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Old 4th December 2014, 12:55 PM   #1
Philfr is offline Philfr  France
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Default LM3886 "fullrange"

Hi,
I said "fullrange" cause my previous builds was for biamping. Here it's was to drive mainly fullrange drivers 4 to 8 ohm..

First, I have to thank AndrewT, Mark Whitney, Tomchr and Danielwritesbac. I'm Sorry if I forgot someone..
I'm collected their advices, and I decided to build a new amp with LM3886 in dual mono setup.
I don't need high power (maybe 25 watts) cause speaker I have is 90 to 97 db, and for a quiet listening. But I hope have a good sounding.


The end result might look like this :

Click the image to open in full size.

Except one pot for this new..
PCB' come from chipamp : Chipamp Electronics

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the heatsink :

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Schematic diagram :

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Two safety loop breaker, one per side :

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dual mono psu with snubber :

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amp schematic :

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components :

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close up Caddock MK132 330 Ohm :

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I hope nothing is wrong or missing !
I work slowly (turtle ?), so it's was finish maybe next year..
Phil.

PS : adding full schematic with labels :

Click the image to open in full size.

Some picts are missing in this tread, you can see them here :
LM3886 "fullrange"

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Last edited by Philfr; 17th April 2015 at 10:02 AM. Reason: new safety loop breaker schematic
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Old 5th December 2014, 02:30 AM   #2
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Best of luck with the build!
I wish my resistors come in fancy box like yours.
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Old 5th December 2014, 03:37 PM   #3
AndrewT is offline AndrewT  Scotland
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Don't connect Cf to -IN.
Connect Cf between +IN and Signal Return.

R2 should roughly match Rf

Connect Ci to R2 and to Cf and to Signal Return at the input socket.

Connect C1 to C2
Connect Cz to the junction of C1/2.
This becomes your Power Ground.

The route from Pin3 through RzCz through C1 to Pin1&5 MUST be VERY short.
The route from Pin3 through RzCz through C2 to Pin4 MUST be VERY short.
All of these components (except the two electrolytics) MUST be kept VERY close to the chip amp package at the Pin1,3,4,5 end.

Connect the incoming PE wire DIRECT to Chassis at the incoming hole/socket (if IEC or similar).
ESP shows a long connection through other components. ESP's method is not good practice.

BTW,
you can add a switch across The Disconnecting Network (Safety Loop Breaker), i.e. parallel to the R & C.
This optional switch becomes your ground lift switch (the safe way !!!!!).
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Last edited by AndrewT; 5th December 2014 at 03:39 PM.
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Old 5th December 2014, 04:40 PM   #4
Philfr is offline Philfr  France
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Quote:
Originally Posted by AndrewT View Post
Don't connect Cf to -IN.
Connect Cf between +IN and Signal Return.
Thank's, it's mistake, I will change schematic
Quote:
Originally Posted by AndrewT View Post
R2 should roughly match Rf
Yo think it's better when R2 is equal (or nearly) Rf, I read somewhere that it was necessary that R2 was always less than Rf
Quote:
Originally Posted by AndrewT View Post
Connect Ci to R2 and to Cf and to Signal Return at the input socket.
Connect C1 to C2
Connect Cz to the junction of C1/2.
This becomes your Power Ground.
It's my fault, C4 is missing, I should write (C3 C4) and not (C2 C3), sorry.
Quote:
Originally Posted by AndrewT View Post
The route from Pin3 through RzCz through C1 to Pin1&5 MUST be VERY short.
The route from Pin3 through RzCz through C2 to Pin4 MUST be VERY short.
All of these components (except the two electrolytics) MUST be kept VERY close to the chip amp package at the Pin1,3,4,5 end.
I will see with pcb, but it's small, not sure I can change many things.
Quote:
Originally Posted by AndrewT View Post
Connect the incoming PE wire DIRECT to Chassis at the incoming hole/socket (if IEC or similar).
Yes, is provided.
Quote:
Originally Posted by AndrewT View Post
ESP shows a long connection through other components. ESP's method is not good practice.
Sorry, what do mean by ESP ?
Quote:
Originally Posted by AndrewT View Post
BTW,
you can add a switch across The Disconnecting Network (Safety Loop Breaker), i.e. parallel to the R & C.
This optional switch becomes your ground lift switch (the safe way !!!!!).
Good idea.
Phil.
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Old 5th December 2014, 05:42 PM   #5
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Philfr View Post
................... Rf, I read somewhere that it was necessary that R2 was always less than Rf
if the BJT input pair have exactly the same Ic, same Vce, same Tj and exactly the same hFE, then they will have the same Ib. Ib passes through those two resistors.
If the voltage drop across those two resistor is different, then the difference is the output offset.
If the two input resistors are not well matched either due to Ic being different, or due to different device parameters, then Ib is usually different. That is the specification value: input offset current.
Your first try assumes Ib will be the same and then adjust one of the resistors to equalise the V at the two input pins, since with a chipamp you cannot adjust the Ic.
Quote:
what do mean by ESP ?
the 5th diagram is a crib from esp with the copyright logo removed and the language changed.
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Old 5th December 2014, 06:09 PM   #6
Philfr is offline Philfr  France
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So I will change R2 with 27 Ko like Rf..
Thank's.
Phil.
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Old 5th December 2014, 07:33 PM   #7
davym is offline davym  Europe
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Hi Phil. I've used those boards with the specified component values and a single PSU, it's an easy build and it works exceptionally well.

