GTA02A5: bug in Headset Amp circuitry, max output power to headset

joerg joerg.twinklephone at
Sun Mar 30 04:45:59 CEST 2008

To make things worse and this a real bug, we have 1kR R4116,R4117 as 
additional load after the Cs, in parallel to the speakers of headset.
(No, these Rs are ok, they just make things worse wrt small capacitors)

> From datasheet, p5:
> > Output coupling capacitor which blocks the DC voltage at the amplifier’s
> > output. Forms a high pass filter(!!!) with the single-ended load RL at 
> > fO = 1/(2π RLCO).
> With e.g. *typical* (->ds, p.12) 32R-headphones, RL = 65 Ohm! Could anybody 
> check the Z for OM-headset? 
> Please calculate f0 for this value!!!!!! 

So even with some headset with "infinite" Z, my result for above equation is 
f0=~160Hz still, which is not acceptable for high quality sound (IMHO not for 
music playback at all!). This means you can't even connect your home stereo 
to the earphone outlet to get usable sound, it also will drop -3dB at 250HZ.
This much for frequency response. Now for volume:
With 3.3V power for amp, we won't even get as few as around 1mW(!) Po(max) to 
the headset speakers for Z(speaker) = 1kR, single ended amp. No matter 
whether there is a 33R series resistor, this limits *maximum* volume to 
possibly <80 dB Sound Pressure Level, depending on sensitivity of headset. 
This also isn't acceptable for high quality sound. A headset amp should be 
able to power any headset with at least 10~20mW.
By choosing a headset with even higher Z loudness decreases, with lower Z low 
cutoff frequency increases (-3db at 300Hz for 1kR headset!!!!!!!!!!!!!!!!!!!).
Ergo: this design is definitely broken. There is simply *no* headset that 
yields _good_ sound quality with NEO. The vast majority of 
headsets/headphones will probably not work at all. In fact the headset outlet 
itself cuts frequencies below 160Hz, this gets worse when you connect 

O== Components we MUST have
6.8uF C minimum for 1kR internal + highZ(>>1k) external load, like amp input / 
active speakers, so we at least have one chance to get good audio signal 
directly out of Freerunner by any means.
O== ...We SHOULD have
For -3dB at 50Hz with a load of 65R (internal 33R + std low impedance headset) we 
need 47uF.
For tradeoff power against low cutoff frequency, we should keep the 33R series 
resistors, thus shifting f0 from 100Hz to 50Hz for the usual 32R-headphone, 
and still having a comfortable 20mW for high sensitive earphones with >80dB 
SPL / mW.
O== ...nice to have
If we can have C >= 100uF, we should use 0R series (1R gets hot(0.5W) on 
short), thus allowing to connect to common passive stereo computer satelite 
speakers, with option active subwoofer, and drive them with a nice 300mW.
With 32R-HP we really get HiFi with C>100uF.

For a future design i suggest to insert a similar highpass (plus f0=1Hz 
lowpass, to compensate offset) to R4112/R4115 feedback path, to compensate 
frequency response. Or switch from U-amp design to I-amp (feedback current, 
not voltage)


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