Lme49830

[nickarsow]

O.K. - I understood the question:
Header2 - pins 2 and 3 = PGND;
Header1 - pin 2 = AGND

 

[peranders]

H2 has 15 poles but I can't see them all in the schematic. Somewhere your board must have ground connected. Where?

 

[AndrewT]

C20 should not be connected to the dirty ground.
Pin3 should not be connected to the clean ground.

Your schematic shows two types of ground connection:- Agrnd (=clean), Earth (=dirty). How do these get off the PCB to the main star ground inside the amplifier chassis?

I would make provision to add a small resistor or inverse parallel diodes between the Aground and it's lead off to the main star. You can use a shorting link or install the diodes (1n4002) or the resistor.

 

[peranders]

O.K. - I understood the question:
Header2 - pins 2 and 3 = PGND;
Header1 - pin 2 = AGND

Since you use Altium Designer it would be a very good idea if the schematics corresponds to 100% to the layout. You have an excellent tool for this. After you have done some change in the schematic, do an "import changes from schematic" after each time you have changed, then you'll have your project in 100% sync.

If you want reviews of your design it would help you and also us if we talk about the same thing.

I would recommend you to not have more than one connection in the schematics at a symbol. A 15-pole part should have 15 visible connections in the schematic also.

 

[nickarsow]

Dear P-A,
Here's the corrected sch

 

[peranders]

That's better but AGND and GND should be tied together with at least one resistor as Andrew points out.

 

[nickarsow]

There is a connecting resistor - R100 = 10R SMD 1206

 

[nickarsow]

So sorry - I missed it in the SCH.
Here is the final one.

 

[nickarsow]

here

 

[AndrewT]

what advantage does the opamp give cf. feeding the 49830 direct?

 

[AndrewT]

I think it was MJL or KP that showed inverse parallel diodes across that ground linking 10r resistor.
After thinking about it, I came to realise that adding the diodes offers a safer solution and that the resistor can probably be omitted leaving just the diodes to isolate the two grounds.

 

[nickarsow]

You are right.
What about the OpAmp - nothing but price and small Cin.

 

[nickarsow]

I mean that the price of LM833 ( could be used instead LME49720 ) is about USD0.3 , and the small Cin=1uF( 25V ) are cheaper than one BIG cap of 10uF/100V

 

[AndrewT]

but you can omit the opamp and eight other components, to feed the 49830 with the same input filters that you show feeding the opamp.

 

[nickarsow]

Then explain the original usage of MKP 10uF/100V which is huge and too expensive?

 

[nickarsow]

Just for fun - see the 3D bottom view

 

[AndrewT]

Hi,
set Zin of the 49830 to 91K and use 1uF as the DC blocking capacitor.
F-1dB ~3 to 4Hz. Add on an appropriate series resistor and grounding pF cap and you have both your input filters. Add another pF cap across the input RCA.
Just make sure that the amplifier has a bandwidth wider than that set by the input filters.

You can now use a small 47uF as the DC blocking cap in the NFB loop for a gain of ~26 to 30 times.

 

[nickarsow]

Thanks AndrewT,
This is very usefull proposal!
When I have a little time I'll modify the PCB - that should be version V2.0.
Thanks again
Nick

 

[jackinnj]

Since you have a buffer, why not try inverted? You won't have to worry about the impedance and it WILL sound better.

 

[nickarsow]

Dear Jackinnj,
Could you draw your idea?