audioguru Posted August 29, 2006 Report Share Posted August 29, 2006 You won't have anything to limit the power to only 40W. You won't even know when the power reaches only 40W. If the supply voltage is too high for the amplifier's load then the current will also be too high. Voltage times current= power. Power makes the amplifier IC get hot. The heatsink with a fan can take away only as much heat as per the power rating of the IC.One inexpensive 25-0-25 transformer is perfect for two LM3876 amplifiers driving 8 ohm speakers. If you use 4 ohm speakers then either parallel ICs are needed and a single transformer per channel, or a lower voltage transformer with two LM3886 amplifiers. Quote Link to comment Share on other sites More sharing options...
frankwas Posted August 29, 2006 Author Report Share Posted August 29, 2006 Ok.I have another quesiton. You said earlier that I can use a 21-0-21 transformer. This will work perfectly for a 4 ohm load then? How many VA's must the transformer be?I apologise if we are going in circles. :P Quote Link to comment Share on other sites More sharing options...
audioguru Posted August 29, 2006 Report Share Posted August 29, 2006 The datasheet shows an LM3886 with a positive and negative 28VDC supply feeding 68W to a 4 ohm speaker.Amplifier ICs are about 70% efficient, so two amplifiers will draw 194VA from a transformer.A 21-0-21 transformer will make positive and negative 28VDC. Quote Link to comment Share on other sites More sharing options...
frankwas Posted August 29, 2006 Author Report Share Posted August 29, 2006 Oh ok.The closest that i can get to that is a 18-0-18. Will that be ok? Quote Link to comment Share on other sites More sharing options...
audioguru Posted August 29, 2006 Report Share Posted August 29, 2006 A transformer that is 18-0-18 will make a positive and negative 23.4VDC supply. The LM3886 will give about 50W to a 4 ohm speaker. Quote Link to comment Share on other sites More sharing options...
frankwas Posted August 30, 2006 Author Report Share Posted August 30, 2006 Ok. Will this transformer do the trick?230V to 18-0-18V 2x4.44A 160VAOr must I get another one?Can I still use Rod's design for the LM3886 just as it is with the PSU I mentioned earlier with two 4700uF's and a 400V 35A bridge rectifier?I also have another question about pots. What is the difference between a linear and log pot and which one will work best for this project?Thanks. Quote Link to comment Share on other sites More sharing options...
audioguru Posted August 30, 2006 Report Share Posted August 30, 2006 Two 50W amplifiers will need a transformer of at least 143VA so the 160VA transformer will be fine.Two 4700uF capacitors would be the minimum value for 1 amplifier. Use two 10,000uF capacitors for the stereo amplifier.The voltage to the bridge rectifier is the peak of 18V which is only 25.5V. It is a waste to use a bridge rectifier rated for 300V. A 50V or 100V bridge rectifier would be fine.A linear pot is for adjusting a DC voltage. A logarithmic (audio taper) pot is a volume control and it matches the logarithmic response to loudness of our hearing. Quote Link to comment Share on other sites More sharing options...
frankwas Posted August 30, 2006 Author Report Share Posted August 30, 2006 I built the first prototype today and it worked really well!Except, I have a lot of noise as soon as signal is fed into the amp. Can the be because the gain is too high? Or what can I do to remoce the noise?Thanks! Quote Link to comment Share on other sites More sharing options...
audioguru Posted August 31, 2006 Report Share Posted August 31, 2006 The noise from these excellent IC amplifiers is so low that you would hardly hear it, and it would be all the time, not just with signal. The gain is only 23 and is recommended.The pcb layout and supply bypass capacitors are very important to prevent the IC from oscillating, which could occur with signal. The pcb ground trace from the speaker should not flow into the input ground trace.Frequently the datasheet has a recommended pcb layout but not this one. Maybe its applications note does or maybe there is a project with it on the web. Quote Link to comment Share on other sites More sharing options...
frankwas Posted August 31, 2006 Author Report Share Posted August 31, 2006 Oh ok.I think that it's my layout that is giving the problems. Do you perhaps know of a pcb layout for this circuit?I really appreciate your input!!! Quote Link to comment Share on other sites More sharing options...
frankwas Posted August 31, 2006 Author Report Share Posted August 31, 2006 The pcb ground trace from the speaker should not flow into the input ground trace.How do I seperate my input ground from my speaker ground? Where do I get ground for my speaker and/or signal?TA. Quote Link to comment Share on other sites More sharing options...
