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Old 12th September 2014, 05:48 PM   #1
conve is offline conve  Germany
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Default BrambleAmp - LM1876 Gainclone

Hello!

As I already mentioned in my introduction Im currently building a 4.1 sound system which will mainly consists of four Dayton RS100-4 (from Klangriese by Udo Wohlgemuth), a Mivoc XAW 210 HC (Micro Cube by Hobby-Hifi) and two LM1876 for amplification. I already did a test with two speakers and the subwoofer with my SymAsym and it sounded really nice although the Micro Cube struggled to adapt with the room at the beginning.

At the moment Im designing the layout for the chips. This is what Ive came up with so far (stand-by parts not implemented yet!):

brambleamp2.png
brambleamp3.png
brambleamp1.png

Now Im very uncertain whether the grounding is good the way I routed it. I read a lot of articles in this forum about this topic lately but still there are very different types of solutions on how to achieve a clean ground wiring/routing. Its irritating! Some suggest to use ground-planes, others star-grounding and even others T-shapes (like I did it in my version).

So this raises a big question for me: What is the best way to do ground routing?

Regards,
Martin
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Old 12th September 2014, 06:51 PM   #2
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You can get into various grounding schemes, but one thing that is very important is to not have noise grounds (like input and feedback loop grounds) going through the same trace back to the ripple capacitor as the power grounds and bypass grounds. It looks like your layout could benefit from even minor layout changes.

Some kind of star grounding is a good idea. Just group the grounds together following the above principles.

Here's an in-depth article that might help you. Grounding Principles - The Signal - Archives - TI E2E Community
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Old 13th September 2014, 05:57 PM   #3
conve is offline conve  Germany
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Thank you for the article! It is really well written and understandable.

I think I will go with a star grounding scheme with two star points. One for signal ground and one for power ground. Signal ground will then be connected at one single point to power ground and from there to the mains PE. Is this one possible way to do it?
grounding1.png

Could you elaborate more what these minor layout changes would look like? (maybe a small drawing or something...)
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Old 13th September 2014, 07:34 PM   #4
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Skipping over most of your drawing, the Main Audio Ground point connects to the chassis near the input jacks. The PE connects to the chassis near the AC power inlet.
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Old 13th September 2014, 10:57 PM   #5
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Below is my understanding of the right way to do it. It's based on the reference posted by Fast Eddie D, and on some circuits by D. Self. The NE5532 is arbitrary, it's there so a complete circuit will be illustrated.

On another subject, I don't think you want to include R16. As drawn it carries the full output of the amplifier, and will have to be 20 watts or whatever.

A related but different subject, in the first circuit you posted I'm guessing R15 and C13 are a zobal network (Boucherot cell). Zobals are intended to react between the speaker and amp, so this should be on the other side of R16 (if present). As it's drawn now the zobal sees the 10 ohms of R16 as part of the speaker, which can confuse things.

Zobals are actually something of a design exercise, not at all trifling, and can do more harm than good when wrongly applied. Just lifting values from another circuit might or might not give the desired result. Key fact: the engineers who designed the LM1875 in the first place include zobals in other suggested circuits, but not the one you're using.
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File Type: jpg grounding_01_e.jpg (37.1 KB, 187 views)
File Type: jpg grounding_02_f.jpg (37.3 KB, 187 views)

Last edited by bentsnake; 13th September 2014 at 11:00 PM.
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Old 13th September 2014, 11:25 PM   #6
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Wait, correcting myself. Are you using a single-supply circuit of some sort? OK, sorry, forget what I said. But be sure to have an output capacitor somewhere around R16. PS in my view single-supply circuits are best described as, "but...why?"
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Old 14th September 2014, 07:53 AM   #7
conve is offline conve  Germany
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Sorry for the confusion. To clear things up:
  • its a LM1876 (not LM1875!)
  • Im using a symmetrical power-supply
  • R16 (10 Ohm) will have a small inductance paralleled, like it is shown in figure 4 in the datasheet
  • R16 shouldnt have to carry that much current since the inductance will do so for the most part
  • the schematic is exactly the same (except for the standby part) as the one in the datasheet (figure 4 again)
Now on the grounding issue:
It seems fine by me to connect things like Speedskater described it above. But how will this look like if I have a chassis which mostly consists of insulating materials (like plastic or wood) and only have a aluminium shield at the rear and the front of my chassis? Do I connect both aluminium parts with a wire, leave it completely disconnected or maybe do something completely different?
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Old 14th September 2014, 11:58 AM   #8
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<< its a LM1876 (not LM1875!) >>

Yep. Typo by me.

<< the schematic is exactly the same (except for the standby part) as the one in the datasheet (figure 4 again) >>

This seems to need clearing up because there must be different versions or different data sheets. Mine is Texas Instruments, "SNAS097C – MAY 1999 – REVISED APRIL 2013"

Below is the figure 4 that I see. Or you meant figure 5, another typo? These foreign keyboards they sell us are really worthless.
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Last edited by bentsnake; 14th September 2014 at 12:02 PM.
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Old 14th September 2014, 12:33 PM   #9
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<< I have a chassis which mostly consists of insulating materials (like plastic or wood) and only have a aluminium shield at the rear and the front of my chassis? Do I connect both aluminium parts with a wire, leave it completely disconnected or maybe do something completely different? >>

You're kinda-sorta taking your chances by not having your circuit in a shielding enclosure--meaning a grounded metal enclosure. Studio and performing environments are the worst for radio frequency noise, but anywhere near computers is bad too, and in fact rf noise is everywhere these days, and can cause strange problems in unshielded circuits.

In any case yes, the correct procedure is to connect any/all metal shielding plates together, and connect them to the ground point, which is actually the third prong of the power cord. Thus rf noise is shunted to ground. "Ground" really does mean ground because the third prong of the power cord is ultimately connected to a 10-foot metal rod driven into the earth near your electric meter. All electrical power installations are done this way, worldwide (with variations according to local codes).

As a matter of interest, a metal housing shields against rf noise because rf energy induces microcurrents in the housing, the same thing that happens on a much larger scale in a transformer. The rf energy is absorbed by this (so to speak), and the microcurrents are conducted to ground.

These microcurrents are the reason for not using a metal housing for an input signal ground. At any given time a metal housing is simply a mess of tiny little local voltages and currents appearing and disappearing. These are too small to affect the much larger power supply, but they can distort the small audio signal.

"Distortion" is relative, of course. The various voltages and currents are simply combining according to the laws laid down by Ma Nature. But as far as we're concerned it's distortion and we want to avoid it.
.

Last edited by bentsnake; 14th September 2014 at 12:48 PM.
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Old 14th September 2014, 04:14 PM   #10
conve is offline conve  Germany
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Well figure 4 was referred to the datasheet from National Semiconductor which I linked to, if youd just followed the link. In the Texas Instruments datasheet its figure 5.

I am aware of the lack of shielding against noise in a non-metal case. The reason for not using a metal case is simply that I dont want to spend that much money. Appropriate ones like the Hifi2000/Galaxy series starts at about 50 going easily up to over 100 which is too much for a student like me. If you know any good looking and relatively cheap chassis let me know of course!
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