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LM3886 Chipamp Kit and optional Ci (feedback capacitor)
http://diyaudioprojects.com/Forum/viewtopic.php?f=10&t=366
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Author:  RiGuy [ 24 Jan 2009, 20:30 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

If you do return go for a bigger transformer. However, I think also that 400VA will be fine for 2. If not get another 400VA and use both :) The voltage you have is fine for LM3886.
Ri

Author:  megamahi [ 24 Jan 2009, 20:58 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

@gmilitano: Would 150VA per channel be more suited? I wouldn't expect anymore than 4-5A per channel going continuously especially on the tweeter channel. Also here is the manual on the speakers: http://www.polkaudio.com/downloads/manu ... Manual.pdf . The manual says they can be wired normally, bi-wired, or bi-amped. If they are bi-amp ready, I'd rather just bi-amp and use 4 single channels, than bi-wire or singe wire and run 2 parallel or bridged channels. From what I understand a servo would have to be built to keep DC offset at bay. I can easily build a XO, I have a stock of parts sitting in the basement at home for an old speaker project that never got built. If they aren't the right values I can source from Zalytron, Parts Express, or my new favorite site Surplus Sales of Nebraska.

@Ri, unfortunately the case I'm using only really has enough room for 1 transformer :worried:. Trying to fit 2 tranfos, and 4 channels while trying to keep heat and hum under control would be a challenge as I can really only 'measure twice, cut once'. I have some ideas going thru my head right now, but I'll have to see what material I have stocked for heat sinking.

Author:  RiGuy [ 25 Jan 2009, 00:00 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

These LM3886 chips are max at 56 watts and less voltage into them means less max watts as the big limiter is the current. 150VA for each channel is great. You will do fine with 150 and you could even go a little little lower. Theoretically you could go with 56VA but you will probably be disappointed with what you get back out of them. These will never sustain 4-5 amps for very long at all. Your 23V will rectifiy to 32 so hitting >2 amps will only happen in sudden jumps in volume in the music.
150 is great.
Ri

Author:  megamahi [ 25 Jan 2009, 01:09 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

Thanks Ri. I'll be returning this tranfo and go for the 600VA 25V (AN-6225). Weird part is Antek doesn't sell a 23V tranfo (AN-4223 is the imaginary, unlisted part number), so this threw me for a loop when I got it, so I figured why the hell not try.

Probably would also be beneficial to up the capacitance (kits are 10mF/rail for 2 channels) as well as the rectifier bridges (MUR860 to MURF1560)? No 2nd tranfo gives me room for slow start and maybe a clipping detector if needed.

Also ordered a Pass B1 for a pre-amp should ship in a week. Can't wait to get this done, set-up, and running. When the quarter is over up here, I'm bringing the set down home and do some comparisons between the clones and my buddies Hafler amp (he's bought nothing but). He's got the turn-table and a copy of Dark Side of the Moon. :P

Author:  the.sabs [ 25 Jan 2009, 05:40 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

Yo mega,

what's your project? from some of your posts it sounds like your building a HT amp. if so i have this penciled in for a future project so would like any info to start planning. Have you got any photos?

Author:  Gio [ 25 Jan 2009, 11:31 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

megamahi wrote:
@gmilitano: Would 150VA per channel be more suited? I wouldn't expect anymore than 4-5A per channel going continuously especially on the tweeter channel.

Unless you will be listening at near max power, I would expect 100VA to be enough. I've had no issues with 80VA.

There is not much detail about those speakers, but I would expect that use of the dual posts with no bypass it.

Author:  megamahi [ 25 Jan 2009, 13:06 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

@the.sabs: It's not a HT, but I figured cause the speakers are bi-amp ready, and the speakon connectors for the back are 4 pin, then go full bi-amp with 4 x 50W, than trying to drive 2 sets of 3 speakers(2 mid, 1 tweeter) with only 50W, realize I need more power, and be disappointed :P . I haven't got the case drilled yet, and I'm trying to see if I can fit 4 connector pairs on the back with out getting to close to the mains, then I can do a HT, but without bi-amping (oh man, now you have me thinking how can I fit 8 x 50W in here!).

@gmilitano: Yup, not much at all. I'm not familiar with Polk as I'd like to be. I know Boston and Focal well, I have a set of Polk reference monitors, but they don't get used much as I'm not back home, and they are connected to a working system. When they show up I'll have to take off the binding post block and look to see if there is a XO in there somewhere.

Here is a pic of the case, and the current placement (ignore the sharpie, I have yet to clean it off).
Attachment:
amp1.jpeg

Attachment:
amp2.jpeg

Author:  RiGuy [ 25 Jan 2009, 14:24 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

8 will not fit in there :)
The caps and diodes are fine as long as you use one power supply board per amp. More than fine on the diodes.
If you get that big toroid singing right next to the amps you will drive yourself mad trying to get rid of the noise because it will end up amplified and a buzz/humm will be in your speakers. You can try separating the toroids electromagnetic field from the amp with a piece of metal if you have room. It might or might not work because that field radiated 3 dimensionally not just straight out. You might be fine if you separate with metal though. You may consider doing your 8 amps in that enclosure and then putting the power supplies and toroid in another case :)
Ri

Author:  megamahi [ 25 Jan 2009, 15:46 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

Something like wrapping the toroid in a layer of Mu-metal? Also to power each amp power supply, I'm assuming you mean take the secondaries and split them with a junction block, to each respective bridge?

Author:  RiGuy [ 25 Jan 2009, 18:17 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

No you need to use the secondaries to get 25V/0V/25V then from there power all the boards with 25 0 25.
Dont know if your mu metal will work or not. The thing is its cramped in there and LM3886 dont do real well with a lot of noise.
Ri

Edit: By "the boards" I mean that you need to take that power to the rectifier boards.

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