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LM3886 Gainclone 2x68 Watt Full Amplifier Project

Project Summary

In this application, we are building a gaincard like amplifier. This application type is named gainclone in the audio world. To take a satisfactory audio response, we are adding a Linkwitz equaliser to the feedback line and adding bass compensation also.

Project Description

Figure:1

Figure 1 

We are using LM3886 which is the revised version of its brother LM3875.Briefly features of LM3886 are;

Maximum Output Power: 68W RMS – 108W Peak
THD: %0.03 at 60W
SNR: 110dB at 60 W – 92.5dB at 1W
PSRR: 120dB
Protection Circuitries: DC/AC Short circuit protection and thermal protection
Output Class: Conjugate AB-A

These are the parameters that we are interested in. Honestly this values are much more better than many of HI-FI amplifiers sold in the market. Especially it is hard to find 110dB signal to noise ratio. And another property is, when there is no input, this amplifier is quite like a dead. It is nearly impossible to hear any noise when you stick your ear to the speakers.

The circuit below is the recommended design by National Semiconductors engineers. As you can see it is a plain and applicable circuit. Essentially we did any change to the input line hence we didn’t add extra noise. Rin is input level potentiometer, Rb is current limiter potentiometer and Rf1-Ri are gain resistors. In this application, gain is 20k/1k=20 (26.4dB). 22uF capacitor provides the gain curve start from about 7 Hz so it filters unwanted low frequencies.

Figure:2

Figure 2 

This is our design. As you can see, negative feedback line is revised. In this way, about 7dB is gained at 30Hz-70Hz band, and reached about 34db at peak point at 30Hz.

Figure:3

Figure 3 

When 26v DC supply is applied to the given design, with 4 ohm speaker load and 2Vp-p input signal it gives about 65W RMS power. For users who want to use 8 ohm, we recommend +/- 38V voltage supply. Of course less than this value may be applied too.

Supply circuit is shown below, It is simple too. There are two KBU series (6A) bridge diodes and 2×10000 uF capacitors. As a general rule, 2200uF stabilization capacitor is used per ampere but we are used to use 4700uF per ampere. In our circuit, 2 Amperes will be drawn per line and we use 10000uF capacitors. We recommend you to use 2×10000uF capacitors per each line if you will use the amplifier in party mode.

Figure:4

Figure 4 

Generally in HI-FI applications, toroid transformers are used. But this kind of transformers are expensive and large in volume. Advantage of this transformers is they have very low leakage magnetic flux so they can be placed in the same box with amplifier. But we choose to place transformer and amplifier in separate boxes so we are using standard transformers. Transformer values should be;

For 8 Ohm – standard mode : 220 / 2 × 24v (with middle out) at least 250W
For 8 Ohm – part mode: 220 / 2 × 24V (with middle out) at least 350W
For 4 Ohm – standard mode: 220 / 2 × 18v (with middle out) at least 250W
For 4 Ohm – party mode: 220 / 2 × 18v (with middle out) at least 350W

As you can see PCB components are tight but as a result it is compact ,small and noise free. Especially short negative feedback line supports this function.

Figure:5

Figure 5 

Assembly is not very clear but it gives perfect results. You should use %1 tolerance – metal strip resistors.

Figure:6

Figure 6 

This is the PCB side of the board. You can see some components are connected to this side to keep the power line very short.Line under the ICs is fed by solder. This process takes time but not so difficult. You should not omit this step.

The PCB file is provided in pdf format. You can apply it to the board by using the ironing method.

Notice that there are two types of LM3886 IC. One of them is unisolated LM3886TA and the other one is isolated LM3886TF.

Figure:7

Figure 7 

Figure:8

Figure 8 

Not to mess with isolation, we used TA model and connected it to the heatsink by using silicon gress. For 8 Ohm and TA users, we recommend 2.8°C/W heatsink. This means about 5cm x 10cm, 8 or more layered simple aluminum heatsink. For TF users this value is 1.8°C/W and this means double size of the heatsink that we said just before.

Placement of circuits in box is given in the downloadable .rar file. You should open aeration holes on your boxes. You can use standart speaker terminals for speaker outputs and RCA terminal for signal inputs.

1 – RCA connector’s ground terminal must be connected to the middle of the AF coded terminal block directly and using the shortest way by using a thin wire.

2 – RCA connector’s right and left signal input terminals must be connected to the rightmost pins ( from PCB view) of the sound level potentiometer by using LIYCY cable.

