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Old 16th October 2012, 08:34 PM   #1
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Join Date: Oct 2012
Default Help needed on a new DIY amp project

Hi,
I am new to this forum and this is my very first post. I am kind of a newbie in eletronics and I am willing to do my very first pcb amp project.
I would like to realise a complete project with the following parts according to the research that I have done on the threads of this forum and on the internet :

- power supply bases on a toroidal transformer
- Tone control based on a LM4610
- active 2.1 filter/crossover+summer
- 1 dual amp for the mid/high and 1 mono amp for the subwoofer

One chip that I have chosen and which seems to be very spread on the internet is the LM3886 which offer quite the power that I am looking for, with its cousin the LM4780. For my first project, I am not sure if I am willing to print my own pcb or at least the minimum so those ones are sold on ebay with the pcb for nothing.
First I would like to know what do you think of this project.

And secondly and here is where my small knowledge of electronics stops me is for the power supply. I have read that a symmetrical power supply is always better for audio amp as well as toroidal so I came to the nearest electronics repair shop of my house and found a beautiful toroidal transformer for my project attached to a subwoofer amp plate for 20bucks. The problem is that the amp needs to be repaired and is a numerical one so no need for me to try I give up already and willing to build a full one my own.
But I think that the toroidal is too powerful for waht I am thinking to do, here it is :

Click the image to open in full size.

it is a 49.5-24-0-24-49.5
so willing to build a symmetrical I would take the 49.5 and the efficient power would be 49.5*1.41=70v
70v is too high for the lm3886. I also found the LME49830 and LM49811 but the schematics seem a little bit complicated with another supply for the regulator which I do not really understand so I am willing to stay with the LM3886 if possible.
So my questions are, what would you do with that transformer ? Here is what I have thought of :
- using a single supply for every LM3886 so I would use 2 amps on every 49.5v powered by 70v approximatively
- create a symmetrical power supply of 84/2= +/-42 volts, the maximum voltage allowed on the lm3886 from the 49-0-49 with some regulators but I think there will be a lot of heat
- splitting every 49.5 branch into a +-35v symmetrical power supply according to these scheme (need to adjust the components)
How to build Symmetrical from a single voltage supply - circuit diagram
- looking for another amp or power supply ?

For the rest, the 24-0-24 could be used to do a 15-0-15 for the tone control and pre-amp and I can reuse a lot of things on the subwoofer plate like the crossover and some capacitors (the big ones)

It is open for suggestions and I really need some help on this from guys better with eletronics ! Thank you
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Old 16th October 2012, 09:17 PM   #2
sreten is online now sreten  United Kingdom
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Join Date: Nov 2003
Location: Brighton UK
Hi,

The windings do not appear to be separate. Normally I'd suggest normal
chips amps for left and right 4 ohm speakers and two chip amps bridged
for an 8 ohm subwoofer all off the same simple dual supply.

I'd really try and fix the sub-amp and use that as the .1 amplifier.
And work out what the 24V taps are for, measure DCR of all
sections to see if they are all the same winding resistance.

Still even with +/- 70V rails you could run an AC coupled chip amp off
each rail, reducing input capacitance for 1st order high pass and lowering
the output coupling cap values to what you actually need for satellites.

rgds, sreten.
__________________
There is nothing so practical as a really good theory - Ludwig Boltzmann
When your only tool is a hammer, every problem looks like a nail - Abraham Maslow
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Old 17th October 2012, 01:20 PM   #3
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Join Date: Oct 2012
You're probably right, I will try today to repair the amp but still a problem, it's a high-input amp so I will still need a mid/high amp preferably on the same transformer so I hope the sub amp doesn't pump all the power.
I will take a look and keep you posted, thank you for your response.


I also found this amp kit :
L25D kit IRS2092 IRFB4020PBF
which works on a +-70v, anyone has had good returns on it ?

Thanks
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Old 19th December 2012, 02:10 AM   #4
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I have the same amp pictured and it's busted. Have you made any progress in fixing it? If so, can you share your troubleshooting process? I verified that the transformer was fine. C60 was bulging so I replaced it only to have C59 pop and spit oil all over the place.
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Old 19th December 2012, 09:37 AM   #5
jaycee is offline jaycee  United Kingdom
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Location: Norwich, UK
For a beginner project, I would advise sticking with the chipamps like LM3886. Discrete amps, even in kit form, are really for when you've got a bit of experience as you may need to troubleshoot and test. I started with a chipamp myself.

The transformer in the sub amp looks like it is wound for multiple taps. As it is just a sub amp, this hints at a more complex design such as class G. These can be complex to troubleshoot.

I would not recommend building an amp using +/-70v supplies as your first amp.

Are you absolutely sure on your requirement for 2.1 ? Usually this is used where the desire is to either a) be cheap or b) use small "satellite" speakers. A nice set of bookshelf speakers will deliver a good amount of bass, if the size is not a problem.
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Old 17th January 2013, 01:42 PM   #6
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Join Date: Oct 2012
I came back on this project and desolder every part needed for a good Power Supply Unit which includes :
- the toroidal transformer 49.5-24-0-24-49.5
For the high voltage part:
- a GBU606 Bridge rectifier
- two 6800uf 80v caps
For the low voltage part:
- a HD04 bridg rectifier
- two small caps

I will try to plug that to obtain a stable power supply and test how much volts I got from that and continue to look for a good +-70v project or maybe a dual 70v single supply!
I gave up on trying to fix the amp since my local eletronic repair shop did not suceed.
I keep you posted !
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