need help with power supply and led wiring

[jimiko]

First of all, I'm new here. I've been reading some stuff posted here, and had built a few things, including some stompboxes and a few little amps, but this is my first time making a topic. I know this is not stompbox-related, but I didn't found a better place to ask this.

I want to build a SS amp based on two TDA7294 in bridge. Also, I'm planning to use a guitar preamp based on Randall RG100 preamp. But I have two problems:

1- I need a power supply that can power both circuits. The amp asks for a 28V+28V/5A transformer, and the preamp works with +24V. So, how can I use the same power supply to power both circuits?

2- In the original Randall preamp, the led is conected to -40V, which I will not have in my build. This is a dumb question, but how can I make the led channel connections?

This is the amp I'm planning to build (it's in portuguese): http://www.te1.com.br/2011/04/circuito-amplificador-audio-dinamico-tda7294-bridge-estereo/#axzz2HbDwxCL5
And this is the preamp: http://spaceryerson.com/pics/img%20RG80-A-SCH234.jpg

Sorry for the dumb questions, I have experience on building effects cased on ready-to-build projects, but I don't know how to project/mod a simple power supply .

Thanks a lot!

[GibsonGM]

Hi Jimiko, welcome! Some good questions there.

1) to provide your preamp voltage from the existing power supply, it sounds like you need to use a 3-terminal adjustable regulator.   An LM317 might be what you need. You can design it to give you 24VDC from your 28V supply, and it is easy to use.   Search for "LM317 circuits" online, or get the data sheet which will show you the basic connections to make to do this.

2) use any convenient power supply to power up your channel LEDs, just use an appropriate series resistor to get the current you want.  I didn't take a lot of time to look at the schematic - you need to go through and be SURE you have the voltages you need to actually RUN things (such as bias voltages and the like), not just the LED, before you try to tackle this.   

It is an ambitious project, but one that I'm sure can be done without too much trouble!  Good luck!

[jimiko]

Thanks a lot! I forgot about the LM317. I got the datasheet, but didn't understood exacly how to do the maths. But I found out about the 7824 regulator, seems to be easier to use. Do you think it will be necessary to use a heat sink for the regulator? And how big the TDA heat sinks need to be?

About the voltages, bias and the like, I will be careful about this. And to bias the FETs, I will use a trim instead of a fixed value resistor.

This is the biggest project I decided to build until now. It will take some time, but I guess I can do it.

[GibsonGM]

Sounds like a good idea. The '24 will give you exactly what you want.  There are some calculations you can get off the net for how big a heatsink you need, I'm not up to speed on them right off. However, I would put one one for grins, nothing special, what I'd call "mioderately sized".  Maybe 1.5" square with thermal compound...I get them out of old PC's, so I have a selection to choose from...hard to tell ya what size to buy when I just use what's here, of varying thicknesses etc.  If in doubt, get bigger...

You're not dropping a ton of voltage, but the power can add up pretty fast.  You should determine what your preamp will draw and act accordingly.  The LM78XX series is thermally-protected, so you shouldn't toast it quickly - it will tell you if it's getting too hot (it MAY get pretty hot, but not terminally hot, if you get the drift).     I'd set up the '24, put a load on there to simulate your preamp, and see what the temp gets to with the above heatsink.  Use properly rated caps, don't forget...nice project.

http://www.designworldonline.com/how-to-select-a-suitable-heat-sink/#_

[jimiko]

Very interesting read! I think I can get a few heat sinks from old pcs from a friend who repairs/sells pcs. Also, I'm planning to drill a few holes at the back and add one of those pc cooler fans to keep the temperatures as low as possible.

I finished the schematic for the pre. I added the 7824 with the caps and connected the LEDs to the master volume pots (which voltage at this point should be about 12V dc, according to the schematic) and to a switch to ground, do you think this will do it? I used a resistor, of course.