Help Needed on DIY LM 3886 based 5.1 Channel AMP for PC Home Theater - diyAudio
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Old 22nd September 2013, 09:47 AM   #1
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Default Help Needed on DIY LM 3886 based 5.1 Channel AMP for PC Home Theater

Hello guys. I am from India.

Have been following DIYAudio since long time and now feeling great to be part of this community and here goes my first post.

I use a PC for my home audio/video needs. Its an Asus M5A97 based PC and right now I am using onboard ALC 887 sound card.

Currently I am using a 6-7 years old DIY LA4440 PCB X 3 = 6 channel AMP to feed my speakers. Current AMP is somehow too much noisy so recently decided to upgrade the AMP.

After reading a lot, decided to go with LM3886, beautiful chip, isn't it !

Found a vendor who has readymade LM3886 PCBs so I bought one AMP kit from there for the testing purpose. Also wanted a more powerful sub AMP so got another 200W AMP kit available at there.

Here is the list of Items I got right now:

1. LM3886 AMP MONO board = 1 unit
2. Dual rail PSU = 1 unit
3. Speaker protection Module = 1 unit

4. 200W Mono Toshiba 2SA1943 / 2SC5200 = 1 unit
5. Active crossover 110hz for sub = 1 unit.

My Queries:


I am noob in the electricals. Ealrier I took help of one radio mechanic to assemble my previous AMP and he knew what is needed and what is not. But now I want to do it myself because its hard to find a good radio mechanic who does the job neatly. I want to make a noise free clean AMP.

Here are my concerns and confusions:

1. Transformers/Power supply :
This is crucial part to chose right transformer/power supply. I tried to read a a lot but it made me more confused.

My requirement is to make a power efficient AMP so that it doesn't keep consuming power all the time, especially when its not used. My usage is such that I turn on the AMP in the morning and turn it off in late night. So power consumption matters to me.

Another thing is I want separate power on/off switches for some channels. eg- I want to keep the front two channels separate from the rest of the channels so that if I want to enjoy stereo, I can turn of rest of the channels including sub.

Now how to know what transformer I need?
Do I need separate transformer for front two channels and separate for center+rear which I want to keep separate from the fronts. Also the SUB AMP's power ratings are different so do I need to use different transformer for the SUB AMP? If so then I will keep the sub amp in a separate cabinet.

Isn't there any easily substitute of the transformer something like computer's SMPS?

Most probably I will go into the market at some transformer vendor to get right transformers for me so any suggestions made today will be really helpful for me.

Overall, What I need to say to the transformer vendor so that he gives me right transformer which doesn't fry my kits.

2. Power Supply Unit (PSU)
I got one PSU with my kit. Now I am not sure if a PSU is really needed or not in case of LM3886. After reading a lot on LM3886 based AMPs I learnt that sometime people use PSU and sometimes don't so confused here. Also my PSU specs are not matching with the LM3886 power specs, here are PSU specs written on the vendor's site :
  • 12-0-12VAC 1A Input
  • 18-0-18VAC 1A Input
  • 27-0-27VAC 5A Input
  • Regulated 12-0-12VDC 1A Output
  • Regulated 18-0-18VDC 1A Output
  • 38-0-38VDC 5A Output
This is confusing for me.


3. Speaker protection module
I also got a speaker protection module. Don't know hot and where to use it. Also not sure if its really required to use a speaker protection module with LM3886 based amp boards.


4. SUB AMP
First of all what transformer I would need for this 200W sub amp board?

How to use the crossover I have? It has fixed cut off at 110hz but there is a gain controller on the crossover board. Is it good enough for me?

How to feed power into this crossover?



Do I need to use some PSU and speaker protection kit with this sub AMP kit?


I am a noob so please explain everything in easy to understand language rather then using some technical words


Seeking for your help


Thanks in advance.
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Old 22nd September 2013, 10:12 AM   #2
AndrewT is online now AndrewT  Scotland
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Do you have 8ohms, or 6ohms, or 4ohms, or 4 to 8ohms speakers.
That will determine the maximum transformer voltage the chips will accept for reliability.

One transformer powering all the channels will consume less power than multiple transformers.

Increasing the number of primary turns reduces the power losses in the transformer. This also allows you to make small voltage reductions of your PSU output voltage.

The 12-0-12 1A and the 18-0-18 1A transformers will ruin the audio quality you can get from the chips.
The 27-0-27 5A transformer may blow up your chips.
Measure the transformer and post the results. Check your speakers and post the results.
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Old 22nd September 2013, 11:04 AM   #3
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Thanks a lot for the replies Andrew.

Quote:
Originally Posted by AndrewT View Post
Do you have 8ohms, or 6ohms, or 4ohms, or 4 to 8ohms speakers.
Right now I have 4 ohm and 4-8 ohm rated speakers/tweeters. Some are 40 W and some are 150W. My speakers are 8"-10" and consume lot of space so maybe I replace them sometime later with smaller and more efficient speakers. Chances are that I get 8 ohm speakers later but its not certain.

Quote:
One transformer powering all the channels will consume less power than multiple transformers.
And I was thinking opposite that multiple smaller transformers will be better in terms of efficiency since I want separate on/off for few channels.

Quote:
Increasing the number of primary turns reduces the power losses in the transformer. This also allows you to make small voltage reductions of your PSU output voltage.

The 12-0-12 1A and the 18-0-18 1A transformers will ruin the audio quality you can get from the chips.
The 27-0-27 5A transformer may blow up your chips.
Measure the transformer and post the results. Check your speakers and post the results.
I don't have any transformer right now, I just want to know what exact transformer I need so that I can get it from the market.