Just be careful not to do what I did! I used screws with too big a head on them to fasten the boards with, it shorted out a track which fried some PSU diodes. I refitted the boards using insulating fibre washers on each side.

Good luck with your build.

Davy



Good luck
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Last edited by davym; 5th December 2014 at 07:43 PM.
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Old 5th December 2014, 08:24 PM   #8
Philfr is offline Philfr  France
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Hi Davy,
I build one (first pict first post), and yes I found some small holes..
I use this plastic brace :

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with screws..
On amp pcb, some hole was to small for output for example..
I will use terminals for psu and amp supply.
Thank's.
Phil.
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Old 6th December 2014, 06:00 PM   #9
tomchr is offline tomchr  United States
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Thanks for the recognition. Glad I could help.

I recommend adding a Thiele network on the output. You can do that between the board and the output connector. The Thiele network is the inductor in parallel with a resistor. It is needed for stability with capacitive loads (think long speaker cables or electrostatic speakers). You can wind the inductor yourself...

These guys have a good air core inductance calculator. You'll want something in the 700 nH to 1 uH range.

The commonly used Thiele network is 10 Ω, 2 W in parallel with 0.7-1 H.

I would build the boards as-is and just add the Thiele network. There's no point in turning this into a point-to-point wiring exercise if you already have the boards. That said, the board layout can be improved considerably. Specifically, the inductance of the ground net is way larger than it needs to be. This reduces the effectiveness of the decoupling network and the Zobel network (R+C from output to ground). The LM3886 needs a decoupling network consisting of a bulk electrolytic cap, a smaller electrolytic or tantalum, and a ceramic or film cap. This is spelled out in the LM3886 data sheet, actually. My recommendation is to use 1000 F electrolytic + 22 F low ESR, ESL electrolytic (or OSCON) + 1 F X7R ceramic. I don't see a way to get those components on the board, cleanly.
It looks like the board was optimized for size rather than performance. Nothing wrong with that, I just personally prefer to optimize for performance. I guess I should look at the positive side of this... When you're ready to upgrade to a Modulus-86, I'll be happy to sell you a couple of boards...

If you haven't already, I suggest that you take a look at my Taming the LM3886 website. I cover stability, thermal design, decoupling, power supply design, and more.

~Tom
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Last edited by tomchr; 6th December 2014 at 06:22 PM.
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Old 7th December 2014, 08:06 AM   #10
Philfr is offline Philfr  France
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Hi Tom,
Thank's for interest..
Quote:
Originally Posted by tomchr View Post
Thanks for the recognition. Glad I could help.

I recommend adding a Thiele network on the output. You can do that between the board and the output connector. The Thiele network is the inductor in parallel with a resistor. It is needed for stability with capacitive loads (think long speaker cables or electrostatic speakers). You can wind the inductor yourself...
Yes I read some advices about Thiele network, I left aside, cause speakers are 92 to 97 db, I don't need much power (maybe 10 watt max for a party). Length cables are 2,5 meter with two Cat 5 wire per side.
At first time, I thought "it's not necessary", and what good resistor I can use for that ? Caddock ?
But I will try this network to hear some difference with sonics..

Quote:
Originally Posted by tomchr View Post
The LM3886 needs a decoupling network consisting of a bulk electrolytic cap, a smaller electrolytic or tantalum, and a ceramic or film cap. This is spelled out in the LM3886 data sheet, actually. My recommendation is to use 1000 F electrolytic + 22 F low ESR, ESL electrolytic (or OSCON) + 1 F X7R ceramic. I don't see a way to get those components on the board, cleanly.
Yes, but like you said I don't see a way to get those components on the board..

Quote:
Originally Posted by tomchr View Post
If you haven't already, I suggest that you take a look at my Taming the LM3886 website. I cover stability, thermal design, decoupling, power supply design, and more.
~Tom
I read about your Modulus 86, great job, maybe next year I try it !
Phil.
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