New design help

[tangmonster]

Hi guys. I am n00b in the diy audio. I have been planning my living room sound system for a while now , and currently have kind of a plan of action. Have spend a lot of time reading the very interesting threads on this forum.

My needs:

a 5.1 system for my living room which will be connected to the audio outs from my mediacentre entertainment pc.

I am tackleing the 5 of the 5.1 stage first. the sub .1 stage i'll tackle after this is done.

So I need 5 channels.

Bear in mind that i live in a small flat currently my 1982 pioneer sa420 amplifier with mid 80's sanyo speakers are actually more than loud enough but i want better. which probably is like 20 to 30W per channel. I don't want to spend huge amounts of money on it.

My current design:

Amplifiers:

Each of the 5 channels will use a lm3886.
Each in a seperate small enclosure.
I will use 8 ohm loads for each channel.

Power:

I want to use one seperate powersupply case. with one powersupply to supply all 5 channels. I want to use a 500VA 25-0-25 toroidal, with one supply resembling this layout:

http://www.diyaudio.com/forums/attachment.php?s=&postid=499002&stamp=1098741354


Questions:

1: For my use at 8ohm loads will one 500VA toroidal do the trick? bearing in mind that i probably won't be driving things that hard and at 30 to 40 Watts per channels max.

2: how big should the filtering caps be for this one supply setup will 2 x 10000uF per voltage rail be enough or will I see much better results with another one or two even for my moderate listening levels?


Any input will be greatly appreciated.

Heinrich

[Minion]

Hi, 500VA will be plenty for five LM3886 chips.....

I generally use 10,000uf per rail per chip , so for a 5Ch system you would need Ten 10,000uF caps, you can use less but I don"t think 2 will do it......

Be very carefull with grounding when useing multiple channels off of the same supply....and you are going to need a bridge rectifier that can handle more than 10 amps......

[AndrewT]

Hi,
you need long cables to get your sound around the room.
Either long speaker cables to reach from the central amplifiers to the speakers.
or
long PSU to amplifier cables to reach from a central PSU to amplifiers located beside each speaker and long interconnects to feed each amplifier.
or
long interconnects to reach from the central 5.1 processor to the each amplifier with it's own PSU.
I favour this last option.

More on the smoothing capacitance. If you want good low bass from your speakers then that requires F-1db <=4Hz. This demands smoothing of +-20mF per 8ohm channel. Using +-10mF moves the F-1db ~=7Hz. The surround channels may not require this wideband response if you are using a sub-bass or bass speaker and amplifier, but the front channels may benefit from very wideband response, particularly if you use them for stereo.

[tangmonster]

Thanks a lot guys for your input ,I appreciate it.

I know there are better ways of doing it. But i will use long speaker cables from centre amplifiers ( want to make a bit of a mission to make cool looking little amplifier enclosures.)

I am going to have a seperate low frequency amplifier and speaker , so yes most of the time the surround speakers won't handle the low frequencies, But i will also be using it in a stereo configuration.So the two front speakers/amplifiers must be able to handle it.


Minion , I you are probably right , i'll rather fork out the cash and get 10 x 10000uF from the start and be sure.


I realise about the 10 amp bridge rectifier diodes.

Minion , about the grounding , if you can give me some quick tips it would help a lot.

In my mind I see a 3 prong connector on each amplifier for power and an rca audio connector not connected to chassis running straight to lm3886.

Will 100uF caps be ok next to ic's ?

Heinrich

[AndrewT]

Originally posted by tangmonster
Minion , about the grounding , if you can give me some quick tips it would help a lot.

In my mind I see a 3 prong connector on each amplifier for power and an rca audio connector not connected to chassis running straight to lm3886.

Can you design the PSU for the worst case failure condition and ensure that mains voltage can NEVER be fed down the DC power umbilical to the amplifier/s?

If you can't see the significance of this senario then you must safety ground each of the amplifiers as well as the PSU.

Once you do that you may be plagued with numerous ground loops.

[pmchoong]

Hi Tangmonster

I am trying to do something like yours. I have a 500Va, 25-25v transformer feeding 5 separate Peter Daniel's LM3875 amps.

The problem is the buzzing sound at the speaker when all is connected. How do i get rid of this and is it damaging to the speakers?

Thanks. Any help is very much appreciated.

[pacificblue]

If you only want

Originally Posted by tangmonster

like 20 to 30W per channel.

why the

Originally Posted by tangmonster

25-0-25 toroidal

that will bring you nearly 60 W per channel?

Originally Posted by tangmonster

1: For my use at 8ohm loads will one 500VA toroidal do the trick?

Yes.

Originally Posted by AndrewT

long interconnects to reach from the central 5.1 processor to the each amplifier with it's own PSU.
I favour this last option.

Me too.

Originally Posted by AndrewT

F-1db <=4Hz. This demands smoothing of +-20mF per 8ohm channel. Using +-10mF moves the F-1db ~=7Hz.

How does the smoothing capacity change the frequency response?

Originally Posted by pmchoong

The problem is the buzzing sound at the speaker when all is connected. How do i get rid of this and is it damaging to the speakers?

It usually does not damage the speakers, except when it is extremely loud, but then you would not be able to listen to music at all.

To get rid of it is not always easy. There are many books dealing with that topic and many threads here in the Forum as well.

If your schematic reflects reality, you are using wires to connect the cases with each other, but there is no separate ground wire as return for the power supply. Do you use the same wire for those two functions at the same time? In that case this is the first thing to change. The wires that connect the cases should not carry current. Add separate wires for the power ground.

The next thing is to make sure that the grounds of RCA in and speaker out are isolated from the case.

Then try, if the hum is already present with no source connected or if it only appears with a certain source. The usually suspects are radios or TVs that connect to an antenna. If you can trace the hum loop to such a device, a ground loop isolator will help. That is a cheap device and can be plugged in between the antenna and the radio or TV. MONACOR INTERNATIONAL:New products

Another thing that is worth trying is to swap the polarity of the transformer's primary. Sometimes that reduces the intensity of hum, too.

Then there is the possibility to isolate the audio ground from the rest with a resistor. Up to 10 Ohm should do. You can see an example in the LM4780 datasheet's reference artwork on page 22 labeled as Rg.

[pmchoong]

Originally Posted by pacificblue

If your schematic reflects reality, you are using wires to connect the cases with each other, but there is no separate ground wire as return for the power supply. Do you use the same wire for those two functions at the same time? In that case this is the first thing to change. The wires that connect the cases should not carry current. Add separate wires for the power ground.

Yes, I use the chassis gnd for the power gnd as well. Now, if I separate these, then there will be two wires running back to the PSU star gnd. Is this what you are suggesting?

I have checked....there wasn't any rf interference from near by electrical appliances.

Thanks

[pmchoong]

Is this what you recommended? (Please note the top amp as all the rest will be the same)
thanks again

[tangmonster]

Hi PMchoog , the biggest difference between your amp and mine is that i have ONE big rectifier and 4 x 10000uF capacitors per rail in the power supply enclosure , so i run DC between the power supply and separate amplifiers.

Thus there is no high current alternating current flowing in wires but steady DC.

this might one of the problems.

Here is a question. (other posters please help to see if my logic works)

In the few seconds after you switch off your power supply you should be able to get music from the speakers. possibly only 1 or 2 seconds depending on capacitor sizes.

my question is. When you switch off the power switch does the hum disappear immediately or does it fade away softly.

My common sense say this will prove if it is high AC current in power wires inducing hum.