2xLM3886 simplifiing - Page 1
Forum section: electronics / projects

author: Wor3d
author_rank: Contributor
timestamp: February 29, 2016, 10:27:48 am
content: Hi guys!I am working on stereo 60W Amplifier using 2 LM3886s, and this is my schematic so far (Recommended application scheme + misc stuff)I have some questions, and I will be happy to all other suggestions to the circuitI am thinking, if I can use just one 2N3904 (with one 100k||1M, 300k||100k and that small snubber 100k||100n) to provide reference ground (pin 7) and offset voltage (pin 10) for both LM ICs.Note: 2x100n caps are supposed to be between pin 5/1 and 4, but I placed them up thereI will be glad to any suggestions, even some that I didn't ask forI am completely self-learner, so..Doing my bestThanks!Edit: Maybe I put the wrong titleMy main goal is simplifying some things, yes, but I also want the quality to be the as good as possible (even atthe cost of adding few components)Edit2: Also, can I use 2N2222 instead of 2N3904? I got 1000 of them at home, they have same pinout, similar purpose and the "noise" of transistor does not interfere with the circuit, or does it?

author: MartinX
author_rank: Regular Contributor
timestamp: February 29, 2016, 06:54:00 pm
content: Why a single supply? If you have no specific reason not to use a symmetrical supply go for a symmetrical supply, then you can skip the output capacitor and the complicated input bias arrangement. The supply transformer needs two secondary windings but that is very easy to get and basically you need one more capacitor in the power supply but that is just the output capacitor moved to another place so if you build from scratch and buy all the parts the cost will be almost the same.

author: Wor3d
author_rank: Contributor
timestamp: February 29, 2016, 06:58:03 pm
content: Yeah, I know, but it's one of the circuit requirements for meSingle supply powered. Otherwise, yes, it is many many times simpler omit that super expensive end filter cap.... So I am searching for optionsOne guy (edit: c4757p) suggested using opamp buffer, kinda understand how to replace the Q (IN+= previous base, IN-=short feedback, output to the "gnd" pins of both LMs), hope it is correct, but I couldn't figure out how to apply this offset on LM3886 IN+. Should I just add same buffer, or it has to be splitted somehow?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 29, 2016, 07:07:31 pm
content: With single supply, the alternative to the coupling capacitor is using a bridge configuration. Thus 2 amplifiers, driving one side of the speaker each.

author: Wor3d
author_rank: Contributor
timestamp: February 29, 2016, 07:09:40 pm
content: Ouch, that would mean find another good power amp :/ I would be OK with this, and the filter cap, if I can maybe improve this offsetting litle bit more simplistic. If not, I will just duplicate those...

author: Audioguru
author_rank: Super Contributor
timestamp: March 01, 2016, 01:59:08 am
content: Your input capacitors C4 and C5 have backwards polarity.

author: Wor3d
author_rank: Contributor
timestamp: March 01, 2016, 05:29:40 am
content: As far as I know, they are supposed to be like that, according to tge datasheet. My rough explanation woul be that I am using Single supply, offsetting the IN+ of the amp, so it is always "higher potential", and the input is allowing the cap to discharge through pot to ground. I can also be wrong, tho..

author: MartinX
author_rank: Regular Contributor
timestamp: March 01, 2016, 05:08:00 pm
content: In the datasheet I am looking at, from TI´s homepage the input capacitor have the positive terminal to the amplifier input which makes sense since the input is positively biased and the volume pot is referenced to ground which is the same as the negative end of the supply. The the input capacitors needs to be reversed in your schematic.

author: Audioguru
author_rank: Super Contributor
timestamp: March 01, 2016, 06:30:13 pm
content: Quote from: Wor3d on March 01, 2016, 05:29:40 amThe datasheet clearly shows that the inputs of the amplifier are biased to a positive voltage therefore they show the positive terminal of the input capacitor at the input and the negative terminal of the input capacitor at the grounded volume control.

author: Wor3d
author_rank: Contributor
timestamp: March 01, 2016, 10:14:23 pm
content: Oh crap! Yes ofcourse, I knew it, and even I thought I have it that way :O So I wrote the message back to you, and I understand why. I just misplaced them in schematic :O Thx