Converting a Guitar amp to battery power

[Alster370]

I've disassembled the amp, but im a little confused regarding the number of wires coming from the transformer. Any information regarding that would be a great help , as well as any other advice/information I should know.

[Crna Legija]

you could get a power inverter and have it connected to a 12v battery but I doubt you could get much more than a hour of play time with a car battery.

[Alster370]

I know that is the easy option but most can be used on DC, I have done it in the past with smaller amps.

[Technician1002]

Most higher power AC amps do not have a voltage inverter inside like car amplifiers. Most AC amps use a + and - voltage to drive the speaker cone both ways instead of using a bridged output where both leads have about 6 volts DC on them like many car decks. (BTL circuit, Bridged, Transformer-Less)

The coloring on the wires on the transformer is black is the common. On the transformer it is referred to as the Center Tap as it ties in the middle of both the red and blue windings. The two reds are the high voltage AC for the rectifier for the output transistors. In that design that will typically be between 15 to 25 volts. This will generate about +25 and -25 volts DC so the speaker hot lead can swing between + and - 25 volts. This supply is not regulated, just filtered.

This color code for the transformer is industry standard which is why I didn't need the schematic or model number of the amp to identify the windings.

The blue wires are the low voltage supply and it functions the same as the high voltage for the outputs. They drive the low current highly regulated and filtered power supplies to drive the pre-amp, tone controls, mixer, etc with a very clean + and - DC source. This voltage is typically + and - 5 volts, 10, volts, 12 volts, or 15 volts depending on the manufacture.

Due to this balanced power supply in the amplifier, there is no easy way to drive that amplifier from a power supply with just +12 volts DC source.

I used to repair high end Audio and do Radio and TV station engineering in my prior job.

[Fnord]

...So basically, unhook the transformer leads, put your multimeter's 'common' to the black wire, and check the voltages of the other wires

If you need a highly filtered low voltage supply, you may be able to do a setup like people use for driving high-powered laser diodes, which are sensitive to voltage fluctuations batteries put out.

[Alster370]

So an inverter is the only real option?

If not , could you explain how you would drive it from a DC source?
thanks

Oh and its a line 6 spider III , 15watts if that means anything to you

[Technician1002]

If you measure the leads on the transformer with a multimeter, you will need to use the AC setting, not DC. To run the amp off DC you will need several DC power sources. On the place the blue wires connect, you will need about + and - 9 volts. A pair of 9 volt batteries connected in series will do nicely. Connect the common between the two series batteries to the place the black connectd and connect the + battery to the + output of the bridge rectifier. Connect the - lead to the - of the bridge rectifier. This will power the pre amp. With two car batteries or two 12 volt gel cell batteries again connect them in series to provide 24 volts with a center tap connection. Connect the center tap to the common connection as before. Then connect the +12 volt battery lead to the output of the bridge rectifier for the main amp supply and connect the -12 volt battery lead to the output of the - of the same bridge rectifier. You will want a 4 pole power switch to connect and disconnect all 4 hot power leads at the same time. I hope this helps.

+ and - 9 volts is ok on the low voltage supply because this is typically regulated. + and - 12 volts is about normal for that power size amp. Car amps claim higher power for the same voltages because they use lower impedance speakers so they run higher current. Most guitar amps use 8 ohm speakers and most car systems are 4 ohms.

High power car amps use an inverter to generate the + and - voltages needed. Cheap car amps don't use an inverter but claim impossible power ratings based on super low speaker impedances.

You can tell the cheap car amps because they often claim in excess of 200 watts out, but have an 8 amp fuse or less. Think about it. The power draw into the amp at the point the fuse will blow is 12 volts times 8 amps current. This is 72 watts in at the point the fuse blows. There is no way that can put out 200+ watts RMS to the speakers.

[Alster370]

Could I have say a 24v power supply for all the different voltages but use potentiometers to get the lower voltages need for the other components? It would simplify things much. I also looked at the capacitors near the output connection coming from the transformer. There are 2 25v caps , 1 16v and 1 6.3v cap. Would this indicate the different voltages?

Another idea, stupid or not im not sure but If I ran 36v DC though the transformer would the output voltages be the same? Im guessing not because its DC not AC.

I think If I did do all these different power sources I would have to have them all with the same Amp rating, but even then the amp draw would most likely be different for each voltage input the amp requires.

Tech, would you mind drawing a simple diagram of your solution? Id really appreciate it.

Thanks

[Technician1002]

I'm at a campout scanning photos from an old family photo album. I'll try to do a drawing later.

[LeMaudit]

Those old photographs are always fun!
Sometimes with a little numeric enhancement you can retrieve a lot of details.

[Technician1002]

Sorry for the delay. we came back with a completly soaked tent. To prevent mildew, it was nessary to put it up right away when we got home to properly dry and then it needed folded and stored again before the next rain. By then I was in the eat sleep commute work mode.

I still don't have a "simple drawing" because driving a balanced amplifier from a single ended supply is not a simple task. Your car electrical system has +12 volts and ground. The amp needs the following 4 voltages..

1 About +12 volts.. That is the easy one.
2 About - 12 volts. This is not so easy. A DC to DC converter is required.
3 About +9 volts highly regulated and filtered. Not hard. A regulator off the +12 volt supply will do the trick.
4 About -9 volts highly regulated and filtered. Not hard if you have the about -12 volt supply up and running.

So in simple terms you need +12 volts to feed the + DC side of the amp and the regulated filtered + supply. That will need to feed a DC to DC converter to provide the -12 volt side of the amp and the regulated and filtered - supply. The voltages given are approximate. You will need to measure the amp when it is running on AC to find the correct DC voltages needed. The high voltage DC could be anywhere from 10 volts to about 16 volts for that size of an amp. The low voltage side could be anywhere from 5 volts to 12 volts depending on the model.

The easy solution is to use an AC inverter so the transformer can provide all 4 voltages with one shot.