[gta02-core] Caps for bass fix (ECN0022)

Rene Harder rehar at saweb.de
Thu Aug 20 17:42:46 CEST 2009

Al Johnson wrote:
> On Saturday 15 August 2009, Werner Almesberger wrote:
>> Joerg and I discussed the choice of capacitors for the bass fix on
>> #gta02-core. Some conclusions:
>> - as explained in ECN0022, the capacitor should have about 100 uF
>> - ceramic is unsuitable because of harmonic distortions
> I'll qualify this in case anyone gets the wrong impression. High-k dielectrics 
> like X7R have extremely high distortion levels. Low-k dielectrics like C0G 
> have exceptionally low distortion, but aren't available in values bigger than 
> about 220nF.

I agree, High-K ceramic caps have higher harmonic distortion that C0G
types. One source of harmonic distortion is that for XR7 and Z5U
dielectrics the capacitance varies with amplitude (and of course
frequency). For XR7 around 30% and Z5U around 70% [1]. Now, you can
limit this distortion with reducing the voltage swing or using caps with
higher maximum voltage. However, increasing the maximum allowed voltage
will increase the size of the cap quite a bit.
Another source of harmonic distortion is the piezoelectric effect of
High-K dielectrics. If you apply mechanical vibrations (e.g. sound
waves) to the capacitor you will get voltage spikes across it which
leads to distortions of your signal.

>> - Tantalum seems to be the best choice
> Distortion from tantalums is typically >10x that of a low cost polar aluminium 
> electrolytic. What makes them the best choice?

Capacitors with aluminum electrolyte are way to big with same
capacitance/voltage, tantalum seems to be the better choice here. I
found this benchmark [2] where AVX measured noise and THD+N with several
different caps for input and output coupling. The THD+N for ceramic caps
is really high compared to the other ones but AVX coupled an acoustic
exciter to the caps to observe and measure the piezoelectric effect.
(see Figure 4). According to this benchmark low ESR tantalum caps are
the second best choice for audio coupling. I should mention, that
surprisingly polymer tantalum caps which have a low ESR exhibit the same
noise than standard tantalum caps.

Why don't we use the 100uF caps from the big-C buzz fix (C4306/4307
GTA02-A7) so we don't have any component change. It's a Tantalum Solid
Electrolytic Chip Capacitor [3] (TLJA107M006R0500)

And at the end we are just designing a cell phone with music playback
ability and no high class audio amplifier so i would not expect an
outstanding THD at all. ;-)

[1] http://my.execpc.com/~endlr/ceramic.html
[2] http://avxta.com/docs/techinfo/tantbench.pdf
[3] http://avxta.com/docs/Catalogs/tlj.pdf


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