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Old 20th June 2011, 12:58 PM   #41
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Hi hurtz, yes ideally you should have separate traces to the common ground point, or as you say separate wiring back to the PSU ground point. A lot of people say to run the speaker returns direct to the psu rather than to the amp circuit board.

I wouldn't call the pin 7 connection low level It is the main power ground for the chip. The input ground is the low level ground (as is the feedback ground)

In my p2p LM3886 I shared the input ground and feedback ground, but had separate wiring for output compensation, power ground and speaker return (load return in the datasheet). Ideally you should have a separate trace on the PCB for each of these, all joining at a common point which should then run back to your PS 0V point.

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Old 20th June 2011, 01:40 PM   #42
hurtz is offline hurtz  Germany
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Ok I think this is as stary as it can get

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Old 20th June 2011, 02:05 PM   #43
AndrewT is online now AndrewT  Scotland
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You have the Signal Ground mixed up with the everything else Ground.

Do not put the common Ground (= Star Ground) (= Main Audio Ground, MAG) on or in the PSU.
The PSU has a Zero Volts point. Connect that Zero Volts to the MAG at some convenient location.
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Old 20th June 2011, 02:24 PM   #44
hurtz is offline hurtz  Germany
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Quote:
Originally Posted by AndrewT View Post
You have the Signal Ground mixed up with the everything else Ground.

Do not put the common Ground (= Star Ground) (= Main Audio Ground, MAG) on or in the PSU.
The PSU has a Zero Volts point. Connect that Zero Volts to the MAG at some convenient location.
Sorry, but I don't understand what you mean! The signal Ground has to go somewhere, and if not the star ground, where else? And also the star Ground is on the AMP and not on the PSU
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Old 20th June 2011, 02:38 PM   #45
Piggy is offline Piggy  Hong Kong
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When I built my first amp, I don't have the star ground on the PCB. I use flying wires to connect the grounds to a "common point". That common ground is connected back to the PSU 0 volt. With this arrangement I can easily reconfigure the grounding topology. When I able to make the amp quiet, I transferred the whole thing to one PCB.
Your common ground is not locate at the connector Gnd point.
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Old 20th June 2011, 04:43 PM   #46
AndrewT is online now AndrewT  Scotland
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Let's give the signal hot and signal return their correct names and forget "ground".

The signal is part of a circuit. The flow half and the return half.
Those are the two wires in your interconnect.
The RCA has two connections to receive the two wires from the interconnect, the signal hot and the signal return.

The RCA passes it's signal on to the amplifier +IN and -IN terminals going via the two input filters that you have put in there.
The Negative FeedBack (NFB) must also feed into -IN and thus demands that this lower leg of the NFB must connect to Signal Return.

The input circuit is now complete. The current can flow around the circuit.

BUT,
there needs to be a reference for the amplifier to determine what to do with the output voltage. A voltage reference, that carries virtually no current is required between the Signal Circuit and the Speaker/Output Circuit. This connection is only a voltage reference it is not really a part of any circuit. A wire/trace, even better with a bit of resistance, is used to reference the Signal Return to the Speaker Return. Note the similarity here to an opamp. It has +IN & -IN and Load Return is off board but voltage referenced to the input signal. The opamp has no "ground". It is designed to to operate and amplify with a ground pin. The reference between output and input returns is done external to the opamp. The same as described above.
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Old 20th June 2011, 07:42 PM   #47
hurtz is offline hurtz  Germany
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Hmm the negative feedback line you're talking about is the one running through Rf?

And the signal return i.e. IN- should be tied in with(colse to) the speaker return thus being the reference line, or do you mean the left pin of Rin being tied close to the speaker return as reference?

Sorry if I'm bugging you >.< maybe its easier for me to understand if you just paint in the connections you're talking about, I still have some probems determining wich net is exactly which.

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Old 21st June 2011, 05:05 AM   #48
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To my eye your starGND does not look that bad...
I think (but I might be very wrong) that you could actually make this work.
But: If I was you, I would loose the two gnd connections between the Vn and Vp.
Instead I would put only one GND connection in the middle of your 'star', right above the Rz left connection point.
To that new gnd-everything (make it BIG) you connect your PSU and the crazy large reservoir capacitors and what ever you use to filter your power supply...
All with separate wires!
Your speaker return would be OK as it is. Your input return would be OK as it is.

Or you go with the Piggy's idea...

Or you go with Andrew...

But as I said, I might be very wrong here now... The grounding is a difficult thing to do perfectly.
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Old 21st June 2011, 11:57 AM   #49
DaveR is offline DaveR  United Kingdom
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Been following with interest as I've got a spare 3886 in my parts drawer. Here's my 2p's worth!.

Regarding the board in post 42 - looks like you've got provision for both a zobel network (Rz/Cz) and, I think, a Thiele network (R and L). Normally L is wound around R so removing L from your board means you've got room for Ci. I think I'm right in saying you don't use a zobel and a thiele, it's one or the other (or at least I've never seen both used together).

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Old 21st June 2011, 01:36 PM   #50
hurtz is offline hurtz  Germany
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Since a friend of mine wants to buy one assembled board from me, I'm eager to deliver a very good sounding build with this chip...

@palstanturhin: good idea, I relocated the GND's to a more convenient position.
I've found that the LM4780 datasheet is actually a much better source concerning the optional components (figure 5). Although I started with the minimal design approach of the point-to-point-lm3886 it seems when transferring this to a PCB the optional components should all be used, and as far as my knowledge goes, they're overall just a series of in-/out-/feedback filters that the p2p-design omits.
So thinkning about this I guess I might as well include Ci, and Rf2/Cf before going through 3 more design changes just to end up there anyways.

@DaveR: Hmm as I understand it, the zobel is "to stabilize the output stage by creating a pole that reduces high frequency instabilities." and the Thiele for "Provides high impedance at high frequencies so that R may decouple a highly capacitive load and reduce the Q of the series resonant circuit. Also provides a low impedance at low frequencies to short out R and pass audio signals to the load." to directly quote the datasheet.
Or in short terms as I understand it: Zobel: remove high frequency instabilities, and thiele to act as an output RF-filter.


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cheers!

Last edited by hurtz; 21st June 2011 at 01:46 PM.
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