Help needed ...LM3886 power amp

[Bismarck]

So I decided to build a power amp for my electric guitar, and through online search I ended up on the LM3886 design at www.generalguitargadgets.com Here is the power amp schematics:

http://www.generalguitargadgets.com/pdf/ggg_lm3886_amp.pdf

The data sheet for the LM3886 states a needed voltage of 28-35V but the PDF asks for a power supply of 18V. Being a complete electronics noob I opted on following the PDF's directions of 18V. Then I needed a 18 bipolar power supply so I found and built this:

http://www.generalguitargadgets.com/pdf/ggg_bipolar_ps.pdf

When I built and connected the whole thing, it only produced a crackling noise, the 7918 regulator of the power supply got EXTREMELY hot (i couldn't even touch the heat sink), and suddenly the 2200uF electrolytic capacitor on the V- end of the amp exploded. Now I know that the capacitor's polarity was correct and it was rated for 26V. And the rest of the caps and resistors are the correct values for the best of my knowledge.

What could have gone wrong? Can you guys please take a look at the PDFs and tell me if they're correct? I'm fairly electronically illiterate but I'm pretty good at following schematics so I don't understand where I've made a mistake. I would appreciate ANY recommendations, corrections, making fun of my noobness etc.

Please help me fix this sucker. Thanx 

[Robin755]

It's not smart to use voltage regulators for your poweramp, it can't give you all the power needed for your poweramp,

just build a supply like this (scroll down to "power supply")
http://sound.westhost.com/project27.htm

make sure you have a proper transformer, probably something about 80VA,
the voltage coming from your supply depends on how much voltage you get from your transformer,
if you buy a 2x25V transformer, you'll get a +/-35V to supply your poweramp,

the 18Volt you have read on the datasheet is the minimum voltage it needs te work properly,
so if you use a larger voltage, it's ok, I will even give you more power

just curious which preamp you will build for it?

[Bismarck]

Robin, thanx for the reply and insight. I am using a 230V primary, 24V secondary (two wires coming out of it) transformer. It is also 1 ampere (I wasn't sure what to get). Do you think that transformer is enough to properly operate the amp? Will that transformer work with the power supply you suggested?

I haven't decided yet on the preamp, probably something simple like a Little Gem or Ruby amp. I didn't try to connect a preamp when I plugged the amp in since it would make troubleshooting even more difficult.

What I really want to know is why the LM 3886 amp didn't work, the 2200uF cap exploded, and the power supply's regulator got so overheated, and if there's a connection between those two.

Thanx again, all help appreciated 

[Robin755]

I believe you have build the wrong supply for the job,
what you need to have as transformer is one with 3 (or 4) wires coming out of it,
as you can see on the link that I've posted there is a +35, a -35 and a GND,
these 3 connections need to be connected to you amplifier,

what you have done is build a single supply, with just a +35Volt and a GND,

also, your transformer is to light for the poweramp you use,
just go out and buy a symmetrical transformer of 2x25V of 80VA,
perhaps print the schematic of the link I've posted and ask the salesman,
they will know what you need if you give him this information,

remember to put a heatsink on you LM3886, or else it will overheat,
and check if the LM in mute posistion or not,

the reason probably why your supply regulator broke, is because your circuit needs to much current which the regulator can't handle, these things aren't build for this,

good luck,

[Robin755]

forgot to tell you that it's possible to make your amplifier with the transformer you've got, (without the voltage regulator)
you just need to adjust your circuit of the LM, look in the datasheet of the LM and search for "single supply application circuit"
it just won't go as loud as the one you wanted to build though,

[Bismarck]

Actually my power supply has a +18V, Ground, and -18V. At least that's the design I followed, u can look at the second PDF that I have on my first post. Do you think the 2200uF cap on the amplifier board could have exploded because of undervoltage?  Or maybe overheated because of the overheating regulator that it was connected to? (they are on separate boards and connected with a wire so i don't know if that's possible)

[Robin755]

just build a proper power supply without voltage regulators, like the one I gave you
it could be that the capacitor exploded because of undervoltage

succes with the building,

[Bismarck]

Robin, I appreciate your help and advice. I'll try and build the power supply you suggested... hopefully it will work out.
Again thanx for the help.

[J M Fahey]

Dear Bismark. An easier option is to  build a power supply with "over the counter" components. You´ll need a 24V center tapped 2 Amp transformer (that means 3 secondary wires), 4 1N5402 diodes (or a 3-4 Amp bridge) and two 2200 or 4700 uFd x  25V. electrolytics. It´ll give +-16Volts (32 total) and will allow your amplifier to supply around 15W, which is an excellent practice power. These are VERY common (and relatively cheap) components.

[n9voc]

Bismark,

Looking at your supply design, I suggest you double check the polarity of the "negative side" power diode - from your description of what happened, I suspect you applied positive voltage to the negative side (by having the diode in backwards)- would cause both effects you mentioned - the extremely hot 7918 and an explosive situation with an electrolytic.  It also would explain the misbehaviour of the 3886 chip.

The power supply you have built, if the schematic is closely followed, is indeed a +/- 18V supply.  However, it has an absolute maximum output of 1 amp (the current limit of the 7818 and 7918 regulators.)  This is good for amplifiers up to about 5 watts tops.

The power supply utilizes two half wave rectifiers to develop the  positive and negative voltages.  To keep within the current rating of your transformer, I wouldn't draw more than 1/2 amp from each side.

(With a 24 volt, 1 amp transformer you have 24 VA out.  24 VA can be likened to an absolute maximum of 24 Watts DC -  at the diodes. With a +/- 18V out, it is 36 volts end to end.  36 V * 0.6666 amps = approx 24 Watts.  This does not include losses in the regulators themselves or the inherent losses in capability when converting VA to Watts DC.  Preceding information brought to you by Ohms Law: WATTS=VOLTSxAMPS) 

This supply is good for things like op-amps and low power amplifiers, but NOT for a higher power amplifier such as the 3886.  Additionally, I wouldn't use LESS than 4700 uF on each side of the supply right after the diode, and given your transformer, you need a primary filter capacitor voltage rating of at least 35 volts to handle the peak voltages coming off the diodes. (24VRMS is approximately 34 Volts peak).

I agree with those who have gone before that a +/- supply from a center tapped secondary is best - also much more efficient than a dual half wave rectifier setup - and that will improve ripple issues and reduce "hum" in the audio.

I hope this helps!

:tu:

[darwindeathcat]

n9voc, I have read probably close to a hundred different explainations about power supply designing and "how to pick the right transformer" for a power amp, but your short post has made clear to me what these other explainations couldn't! Thanks for this!