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Old 22nd June 2014, 10:08 PM   #1
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Default WTbuild 5 ch. smallest audio amp

Hi guys, I'm looking to build the smallest 5 channels audio amp for a home theater system.

I have decent regular speakers (4 x KEF Cresta II and 1 GALE center) and a AE dolby decoder.

Now I'm using a huge H-K home theater amp. which is not passing the wife test after renovating my living room...

So I would like to build a decent somehow 15 to 20 watts per channel 5 channels amp. I've been working and fixing many vintage stuff for some years now and am perfectly able to build this kind of amp. (I've MOSFETTED a H-K CT12 per Mr Nelson Pass design using old Hitachi MOSFET and am very happy with the results BTW).

I'm looking for a good design that would be small. That brought me to the chipamp area. I have many toroidal trannies, bridges, capacitors, etc. Power supply is not a problem. I'm just looking for the amp section. As I wrote, I'll be using an AE dolby decoder who will serve as preamp. No treble neither bass control. And I'm not putting in a sub. so bass will have to come in front L + R speakers.

I've tried looking into this section but thought I could save time asking directly.

Thanks for any help!!!
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Old 24th June 2014, 05:36 AM   #2
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For 20watts you could do some Chipamp's or you can do a current boosted opamp design like I have done here,

opamp plus driver stage and output stage with its own supply, what could happen?

Although that one is not a final design but it worked great!!

If you wanted extreme performance at 20watt's there is a design in these threads that uses something like 4 or 8 LT1210 that are in parallel, this would be costly though.

Here is a great circuit I made using a LT1210,

How to measure capacitor impedance graph

A Mini Aleph Class A amp comes to mind including Elvee's Class A amp that uses LM317/337 regulator chips is a possibility for that power range.

I once had a small comercial 5ch. HT amp that use the TDA2030's or TDA2040's in it and it sounded okay.

The highest power TDA2050 is a 32watt (50watt) amp in that series.
It just went out of production but there are plenty to be found inexpensively.
It is in a nice and compact 5 pin TO-220 package.

The possibility's are endless.

Maybe some thing like A MY-Ref amp would be good as it is in the 50 to 70 watt range, it uses the LM3886.
The LM4780's are dual Lm3886's for less space would work good.
A My-Ref design could be made with the LM1875 20watt chip as well.

Just a few thoughts,
FWIW

jer

Last edited by geraldfryjr; 24th June 2014 at 05:49 AM.
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Old 24th June 2014, 12:55 PM   #3
Mihkus is offline Mihkus  Estonia
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That is a quite efficient and good sounding class AB amp indeed.
It can be made very small. I even have PCB designs that could fit your needs.
Got lots of measurements of THD IMD and so on. But i have to find them first.
I have currently listening to this: Posr#42 opamp plus driver stage and output stage with its own supply, what could happen? But I used NJW0281G / NJW0302G as outputs and MJE340/50 as drivers.

You may try this outputstage too because this one is ultra efficient class AB and delivers 1W at 0.0007% THD+N
The design belongs to Mr palstanturhin.
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Old 24th June 2014, 01:04 PM   #4
sangram is offline sangram  India
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LM4781 has three amplifier per channel, roughly equivalent to 3xLM1875 in a single package.

I think two of those will be enough for an HT system, the additional channel can be used for the .1 channel.

With decent sinks, you should get about 25W per channel out of them. They say 35, but I wouldn't bet on it. Keep the rails below 25V if possible for 6 ohm speakers, for 4 ohms you may need to drop it to 20V.

No PCBs are available so you have to roll your own. There's a nice Italian site with a single-sided design.
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Old 24th June 2014, 03:17 PM   #5
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Well thanks a lot!

After quick looking, the LM4781 seems the smallest and easiest way to go... Thanks sangram!

On the datasheet here:http://www.ti.com/lit/ds/symlink/lm4781.pdf they show a double sided PC board layout. Can one of you tell me where I could have 2 made?

And Mikhus, can you show me how big is your PCB? Can be quite small too...

But I'm still open to opinions! Ain't decided yet...

Thanks again!

Last edited by electrochap; 24th June 2014 at 03:19 PM.
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Old 24th June 2014, 04:38 PM   #6
Mihkus is offline Mihkus  Estonia
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45x63mm
Its quite large compared to LM4781 but has better perfomance and is well balanced.
I would like to see that single sided board for LM4781. How could you possibly fit that on single sided board?
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Last edited by Mihkus; 24th June 2014 at 04:41 PM.
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Old 24th June 2014, 04:59 PM   #7
sangram is offline sangram  India
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^^ here it is: Servisystem • Ver Tema - LM4781 - 3 X 35W (L + R + Sub Woofer) [DIY]

I'm quite amazed at it too, and can't verify that it works. I had a buddy print out a board but haven't yet tested it.

There's a link to a pdf.

I might want to add that these chips - LM4780, 4781, 4766, with more than one amp in the package - can get super toasty. The board may be small, but don't assume the heatsinks will be tiny. If you want it really compact, you'll get more savings by using lower voltage and smaller sinks than trying to find the most physically compact board.
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Old 24th June 2014, 07:23 PM   #8
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Yes, for a small footprint and high quality you can't beat the current boosted opamp design.
And you don't need anymore than a +/-18v power supply either for 20 watts in to 4 ohms.
Although it does require a 17.88Vpeak signal in to 8 ohms so a +/- 22V supply may be in order.

Make sure you use an opamp rated for that high of a voltage as there are many to choose from in dual and quad packages for that range.

A bridged configuration is another way to get a higher voltage swing to the load with a lower supply voltage as well.

Take a look at this LM1876 My_Ref design,

MiniRef Schematic and PCB layout

Read through the whole thread as there might be more later revisions.

jer

Last edited by geraldfryjr; 24th June 2014 at 07:31 PM.
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Old 24th June 2014, 07:52 PM   #9
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The nice thing about the current boosted opamp designed is that you can bias the output stage high so that it runs in to the class A region for the First watt or so.

jer
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Old 25th June 2014, 03:59 PM   #10
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Hey Mihkus, the LM4781 use a double sided board. Not single.

Do you have boards of this cct. made I could buy ? I tend to have more confidence to regular NPN/PNP stuff... Strange in the chipamp section...

If I use the LM4781 I sure would use a heatsink wall for it... with some of this nice white paste.
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