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

[Al Johnson]

On Thursday 20 August 2009, Rene Harder wrote:
> 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

> >> - 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.

Werner's comparison of sizes demonstrates this nicely. 'Nothing better will 
fit' is always a good reason.

> 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.

THD+N is next to useless as a predictor of audio quality. It's the measure 
that caused so much trouble with the early transistor amps because it fails to 
distinguish between relatively pleasant 2nd order harmonics and unpleasant 
high order harmonics, among other failings. It's the character of the 
distortion that makes X7R entirely unsuitable for audio use rather than its 
level; just compare the distortion spectra.

> 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. ;-)

True, but why make it worse than it has to be?

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