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Old 13th December 2011, 11:49 AM   #11
diyAudio Member
Join Date: May 2008
The equivalent schematic shows pin7 connected to four components.

One provides the ground reference for internal feedback/compensation, which makes it audio and compels its connection to audio ground. Being a transistor's base current limited by a 10k resistor, whatever flows there should not be much of an issue.

The second is the reference for the mute circuit, which should be DC as long as the amp is unmuted and should not have bad effects due to its linear character and low signal level.

Third and fourth are protective diodes that should only pass current when too high voltage is present at the output, e.g. due to back EMF. Current will only flow, when too much reactive energy is present, and if that situation ocurrs, having a clean audio ground or not won't make much of a difference.

I'd connect pin7 to audio ground and rather worry about the missing DC blocking cap, missing Thiele network and the limited size of C555.
If you've always done it like that, then it's probably wrong. (Henry Ford)
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Old 16th December 2011, 02:00 AM   #12
benb is offline benb  United States
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Here's a reference (seen in another chipamp thread) to the Pin 7, or Ground pin polluting the input/audio/"signal" ground, with further down a statement that the LM4780 (which the article is about) is just a dual LM3886 and has two LM3886 chips in the package:

LM4780 Dual 60W Audio Power Amplifier

I note there's a 2.7 ohm resistor between the input ground and the output-and-power ground, and the chip's ground pin is shown as "better" going to the output-and-power ground. That is further explained in this discussion:

LM4780 Audio Power Amplifier - Glenn's Design Log

I tend to agree with pacificblue on this, on the other hand, looking again at the layout in the OP, there's no cost in connecting Pin 7 directly to the "power" ground through a PCB trace or (as looks easier on this single-sided layout) a jumper wire.
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Old 16th December 2011, 06:27 AM   #13
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National's reference design shows the ground pins connected to audio ground and they also use a resistor to separate audio from power/speaker ground.
If you've always done it like that, then it's probably wrong. (Henry Ford)
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Old 16th December 2011, 10:54 AM   #14
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
Those two links show the same message about the GND pin of the chipamps.
I saw a similar report by a different author.
He went into a bit more detail of the current variations (noise) that came out of that pin. That's what convinced him and me that GND PIN must not be connected to Signal Return (Signal Ground).
There is no harm if the GND PIN is connected to the Main Audio Ground. That's where all the other grounds get their reference from. That difference in tapping point is shown as "OK" and "better" in the diagram
regards Andrew T.
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Old 16th December 2011, 03:05 PM   #15
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According to the links in post #12 the ground pin is not a signal reference, but the equivalent schematic shows the opposite to be true. The current draw of 3 mA does not coincide with what must be expected to flow into the bases of two small-signal transistors that only drive mAs of their own. The assumption that modulation currents flow and interfere with the amplifier's sound quality is not proven in any reproduceable way on that homepage.

Maybe you can come up with that more detailed report from the different author with hopefully more convincing measurements.

Until then I can only advise to follow proven practice and use the same reference for the internal and external feedback.
If you've always done it like that, then it's probably wrong. (Henry Ford)
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