Improving the Non-Inverting chipamp

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loong

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carlosfm said:
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.
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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?
 
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carlosfm

Banned
Joined 2003
Re: Re: Improving the Non-Inverting chipamp

loong said:
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?
Click to expand...

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.
 
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carlosfm

Banned
Joined 2003
XELB said:
How does it sounds ?
Click to expand...

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

XELB said:
I saw the picture of the inside of your amp. Do you have the PSU outside the amp ?
( I will put my PSU outside )
Click to expand...

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

XELB said:
Carlos, can you tell me where to buy a nice and cheap 10k pot ?
Click to expand...

You want cheap and very good?
Get these Alpha pots from Jaycar UK:
http://www1.jaycarelectronics.co.uk...d2=&pageNumber=&priceMin=&priceMax=&SUBCATID=
 
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jleaman

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Re: Update: a hell of an amp

carlosfm said:
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...:eek:
:bawling:

This is it!:cool:
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:
Click to expand...

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

Banned
Joined 2003
XELB said:
Carlos, have you ever tried to annul the DC OFFSET and take the capacitor C1(2.2uF) out of the LM3886 input?
Click to expand...

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.:dead:
 
X

XELB

Member
Joined 2004
To null the DC-offset of my "pre" SE to differential converter, I used this technique :

(the psu will be regulated(12V))

An externally hosted image should be here but it was not working when we last tested it.








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


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



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

Banned
Joined 2003
XELB said:
To null the DC-offset of my "pre" SE to differential converter, I used this technique :
Click to expand...

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

XELB said:
I think this way I will not need the cap C1 in the amplifier input :D
But I still have to test it :dead:
Click to expand...

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).:hot:
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.
 
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XELB

Member
Joined 2004
I will use in the positive rail 2x LM78M12 and in the negative rail 2x LM79M12, I think this should do it.

The main reason I am doing this "pre" is because I will use a bridged chip for each channel.
I think you only take the most of bridged amps when you have balanced input. Don't you agree ?


My amp will have a 20x gain.
I will only connect this amp to the same cd player.
I will tune with the cd player connected(input) and then with the input and output connected.
I will not take chances! I love my speakers :bawling:


Have you haver tried to do this technique but with automatic regulation ? In other words, the "pot"(or chip pot) regulates him self to null the DC-offset.
 
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carlosfm

Banned
Joined 2003
Xelb, you don't need that circuit to null DC-offset for the DRV134.
You will have very low DC-offset with that chip.

I did use it on my AD815 as it's a current-feedback op-amp, with quite high input and output DC, and I didn't want to use caps.

I don't use input or output coupling caps with preamps, and even more so with normal op-amps (specially fet-input).
With my OPA627+BUF634 pre I had 0.2~0.3mv dc-offset.
 
X

XELB

Member
Joined 2004
Carlos, one day we should test our amps.... drink some beers, listen to some good music and in the end, you will tell me that my amp full of crazy things sounds much better :p



Ps: I think it's better to test in your beloved speakers, I don't want to burn mine :dead:





joke ;)
 
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XELB

Member
Joined 2004
Upupa Epops said:
ToXELB : DRV 134 is not good sounding circuit. Try to make it with another opamps. ;)
Click to expand...


Thanks for the advice but I am only using the DRV134 to convert a Single Ended signal to Balanced.
(the convertion will give me a 6dB gain, that why I call it "pre")

I don't know other way to convert...
I have tried to find analog devices products that could do this, but i could not find them!
I know they have Balanced to single ended but I don't know if they have the apposite.
 
U

Upupa Epops

R.I.P.
Joined 2003
DRV 134 have little bit " synthetic " sound, for realy high claims isn't OK. Is easy implement, if you can simply solution, but it's all. If you can compromisless solution, circuit will be not so easy to build ;) . By my opinion ideal solution will be next : Input amp : AD 8065 ( or OPA 627 ?). Output amps : TPA 6120A, one channel noninverted, second channel inverted. These both channels with DC servo. Both stages with own voltage regulators, dual mono construction. I mean, that this connection give best results, but will be much more expensive than first one :cool: and PCB must be very carefully designed :cool: .
 
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