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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:  the.sabs [ 05 Jun 2009, 11:48 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

RiGuy wrote:
what is the efficiency of your speakers? Dont know if you posted this before.

I'm in the process of building a pair vented FE127E speakers with a BSC. So i will use this as a start. then finally to hook up to a pair of 2-way HiVi i'm in the early stages of researching. I do have a pair of jordans, but i want a good small design, as i'm a bit space limited.$*

I think if i'm on the limit, i'm going to use the 2 sinks. This way i can push the amp a bit harder.
How far would the chipamp pcb need to be away from the psu, to avoid interference?

Author:  RiGuy [ 05 Jun 2009, 12:05 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

Here is my philosophy:
As far as you have room for :)
So put the transformer at the front of your chassis. If you have some ferrous material you can put that behind the toroid to stop interference. Twist all wires. All of them. Put the amps at the back of the amp with the heatsinks facing the toroid. Put them real close to the RCA jacks. Now your signal wire is super short and less able to pick up stray frequencies. The heatsinks hopefully stop some interference as well and if you are using the the TF version of the LM3886 this is even better as there will be no electrical relationship between the sink and chip to transfer signals the sink picks up.
If you put the RCAs in one corner of the chassis then you can put the trafo in the opposite corner for even more distance.
Ri

Author:  booangler [ 18 Nov 2009, 07:44 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

Morning All,

I started building my ChipAmp.com Boards last night. I was able to complete the power supply in one sitting, not to hard to do. When I got to the Amplifier boards I was surprised to find that all the caps, with exception of the snubbers, are electrolytic. I have noticed that not everyone does this. So, should I make Ci, which I am planning on using, electrolytic or change it to a metalized cap of some kind?

Cheers,

Alan

Author:  booangler [ 18 Nov 2009, 07:54 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

Morning All,

I started building my ChipAmp.com LM3886 boards last night. I was able to complete the power supply in one sitting, which is not to hard to do. 8-) When I got to the Amplifier boards, I was surprised to find that all the caps that came with the kit, with exception of the snubbers, are electrolytic. :confused: This goes against the schematics that indicate that Ci is not Electrolytic. I have noticed that not everyone follows the schematic so, should I make Ci, which I am planning on using, electrolytic or change it to a metalized cap of some kind? If so, what impact will I notice based on the material type of cap (i.e. polyester, polypropylene, Teflon, etc)?

Cheers,

Alan

Author:  RiGuy [ 18 Nov 2009, 09:37 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

Alan,
most of us use electrolytic for Ci. 47uf Panasonic FC or FM. You can find a bipolar one, they do sell them.
Uriah

Author:  Gio [ 18 Nov 2009, 13:32 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

Hi Alan,
Actually, you can eliminate this capacitor. It may improve sound, but the trade off is that the DC offset will be higher. So be careful if you eliminate it and with all chip amps, always check the DC offset before you attach it to a speaker. Matching Rf and R2 as close as possible will also help with the offset.

Given the cap is 47uF, you will have a hard time finding something film that will fit. The caps that come with the kit are decent so don;t worry about it too much.

Another warning, the amplifier is DC coupled so make sure that the source you connect it to is free of DC. Otherwise the amplifier will amplify the DC from the source and it won't take much DC to kill a speaker.

Cheers

Author:  booangler [ 18 Nov 2009, 16:51 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

Cool beans Gio,

I will match up the resistors a tight as I can.

A questions for you. Do I just put a voltmeter to the output of my preamp to measure the DC?

Alan

Author:  Gio [ 18 Nov 2009, 23:12 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

Yes, set it at the lowest DC setting. Check your source too.

Author:  booangler [ 19 Nov 2009, 10:18 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

Gio wrote:
Yes, set it at the lowest DC setting. Check your source too.
Gio,

Thanks, I will do that!

I matched the resistors for R2 and Rf as you suggested. It turned out to be very simple as the 4 resistors were withing a .1 of an Ohm of each other. :mrgreen: So the boards are all populated. I stopped there to go watch some Hockey on MSG, which was a waste as the Sabres played like crap last night but that is a different story. So tonight I am hopping to do a bit of assembly and DC testing.

I have another question for you though. How do you clean your PCB's after assembly? Do you use an aerosol like Big Bath or DEOXIT D5 SPRAY? Or a bath of isopropyl alcohol? Just curious has to what process people use?

Cheers,
Alan

Author:  Gio [ 20 Nov 2009, 00:48 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

Hi Alan,
I typically scrape off the excess (visible) flux with my whatever - fingernail, small screwdriver, knife. Then I use 99% isopropyl alcohol (rubbing alcohol is only 70%) and a toothbrush. It does a decent job and is environmentally friendly. But keep in mind it is highly flammable. :hot:

I have not tried any of the chemical sprays.
Cheers

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