LM3886 component selection
First, I have read through a lot of different threads, but haven't found exactly what I was looking for.
As I didn't want to hijack several threads, I decided to start a new one.
I already have successfully designed and built some microcontroller circuits, but I'm quite new to audio design.
Now I want to build a small amp do drive (diy) PC speakers, and I have chosen the popular LM3886.
I generally like my projects being reusable, so everything that could be of use later on should be included.
I hope this was enough background information, so let's start with my questions:
PS: I'm not into that high-end-voodoo-thing :D, I just want to build an optimal, reasonable and stable design.
Ordinary polarised electrolytics are good enough for Ci. But, you must ensure they have virtually no AC across them. i.e. make them big enough (low AC impedance) and any distortion becomes unmeasurable.
If the amplifier never oscillates then the Rsn can never get hot.
If you intend "testing" the amplifier at high frequency, such that you end up "driving" a signal through Rsn, then you may want to make it big enough for however much power you intend to dissipate in it. That could be 60W of continuous 100kHz !
The Rsn + Csn = output Zobel.
Thiele proposed a different stability enhancing Network that had both the Zobel and an output inductor. I refer to this as the Thiele Network. Dr Cherry did more work in explaining the Thiele Network and the infinite variations that can exist.
The amplifier can only heat up the Thiele Network for similar abuse as explained earlier. Do you plan to amplify >100kHz signals?
R//L pure air core. Keep the air core away from the amplifier stage and if possible away from steel chassis. Run the R outside the coil, not through it. I have not seen data for the effect of running the resistor inside the coil, so I'm recommending "play safe".
BTW I've never been convinced by that PS, read here.
I thought of using a bipolar or un-polarized cap because the output swings around GND, and so will the voltage the cap sees.
I planned to make Rin=22k and Cin=4.7µF, which results in a corner frequency of ~1.5Hz.
Ri will be 1k and Ci=180µF, which results in a corner frequency of ~0.9Hz.
I hope this will be big enough, or am I wrong?
But how can I prevent the amp from oscillating?
Is it just the component placement?
Or can I do anything more to achieve this, like the optional components Rf2 and Cf?
By the way, the formula for the corner frequency of that network is specified in the datasheet as follows:
fc=[Rf1 Rf2 (s + 1/Rf2Cf)]/[(Rf1 +Rf2)(s + 1/Cf(Rf1 +Rf2))]
What is "s"?
The (worst case) 60W of dissipation, when the snubber becomes active, has lead to my question of a reasonable size.
But as I think about it, IF the amp oscillates, it fries the resistor whichever power rating it may have.
So it is quite important (at least for me) to prevent the amp from ever oscillating, as I want an audio amplifier and no RF source.
Those Elna RJH seem not very well available...
I'll have a look at the other types you suggested, and at your thread.
The NFB DC blocking cap has a very small DC voltage across it.
This DC voltage is due to the input offset voltage of the amplifier. This input offset voltage exists at BOTH the +IN and -IN inputs.
The AC voltage across the DC blocking cap should be very small and the peak voltage is likely to be less than the DC bias across it. This addition of two very small voltages is usually never a -ve bias across the polarised capacitor.
In addition some of the polarised capacitor manufacturers state that a reverse voltage of < ~1Vdc is OK for their capacitors.
I can see that when the DC blocking capacitor is too small and thus develops a significant AC voltage across it that the added distortion of that too small capacitor will have an audible effect. I suspect that many have heard this and without realising the cause concluded that the NFB DC blocking capacitor has a "sound" effect.
When the capacitor is sized such that there is virtually no signal voltage across it then the distortion added by the capacitor is inaudible.
This is something I have found in practice. I cannot hear the "sound" changes alluded to by other builders.
Thank you for the explanation AndrewT, I really appreciate your help!
I have read a lot from you in other LM3886 threads.
Here is what I have done until now.
The schematics is still a bit messy, because I have changed it several times after studying several design "guidelines".
The color codes for the layout are:
Red: top copper layer
Green: bottom copper layer
Cyan: top silkscreen layer
Magenta: bottom silkscreen layer
The parts were selected from the available types at Conrad and Reichelt, two popular german parts suppliers.
They may still change, but here is what was used for creating the current layout:
Cs (C1, C4): Panasonic FR 470µF 50V; (C3, C5): ceramic X7R 100nF
Ci (C2): Panasonic FC 180µF 50V
Cin (C10): Wima MKS-2 4.7µF
Cm (C6): Panasonic FC 100µF 50V
Cc (C8, C11): ceramic NPO 220pF
Cf (C9): ceramic NPO 47pF
Csn (C7): ceramic X7R 100nF
Rsn (R6): Yageo RC1206 2.7R 0.25W
Output resistor+coil (R9): will be removed from PCB
other resistors: metal film 0805
What do you think of my attempt?
Something like that:
As you can see V+ doesn't overlap OUT ;)
Do they stock the other types you mentioned as well?
The site is down at the moment...
But I could trim the ground polygon under the IC, and place the V+ track under the IC (where the ground trace is at the moment).
How would that be?
I don't like the crossing of V+ and Output either.
But sometimes I need to get a hint from someone else to see a solution.
They stock Wima FKP2 (use 2.5% tolerance -> copper leads, better if 100V rated)
Has the rail to rail bypassing any advantages over bypassing each rail to ground?
But sadly they don't have a 47pF or 56pF type, as is needed for Cf...
And SMD types like Panasonic EHCU are also not available, but Reichelt has them only in 330pF and 470pF, and also 100nF.
Will the 100nF EHCU be ok for power bypassing and Csn?
Are Wima MKS-2 ok for NF input?
And are the Elna RJH also ok for Ci?
Because Distrelec doesn't have any Nichion caps.
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