Choosing battery for guitar amp

[nareto]

Hello, I have a small (24 Watt) guitar amp which I'd like to retrofit with a
battery for outdoor playing. I actually study mathematics and do electronics
mostly as an hobby, so I may miss some fundamental things (i.e. bare with me). Basically, I have two little problems which I'd like help with:

1) Is the following reasoning correct and appropriate?

I need to calculate the current the amplifier absorbs in order to choose a
battery capable of providing the same current. So, given the power consumption
is 24 watts at 220 V RMS (I'm in Italy), the current is 109 mA RMS. But since
I'd attach the battery after the rectifier that's inside the amp, I actually need peak to peak
amperes, so I'd need 154 mA from the battery. So, given I'd like the battery to last at least 1
hour, I'd need one with "C" greater than 1 and 154mah (for one hour),
154*2=308mah (for two hours) and so on.

2) if I know little about electronics, I know next to nothing about battery types
(lithium, lithium-polymer, nimh and so on). What would be a reccomended choice
for my project?
Ideally I'd like it to be cheap (the DC after the rectifier is
35V, so probably I'll need many of them), have many charge/discharge
cycles, have a simple (if any) recharging circuit, which I could power from
the 35V provided by the rectified "wall" power supply (I'd add a rotary switch so the amp
would either be off, running on battery, running on 220 or charging the battery
from the 220)

There's actually another little problem, that the rectifier provides a dual
supply (+-17V) to the amp circuit, but I allready found that I could achieve
that (with the battery) with a simple op-amp circuit (found here, for future
reference: http://tangentsoft.net/elec/vgrounds.html )

any help is very much appreciated
cheers

 

[nareto]

ok, done some research and I found out it's way more difficult than I thought. There's no circuit I could find to charge the 30 nimh or 9 lithium batteries I'd need to get close to the 35V, and anyhow they are not "easy" circuits (lithium batteries catching fire if you make the slightest mistake etc.)

So, I'm abandoning this and instead I'm going to build from scratch a 9-12V amp based probably on the tda2003 or similar unit

 

[vk6kro]

If you wanted to use the amplifier you already have, you might be able to find a small inverter to get 220 volts from a 12 volt car battery.

This would mean you could just leave the amplifier intact. 12 volt chargers are readily available and your amplifier would last several hours before the battery needed recharging.

Amplifiers are usually rated on maximum output power per channel, so yours may not use 24 watts all the time, but its input power could be higher tnan 24 watts when the amplifier is being used at full output.

Probably, you would need a 100+ watt inverter but it would need to be rated for low noise and varying load. You wouldn't want strange whistling noises accompanying your performance.

 

[nareto]

hey, thanks that's a good idea.

unfortunately cheap inverters I find around (searched for 2 mins though) like this one
http://www.pc210.com/productshtml/2008-11-4/DY-8102_20081147680.html

don't specify if they're for varying load and low noise... I should assume they are not, right?

But regarding the noise, shouldn't it be more or less canceled by the rectifier, which has capacitors in parallel?

 

[turbo]

I have an inexpensive [STRIKE]rectifier[/STRIKE] DC-AC inverter that I use to supply my telescopes drive from my vehicle's 12-volt battery when I am not near AC. The right-ascension drive is nice and quiet when it is plugged into real AC, but becomes noticeably noisier when it is plugged into the [STRIKE]rectifier[/STRIKE] inverter. For this reason, I have to think that the AC out of the [STRIKE]rectifier[/STRIKE] inverter is square-wave with sharp transitions. This could pose noise problems for you.

People who go camping, tenting, etc, may have [STRIKE]rectifiers[/STRIKE] inverters so they can enjoy some of the comforts of home when they have access to their vehicle's 12V system. You might ask around to see who owns such [STRIKE]rectifiers[/STRIKE] inverters, and ask if you can plug your amp into them to see if input noise is a problem... Probably a good way to get into this cheaply and avoid having to hack up your amplifier.

 

[vk6kro]

hey, thanks that's a good idea.

unfortunately cheap inverters I find around (searched for 2 mins though) like this one
http://www.pc210.com/productshtml/2008-11-4/DY-8102_20081147680.html

don't specify if they're for varying load and low noise... I should assume they are not, right?

But regarding the noise, shouldn't it be more or less canceled by the rectifier, which has capacitors in parallel?

You really need to take your amplifier and a speaker and a battery to the store and try it.

Unfortunately, there are inverters that put out a lot of harmonics as well as the wanted 220 volt sinewave.
This one says it puts out a "corrected sinewave" which is a polite way of saying it is a pretty rough sinewave and may contain a lot of harmonics.

The electrolytic capacitors may be able to correct some of this or you may be able to add extra filtering, but it is better to choose a good inverter with "true sinewave" output if you can.

 

[nareto]

thanks for replies. I see, I'll have to find an inverter from someone and do some tests :)

turbo-1 I think you were saying "rectifier" in your post but meant "inverter"?

 

[turbo]

thanks for replies. I see, I'll have to find an inverter from someone and do some tests :)

turbo-1 I think you were saying "rectifier" in your post but meant "inverter"?

Yes, sorry. DC-to-AC inverter. Unfortunately, mine is quite cheap, and though it is accurate enough for a 'scope drive, it appears to have a square-wave output that would potentially create noise in your amp. For some reason, I'm having brain-cramps today. I have been building, restoring, and renovating tube amps since the early '70s, so I certainly know what a rectifier (tube or solid-state) is. Just being Duh! Original post is edited with strike-outs to highlight the Duh!