First DIY project - LM4766

[meche]

I've been bitten by the DIY bug, and now I'm taking the plunge.
I've got a couple full-range speakers laying around (4 Ohm, 87dB sensitvity, coaxial tweeter). They can handle up to 50W RMS and 120W peak. For my first project, I'd like to build a simple amp to drive them, and I've got a couple of LM4766 chips on the way (the isolated packages). I'm planning to do p2p wiring on perfboard.

Being new to DIY audio, I have some questions that seem to be pretty basic, but I haven't been able to find good answers from searching past posts.

First, what determines the power output of an amplifier? It seems to be related to the supply voltage and the load impedance, so is there a good back-of-the-envelope calculation I can do?

Second, on power supply sizing. Assuming the amp actually does put 50W each onto two channels, a 150VA TF seems to be plenty. My initial plan is a 25-0-25 EI TF mated to a bridge rectifier rated at 6A and 100V, output through 10000uF caps, with 0.1uF caps to prevent high frequency feedback on the power lines (recommended by the chip datasheet). I'll fuse the mains supply at 1.25A. Are these numbers appropriate, and what are the general guidelines & calculations for designing a power supply?

Third, How should I determine the required power, voltage, etc. ratings for the other resistors & capacitors in the circuit (based on the typical circuit in the datasheet)?

Finally, and possibly superceding the above, is there a good web page, textbook, etc. that will give a primer on the "basics" of audio amplifier design & construction, so I don't bother the experts with simple questions?

I'm a grad student in mechanical engineering, so although I have minimal electronics background, I'm not afraid of the technical stuff, so don't hold back!

Thanks in advance.

[SpittinLLama]

First, download the Overture Design Guide from the National site, found here: http://www.national.com/appinfo/audio/

You might have to scroll down a bit and it is in the middle column. It will let you do some power calculations with supply voltage and thermal info. The LM4766 is not a good chip for 4 ohms, not enough output current. Since it is your first DIY project, use what you have but your voltage is too high. For 4 ohms you will want a +/-21V supply and only get about 28W. With those isolated packaged they will run hotter too so may need to reduce supply voltage further. Play with the design guide and see what it tells you to do. There are also instructions for the guide on the site.

For 4 ohms you really only want the LM3876 or LM3886 or the stereo LM4780. These chips can do more of the power range you might want. Keep in mind, 28W is not too bad.

All resistors are fine at 1/4W, nothing special needed.

Good Luck
-SL

[YFW]

The power output will depend on the supply voltage and load impdedance.For example,if you supply +/-28V to the curcuit and the load is 8 ohms,the output power is 20*20/8=50W.

20Vrms could be obtained by 28/1.414.

In fact, the real output power will lower because of the efficiency and design structure.

[meche]

I really appreciate the advice. I found the design spreadsheet, and that was a huge help.

I also ran across a site with some good information for beginners:
http://sound.westhost.com/articles.htm

After learning a little more, I think I'll choose a different chip to built the amp around, since the chips are free.

How low do I need to get the power supply ripple for good performance with the Overture chips?

I may be stepping on a land mine here, but I have a limited budget, so how much difference do "premium" components make over the more generic ones I can get from Digi-Key, Radio Shack, etc. Do they really perform *that* much better, or is this more a matter of the qualitative characteristics like "brightness" and "warmth?"

[YFW]

ripple?It needs some experience.
For the OP you chose,I think the capacitors need at least 6800uF*2 for each chanel to achieve better performance.

[SpittinLLama]

My advice, keep is easy and low cost for a first time project. It will sound pretty good. Then with what you learn you can go on to a better project. My first 2-3 amps were not very good but what I learned was very valuable so my later amps were much better in looks and function.

-SL

[Ponz]

if you want to lower rippe but not want to buy large capacitor.
maybe you can try to use C-R-C filter network.
for example, prepare four 2200uF ~ 6800uF capacitor and two 10R/5W resistor(the 5W resistor is very cheap).
Than connect 5W resistor between two capacitors. it makes a simple
C-R-C filter network and get great effect than juse use one large capcitor.

[AndrewT]

Hi,
10r for the R in the CRC is a bit high for a power amplifier.
Somewhere in the range 0r1 to 1r0 will drop an acceptable voltage between the capacitors.

[Ponz]

thanks AndrewT, it's typo, 10r for C-R-C filter is truly too high for 40W
power amplifier.

the resistor value should be 1Ohm for usual power amplifier, and power rating needs 5W above.

[jrdmedford]

Heyho. Also in Boston. I'm a tube guy, but I have done some solid
state projects for friends on very tight budgets.

You should also check out a number of surplus parts vendors.
I like: www.allelectronics.com www.smcelectronics.com there
are others as well. For simple stuff like capactors, heatsinks,
IC sockets and what not you cant beat the prices. Digikey et. al. has
minimum orders, and shipping costs, and parts picking costs for low
quantities. I tend to avoid Digikey unless I really really need something
special - like an IC or some fancy transistor.

Another project to think of is the JLH Class A aka Death of Zen.
The transistors are dirt cheap, so if you explode them by accident, its
no biggie. More important, these use an Output Capacitor to keep DC
off your speakers. I highly recommend your first projects use a
blocking capacitor so to avoid the consequences of beginners - i.e.
blowing your speaker drivers from DC...

A nice TO-3 package transistor amp is also friendlier to wire because
the parts are big. Chips are a pain. You cant find anything in DIP
package and the new SMD stuff is so small its only made for machines to solder to boards.

-- Jim