they have in store, so it takes time to get the stuff from their sub-contractors. That is often the real cause for delays.

Roger sez: # That is very irritating. I hate when that happens. # The company should tell all customers about such delays at their web # site, so the customers know about it before even considering placing an # order.

Definately. I wish i had known that up front, and if i did i would still have ordered from them, it's just that the first couple of weeks of it sitting in "Order Submitted" status and being left in the dark was starting to make me a little bit nervous... ;-)

Roger says again: # Maybe they are not storing all the products they say they have in store, # so it takes time to get the stuff from their sub-contractors. That is # often the real cause for delays.

That's my guess too. Again, some notification on the website ("May delay order shipment by 3-4 business days") would be nice. I don't think Futurlec is trying to be malicious or deceitful, they've just got a rather sparse website. To their credit, it looks great and works great in every web browser i've hit it with. Jameco's website used to be slow as molasses, then they revamped it and for awhile there i couldn't do anything at all with it using Firefox. They've since improved the compatibility and it's 100% functional, though occasionally the forms get all jumbled up and ugly. Everything works now, so that's good enough.