Can we fix the audio for A5/A6/A7?
andy at openmoko.com
Wed Apr 2 14:25:07 CEST 2008
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Somebody in the thread at some point said:
Hi Joerg -
Thanks for finding this.
|> With e.g. *typical* (->ds, p.12) 32R-headphones, RL = 65 Ohm! Could
|> check the Z for OM-headset?
|> Please calculate f0 for this value!!!!!!
They're ~32R apparently, with 33R in series (R4405 / R4407) so it is ~65R.
|> If we can't resolve the issue,
| The solution is plain simple: have bigger C (at least 47u).
| Or completely redesign the audio circuit. :-(
The caps shown in the typical application circuit is 100u, not 1u. But
the datasheet says -- it is about Cin but the last part is true for Cout
and the transducers --
''Selecting Input and Output Capacitor Values
Besides gain, one of the major considerations is the closed-
loop bandwidth of the amplifier. To a large extent, the band-
width is dictated by the choice of external components
shown in Figure 1. The input coupling capacitor CI and
resistor RI form a first order high pass filter that limits low
frequency response. CI’s value should be based on the
desired frequency response weighed against the following:
Large value input and output capacitors are both expensive
and space consuming for portable designs. Clearly a certain
sized capacitor is needed to couple in low frequencies with-
out severe attenuation. But in many cases the speakers <============
used in portable systems, whether internal or external, have
little ability to reproduce signals below 150Hz. Thus, large
value input and output capacitors may not increase system
So the impact is we lost some bass end... maybe not a noticeable amount
to what we would have with 100u due to the transducer performance. If
there's no room for adjusting the value of the caps on the A5 or A6 PCB
then I would just accept this.
Maybe in future we can improve bass performance but it seems we have to
do it by premium headphones and lower frequency output filter at the
Still, it's good spotting :-))
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