LM3886 Pin7(gnd) current?

[Dxvideo]

Dear Andrew,

Sorry for late answering. But I am working on my new Class A project as you know.
Anyway,
Yes as you see CS1&2 is on bottom layer. I saw a lot of benefits to keep there close to the power pins, especially on HF interferences.
For rail to rail caps; I've tried that with many different values of caps; 47uF + 100nF, 100uF + 470nF however I couldnt hear any difference on sound. Some diyers recomment that way very strongly but I cannot. For example in my LM4702 project I applied the original factory design for MOSFETs. And in this configuration there was a 100nF poly between rails. I tried the circuit first with that cap and I feel some bass weakness, after then tried without it and the same bass weakness was still exist. But when I put extra 1000µFs on each rail to gnd then bass response was improved A BIT but not much.
And for the unused pins. Generally I trim that pins, so they never be soldered to the PCB anyway.
...

Dear Glenn,
I havent noticed that pin 7 is sinking 1mA current. Thats interesting. Is this the main difference between LM3875 and 3886? Because as you know, there is no GND pin on LM3875.
Thx for the reminding.

[AndrewT]

Originally posted by Dxvideo
I saw a lot of benefits to keep there close to the power pins, especially on HF interferences.
For rail to rail caps; I've tried that with many different values of caps; 47uF + 100nF, 100uF + 470nF however I couldnt hear any difference on sound. Some diyers recomment that way very strongly but I cannot. For example in my LM4702 project I applied the original factory design for MOSFETs. And in this configuration there was a 100nF poly between rails. I tried the circuit first with that cap and I feel some bass weakness, after then tried without it and the same bass weakness was still exist.

I would not expect any change in handling bass signals.
It's main (only) advantage is the way it allows the chip to handle fast transients.

[digi01]

connect to sg,or through a 10uH inductors to sg.

Zang

[KSTR]

Originally posted by glennb
It would be interesting to add a 10 Ohm resistor between the clean signal earth and the dirty power supply earth, and then measure the output THD etc when connecting the GND pin to either the clean or dirty earth. It would prove the point one way or the other and provide a magnitude of the effect. It may vindicate or eradicate my assumptions

Maybe more effective might be to couple some small test signal (100Hz bw-limited square) directly into the GND pin and see how much feeds through to the output, with no signal at the input. Most easily seen with a high noise gain setup (like 100 or so).

Makes me think about the option to generate a seperate GND potential for that pin that is stable with reference to the neg supply. When there are internal current sources etc. attached to the GND pin that also might explain the weak -PSRR curves of the chip, in case this isn't mainly due to the miller cap -- creating the amps dominant pole -- which semms to reference to the V- supply. And maybe the Miller cap is actually hooked to the GND pin. A lot of vague guesswork...

Regards, Klaus

[KSTR]

Originally posted by Mick_F
Nice to see you here Klaus. The Forum will benefit from your knowledge!

Hi Mr. Dogbreath, I'm pleased. One thing for sure, I will (and did already) benefit way more from the forum that it will ever benefit from my modest contribution. There's a lot of very competent people around, let alone ubertechs like Mr. Pass et al.

Regards, Klaus