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Old 7th July 2013, 08:01 PM   #21
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I have another question.

At present I am using only one transformer for all the 6 set of LM3886 chips.

I just added the 3 way active X over with a 100K dual volume control in front of it.
The X-over is getting its power from a 20-0-20 Vac tapping from the same transformer.
It has a built in +-15V regulated supply on board which is giving +16.8 and -16.2 Vdc.

I'm using an Mp3 player with some 320 kbps test samples. I tested with some Flac Samples too.

When I removed the Lamp in series I see the following.
The big heat sinks can handle the heat very well now.
The output is very clean and in fact way more than enough for my listening room.
It even damaged a tweeter with its high output due to a silly mistake of mine.

Bass response is very very good. Excellent Mid range. Great sound stage/ stereo imaging.
No hum, hiss or any kind of distortion.

The transformer alone can handle all the amps with X-over for my personal requirements as it seems .

Do I still need to use the other transformer and the those 6 X 4700 uFd caps for the second channel?
Will there be a great difference in sound quality even a low volume if I use the individual PSU for each channel?
Kindly explain.
Thank you.
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Last edited by csom; 7th July 2013 at 08:06 PM.
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Old 7th July 2013, 08:25 PM   #22
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The main transformer is giving an output of 26.6-0-26.6 VAC @ 4 Ampere
And +-33.5 VDC after rectification and filtration with 6 X 4700 uFd caps ( when all six set of LM3886 amps are connected).
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Last edited by csom; 7th July 2013 at 08:39 PM.
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Old 8th July 2013, 03:53 AM   #23
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Do I still need to use the other transformer and the those 6 X 4700 uFd caps for the second channel?
Depends on whether you plan to house the electronics in the speaker boxes or not.
Quote:
Will there be a great difference in sound quality even a low volume if I use the individual PSU for each channel?
No, and anyone who says otherwise has an over-active imagination.

I will post pics in the next few days. They are not attractive but that isn't important.

Last edited by Absconditus; 8th July 2013 at 03:55 AM. Reason: Wrote another line.
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Old 8th July 2013, 04:45 AM   #24
gootee is offline gootee  United States
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There shouldn't be a GREAT difference in sound quality just from using one transformer instead of two. There might be a little.

But you will probably only be able to get 50 or 60 watts output, TOTAL, with only one of those transformers. Amplifiers like these are usually not too much more than 60-something percent efficient. The rest goes out through the heatsinks. And only one rail plays at a time, with push-pull (class AB) amps.

So if you get enough output power with one, then it's fine. But make sure that you will be satisfied with the maximum output power level when using only one transformer to power both channels. Also, clipping might occur much more easily, if there's not enough power available. That can damage tweeters. You might need to use all of the caps from the other transformer, too, with the one that you use, to mitigate the clipping potential, depending on how loud you might want to have it and how you have everything configured (Too lazy to look, right now. Sorry.) I think that I would prefer to have power available that I never used, rather than possibly not having enough, unless I needed the other transformer for something else.
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Old 8th July 2013, 05:57 AM   #25
sangram is offline sangram  India
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I would recommend using a small, separate transformer for the XO board. I suspect the majority of any ground issues will be linked to using a tapped secondary for a low level circuit, unless it is a totally separated winding.

I have successfully powered five amplifier circuits off one transformer but the grounding ended up being a lot more complex than I imagined. This is for a surround amplifier with a total of seven amplifier chips and a mix of chips and topologies - a parts bin project, if you will.

I had to route each power ground separately, and the input grounds were even more complex given they were coming in from outside the chassis from a computer soundcard - not the nicest source. All the output grounds are returned to the power ground instead of the chip outputs to help me a bit, and this worked well. I have 3 LM3886, two 1875 and two OPA549s in bridge mode for a sub in one little box. It's a miracle it even works
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Old 8th July 2013, 09:03 AM   #26
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by csom View Post
Thank you sir.
I tried my best as I understood from your direction . Kindly have a look.



Done.


