DIY LM3886 Amplifier not working

[Carel]

Hi,
can anybody maybe help me with a little problem... I am trying to build an LM3886T based amplifier with the diagram provided in the datasheet: http://www.national.com/ds/LM/LM3886.pdf but the amp sends 12V straight to the speaker? I've got no idea why... I've asked for help on another forum (more details about my problem): LM3886 68W Amplifier
I think somewhere some of my connections are wrong, I've built the amp exactly like in Fig 1 of the datasheet. Is there anyone who can maybe post me a fully working diagram that shows EXACTLY where to connect the Speaker + & -, where to connect the input + & - and where the PSU is connected, because I'm doing something stupid and I can't figure out what I'm doing wrong... And then my speakers get 12V...

ANY help will be GLADLY appreciated
Thanks!

[AndrewT]

are you able to post pictures?

[ic4ru5]

pcbs diy to or bought? i remember that i managed to sc my opa2132 due my bad quality pcb. check pins opamps have output offset regulation pins.

[jaudio]

check Rin,make sure one end is connected to ground. My first lm3886 amp had the same problem.

[Carel]

Ok, unfortunately I'm not able to post a picture, because I'm building the circuit without a breadboard... Look, I've built an amplifier with a TA8205AH chip before. This chip (TA8205AH) worked without any components attached to it. Ok, I wasn't planning to use it like that, but I just wanted to test the chip first (it was a chip I got from an old radio). The chip gave me nice audio, even with only 3 components (The Chip, PSU & Speaker). Yeah, well once I figured out the chip was in working order, I disconnected it and built the complete amp. It has very, very clear sound and has got nice bass. The chip can output about 9A per channel, so I can even connect 2x speakers per channel... With this chip there was no Gain adjustment. Gain was 1:300, so I had to terminate the input signal to input ground to reduce the Gain.

Now I suspect that this is why I'm having problems with my test setup. I havent connected anything yet, just the Chip, Speaker & PSU. There are no Rin, Ri, Rf1 resistors... Yeah, I know I'm a moron, but I thought that the LM3886T chip would send noise to the speaker, like the TA8205AH, without any components attached. So, I'm going to build the complete device tomorrow & then I'll let you guys know.

I just want to make sure of a few things referring to this image (http://i578.photobucket.com/albums/s...pplication.jpg):
1.) GND of the chip is connected to the GND of the PSU? (someone else told me something different)
2.) Where do I connect my Input's Ground, to the PSU GND?
3.) What value can I use for Rm to disable the mute function permanently?

[Minion]

The PSU Ground and the Chip ground are the same but you should have all the audio grounds comeing together before they connect to the PSU ground , some poeple also isolate the audio ground from the PSU ground with a Low value resistor ...... The speaker ground should connect to the ground between the big filter caps....

For the mute function I use a 10k resistor .....

[sregor]

One minor caveat - on the 3886t the tab is internally connected to V- (pin4). If you mounted it on a heat sink, you would need a mica or silpad insulator. Good luck with it.

[Carel]

Ok, you guys were right. This chip works COMPLETELY different than all the other chips I've came across before. After I've connected some resistors, Ri & Rin, the 12 ~ 20v at the output is gone... So for this chip it's normal to pass the V+ straight to the output if Rin & Ri is disconnected from the ground, not a very good design from National, becase if an amp fail or if Rin / Ri should get disconnected from some reason, then the chip will blow the speaker and I've seen speakers taking flame due to this before. Toshiba's chips work very differently though.

