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

[Werner Almesberger]

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

- tolerance is only a minor concern, given that parts from the same
  reel are likely to be similar

- the capacitor should be rated at 4 V or higher

- ceramic is unsuitable because of harmonic distortions

- Tantalum seems to be the best choice

- due to the frequency-dependent ESR of tantalum caps, the ESR should
  be low (<< 1 Ohm)

- component size and height are critical. The tallest component we
  have inside the GTA02 main can is C1707 with a height of 1.6 mm.
  We can place the capacitors outside the can, but given that we now
  have plenty of room inside, that may not be optimal.

Here's a document that compares various capacitor technologies:

http://industrial.panasonic.com/www-data/pdf/ABE0000/ABE0000TE1.pdf

Particularly the ESR chart on page 5 is interesting.


With this data, I went to Digi-Key and did a search with the
following parameters:

- SMT, in stock, lead free, RoHS compliant
- available as cut tape
- 47 uF ... 120 uF
- 4 V ... 6.3 V
- ESR <= 200 mOhm
- case EIA 3528-21 or smaller

The leaves a choice of products from AVX, Kemet, and Nichion.

Possibilities for inside the can, maximum height 1.6 mm:

- AVX TCJT686M004R0080, 68 uF, 4 V, 20%, 80 mOhm, EIA 3528-12
  Digi-Key 478-3480-1-ND, USD 2.58/1
- Kemet T520T107M004ATE150, 100 uF, 4 V, 20%, 150 mOhm, EIA 3528-12
  Digi-Key 399-5288-1-ND, USD 1.55/1
- Nichicon F320J476MAA, 47 uF, 6.3 V, 20%, 180 mOhm, EIA 3216-16
  Digi-Key 493-3149-1-ND, USD 0.84/1

Note the smaller footprint of the Nichicon part. It could probably
still be considered a viable 3rd source, though.

Outside the can, height unlimited but trying to minimize footprint:

- AVX TCJA107M004R0200, 100 uF, 4 V, 20%, 200 mOhm, EIA 3216-18
  Digi-Key 478-3453-1-ND, USD 2.48/1
- Kemet T528I686M004ATE200, 68 uF, 4 V, 20%, 200 mOhm, EIA 3216-10
  Digi-Key 399-5243-1-ND, USD 4.85/1
- same as above

So it seems that we should plan on using a 3528 footprint, if possible.
If we can place it outside the can, that would be even better.

Comments ?

- Werner