Welcome to Solid State Guitar Amp Forum | DIY Guitar Amplifiers. Please login or sign up.

March 28, 2024, 04:36:40 PM

Login with username, password and session length

Recent Posts

 

Pre-amp power question

Started by Dio666, December 21, 2008, 03:11:59 PM

Previous topic - Next topic

Dio666

I've heard of people building LM3886 power amps and using runoffgroove amp-in-a-pedal circuits as pre-amps.  I'm thinking it would be cool to build a sans-amp GT2 circuit as the pre-amp.  So if I were to put the power amp and pre-amp into a single chassis for a guitar amp head, how would I power the preamp from the power supply?  Would I create a voltage divider from the high voltage DC line to put out the 9 volts?  I'm hoping I wouldn't need a lower voltage secondary from the power transformer.

I'm sorry if this is a stupid question.  I built a tube amp recently so I'm not a total noob, but I don't have any experience with solid state amps.

darwindeathcat

When I was just learning how to build my first amps, I too was baffled at how to power preamp sextions which, if you are building anything over 12-15 watts, require lower voltages than the poweramp section will. The best solution I've found is to use a voltage regulator chip to supply the preamp. These are readily available (even from radioshack), and are very easy to use. You attach one pin to the input votage (higher voltage), one pin to ground, and then the specified output voltage up to the specified number of amps (500 milliamps is all you need for most preamps) comes out the third pin. I've successfully used these chips to power the preamp in all the amps I've built (three).
__ ------------------- __
__ | | | | | | | | | | | __
    --------------------

armstrom

regulators work very well as long as your power amp supply voltage falls within the limits of the chip. Most of the standard 78XX series have a maximum input voltage of 35V. If you need to go higher than this you may need a custom "regulator" built from a zener diode, resistor and capacitor. Take a look at the power supply section of the preamp on this page: http://sound.westhost.com/project27.htm Note: You will need different parts for a 9 or 12V pedal and only half as many rails (that preamp uses a bipolar supply).

darwindeathcat

Yes, Armstrom is correct there. You will definately blow the regulator chip if your supply rails are too high for its specs. By the way, I failed to mention that if your power amp uses a bipolar supply, you should attach the regulator between the positive supply rail and the center tap ground, not between the positive and negative rails.
Cheers,
DDC
__ ------------------- __
__ | | | | | | | | | | | __
    --------------------

n9voc

 8)
Another option for a regulator is to use an LM317 style "adjustable" regulator. Using it, the tab and "ground" termination are "lifted" from ground and thus not an issue for higher than 35Volt input.

Check out the data sheet from national semi for ideas

Cheers!