Improving the Non-Inverting chipamp

[carlosfm]

Originally posted by Nuuk
I'll be ordrering parts today (so if you have any more ideas please post them before noon )!

Still before noon.

Sorry, on my yesterday late schematicing I forgot to include two components on the input.
Here's the revised schematic.

[carlosfm]

Originally posted by peranders
Carlos, if you _do_ read the datasheet you'll realize that your claim isn't right. If you are unlucky you may get a LM3886 with 10 mV and that gives you 300 mV offset regardless of resistor values.

Notice that I'm not claiming.
30mv is what I have on this working amp, that is
in-in on my main system right now.
Also, if you change R1 (15k) for a 10k resistor, DC-offset will be a little lower, at around 25mv.
I used 15k because I had some good 2.2uf caps I wanted to use.

Originally posted by Upupa Epops
All, what Carlos " develope " now, you can find on my PA 03 ( and my version is still better ).

I wouldn't say that, but you are free to sell your fish.

Originally posted by Upupa Epops
Not a long time ago, Carlos had said about my version, that it is s...

Yes, because of the PSU.

[carlosfm]

As I say on the schematic, this is a power amp.
And for that role, it doesn't need an input buffer.
What it does need is a good active pre.
Notice that I give the option for a "minimalist integrated amp", which is just using a pot and increasing the gain of the amp.
I thought I should mention this option, even if it's not my favourite.

For an integrated amp I would use a gainstage at 2~3x gain and keep the gain on the LM3886 the same.
This is the way to go.

I have a preamp, with a gain of 3x.
This NI power amp doesn't need an input buffer if it's used as a power amp.
Even as a power amp, with the Inverting topology, it is recommended (and beneficial) to use an input buffer.

So, in conclusion, using the chip as a dedicated power amp as I do, I'm having very good results with this amp (exactly as on the schematic), better than what I tested before with the NI topology and the commonly used values.

[carlosfm]

Originally posted by peranders
Why don't you use mute, pretty convinient and has no drawbacks.

Aha!
At last a good question.

I just use a 10k resistor to the negative rail, never used the cap.
And there's absolutely no turn-on thump, with the high cap. snubberized PSU.

Why don't I use the cap on the muting circuit?
I don't feel very well inserting more capacitance on the negative rail, even it it's after a 10k resistor.
Maby it's just me, but I don't do that.

[GregGC]

Carlos,

You know, I've been looking at the 0.27 Ohm res at the output for so long and never actually implemented it until now (didn't believe it would make a big diff.). So, looking at your schematic I decided to connect it today. I used an ugly ceramic 5W resistor because it has some extra inductance and I think it's perfect for that place. Well, It did improve the HF. It looks like the amp has had some slight instability problem and that resistor made it sound more balanced, not as forward in the HF area. I did have the Zobel network before, but I guess it's not enough in some cases. Now I have to revisit some other variations of GC including the t-network (the advantage of much lower DC offset). One never knows....

Greg

[carlosfm]

2 // 0.47R = 0.235R.

Yes Greg, you did well.
MY suggestion for the zobel includes these.

[shusha]

I used an ugly ceramic 5W resistor because it has some extra inductance and I think it's perfect for that place. Well, It did improve the HF.

Well, I've done the same thing. Also put some 0R22 ugly ceramics (5W) and I think that sound in HF changed somehow, but will know more after few days/weeks.

BTW, just finished snubber this weekend, had no chance to implement it yet. I have listened it while I was working. LM3875 NI was connected to my mini monitors in workshop (Visonik David) and there were certainly difference in sound between my 338 reg PSU and snubber. Will try next week on main system.

[Vix]

Hi

Carlos, your Gc now gets me thinking into more options, as:

1. Use it with a Bride of Zen preamp, it should be more than enough,

2. Try to use a tube, but not only as a buffer, but also with some gain??

3. Since 0,22 ohm resistor has been added , maybe Zobel can be removed.

If I am not mistaken, Peter Daniel does not use a Zobel...

Regards

Vix

[cjd]

Carlos:

Have you experimented with the output resistor value? Or more specifically, why ~0R22?

I am still gathering parts for that SUSY amp but you now have me pondering component values again. Fortunately, a good selection of the small wattage resistors in various values is cheap relative to the other parts so I can easily afford to experiment. The larger output resistors are a tad more, so I'm curious. Since my amp will use the LM4780 with the amps paralleled I already had plans for 0R1.

And, of course, I have to know - how do you think this compares to the inverted amp?

C

[carlosfm]

Originally posted by cjd
Have you experimented with the output resistor value? Or more specifically, why ~0R22?

I didn't test 0.1R, but it seems very small to have any effect.
A resistor in series with the output, as with any op-amp, makes the chip stable driving capacitive loads.
The amp is much less picky with cables.

Originally posted by cjd
And, of course, I have to know - how do you think this compares to the inverted amp?

Right now the only inverted LM3886 I have has a regulated LM338 PSU.
The non-inverted, non-buffered LM3886 as per this schematic is sounding better, but the PSU is also different, so I can't make a direct comparison.