LM3886 minimal vs. additional components

[kgb_m3]

I've put together LM3886 following this guide Mick Feuerbacher Audio Projects

It sounds great, but before I'm going to finalize the build inside an actual enclosure, I would like to ask those of you with some experience about the following missing parts:

Input capacitor - what value? do I even need one?
Zobel network - do I need one? If so what values? 2.7R with .1uF seem common, but I keep seeing some with caps to ground and some with resistors to ground. Which way is better?
Input resistors - I'm planning on running a dual gain log pot, what value? and do I still need 1k and 22k input resistors?
Last I would like to know if a 220pF cap between pins 9 and 10 should be used or isn't needed.

My build is here: My first step by step LM3886 illustrated build and I would really like to figure out the things above prior to continuing on any further.

I'm using a high end PC sound card Auzentech X-Fi Forte as a preamp/input source along with Polk Audio Monitor 30 bookshelf speakers (for now)

Thanks in advance!

[tiltedhalo]

1st, the input capacitor is suggested, but if you know your preamp is cap coupled or has 0 or very low offset you can get away being DC coupled. Remember offset will be multiplied by the gain factor of the amp so a cap is the safe way to go. Go with a good quality poly 1uf will work with the input impedance of 22K, if you want to get as close to DC as possible use a 4.7uf and bypass it with a close tolerance 1% .01uf film and foil.
2nd, depends on the stability and load, I would include a zobel just because. The standard 2R7 and .1uf will not affect upper end at all until about 30K, the order most common is resistor, capacitor, ground.
3rd, the 220 or 330pf is for RF rejection, if its quiet and does not pick up RF hash you can live without it, if you need it use a high quality silver mica.

[kgb_m3]

Thanks a lot for your quick help. Everything make sense except for the input resistors. Since I'll be running the pot, I'm still not sure what value 10k, 25k or 50k, it confuses me with the rest of input resistors. Some schematics show the 1k ohm to 22k grounded to input, some don't have 1k at all, and have the input cap in the place of it. So I'm not sure which ones to use and what values. The sound card has 330R output impedance that should be matched quite high on the amp input, it also has the dc coupling output caps so I'm ok running it without the input cap. What if I want to hook up an ipod or similar, can that push dc voltage into the amp?

[tiltedhalo]

Oh sorry, Your using a pot you will cap couple 50K is a happy medium and your input cap value will remain the same.

[ratza]

In order of importance:

- Keep the 1k and 22k if you don't want nasty behaviour. If the cursor of the pot doesn't make contact anymore, the output can go either to +VCC or -VCC, killing the speakers.
- Input cap is also something I wouldn't remove. Any DC on input will go to output, again, killing the speakers.
- The Zobel network makes the output impedance more linear. It also works without it, but it's good to have it.
- 220pF cap between input and ground and/or between inverting input and non inverting input will kill any RF noise which may be picked up by the IC, e.g. pops and clicks when other appliances are switched on and off. I only have it between the IC's inputs.

[AndrewT]

Originally Posted by ratza

In order of importance:

- Keep the 1k and 22k if you don't want nasty behaviour. If the cursor of the pot doesn't make contact anymore, the output can go either to +VCC or -VCC, killing the speakers.
- Input cap is also something I wouldn't remove. Any DC on input will go to output, again, killing the speakers.
- The Zobel network makes the output impedance more linear. It also works without it, but it's good to have it.
- 220pF cap between input and ground and/or between inverting input and non inverting input will kill any RF noise which may be picked up by the IC, e.g. pops and clicks when other appliances are switched on and off. I only have it between the IC's inputs.

I agree completely.
It is false economy to omit the optional components.
I would also AC couple the amplifier by adding a DC blocking cap in the feedback line.
I would much rather follow the logic that a working system with some defined sound quality compromises is far better than a broken system that plays nothing.

If your source can drive 10k//cable capacitance, then use that as a passive pot. 25k and 50k are too high for passive pot.

Aim for an input high pass filter F-3dB<4Hz and preferably <2Hz.
This require a 5u6F to 10uF, MKT or MKP, DC blocking cap in either the source equipment or in the receiving equipment. Note either, not both.
You can move the DC blocking cap to after the passive pot and this allows a lower value capacitor since it sees a higher impedance after it.

[wwenze]

Originally Posted by AndrewT

If your source can drive 10k//cable capacitance, then use that as a passive pot. 25k and 50k are too high for passive pot

Why so?

[AndrewT]

the maximum output impedance of a 25k pot is 6k2. The usual recommendation is that the input impedance of the stage following any source is >=10 times the source impedance. I prefer >=20times to reduce the interaction between the pot and the receiver. That leaves you with an input impedance >62k and a source impedance varying between 0r0 and 6k2 joined with a cable that has reactance. This will interfere with the audio signal.
A better solution is a 10k pot and an AC coupled input that has an input impedance of 25k to 51k combined with very short cables/traces that are optimised to minimise reactance between the pot and the input stage.

[ratza]

Jeez, I forgot another important part: the feedback capacitor. I would say to try the both configurations, with and without capacitors, you'll see that there's not a BIG diffference, if any is perceived at all.

[a.wayne]

Has anyone here tried running 4 LM3886 per side ? what would be required to do so ?