# Using Batteries For Odd Voltages

Discussion in 'Electronic Basics' started by phaeton, Mar 25, 2005.

1. ### phaetonGuest

I'm looking at a small (simple (beginner)) circuit that I intend to
build. It is a portable guitar amplifier, designed for use with
headphones (think Sony Walkman) or even a small speaker (think
Pignose).

In any event, the circuit calls for 12V DC, which is pretty simple to
wrest from a standard universal wall-wart, but for the sake of
portability, batteries are the key. There is such a thing as a 12V
battery i've seen (i think it's an "N" battery) but since it's just a
little larger than a AAA, i'm going to guess that it will go dead
quickly (not a lot of total mass). I also haven't been able to dig up
a holder for it anywhere...

I know that i could use 8 AAs, or 8 AAAs, or a 9V and 2 AAs, but that
all seems excessive or otherwise strange. There's a lot of ways to do
this, but what would be the universal *best* method to acquire 12V from
smaller batteries (not really interested in strapping a Sears DieHard
to my belt, especially since i have a nice little tube amp that weighs
less )

How about 2 9V batteries with a voltage regulator circuit? Would that
be wasteful or otherwise silly?

Thanks!

2. ### Roger JohanssonGuest

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
500mA for 6 hours, etc..

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.

3. ### Lord GarthGuest

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.

4. ### John FieldsGuest

---
Yes, but...

The capacity of the battery is specified at a certain rate of
discharge, usually C/10 or C/20. That means that, say, a 12V, 3AH
lead-acid battery rated at C/10 would discharge from, say, a nominal
12.6V when fully charged to 10V when fully discharged, if current was
taken from it at a rate 300mA for 10 hours. If it was dicharged at a
rate of 3A it would probably discharge to 10V in 45 minutes or so, so
the capacity would suffer. OTOH, if it was discharged at a rate of
100mA, its capacity would increase and it would still have some charge
left to give after 30 hours.

5. ### tempus fugitGuest

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.

6. ### Robert MonsenGuest

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:

http://www.weirdscience.us/Personal/GuitarAmp/GuitarAmp.htm

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

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.

7. ### Rich GriseGuest

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

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

8. ### phaetonGuest

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

9. ### Fritz SchlunderGuest

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

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

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.

10. ### Roger JohanssonGuest

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?

11. ### Rich GriseGuest

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

12. ### Robert MonsenGuest

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.

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.

13. ### Roger JohanssonGuest

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

14. ### phaetonGuest

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

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

15. ### phaetonGuest

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

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

16. ### Roger JohanssonGuest

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.

17. ### phaetonGuest

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.