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Old 29th August 2006, 01:12 PM   #1
Dxvideo is offline Dxvideo  Turkey
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Default Dual LM4780 Kit Supply Question

Hi all,

I want to buy the AudioSector's LM4780 Amplifier Kit (non-inverting) kit (it includes two stereo channel amp.) but i'm not sure that what kind of transformer i will need for them.

I will build a good 2+1 system with them and i expect to have 120W + 2x60W RMS power from this system.

Please help me to determine a correct transformer for that system. Thanks a lot in advance for your helps and best regards,
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Old 29th August 2006, 02:48 PM   #2
traw is offline traw  United States
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one board providing 2 channels (4780 in stereo) and other paralleled?

what's the target impedence of the 60w recipients? could use 28v rails if 4ohm but the parallel config would likely be 80-90w ... however, would just be wiser to just run 3 parallel boards if 4 ohm, despite having to likely aquire 2 sets/4 boards, on higher voltage like 33-35 volt rails.

rule thumb i've seen around is 80va per logical chip. so, 4x80 gives 320 so around 300va would be about right. others will say more and others say good results with less (as I). a 400va in various voltage offerings is often available via antek on ebay and gives plenty of comfort, around 50 bucks shipped. image one could get by with smaller transformer if scouring used or surplus market but getting a good flexible transformer, capacity wise, will save mental anxiety later - not sitting there wondering "gee, sound better with bigger transformer"

back to the voltages... if running 8ohm for the 60w, get 23-26vac dual seconardary or 45-50vct. if running 4ohm, get 18-22 vac secondaries or 36-44vct (but your parallel config will be closer to 80-90watts)
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Old 30th August 2006, 05:55 AM   #3
sangram is offline sangram  India
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Like traw said, try and first figure out how you want to get the 120 watts for the .1 channel

Two parallel amps will deliver 120 watts into a 4 ohm load at 35 volt rails.

Two bridged amps will deliver 120 watt into 8 ohm at 27-28 volt rail, more than will cause overload.

Of course, these voltages will determine how much power you get out of the stereo amp, unless you plan on a separate transformer for them.

I use two of the boards Peter sells, for total 8 channels in bridge-parallel, and I use 100 VA per chip half (800 VA total). I would say you can probably get by with 80 VA, but 120-150 VA would probably be better.
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Old 7th September 2006, 06:38 AM   #4
Dxvideo is offline Dxvideo  Turkey
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Thank you for your replys.
I plan to use first board in parallel and of course the driver will be 4 ohms, so i need to have 120W rms from that rail. The other hand for stereo channels i will use 8 ohm drivers per channel.
The question is; with same power supply (for ex: 2x35v DC) may i supply all this channels?
The other question is; what secondary voltage i will need for that DC voltage?
The third question is; what power will i need for transformer?
Thanks a lot,
With my best regards,
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Old 7th September 2006, 07:06 AM   #5
sangram is offline sangram  India
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You may use one power supply of +-35 volt to power all the amps, without any danger to the chip. Each chip in the parallel board will see and effective 8 ohm load, so the load on all 4 amplifiers is the same.

To get 35 volt rails, you need a 24-25 volt transformer secondary voltage.

I would feel safe running 4 amplifiers from a 400-500 VA transformer. At the lowest limit, 300 watts is probably bordeline. more than 600 watts would be overkill IMO.
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Old 7th September 2006, 12:40 PM   #6
Dxvideo is offline Dxvideo  Turkey
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Thanks a lot for your reply. I'll buy a 450W and 2 x 24v sec. transformer. But i have another question;
In Audiosector's web page there is a part list of 2xLM4780 kit. But in list ican see only 1500F big caps. Do you know if they are for power supplys stabilising capacitors or the LM's supply pin caps?
In that case (if theyre power supplys stabilising caps) is it enough for power supplys ripple rejection?
Or if theyre not; should i use a couple of bigger (f.ex; 10,000F) stabiliser cap?
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Old 7th September 2006, 02:02 PM   #7
sangram is offline sangram  India
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Let me put it this way.

The 10 uF + 1500 uF is enough for me.

I tried bigger caps, didn't help (actually the sound got worse). Snubbers didn't seem to fix it.

I finally settled on 4700uF on the rectifier PCB and 1500 uf on the amp boards.

AFAIK the sound with the first version was way better, in line with Peter's observations. With bigger caps some of the midrange 'magic' is lost.

I'm not changing them now, I guess I'll rip the boards apart if I do any more (de)soldering on them. Already most of the tracks on the rectifier are peeling off.
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Old 8th September 2006, 06:10 AM   #8
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So you say, i can use a 4700F for rectifier and on amp board 1500 + 10 is ok?
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Old 8th September 2006, 07:57 AM   #9
sangram is offline sangram  India
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No, use 10uF on the rectifier and 1500 on the amp board.

If you don't like the sound, try replacing the 10uF with a 4700 uF.

I started with a 10uF and IMO it was the best sound of all the variations. And not light on the bass either, just different. I guess it depends on your speakers and how you like your music.

Out of pressure I moved to 10,000 uF and now 4700 uF. I haven't touched the caps on the amp board throughout.
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Old 8th September 2006, 10:52 AM   #10
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Really weird!!!
I used to use big cap on PSU and smal one on amp board... Until now!
I will try that. And thanks a lot for your helps.

With my best regards,
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