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Old 4th January 2012, 09:22 PM   #41
epilot is offline epilot  United States
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Quote:
Originally Posted by AndrewT View Post
Are you aware that an aerial that picks up interference is a loop?

Now start at PIN9 and follow the traces and components: to R701, to R702, through R702, to C555, through C555, to star ground, to Input PIN1, to PIN2, to R700, through R700, to PIN10.
Now shade in the area enclosed within that route.
That is your loop area. That is your aerial picking up interference.

You must make that group of components and traces and pins as small and as compact as possible and try to minimise the loop area.

Repeat for the output.
Repeat for the Zobel.
Repeat for the PSU rails.
Andrew,
Thanks a lot for your reply.

Yet I do not have a good point of view about the aerials which make LOOPS!
I did know that the inputs can act as an aerial if the input wire/trace happens to be long. But yet I have no idea what you do mean here.
Plz take a look at the WHITE sign here in the below pic and let me know if I have drew it the right place to show the aerial loop you are talking about?

Thanks

P.S is the aerial loop defined just for INPUTs?
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Old 4th January 2012, 09:54 PM   #42
AndrewT is offline AndrewT  Scotland
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your white line shows the loop.
The area inside the loop is you loop area.
Try to get that area as close to zero square millimetres as possible.
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Old 5th January 2012, 06:11 PM   #43
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Quote:
Originally Posted by epilot View Post
Regarding the optional components actually I want to makes a prototype based upon the schematic in the datasheet and then I will improve it and add the optional components just if needed.
Shouldn't the PCB provide the space for all those components then? Or will you let the additional components dangle about in free air?

Those 1000 F capacitors look pretty small. Do they have sufficient voltage rating for that amp?
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Old 9th April 2013, 02:34 PM   #44
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Default Newb in need of a bit of help

Quote:
Originally Posted by AndrewT View Post
RF attenuation is omitted.
Very easy to correct: add a parallel cap across R5 either on top of the PCB, or underneath the PCB.

Output Thiele Network is omitted.
Very easy to correct: remove the output terminal block. Connect the Zobel capacitor in the spare location.
Tap off the speaker output from the top end of R1.

The other half of the Thiele Network (L//R) can be mounted offboard in the line that joins the amp PCB to the speaker terminals fitted to the chassis.
The speaker return connects to the Main Audio Ground, not to the amp PCB.

Finally,
check to see how R5 and C4 are connected to Signal Ground. They must not connect to Power Ground on the underside of the PCB.
I've got a set of these boards to build up and I'm okay with it appart from one thing which confuses me. Setting aside the attentuation and Zobel amendments (&argument) for a moment, what is the purpose of R1 in the circuit as it stands? I have seen other 3886 circuits which use either an 0.7mH inductor here or an inductor in parallel with a a 10 ohm resistor resistor. Indeed the datasheet itself in one schematic has a 4700mf electrolytic cap in series with the parallel inductor/resistor, although I've yet to see any implemented design using this cap. I'm thinking to leave the R1 out , insert a link and do the zobel off board. Does that make sense?
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Old 10th April 2013, 08:39 AM   #45
AndrewT is offline AndrewT  Scotland
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no sense.

The AC coupled ex sch in the Datasheet is a far better circuit. It adopts the optional components that should never be optional for a beginner project.
Just convert that sch back to dual polarity supply.
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Old 10th April 2013, 03:23 PM   #46
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Quote:
Originally Posted by AndrewT View Post
no sense.

The AC coupled ex sch in the Datasheet is a far better circuit. It adopts the optional components that should never be optional for a beginner project.
Just convert that sch back to dual polarity supply.
Thanks. I realized the error of my ways after posting
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Old 6th October 2013, 07:24 PM   #47
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Hey guys.

I bought a couple of these boards a few months ago and haven't really gotten to them. Today I got my hands on a pair of speakers (for free) that would really go well with this amplifier.

Thing is I don't really know what components I should use. The PSU PCB has 4 capacitors with no values printed, and also the actual amplifier PCBs have no values at all, just numbered resistors and capacitors.

Can someone point me to the components I should use?
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Old 7th October 2013, 08:28 AM   #48
AndrewT is offline AndrewT  Scotland
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You need to go back to the retailer and ask for the schematic and/or bill of materials to match the PCB they have sold to you.
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Old 11th October 2013, 09:15 PM   #49
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Originally Posted by maxpaynner View Post
Hey guys.

I bought a couple of these boards a few months ago and haven't really gotten to them. Today I got my hands on a pair of speakers (for free) that would really go well with this amplifier.

Thing is I don't really know what components I should use. The PSU PCB has 4 capacitors with no values printed, and also the actual amplifier PCBs have no values at all, just numbered resistors and capacitors.

Can someone point me to the components I should use?
I used CharlieLaub's circuit and referred to the National datasheet and various other published designed to arrive at the following values for the same board.

R1 0R15 5w
R4 680 0.25w
R6 1K 0.25w
R2 10K 0.25w
R3,5 22K 0.25w

C5 1-3.3uF 100v DC block. Polyester
C1,2,3 100uF 50v electrolytic
C4 22uF 50v electrolytic

Mine work very well powering 6 ohm mid bass and 8 ohm treble units fed from an active crossover and powered by 22-0-22 traffo. The mid reproduction is superb and in my view compares very well to the much vaunted rev c which I also built. Neither give much deep bass slam.Most designs have a zobel at the output which I omitted for 2 reasons. Firstly it wasn't the designer of the pcb's intention but also I came across some designs where the designer added a zobel and then removed it because it had a deleterious effect on the sound. Likewise most design have a 220pf cap across pins 9 and 10 for rf attentuation. I also omitted this primarily because I wanted to see if I could hear any rf on the circuit as it stands, which I can't. If you want to add a zobel the followinglink may prove helpful.

http://www.eetimes.com/document.asp?doc_id=1274906

Last edited by piggybladder; 11th October 2013 at 09:31 PM.
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Old 11th October 2013, 09:30 PM   #50
udailey is offline udailey  United States
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Quote:
Originally Posted by epilot View Post
Andrew,
Thanks a lot for your reply.

Yet I do not have a good point of view about the aerials which make LOOPS!
I did know that the inputs can act as an aerial if the input wire/trace happens to be long. But yet I have no idea what you do mean here.
Plz take a look at the WHITE sign here in the below pic and let me know if I have drew it the right place to show the aerial loop you are talking about?

Thanks

P.S is the aerial loop defined just for INPUTs?
I worry that you will not be able to get your LM3886 flat against a heatsink unless you place the bottom of the heatsink above the board so the board slips under the sink a bit. Dangerous- as in for sure you will have a short. Of course I suppose you might have the copper on the bottom side but still to me its odd to have the board sitting under the sink.
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