I've bought a BoomBox 150w 2-Channel car amp, which I thought would give a lot of power, but my Hi-Fi (which has got a 70W chip beats this BoomBox by like 900%)... I then opened my BoomBox and discovered that they used 2x 20W chips, TDA2005. And to make things worse there are 2x chips which are bridged. So it can only like provide equivalent MAX 10W, which is a huge Ripoff. I'ts funny how they cheat without cheating when advertising products. And my previously self-built TA8205AH amp gave out waaaaaay more bass & power. I was even planning on rebuilding that amp (because it really has got nice sound and bass responses, even with 2ohm loads are very impressive) and just replace the chip with TA8254BHQ. These chips don't clip any sooner if you adjust the TruBass at the input. And you can configure the chip in two ways:
1.) High-Bass, Low output or
2.) Lower-Bass, Loud Output

I was just wondering about another thing(s):
1.) Why is it a bad idea to use a Switching Power Supply instead of a transformer for an AMP?
2.) What is the maximum amperage this chip will ever draw from the power grid?
3.) How hot does it get in heavy loads?

[AndrewT]

Originally Posted by Carel

.......National...........
3.) How hot does it get in heavy loads?

depends on how long the heavy load needs to be driven for. Short term: no problem. Spike will prevent overcurrent.
Longer term:you must follow the National quidelines for heatsink and voltage supply. I recommend you double the sink size, cf. National's guide.

[gootee]

SMPS can work just fine. Many like them much better than linear supplies (i.e. transformer/rectifiers/caps), for audio amps. [And certainly it's easier to filter out an out-of-the-audio-band SMPS high-frequency switching "ripple" than it is to filter out an in-the-audio-band linear supply "ripple", IF you think that you need to do either one.] Use either one, if it's handy, at least to experiment.

[You probably already know all of this already, but...] Some of the main things, though, are to 1) mind your grounding scheme and 2) to minimize the "loop area" formed by each conductor pair.

For the former, use a "star grounding" scheme, because large dynamic ground-return currents sharing a ground conductor with, say, your input's ground reference, WILL induce a bouncing ground voltage at the "grounded" end of your input resistor, which will arithmetically add to your input signal, which is "a bad thing". So run separate ground-return conductors back to the star ground point; usually the ground end(s) of a linear supply's smoothing cap(s). [The bouncing ground voltages are induced because every conductor has resistance and inductance, and currents give V = IR, and, for time-varying currents, V = L(dI/dt), which can be a real killer because the amplitude only depends in the current's rate-of-change, dI/dt, times the inductance. The R and L are "distributed", incrementally, along any conductor. But the resulting voltage sum ends up back at the non-ground end!]

For the latter, just keep all conductor pairs close together, for as long as possible, so that Faraday's Law doesn't come into play and induce unwanted currents in them (a time-varying [AC, perhaps? H(u)mmmm?] magnetic field will induce a time-varying current in a loop), which would induce voltages across places you wouldn't want them (like your chipamp's input resistor). So, for wires, twist together pairs like input signal and ground, AC power, output signal/ground, DC + and -, etc etc. On PCBs, keep each pair's traces close together as much as possible. (Actually, I like using shielded cables [two conductors plus shield] for inputs, with the shielded connected at the input end only, usually to the chassis.)

Another important thing is using "bypassing" capacitors to ground wherever a DC supply line enters a chip of any kind. And don't use low-ESR (Equivalent Series Resistance) capacitors there. For a little opamp, 0.1 uF ceramic (not high-quality C0G or NPO) and 10 uF electrolytic in parallel are common. You can think of them as low-pass filters to get rid of junk on the DC lines. But you can also think of them as a small local power supply/reservoir, to supply the current for fast transients. Power chipamps will need much more than 10 uF.

And remember that "the signal path" STOPS at an amplifier chip's input resistor. After that, it comes straight from the power supply and goes to the chipamp's or opamp's output.

And always remember that every conductor has resistance and inductance. So you NEED some local capacitors on each chip's power supply pins (and on each board's DC inputs, in larger systems), to more-quickly satisfy any demands for fast current changes (which, especially, the upstream conductors' inductances will tend to try to prevent).

OK, I'm just rambling. Sorry.

Hey! It's not A.D.D! It's "Agile Focus"!

Cheers,

Tom Gootee