LM3886 optimisation and power supply review

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Hello all!

I need an amplifier for bedroom or small room to listen CDs and TV. I've decided to build a LM3886 based small amplifier.

Very good sound quality is a must, but I am not willing to sacrifice everything for the best in class. The size of the amp (and also the cost) was on very high scrutiny. :)

After reading some posts here, the result is on the next two diagrams:
lm3886_additional_components -> optimization of the original
power_supply -> passive and simple enough

Based on experience, I would like your help to review the above schematics and also I’d appreciate any advices or ideas.
Thank you
 

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Cin is in the wrong location. It must be after the pot.
The 1uF // to the 470 nfb cap is not required.
The 1uF on the PSU are not required.
All the 100nF caps around the secondary and the rectifier must be converted to R+C snubbers.
The 1nF caps on the primary must be Y rated.
You can add an X rated cap from N to L.
Cs can be simplified back to the National recommendation. That works well.

The 100r//100n could kill you, or another operator/user if left in the location shown.

You must ensure that the route from Main Audio Ground can pass mains Fault current to Chassis and then to Protective Earth (PE)
 
Last edited:
Thank you Andrew for your help: some answers below:

Cin is in the wrong location. It must be after the pot.

- please elaborate, what is the difference? the input impedance would require bigger Cin

The 1uF // to the 470 nfb cap is not required.

- it is there for the second variation, the one with polarized cap

The 1uF on the PSU are not required.

- You're right, it its a general PSU with the 1uF there

All the 100nF caps around the secondary and the rectifier must be converted to
R+C snubbers.

- as above, space is a premium, R at this power needs some cooling

The 1nF caps on the primary must be Y rated.

- they are Y

You can add an X rated cap from N to L.

- it is already there

Cs can be simplified back to the National recommendation. That works well.

- Ok, I;ll try the simple way

The 100r//100n could kill you, or another operator/user if left in the location shown.

- aware of that, this will implemented only if the case will be metal. On the other hand, space for a bridge....

You must ensure that the route from Main Audio Ground can pass mains Fault current to Chassis and then to Protective Earth (PE)

Do you have any other ideas or advices?
 
See my thread "LM3886 without electrolytic feedback capacitor and (hardly) no DC offset voltage". You can save an electrolytic capacitor and good ones are expensive.

The pot must be placed before Cin. The DC regime is not destroyed because of the R of 20 Kohm after the Cin.

There is no need for Cc of 220 pF. See my thread "LM3886: Stability at high frequencies and output level".

Marc.
 
Thanks for feedback, I would like to point out that the cap in the feedback is not electrolytic, most likely is a non polarised and very affordable price. Could be used MUSE 100µF - 1,60 € or MUSE 470µF - 2,70 €.

Condensateur radial audio MUSE - 25V - 100µF - Condensateurs

Condensateur radial audio MUSE - 25V - 470µF - Condensateurs


See my thread "LM3886 without....". You can save an electrolytic capacitor and good ones are expensive.
.........
Marc.
 
Dear cvpho1, When you place the C after the pot, you avoid the change of R through which the bias current flows. Otherwise it will flow via lower part of the pot and this will cause additional offset problems. A resistor +in to earth is a must of course.
 
Good table!
I could suggest to estimate the offset expectations in advance, using data sheets and circuitry. If you achieve very different results in practice, you've certainly made some mistakes.
You should look for explanation of every offset you got. Not just to say "It's admissible"
 
Measure the change in output offset as you change volume setting.
Measure the change in output offset variation (for a fixed setting of volume) as the amplifier warms up from cold to operational temperature.
Repeat this at extremes of volume pot settings. I suggest Top (-0dB), half volume (-6dB), Bottom (-infinity dB) and at the volume setting that matches +IN resistance, to -IN resistance (this maybe around -30dB).
 
First measurements, running idle

Using 100k log ALPS pot and 2 x LM3886 (just bought two new LMs, not paired or tested before) here are the first results:

COLD
L R
3mV 2mV min 0%
1mV 0mV med 50%
-2mV -4mV med 75%
-9mV -10mV max 100%

WARM after 30 min
L R
3mV 2mV min 0%
1mV 0mV med 50%
-2mV -4mV med 75%
-8mV -10mV max 100%

WARM after 60 min
L R
3mV 2mV min 0%
1mV 0mV med 50%
-2mV -4mV med 75%
-8mV -10mV max 100%

Note 1: the "warm" measurement is done after a period of running without any input signal and no load connected (temp went from 29C to 38C).

Note 2: above is mentioned (in %) the position of the pot, not the value of its resistance



Measure the change in output offset as you change volume setting.
Measure the change in output offset variation (for a fixed setting of volume) as the amplifier warms up from cold to operational temperature.
Repeat this at extremes of volume pot settings. I suggest Top (-0dB), half volume (-6dB), Bottom (-infinity dB) and at the volume setting that matches +IN resistance, to -IN resistance (this maybe around -30dB).
 
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