Done too sir.
no.
The first pair of caps take the biggest charging pulse.
This circuit: transformer to rectifier to cap pair MUST be separate from the next stage of smoothing.
Use ONE wire link from first stage smoothing to second stage.

The use ONE wire from second stage to Main Audio Ground.

DO NOT put the Main Audio Ground between the smoothing caps.
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Last edited by AndrewT; 8th July 2013 at 09:06 AM.
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Old 8th July 2013, 09:18 AM   #27
AndrewT is offline AndrewT  Scotland
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Two sets of 3channel active amplifiers can easily tick over from a 200VA transformer. 6channels of 70mA quiescent equals ~420mA.
A 200VA 25-0-25 has a maximum continuous DC capability of 2Adc.
420mA is just 21% of it's continuous capability.

It's what happens if you can get some of these amps running at near maximum power that a smaller than optimum transformer will show significant voltage sag.

Assume the bass amps are giving 50W and the mids amps are giving 30W and the treble amps are giving 10W on a prolonged peak, but the average is 6dB lower than that.
The total peak load on the 200VA transformer is 2*(50+30+10) * 1.5 for amp efficiency i.e. ~270W peak during the crescendo and returning to your average of ~80W. The transformer can easily manage that.
The smoothing capacitors meet the peak demand. The transformer recharges them.

However I always recommend locating the amplifiers at the speakers.
An active system benefits from this even more, since you are now shortening 6pairs of speaker cables. Two 200VA located as two 3channel amplifiers at the speakers makes more sense. I would not put them in the speakers. Too much vibration.
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Last edited by AndrewT; 8th July 2013 at 09:30 AM.
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Old 8th July 2013, 11:06 AM   #28
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I would not put them in the speakers. Too much vibration.
I did, three years ago, and have not had a single problem since. I understand the concern but it is, I believe, overstated. if you are worried, put the electronics mounting plate (I used 3mm aluminium) on some foam.

Each of those speakers uses a 160VA toroidal transformer, about 40VA less than yours, to run the three amps (in one box, there are four because the woofer runs bridged), the crossover, speaker protectors and the balanced driver for the bridged amps. I have not ever been short of power, never. It can play loud enough to be heard loudly 40 metres away. So I re-iterate what I said in post 23, you will not experience any shortage of volume, assuming drivers with 87 to 90 dB/W sensitivity. Mine can play deafeningly loud when the mood or beverage causes an interest in loud rock.

Generally though, you won't play them loud all the time because they will disturb your neighbours too much. What gootee has overlooked is that the amps drive the speakers directly and there is no loss, as in a passive crossover. If mine can do this with 160VA, 200VA will be more than enough.
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Old 8th July 2013, 11:21 AM   #29
gootee is offline gootee  United States
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Yes, you are correct that I did not consider that a line level crossover might make a difference in available power. My main concern was that there might not be enough power to go loud enough without clipping, if using only one transformer to power all six amplifiers. But since he can simply test it and see, first, I still think he should just test it and find out.

Last edited by gootee; 8th July 2013 at 11:24 AM.
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Old 8th July 2013, 01:43 PM   #30
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Default Work in progress

This is the update of my work.
1. caps around the rectifier removed now. No problem, no perceptible change in the sound quality, no hum, hiss or any disruption.

2. Each amp is connected to a common connecting point with heavily twisted wires using insulating tubes.

3. Now connecting the amps to directly to the AC 220V. Removed the bulb in series. Pretty loud thundering output with exceptional clarity even with one four amp transformer and 6 X 4700 uFd.
when the bulb is in series with six amps, the amps are not working now but with direct connection from the mains, the sound is magical.

No clipping at all when the volume ( 100K dual Volume control ) is at the maximum position. I have not used the preamp this time. Just connected the output from an MP3 player (with maximum volume) to the input of the X-over.

But I have more questions/ doubts. So to all, kindly get in touch.
Thank you.
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File Type: jpg Triamping-in-progress.jpg (247.0 KB, 152 views)
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