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Old 8th October 2013, 06:47 PM   #11
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Quote:
Originally Posted by Michael Bean View Post
Opamps need a DC path for input bias on inverting and non-inverting inputs, otherwise you get instability and all that goes with it.
Quote:
Originally Posted by vacuphile View Post
It is not for bias because it is before the decoupling cap, and furthermore 1M would be too large a resistance to provide this bias current; this is where the 22K after the decoupling cap is for.

I haven't any explanation for this phenomenom. The same circuit (without de 1M resistor) works OK when using a TDA2040, but it becomes oscilating with the LM1875 on-board
Anyway, as the datasheet recommends the 1M resistor... just put it.
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Old 9th October 2013, 02:35 PM   #12
johnr66 is offline johnr66  United States
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These high value resistors (1 Mohm in this case) are sometimes used on the input or output to allow the the coupling cap to charge up. If you have the amp on and grounded the input or connected something and the coupling cap was not charged you get a nice loud thud as 1/2 the supply voltage would dump through the cap. Of course, it is not needed if a volume control or signal source is permanently connected to the input.

I do like to see a small value cap across the input to keep the amplifier from receiving its own output and oscillating at (usually) a high frequency.
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Old 12th October 2013, 07:31 AM   #13
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Quote:
Originally Posted by AndrewT
What were the changes?
With regards to post #7 and post #8:
The paralleled 220u caps (instead of solo 220u or solo 470u caps)
The diodes for series element (works better than creativity with cables)
The plausible sized 100u nfb-shunt cap (instead of a wrong size 22u or 47u)
The 100k feedback resistor for 38X (instead of 114k for 43X)
And the input coupling cap value set at 2u2 (instead of 1u)
Quote:
Originally Posted by Michael Bean View Post
Hi Daniel, I'm curious, how do those diodes "filter" DC?
Mike
There's only DC on an audio amplifier power circuit when there is no audio; so, I probably wasn't talking about filtering DC.

A voltage drop followed by a stiffening cap can serve as a filter.

It is more typical to see it at a discrete amplifier's small signal area; however, for the little LM1875, we can assume that the entire amplifier board is a small signal area.
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Last edited by danielwritesbac; 12th October 2013 at 07:47 AM.
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Old 12th October 2013, 09:25 AM   #14
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Daniel, do you have pcb layout for this circuit?

Thanks
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Old 12th October 2013, 10:30 AM   #15
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Sorry, I don't have that layout.
Getting a second set of 220u onto the K50 board takes some effort; however, the rest is almost plug and play. It could use a layout that looks very much like this: http://www.electronics123.com/kits-a...-PCB-only.html
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Old 12th October 2013, 12:19 PM   #16
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by danielwritesbac View Post
..........A voltage drop followed by a stiffening cap can serve as a filter...........
No.
You have been told this before !
Why do you insist on inventing your own version of non science?

Stated correctly it should say:
A series impedance followed by a shunting capacitor can serve as a low pass filter.

The voltage drop has nothing to do with the filtering action.
The dynamic impedance of the diode to the applied frequency is what creates the impedance that is one half of that low pass filter.
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Old 12th October 2013, 05:00 PM   #17
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Is the output resistor 0.5 or 1W? Please reply ASAP please

Last edited by riko4628; 12th October 2013 at 05:08 PM.
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Old 12th October 2013, 06:20 PM   #18
gootee is offline gootee  United States
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Hey Daniel,

Did you ever try taking the positive rail's decoupling caps to the negative rail instead of to ground (with the negative rail's decoupling still going to ground)? For many chip amps, that makes more sense. For the reasons why, see Klaus's post, at:

Resevoir capacitors for Chip Amps

Edit: And you can see the effect on the rail voltage ripple in simulations, in my post at:

Resevoir capacitors for Chip Amps

Cheers,

Tom
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Last edited by gootee; 12th October 2013 at 06:30 PM.
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Old 12th October 2013, 07:15 PM   #19
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And second thing, is the 2u2 capacitor electrolytic? Do you mean 2,2uF Ceramic?
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Old 12th October 2013, 07:21 PM   #20
gootee is offline gootee  United States
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riko4628,

I think that the 2.2 uF input coupling capacitor should be a film type, e.g. polypropylene or polyester.

Cheers,

Tom
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