3 – Take two output cables (again use LIYCY cable) from the middle pin of the potentiometer and connect them AF coded terminal blocks’ right and left terminals.

4 – Don’t use the amplifier without the potentiometer.

Author: Dr.Hardware

Circuit-Projects & Diyaudiotr

Project Files

File NameFile Size
PCB, Layout, Connection Diagram Files554.17 KB
Tags: LM3886, gainclone, watt, full, amplifier

Comments on this Project:

By DIY Audio Turkey 0Score: 

6 years ago:  Thanks for the English translation of our project.
-Hazar

By Student 0Score: 

6 years ago:  awesome project dude, I'm on it right now!
I'm wondering to have links for preamp (I need it?)I want to transfer signal from computer...( i have integrated soundcard)
Thanks, jaan

By JAYAN 0Score: 

6 years ago:  Hey,

I downloaded project.rar and then extracted it and found that pcb design of amplifier is blank page. Please cure this matter and send one copy to me.

Thanx

By sam 0Score: 

6 years ago:  Hi Jayan, PCB files are available in PDF format in the provided rar archive. You may have some problems with your PDF reader.

By yahwant 0Score: 

5 years ago:  wow!!!!!!!! superb amplifier..it sounds so nice

By dhyne 0Score: 

5 years ago:  the components are only 19 pcs.. my teacher said that i need atleast 20 opponents for a project.. please help

By toni 0Score: 

5 years ago:  thanks for information....

By Elmer 0Score: 

5 years ago:   Is there a pcb already made up for this 3886 amp?
Thanks
Elmer

By dex 0Score: 

5 years ago:  The pcb and shematics are not compatible! On the pcb R1 are equal to R6 and so on? ?n the shematics R1 and R6 have diferent values... Please help!!

By dex 0Score: 

5 years ago:  All so in the schematics i dont have R9, R10, R11??? Can enyone please send me copmlement schematics and PCB layout files.

By royazana 0Score: 

5 years ago:  I'm having trouble with my laptop, could not open pdf files, so, could you kindly send me thru e-mail the pcb layout/traces? I would want to start building the 3886 amp as soon as possible. Thank you very much. -Roy Azana

By royazana 0Score: 

5 years ago:  Ooops, I forgot to state my e-mail, sorry.

My e-mail: .(JavaScript must be enabled to view this email address)

Thanks you very much.

By rohit sharma 0Score: 

5 years ago:  i want ic lm 3886 tf i want india address

By rohit sharma 0Score: 

5 years ago:  i want ic lm 3886 tf i want india address
.(JavaScript must be enabled to view this email address)

By bentong 0Score: 

5 years ago:  Thanks for the complete details of the circuit. I did it exaclty the same replica of what you had in the picture :)

By kapilasf@gmail.com 0Score: 

5 years ago:  this ic is not to buy in Sri Lanka so how can I made this amp

By mzeeeshankhan@gmail.com 0Score: 

5 years ago:  hi,
i have alot of information about this ic but ,i have failed to built it..plz guide me that how to make it?
i shall be thankful to u if u make yhis effort.

By kevin garnett 0Score: 

5 years ago:  the assembly of the circuit was very neat but why was the capacitors soldered on the other side? I think we can still fit it together with the components so all of them will be on one side aight?!

By Sampath 0Score: 

4 years ago:  Do I need a separate pre-amp before this circuit's input? How can I add Bass, Treble controls for this circuit ?

By sabir 0Score: 

4 years ago:  lots of probs facing lm3886t can have some guidance

By guest 0Score: 

4 years ago:  May you explain me why you are using two bridged rectifiers? Do you want to loose more power or do you want to heat your room in winter?

Connect pins 6 and 7 of your transformator to get GND and use one bridged rectifier to generate your symmetrical output voltage refered to GND.

By asdf 0Score: 

3 years ago:  Works fine for me, but I can hear the 50Hz noise. I've got the amplifier and the supply circuit on the same PCB (with toroid transformer), it could be the problem. Another problem is the ic reaches high temperatures about in a minute. I have to get a larger heatsink...

Anderson Ziede

2 years ago:  Is there a part list? I cannot see the entire list of parts due to the very small size of the circuit picture.

Thank you.

Cybercafe
By Cybercafe (0) 0Score: 

1 year ago:  If any Indian friend has been made this amplifier, plz share your project's video and photos with me, i am also interested to built this project.

Cybercafe
By Cybercafe (0) 0Score: 

1 year ago:  Sorry I forgot My Email ID : .(JavaScript must be enabled to view this email address)

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