Simple LM3886 Amplifier

[lifewater]

Hi I have a friend who has built a pair of Amp using LM3886 which he bought off ebay.in along with its PCB. Components he arranged locally. Now the problem is both the amps have no output and he brought them to me to see I did a cursory check of the components and soldering according to the schematic and find that :-

1.) All components according to the schematic used.
2.) Soldering and connections are perfect.
3.) Voltage to amp is 33-0-33 volts (+ve and -ve)
4.) Heatsink is slightly warm.
5.) Speaker output Dc varies but is below 100mv
6.) No OUTPUT if fed signal.
7.) Mute pin (8) run to -ve supply using a 22K X 2 nos resistors.

Checked and rechecked the above. Now friends what to do ?? can I recieve your guidance.

Thanks a ton in advance,
Lifewater

 

[saheb11123]

Please post pictures of the boards , both bottom side and top side...
and also the link of the page from where he bought.
Also post pics of power supply..

 

[saheb11123]

Remove the resistors from the mute pins. See if it works.

 

[linuxguru]

Reduce the value of the resistors from mute to -ve to 10k..15k - about 12k is optimal.

 

[lifewater]

Please post pictures of the boards , both bottom side and top side...
and also the link of the page from where he bought.
Also post pics of power supply..

Remove the resistors from the mute pins. See if it works.

Hi Saheb, Thanks very much for your prompt reply. Removing resistors from the mute pin and connecting directly to -ve would fry the chip. However after reducing value of the mute to -ve to 14K, the amp started working. Thanks once again Saheb for your response.

Reduce the value of the resistors from mute to -ve to 10k..15k - about 12k is optimal.

Hi Shiva, thanks a ton brother. Just did what you said and the amps started singing !!!. Surprised with the sound quality though. Would need to implement the power supply properly, would using 0-33 X 2 nos transformer and after seperate rectification using Bridge rectifier and connecting the +ve of Trf 1 to -Ve of Trf 2 to be used as GND and using the balance (-ve of Trf 1 and +ve of Trf 2) as positive and negative power supplies be a wrong way to go ?? I am saying this because I have got 2 nos 0-24 volts torroid transformers of 240 Va pulled from our company's junked DRS 300 computer (Yes Computer) manufactured by ICL (International Computers Ltd ??).

Regards,

Lifewater

 

[saheb11123]

Hi Saheb, Thanks very much for your prompt reply. Removing resistors from the mute pin and connecting directly to -ve would fry the chip. However after reducing value of the mute to -ve to 14K, the amp started working. Thanks once again Saheb for your response.



Hi Shiva, thanks a ton brother. Just did what you said and the amps started singing !!!. Surprised with the sound quality though. Would need to implement the power supply properly, would using 0-33 X 2 nos transformer and after seperate rectification using Bridge rectifier and connecting the +ve of Trf 1 to -Ve of Trf 2 to be used as GND and using the balance (-ve of Trf 1 and +ve of Trf 2) as positive and negative power supplies be a wrong way to go ?? I am saying this because I have got 2 nos 0-24 volts torroid transformers of 240 Va pulled from our company's junked DRS 300 computer (Yes Computer) manufactured by ICL (International Computers Ltd ??).

Regards,

Lifewater

Actually, I was trying to say was to put a 10uf cap in place of the mute pins, that would actually mute the speakers when the amp is turned on and will let it function after 1s/2s , this is what i use, but at the time of typing this, maybe i got sleepy and this didn't post.

The boards are not so good actually, I have the same boards , used them but they sound meagre, now I use a separate designed board which has lesser part count but is sonically superior. Ofcourse there are high-grade components.

 

[lifewater]

Actually, I was trying to say was to put a 10uf cap in place of the mute pins, that would actually mute the speakers when the amp is turned on and will let it function after 1s/2s , this is what i use, but at the time of typing this, maybe i got sleepy and this didn't post.

The boards are not so good actually, I have the same boards , used them but they sound meagre, now I use a separate designed board which has lesser part count but is sonically superior. Ofcourse there are high-grade components.

Ok get it, Saheb can you let me know more details of the implementation you are using now also any idea if the power supply method I have mentioned above would work ? what about two Laptop SMPS ??

Regards,

Lifewater

 

[saheb11123]

Ok get it, Saheb can you let me know more details of the implementation you are using now also any idea if the power supply method I have mentioned above would work ? what about two Laptop SMPS ??

Regards,

Lifewater

Laptop smps/adapter theory works, a FM linuxguru already has posted that (check the first few posts). Using two traffo would work, but it is too much of an hassle. I find it easier to use a single dual output traffo, i.e, 20-0-20 or of that sort.
For the Psu, I used non-regulated method, which does the job more than enough.

You don't even need more than +/-40v .
Use a traffo with dual output as I said and rectify and put large caps , and done. I also did a bit of snuberizing .
But I'm no expert, there are more experienced people on this forum, who can guide you..
Anything more you need just ask.

Cheers

 

[linuxguru]

Hi Shiva, thanks a ton brother. Just did what you said and the amps started singing !!!. Surprised with the sound quality though. Would need to implement the power supply properly, would using 0-33 X 2 nos transformer and after seperate rectification using Bridge rectifier and connecting the +ve of Trf 1 to -Ve of Trf 2 to be used as GND and using the balance (-ve of Trf 1 and +ve of Trf 2) as positive and negative power supplies be a wrong way to go ?? I am saying this because I have got 2 nos 0-24 volts torroid transformers of 240 Va ...

You're welcome.

