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LM3886 Chipamp Kit and optional Ci (feedback capacitor)
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Author:  the.sabs [ 08 Jan 2009, 08:13 ]
Post subject:  LM3886 Chipamp Kit and optional Ci (feedback capacitor)

another quick Q. When i received the lm3886 kit, i did a quick check and noticed that an optional piece (the Ci) feedback cap, was only delivered for one pcb and not two. i know i can build the kit by omitting this cap, but is it better to include it? taking into account that either it gets forwarded to me or i have to source it myself.

cheers

Author:  Gio [ 08 Jan 2009, 13:38 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

Hi Sabs, here is my LM3886 Chipamp / Gainclone that I built from a chipamp.com kit. I did not use the optional feedback capacitor. There are a couple things to note. If you are getting DC offset, the feedback capacitor will likely lower DC offset by quite a bit. But it does have effects on the frequency response. I went without it and DC offset was acceptable so I left it out.

Author:  the.sabs [ 08 Jan 2009, 16:10 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

Yo Gio,

i think i'm gonna do like you because mainly i don't want to sacrifice the lm's freq response. if it's an issue i'll just add them and replace the R3. thats the wonder of DIY!

Author:  the.sabs [ 17 Jan 2009, 08:59 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

Hi guys, splitting my time between this and the tube amp.

Just a thing whilst mounting the power supply board for the chipamp. do i need to add the two contact terminals for the AC or is it easier to solder the wires direct? I only ask cause on the instructions it says to do then last, but with the transistor diodes in the way it look a bit tight?

Author:  RiGuy [ 17 Jan 2009, 12:37 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

Well both ways are easy. What I would consider is will you ever remove this power supply to work on it or use it elsewhere or to use a different supply in its place..?..
I would say dont solder direct. Using faston connectors or terminal blocks is a better long term solution. Thats my opinion. Heating up those wires to remove them can take time and leave your board a bit messy as well as sometimes lifting a trace which is just no fun.
Have not used that board but I have a real tight fit getting my Fastons on my amps power board as well. I use needle nose pliers to remove and apply them.
Ri

Author:  the.sabs [ 18 Jan 2009, 13:35 ]
Post subject:  LM3886 Chipamp and Phono Preamp Kit Construction

So got a bit of time in to day, a,d started to build the two kits for the amp. The jaycar phono preamp and the chipamp.....er..... chipamp!
both have been a pip so far, i took riguys advice and added the terminals, just in case later i want to upgrade. the chip amps and power supply are all finished. and the jay car is midway. here is a photo so far. A lot will know the score but here they are again for the cheap seats......
Attachment:
P1180028.jpg

So the preamp will have the most adjustments. i am replacing all stock components to others i have sourced. I have decided they will both be housed in the same case so the signal will be in through the preamp with the output connecting directly to the chipamp inputs, both will have ac from the same transformer.
I have decided to leave the pot on the preamp and wire this to the rear side of the case, this way i can control the gain directly from the preamp and have a decent strength source for the amp.

i'm still searching for a ideal sized case but aim to have everything ready so when i have found it it will be a quick mounting job.
More later.........

Author:  megamahi [ 20 Jan 2009, 23:27 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

Just started my LM3886 amp as well. Putting the 12AX7 pre-amp on hold till I can get a break with school work and can get parts sourced and can get a buddy from back home's help (he knows high voltage tube stuff better than I do).

Ordered 4 amp PCBs and 4 chips, 2 channels of which were part of the stereo kit. The other 2 I'm leaving up for experimentation, testing different caps and what not.

Other than the transformer (ordered a 25V 300VA from Antek on monday, I'm driving a 8Ohm load) all the parts are in, and my case is going to the shop next week to get its back panel drilled.

Pic of the amps with the heat sinks strapped to them with spring-metal clips I sourced them from a dead amp I had laying around. Oh and the 15$ butter-stick of an EMI filter I found :mrgreen: .
Attachment:
amp3886.jpg

Still need a pre-amp for the whole thing, thinking of going PassLabs B1, but they seem to be sold out right now online, so I may try my hand at experimenting with an op-amp and voltage controlled attenuator setup (have some old dbx VCAs I pulled from a 1982 DAC of some sort).

Author:  Gio [ 21 Jan 2009, 00:17 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

Here is an idea for the attenuator: Lightspeed Attenuator - A passive attenuator. 2 resistors !

Author:  megamahi [ 21 Jan 2009, 02:30 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

gmilitano wrote:

Yup, I read that as well, unfortunately the day after i placed my mouser order :P.

I bread-boarded one of the VCAs using the application notes that I can find. Seems the only people who make/made this chip is/was THAT Corp. The part is dbx2150A, but there's no info on it other than a cross-reffed datasheet for the THAT2150A. I'm gonna bring it into the lab and see what it does as I don't have a +/-15V source in my apartment. If all goes well (and i don't release the magic smoke trapped from 1984), I'll see about spinning a board and giving it a more thorough evaluation and I'll post some more info in a new thread.

Author:  RiGuy [ 21 Jan 2009, 07:39 ]
Post subject:  Re: LM3886 Chipamp Kit and optional Ci (feedback capacitor)

I posted that attenuator Gio mentioned. Just to throw my 2 cents out there and not to threadjack I wanted to say it is awesome and would work great with your chipamps. All reports come back that is sounds better than any other passive preamp and that it beats most actives in ABX shootouts.
Ri

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