MP Posted August 31, 2006 Report Share Posted August 31, 2006 You should post your schematic and board layout if possible so that you can get more exact help. MP Quote Link to comment Share on other sites More sharing options...
frankwas Posted September 1, 2006 Author Report Share Posted September 1, 2006 I would just like to say thanks guys for all your input!!I have built one of the amps so far and it sounds great!!!! I will post some pics when I have completed the whole project, and ahen I get my bro's cam :) Thanks once again! Quote Link to comment Share on other sites More sharing options...
audioguru Posted September 1, 2006 Report Share Posted September 1, 2006 I have built one of the amps so far and it sounds great!!!!Nice going, Francois. ;DHow did you fix the noise problem? Quote Link to comment Share on other sites More sharing options...
frankwas Posted September 2, 2006 Author Report Share Posted September 2, 2006 How did you fix the noise problem?I got a pcb from a site and had it made for very cheap. I actually had two made for R180, which is about $25.You would believe it, I finished the velleman kit, and it sounds pathetic in comparison and it is twice as expensive!!! Quote Link to comment Share on other sites More sharing options...
audioguru Posted September 2, 2006 Report Share Posted September 2, 2006 I finished the velleman kit, and it sounds pathetic in comparison and it is twice as expensive!!! The cicuit for the Velleman kit amp is simple and basic. The National amp IC is the best they can make. No wonder Velleman don't say what is its distortion at anywhere near its full output. Quote Link to comment Share on other sites More sharing options...
frankwas Posted September 3, 2006 Author Report Share Posted September 3, 2006 Yeah. I am actually disappointed in velleman for that.I have another question: What is the purpose of the 2R7 resistor and the 100nF cap on the output?Also, what would you recommend I use for a low-pass crossover for a sub? An inductor or a electronic crossover. If an inductor, how do I make one?Thanks in advance! Quote Link to comment Share on other sites More sharing options...
audioguru Posted September 3, 2006 Report Share Posted September 3, 2006 Hi Francois,Speakers are inductive which causes them to have a high impedance at high frequencies. The RC networks at the amplifier outputs are "zobel networks" that are a low impedance at high frequencies to prevent the amplifiers from oscillating.An inductor and capacitors to make a passive crossover network for a sub would be huge and have a high resistance which would ruin damping of the sub.Use an active crossover circuit instead. There is a second-order sub crossover project in our projects section, but I think a crossover should be 3rd-order instead. Rod Elliot has a 3rd-order crossover project but it isn't for a sub, it is for 3-way speakers. You could re-calculate its parts for your sub. You should also have a highpass for the main speakers as part of the crossover network. Quote Link to comment Share on other sites More sharing options...
frankwas Posted September 3, 2006 Author Report Share Posted September 3, 2006 Ok. Because I forgot to put that in but it still sounds ok. But I'lll have to put it in. Do you think it's wise to buy a active car audio amp, since all my speakers are car speakers anyway? Quote Link to comment Share on other sites More sharing options...
audioguru Posted September 3, 2006 Report Share Posted September 3, 2006 Do you think it's wise to buy a active car audio amp, since all my speakers are car speakers anyway?What is "an active car audio amp"? Quote Link to comment Share on other sites More sharing options...
frankwas Posted September 4, 2006 Author Report Share Posted September 4, 2006 Sorry. I meant to say active car crossover. :P Quote Link to comment Share on other sites More sharing options...
audioguru Posted September 4, 2006 Report Share Posted September 4, 2006 An active crossover circuit for a car might use a single 12V supply and an output coupling capacitor. When power is turned on or off then the coupling capacitor will charge or discharge and make a loud "BANG" in the sound system.A home active crossover circuit will probably use a positive and negative supply, so its coupling capacitor won't charge and discharge and won't make a noise. Quote Link to comment Share on other sites More sharing options...
frankwas Posted September 5, 2006 Author Report Share Posted September 5, 2006 Oh ok.I just blew my velleman amp!! Just couldnt keep up. Now I need to find an amp that preferably uses two 3886's in parallel and that has a single sided pcb design to go with it. ;) Do you perhaps know of any? I looked at diyAudio, but have not found anything yet. Quote Link to comment Share on other sites More sharing options...
audioguru Posted September 5, 2006 Report Share Posted September 5, 2006 A guy called Gevv joined our forum a few months ago and posted many projects with the RAR file type, and having a password I think is gevv. I saved some of his projects and many are amplifiers using the LM3886, some paralleled.EDIT: This file is without a password and has a 100W into 4 ohms amplifier project using two LM3886 ICs. Its pcb is double-sided.PA100_2x_LM3886_in_parallel.rar Quote Link to comment Share on other sites More sharing options...
frankwas Posted September 5, 2006 Author Report Share Posted September 5, 2006 Thanks.I was really considering that design, but I can't get double sided PCB's made as of yet. Do you perhaps know of any other amp, even if it is not a 3886 design that will have rms 100W output or more?Thanks a lot for all your input!!! Quote Link to comment Share on other sites More sharing options...
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