2x 0-24 VAC Toroids should be fine, but 2x 0-33 is too high (assuming bridge rectification as stated) for the LM3886.

Laptop SMPSes should also work in most cases, but with lower power due to lower rails (typically +/- 19V..24V is feasible).

 

[saheb11123]

The smps's way will work great at the cost of reduced power output, but you will get compactness, less hassles and managed supply.

@linuxguru isn't it easy to just get a single traffo instead of two?
@lifewater 0-33 is too high and would result in +/- 70V , which is risky.

 

[saheb11123]

@linuxguru

I wanted to ask a question, which I forgot,
well is a shunted volume control better than non-shunted?

 

[linuxguru]

@linuxguru isn't it easy to just get a single traffo instead of two?

Sure, 24-0-24 will work just fine. The query was about 2x 0-24 toroidals, which should also work if the secondaries are connected with the right polarity.

Shunted volume controls: many people prefer the sonics, but I can't say how it will sound on an LM3886 gainclone. I haven't done A-B tests versus a conventional voltage-divider volume control.

 

[saheb11123]

A slight update, have started working on a cabinet , the protoype is with the wood working guys right now.
Will get on hand tonight.
once i receive it, i will post the worklog pics along with concept design.

 

[lifewater]

A slight update, have started working on a cabinet , the protoype is with the wood working guys right now.
Will get on hand tonight.
once i receive it, i will post the worklog pics along with concept design.

Lovely, waiting with batted breath for the outcome.... looks I mean. Best wishes,

Lifewater

 

[saheb11123]

Thanks.
Due to heavy raining here, the build is a bit delayed as I have to restock wires and clamps and other components and cannot do that cause of the rain. The woodworking guy had to go to his ancestral home he will return today night, will meet him tomorrow(maybe).



Here is a short description :


The size of the cabinet will be - 1.5ft x 1ft x 1/2ft . The cabinet will be made of wood and will be primered and colored. The top baffle and front baffle will be made of smoked black acrylic. the insides of the cabinet will have small lights which will give an ambient like effect to the cabinet when glowing. There will be holes for fan at the back and other connectors on the back. Primarily i will be using a alps pot for now but in future will add a remote volume control with display in future.



regards

 

[lifewater]

Well let me ask you all a few questions which I hope would force many of us to put on our thinking cap !!
1) Is powering an LM3886 amp using SMPS bad for the sound quality ? SMPS are of 24 Volts 2.1 Amps. Now I am using two of these (connecting +ve of one to the -ve of the other to get 0 Volts) without any problems BUT am not sure of the sound Quality through a transformer PS would be better.... how it is done and the problems you might encounter ... a bit later.

2) Now what is the bare minimum current (Amps) to be used for powering a LM3886 amp in my case 2X(24 X2.1) (say 2amps) = 96 Va and I am not facing any problems.

Would like to have inputs from you all as this would help many of us to try GC amps.

Regards,

Lifewater

 

[saheb11123]

To the better of my knowledge, I have heard that using regulated supply is actually better, given the fact the regulation is done properly. If the smps are of good quality then don't fret. But it also depends on the design on which you are implementing the design. 2*2 amps is okay enough but you will have to increase the current if you want it louder which low distortion. I have used a 20-0-20 and getting around +-30 acter rectification with 5 amps current rating so the distortion figure is low. And I don't push the music too loud though.

If You are happy with the sound then don't fret it is quite okay to power it via smps.

 

[lifewater]

To the better of my knowledge, I have heard that using regulated supply is actually better, given the fact the regulation is done properly. If the smps are of good quality then don't fret. But it also depends on the design on which you are implementing the design. 2*2 amps is okay enough but you will have to increase the current if you want it louder which low distortion. I have used a 20-0-20 and getting around +-30 acter rectification with 5 amps current rating so the distortion figure is low. And I don't push the music too loud though.

If You are happy with the sound then don't fret it is quite okay to power it via smps.

Thanks Saheb, the sound I hear is very satisfying ..... by no means I consider myself an audiophile but am used to hearing good sound. I have the Lyrita tube amp which I listen to regularly. Reason why I am trying to get answers from Gurus here is that there are a lot of members among us who are willing to build themselves an amp but are daunted with Power supply / traffo cost / complexity etc. Also somewhere you have mentioned that the ebay.in PCB is not that good so you are using an alternate desigh, can you please share it with us ?

Thanks once again.

Lifewater

 

[linuxguru]

SMPS for gainclones: Works fine up to modest power levels, i.e. anything below +/- 24V rails into say 8 ohm loads - which translates to ~3A peak current. You can get away with lower rated SMPSes if you use a moderate-sized electrolytic (~4700uF) on each chipamp rail to take care of peak excursions.

For higher power, the price of SMPSes becomes higher than conventional EI trafos, so YMMV. They're still lighter and more compact, so the tradeoff is still viable.

Chipamp PCBs: Search for MyRef Rev C and MiniRef 1875 layouts - there are several different layouts, and various configurations (monoblock, stereo, etc.) and tradeoffs. At the moment, the MiniRef 1875 with dual SMPS is the most compact audiophile-quality medium-power (2x 20W) chipamp solution in the world. There are smaller implementations possible, but they're Class D; and there are higher power implementations possible, but they're much bigger due to the heatsink & trafo requirements. The topologies of both are more complex than basic gainclones, so they aren't recommended as first projects.

 

[saheb11123]

@linuxguru if possible you can help out lifewater with your myref pcb's , given you supply a whole kit, and also advice on the power supply he will be pleased..




Regards
saheb11123