Power Supply filter capacitors
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DF96
diyAudio Member

Join Date: May 2007
Quote:
 Originally Posted by ggidzinski I assume since there is not a single response in support of less than 1,500uF total capacitance (all functions) between the bridge and the chip amp power pins that this approach is no longer considered for new designs? I am going to assume that 10,000uF for smoothing is standard and then you decide on decoupling/snubberizing/etc. In this area I will keep reading.
You could try calculation.

A reservoir capacitor C fed from mains frequency f via full wave rectification (e.g. a bridge) with current draw I will drop by approximately
I/(2 f C) volts
between charging pulses.

The worst case is having to reproduce a low frequency square wave. Then peak current is V/R, where V is supply rail voltage and R is speaker resistance. You don't want the supply rail to droop very much, so say you can cope with a fractional drop of k. So
I/(2 f C) < k V, but I=V/R so
V/(2 f C R) < k V so
1/(2 f C R) < k

For f=50Hz (I am in Europe) and R=8 ohms this means that
1/(800 C) < k
It is up to you to pick a value for k. In the past people might have been happy with k=20% but nowadays k=10% might be better. It depends on the amplifier PSRR, and how much hum you can tolerate. k=10% gives C > 12.5mF. (10mF in 60Hz countries)

 6th December 2017, 01:08 AM #22 FauxFrench   diyAudio Member   Join Date: Dec 2017 LM3886 decoupling Some speak about what capacitance to use after the rectifier, other speak about magic. I like magic and LM3886 can supply a magic sound from a symmetrical power supply. But, let us face facts. If the power supply is a traditional unregulated type, the 100Hz ripple is determined by the size of the total decoupling capacitance (when more capacitors are used in parallel). I believe that a 10000uF capacitor looses 1V in 10ms (the double rectification rate) with a discharge current of 1A. Thus, on each rail you should use some 10000uf-22000uF. Such good-size decoupling capacitors can be bought in low ESR versions (for instance from ELNA) and if the PCB copper-tracks to the LM3886 power pins can be kept short, nothing more is needed. But, another reasonable option exists: Use ordinary electrolytic capacitors (of the same capacity) with two low ESR capacitors (470uF-1000uF) connected in parallel and eventually also two 1uF ceramic/foil capacitors connected in parallel. Then you only need to arrange the two smaller capacitors close to the LM3886 power terminals and the big ones further away. The parallel coupling of the three capacitors cost less than one high quality ELNA and still performs well because the two smaller capacitors reduce the ESR. This trick is frequently used for decoupling of class D amplifiers. If the power supply is of the regulated type, the big buffer capacitors may be less in capacity. The capacitance across the LM3886 power terminals should always be enough to ensure that the LM3886 remains stable (and the copper-tracks should be short). Some 1000uf should be sufficient. For the rest, the regulator must be quick in response and have good surge current margins. These two parameters are a matter of the regulator design I often see amplifier datasheets where the power decoupling capacitors are suggested to be surprisingly small (100uF-1000uF). I assume that these are values that ensure the amplifiers remain stable and do not oscillate. Such small values are clearly not sufficient with just a 50Hz/60Hz rectifier bridge connected upstream.
 6th December 2017, 01:29 AM #23 LinuxGeek diyAudio Member   Join Date: Jul 2015 After trying many different ways to power the LM3886, i found out, that less is more. Mick Feuerbachers 3 resistor copper amp sounded best to me. It uses only 2200uF caps per rail, nothing else. This amp delivers very much detail and very fast response. Mids and highs are beautiful. Forget 10000uF and more smoothing caps if you dont snubberize them. High cap values make the sound very muddy with the lm3886. I was not sure if a single lm3886 is enough for me and use 2 225va torroids, but this is to have the option to go parallel with 2 lm3886s. My torroids are way too strong, but dont go below 100va per lm3886 if you want a punchy nice low frequency.
 6th December 2017, 03:05 AM #24 Panelhead diyAudio Member   Join Date: Dec 2002 Location: Houston What works for me I put snubbers on the large filter caps (10K ufd.). The snubber is a 0.1 ohm resistor and a 47 nfd stacked film cap in series. Then 470 ufd caps a few inches away from the chip. Then 22 ufd caps soldered right to the power pins of the chip. And a 0.22 ufd film connected rail to rail close to the chip. The 22 ufd caps soldered to the legs of the chip make a big difference. __________________ Cheapest is bestest
dotneck335
diyAudio Member

Join Date: Dec 2012
Quote:
 Originally Posted by Panelhead The 22 ufd caps soldered to the legs of the chip make a big difference.
Make a big difference in WHAT, exactly?

phase
diyAudio Member

Join Date: Oct 2004
Quote:
 Originally Posted by scottjoplin It is common practice to use decoupling caps close to the output devices, often in the region of 100uF and 100nF
Seemed like a mystery here so I thought I would bring it up...

The 1000uf parts add a nice sound when close to the outputs, but the 100uf on many amps are usually closer to the input stages from what I’ve noticed, am sure they help the outputs as well though.

Last edited by phase; 6th December 2017 at 04:10 AM.

ggidzinski
diyAudio Member

Join Date: Nov 2016
Quote:
 Originally Posted by LinuxGeek After trying many different ways to power the LM3886, i found out, that less is more. Mick Feuerbachers 3 resistor copper amp sounded best to me. It uses only 2200uF caps per rail, nothing else. This amp delivers very much detail and very fast response. Mids and highs are beautiful. Forget 10000uF and more smoothing caps if you dont snubberize them. High cap values make the sound very muddy with the lm3886. I was not sure if a single lm3886 is enough for me and use 2 225va torroids, but this is to have the option to go parallel with 2 lm3886s. My torroids are way too strong, but dont go below 100va per lm3886 if you want a punchy nice low frequency.
What are examples of other LM3886 implementations that you felt were inferior to the MF 3 resistor design?

scottjoplin
diyAudio Member

Join Date: Dec 2015
Location: Penrhyndeudraeth
Quote:
 Originally Posted by LinuxGeek After trying many different ways to power the LM3886, i found out, that less is more. Mick Feuerbachers 3 resistor copper amp sounded best to me. It uses only 2200uF caps per rail, nothing else. This amp delivers very much detail and very fast response. Mids and highs are beautiful.
Distortion and noise can be perceived as detail and "sparkle" etc
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LinuxGeek
diyAudio Member

Join Date: Jul 2015
Quote:
 Originally Posted by scottjoplin Distortion and noise can be perceived as detail and "sparkle" etc
No, my oscilloscope tells something different. Distortion is rather low, dont know the exact numbers any more. If i find some time i will measure once again.

Last edited by LinuxGeek; 6th December 2017 at 11:24 AM.

LinuxGeek
diyAudio Member

Join Date: Jul 2015
Quote:
 Originally Posted by ggidzinski What are examples of other LM3886 implementations that you felt were inferior to the MF 3 resistor design?
I tried the datasheet implementation before and the carlos version with snubberized high capacity. Micks version sounds more „honest“.

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