Isn't the PSU I got useful for me here? Or I don't need a PSU at all in first place?
What about the speaker protection module I got?
Also there is another sub amp kit which has different power ratings. I would also be using the crossover which also needs some power. Not sure what to use here?

EDIT:
Adding some pics of the kits I got:

20130921_144259.jpg
Description left to right:
1. Active crossover @110hz
2. 200W Toshiba 2SA1943/2SC5200 mono amp for sub
3. LM3886 mono board
4. Speaker protection module
5. Power supply module, here is the pic:

20130921_144840 PSU.jpg

Last edited by Jack Sparrow; 22nd September 2013 at 11:23 AM. Reason: Added pictures and spell correct
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Old 22nd September 2013, 11:47 AM   #4
AndrewT is online now AndrewT  Scotland
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A 22Vac + 22Vac transformer is probably what you need for your 4ohms speaker loaded chipamps.
6 channels @ around 50W equates to very approximately 300VA of transformer. This size is not critical, you could probably get away with anywhere from 200VA to 2000VA
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Old 22nd September 2013, 01:07 PM   #5
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Quote:
Originally Posted by AndrewT View Post
A 22Vac + 22Vac transformer is probably what you need for your 4ohms speaker loaded chipamps.
6 channels @ around 50W equates to very approximately 300VA of transformer. This size is not critical, you could probably get away with anywhere from 200VA to 2000VA
But the specs sheet of LM3886 board saying +/-28v for 4 ohms load.

What is the needed ampere here? When I emailed my amp kit vendor, he recommended 650VA 12 to 13 ampere for 5 X LM3886 and 500-520VA / 9A for 200W Toshiba kit. And you are saying just 300VA. Which one to pick?

My major confusion is what I need to say to the transformer vendor when I go at there so that he can give or prepare me the right transformers for both applications. Also, I would need 18-0-18VDC 500mA tapping for the sub crossover so how to get that? Will there be any provision in the sub transformer?

This is main power supply for the AMPs so a crucial thing and I am worried about it. More I read on the transformers, more I get confused.

First of all I want to buy all the required transformers, will step into next stage after getting all the transformers.
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Old 23rd September 2013, 05:55 AM   #6
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20130923_111932.jpg

This is my laptop's power adaptor. I am curious why can't i use something like this?
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Old 24th September 2013, 04:26 AM   #7
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Location: India
Hi JACK,

The whole idea of 5.1 seem cool..

You have also started collecting part.
That power supply PCB is not good for LM3886 x 5. You need good caps to product nice sound from LM chips..
650VA and 12 Amp is totally overkill for 5 x LM 300AV will be sufficient. And if you are planning of power manage each channel,, then go for 5 small transformers with 5 separate PSU and switches.
Subwoofer amp can be mounted inside the sub box.

I am from Jodhpur Rajasthan and have made 7.1 AMP, if you need any help can call me.

Link of my latest project..
(Case Mod) HTPC 7.1 AMP | TechEnclave - Indian Technology Forum

Regards
Rakesh Sharma
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Old 24th September 2013, 10:53 AM   #8
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Quote:
Originally Posted by Jack Sparrow View Post
Attachment 372700

This is my laptop's power adaptor. I am curious why can't i use something like this?
You have to be sure that the -ve & earth are not joined internally ( they are joined in case of PC SMPS) i.e. they are isolated . I've seen laptop adapters (4 units) used in series to generate +/-38V on diy sites but don't have link.

SMPS for the Gainclone chip amp

Laptop power supplies for amps

I'm using industrial grade SMPS +/- 21V 5A for my LM1875 chipamp. No audible RF or hum problem. Got it cheap directly from manufacturer.

BTW you will require 2 adapters(preferably same make/specs) in series to generate +/- V.
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availlyrics

Last edited by availlyrics; 24th September 2013 at 11:20 AM.
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Old 24th September 2013, 11:07 AM   #9
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A transformer is almost 95% efficient, very little power is used to simply magnetise the core.

If you were to use a single transformer into multiple rectifier / capacitor banks, the individual amps could be switched on and off at the input to the rectifiers.

When two amps are being used, the transformer will only be using enough power to satisfy the two amps plus a small amount of power of its own.

When you turn on more amplifiers the current draw from the single transformer will increase and so will its draw from the mains supply.

Give-or-take a few tens of mA for the hysteresis of the transformer and losses within the windings and the core, if you provide a 10VA load to a 500VA transformer, you would only expect to use slightly more than 12VA from the mains supply.

The exact figures depend on the transformer, but what I am saying is you only need one big transformer.
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Old 24th September 2013, 08:04 PM   #10
Minion is offline Minion  Canada
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Yes you need a PSU , almost everything needs a PSU .. Your standard PSU will consist of a power transformer , a Bridge rectifier (to convert the AC from the transformer to DC) and Capacitors large enough to supply enough low ripple dc to the amplifier(s) .....
The PSU board you posted might work accept for the fact that
A) the bridge rectifier is too small for 5 LM3886 and the 200w sub amp.
B) there isn't enough capacitance and not enough space on the board to install more/larger capacitors. (though it would be useable to power your crossover but a bit overkill)

The non-scientific rule of thumb I use is to use at least 4700uF per rail for every 50w power output though I usually use 10,000uF per rail , so a PSU with 40,000uF to 50,000uF of capacitance would be suitable ....
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