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Old 3rd April 2005, 12:41 AM   #1
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Lightbulb Improving the Non-Inverting chipamp

Basically, you should once and for all forget the datasheet circuit.

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.

Also, regarding the use as a minimalist integrated amp, there's a very important point that it seems few care about: using a (good) input coupling cap after the pot, DC-offset will not change at all depending on the position of the pot.

This is the way to go:
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Old 3rd April 2005, 01:59 AM   #2
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Hi Carlos,

Nice to see you onece again pushing the envelope!!!

Will it work also with 3875's?
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Old 3rd April 2005, 04:06 AM   #3
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what is the point of the resistors in series with the output? was one of them supposed to be an inductor instead?

And what is the point of shielding the output from the amp-chip? is that to reduce interference that it might contaminate the input with?

WHy not add a capacitor to the mute pin to give a delay to the turn-on (to remove any possible thumping, especially if part of an integrated amp, when the preamp might not have stabilized yet).

depeding upon the preamp being used in an integrated amplifier, the coupling cap from the wiper of the pot to the amplifier input would not be needed. I would just remove the input resistor to ground and the input capacitor and connect the + input directly to the wiper of the pot, making sure that there will be no DC on the pot. If the pot is driven by a typical transistor/FET stage, the coupling capacitor from the stage to the top of the pot should take care of the DC. IF driven by op-amps, there should be a coupling capacitor in the amp somewhere to take it off. adding a capacitor to the feedback network to bring the gain down to unity at DC is also helpful, too.

I think the zobels snubbing the power supply raisl wouldn't be needed. I would install some smallish film capacitors right at the pins of the IC, though.
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Old 3rd April 2005, 04:29 AM   #4
GregGC is offline GregGC  Canada
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Quote:
Originally posted by Dr. Photon
I would just remove the input resistor to ground and the input capacitor and connect the + input directly to the wiper of the pot, making sure that there will be no DC on the pot. If the pot is driven by a typical transistor/FET stage, the coupling capacitor from the stage to the top of the pot should take care of the DC. IF driven by op-amps, there should be a coupling capacitor in the amp somewhere to take it off. adding a capacitor to the feedback network to bring the gain down to unity at DC is also helpful, too.

I think the zobels snubbing the power supply raisl wouldn't be needed. I would install some smallish film capacitors right at the pins of the IC, though.

You absolutely need the res. to GND in case the pot (with time and depending on its quality) decides not to conduct. You'll get a nice rail DC on the speakers or as a min. nasty cracking (DC/ spikes).

The res in the output are in addition to the RC network and improve the stability of the amp in case the impedance of your cables/speakers have high capacitance component.

I'm 100% convinced that you need the snubbers on the PS pins. It made a huge diff. on my amp. Just the smallish caps wont do anything (personally tried it on my GC). How much difference it'll make depends on the type of the PS caps and how revealing your speakers are.

Greg
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Old 3rd April 2005, 04:43 AM   #5
GregGC is offline GregGC  Canada
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Default Re: Improving the Non-Inverting chipamp

Quote:
Originally posted by carlosfm
Basically, you should once and for all forget the datasheet circuit.

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.

Also, regarding the use as a minimalist integrated amp, there's a very important point that it seems few care about: using a (good) input coupling cap after the pot, DC-offset will not change at all depending on the position of the pot.

This is the way to go:
Carlos,

The only worry I have is the DC offset. I went the same course at one point and got offset more than 65-70mV, so had to increase the res. to GND. It would be OK if no input cap is used, so that at moderate volume levels the DC offset is fairly low. But I wanted to not have DC going through the pot (noise issue) and not to depend on the pot for the DC offset. With low out impedance preamp that kind of configuration will rock... Of course servo would solve the DC offset too, but I have never tried it and no time for now.

I tried the T-network, which actually does exact opposite (increases the ser res. to -IN) and to be honest didn't like the sound too much. It created a bump in the highest frequencies. My speakers are quite revealing and didn't sound as open as the low -in res. classic version. Also I found it's good to connect the FB res from -In to GND to the same star GND where the speaker is connected. The +in to GND res should also go there. And the input GND should go to the same GND-star. The lower the res. values of the NFB the more important that becomes.

Nice work!

Greg
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Old 3rd April 2005, 08:01 AM   #6
Nuuk is offline Nuuk  United Kingdom
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I'll give it a go Carlos.

My 'findings' to date are that I prefer the single feedback resistor to the T-network and I prefer the inverting circuit to the non-inverted.

It may not be obvious to newbies but we should add that this circuit would be used with the snubbered PSU.

I'll be ordrering parts today (so if you have any more ideas please post them before noon )!
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Old 3rd April 2005, 08:14 AM   #7
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Very interesting, as always, Carlos.

I have found that DC offset does seem to make a difference to the sound, and I usually spend a little time fiddling to get it as low as possible. However, this is mostly with IGCs, like Nuuk, I prefer them overall.
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Old 3rd April 2005, 09:04 AM   #8
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Default Re: Improving the Non-Inverting chipamp

Quote:
Originally posted by carlosfm
Basically, you should once and for all forget the datasheet circuit.
It's no harm to have it in mind though.

Quote:
Originally posted by carlosfm
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, 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.

Once again: C2, C3 and C4, C5 does form a parallel resonance circiut so you should get rid of C3 and C5. Why don't you use _only_ C2 tight together with C7+R4?

The IC have it's return currents to ground so I'll suspect that C6 doesn't do much good but not harm either.

Why don't you use mute, pretty convinient and has no drawbacks.

What is the advantage of using 0.25 ohms as output impedance compared to have much lower (using an inductor)?
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Old 3rd April 2005, 10:20 AM   #9
Tor M is offline Tor M  Norway
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Nice woork Carlos!
Do you prefer this amp over your buffered inverted amp?


Tor Martin
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Old 3rd April 2005, 10:43 AM   #10
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All, what Carlos " develope " now, you can find on my PA 03 ( and my version is still better ). Not a long time ago, Carlos had said about my version, that it is s...
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