step up voltage

segasonicfan

Member
2005-08-13 11:41 am
what do you mean they have a switch mode power supply built into them? My goal is to power some really nice amps in my car like the LM3886 and the LM4752. However, to get the full output for them I need 25-30v (or that's what I understand from the schematic).

-Segasonicfan
 
Most any car audio amplifier uses a switch mode power supply to boost 12V up to useful rail voltages for the output transistors. Using a FET, the input 12V is pulsed into a transformer, and then rectified back to a higher positive and a negative DC source for the amplifier’s rail voltages.

If you need 25V from 12V, a switch mode power supply would get you there.

Do you require positive 25V, and negative 25V for your project? If so, then you could either build your own power supply, or you could part out half of a preexisting amplifier to use as your source.
 
The following power supply will do what you need:

http://sound.westhost.com/project89.htm
(figure 2)

The voltage will have to be set to ±28 volts for the 3886. It would probably be best if you used the same ICs for all channels since different voltages are required for each.

They recommend using a core from an inductor. While this may work, it's best not to use a core that has unknown properties. The F-193A-F or the F-140A-F from the following company should work properly with the supply above:

http://www.cwsbytemark.com/prices/toroidal.php

The supply above has a grounded secondary center-tap. This means that you'll need some sort of isolation for the shield ground (to prevent ground loops). An audio isolation transformer is the easiest form of isolation. For a cheap transformer, you can use a ground loop isolator. Better transformers can be purchased from companies like Jensen Transformer (relatively expensive).

http://www.jensen-transformers.com/ln_in.html
 
I'd recommend using either the ESP project, or just using the power supply from a cheap and or blown up car amplifier. The ESP project and the internal PSU in a car amp work in the same way.

The "cheap" option would be to make a regular home power supply and use a cheap-o inverter (and filter enough to get the cheap inverter crap out of the signal). This is a "ghetto" solution.
 

segasonicfan

Member
2005-08-13 11:41 am
thanks for your help guys. The schematic to make my own looks albit overly complex compared to other DC-DC circuits I've seen. And to build it would cost a damn fortune in parts. I have 2 scrap audio amps from cars and I'm going to the auto junkyard tomorrow to get some more. It seems like the easiest solution would be to pull out the step up voltage conversion from these.

So far I can't find the step up on these two but maybe some at the junkyard will have it.

-Segasonicfan
 
hrmmm it looks like I solded my problem :) Looking over the LM8336 schematic I saw there are two input pins for positive voltage-pins 1 and 5. Further down the schem it says:

"The optional external components shown in Figure 2 and described above are applicable in both single and split voltage supply configurations."

So it looks like you can input +12v (or 13.8v on a full car batt charge) to each pin to get a cool 24-27.6v power supply!

Has anyone tried this or think this may not work?

-Segasonicfan