Improving the Non-Inverting chipamp

[loong]

Originally posted by carlosfm
There are advantages in using very low value resistors, and also with (consistently) manageable DC-Offset, so that the Ci cap can be discarded.

It works, it's simple, it sounds very good.
Even without matching impedances (because I use very low values, and the amp would need a buffer), DC-offset is around 30mv.

Carlos,

I did some quick experiment on LM3875 (24Vdc) using the value of R1 R2 R3 as 22K, 301 and 10K respectively. I am afarid the difference is not noticable in my system but the DC offset is very high ~600-900 mV. Is there something wrong with using these value of res?

[carlosfm]

Originally posted by loong
Carlos,

I did some quick experiment on LM3875 (24Vdc) using the value of R1 R2 R3 as 22K, 301 and 10K respectively. I am afarid the difference is not noticable in my system but the DC offset is very high ~600-900 mV. Is there something wrong with using these value of res?

What do you have before R1?
Input coupling cap?
A volume pot?
An active preamp?
A buffer?

If you respect the values of my schematic and only increase the value of R3 (feedback), if you need more gain, it should be ok.

[XELB]

Originally posted by carlosfm
No, I'm not joking.
I have this at home.

How does it sounds ?




I saw the picture of the inside of your amp. Do you have the PSU outside the amp ?
( I will put my PSU outside )




Carlos, can you tell me where to buy a nice and cheap 10k pot ?




Thanks.

[carlosfm]

Originally posted by XELB
How does it sounds ?

The Krell?
Powerful.
After the mods, a very good integrated.
Before?

Originally posted by XELB
I saw the picture of the inside of your amp. Do you have the PSU outside the amp ?
( I will put my PSU outside )

Yes, I use an external PSU with this amp.
Regulated, snubberized.

Originally posted by XELB
Carlos, can you tell me where to buy a nice and cheap 10k pot ?

You want cheap and very good?
Get these Alpha pots from Jaycar UK:
http://www1.jaycarelectronics.co.uk/...Max=&SUBCATID=

[XELB]

I have to order at least £20 in components


But I was needing some new parts


BTW, thanks for the link, I didn't know this online shop

The Krell?
Powerful.
After the mods, a very good integrated.
Before?

You just never stop "cooking" ......

[jleaman]

Originally posted by carlosfm
With the second snubber near the chip, there are advantages in using bigger caps than 100uf.
I started testing with 470uf and yes, extreme bass gets tighter.
Then I tested 2,200uf and...


This is it!
Midband and treble are gorgeous, as ever, but the bass is really on another league.

PS: If you are wondering, I do not test amps with choral music.
There's a good recording from the beginning of the 90's (!) that always was a good test: Counting Crows - "August and everything after".
Believe me, very few amps can drive my Epos speakers with this disc.
This one does it:

Really good band / Artist. i like the song Ohma : O ) Or that whole cd for that fact : O )

[XELB]

Carlos, have you ever tried to annul the DC OFFSET and take the capacitor C1(2.2uF) out of the LM3886 input?

[carlosfm]

Originally posted by XELB
Carlos, have you ever tried to annul the DC OFFSET and take the capacitor C1(2.2uF) out of the LM3886 input?

C1 is there to protect eventual DC-offset coming from the source, it has nothing to do with nulling DC-offset of the amp.
To null DC-offset to 0mv on the NI amp you can insert a cap between R2 and ground.
But as I use (and recommend) very low value resistors, that cap would have to be big and it would have impact in the sound.
For 100R (R2) the cap would have to be 470uf.

[XELB]

To null the DC-offset of my "pre" SE to differential converter, I used this technique :

(the psu will be regulated(12V))









The problem is that I have to tune the pot.... with some regularity


I think this way I will not need the cap C1 in the amplifier input
But I still have to test it... don't know if will work



Edit: I don't know if the symbol of the pot is correct

[carlosfm]

Originally posted by XELB
To null the DC-offset of my "pre" SE to differential converter, I used this technique :

I used a similar technique on my AD815 pre.
But you must have a well regulated PSU.
At least double regulation for that voltage.

Originally posted by XELB
I think this way I will not need the cap C1 in the amplifier input
But I still have to test it

I don't dispense the input cap on the amp.
Gives me free of mind to connect anything to my system.
Some sources have (not negligible) DC-offset.
In your case that trimmer drifts, and DC-offset on the output of your pre will be amplified by the gain of the power amp (20~22x).
A source component that hits 10mv DC-offset on the outputs (even if it's only with music playin') will give you 200~220mv DC on the output of the amp.