Some questions about the Szekeres headphone amp:

[Bricolo]

IRF510: Input capacitance is 135pF (Vgs=0 Vds=25V f=1MHz)
2SK216: Input capacitance is 90pF (Id=10mA Vds=10V f not specified)

[halojoy]

I would put C1 after the pot and R1.

If you have single supply, using 0 volt as GND,
C1 before pot and connected to GND, would
change the current in MOSFET, when you turn pot.
The potentiometer would be, like R3, connected to ground.
So the working point is changed.

That is the purpose of C1, to block DC-current.
So that only R2 and R3 set the working point,
to about half voltage at Source pin of MOS.

Also if you put C1 before the pot, the highpass roll off point
is affected by the pot setting.
The RxC set by C1+pot, is something like 10 Hz.

When you have it at full volume R= pot//(R1+(R2//R3))
at Zero volume R= pot
So the RxC constant is changed by volume setting

If you put C1 after pot,
in RxC1
R= R1+(R2//R3) all the time.

[Bricolo]

ok
I've understood now



now, does someone know why we change C2's value in the CCS modif?

[halojoy]

C2 doesn't need to be changed.
The value of C2 and the impedance of Headphone, 30 Ohm
is the RxC constant of the high pass filter,
that sets how low freq that can pass to the headphone.

If C2 is too little, say 10uF,
the lower bass tones can not pass.
RxC is then: 10uFx30ohm= 300uFohm

if you use headphone with 600ohm
it will be 10uFx600ohm= 6000uFohm
and lower bass tones can pass.

The higher the RC (RxC), the lower freq you pass out; can hear.
So you can say a headphone with 20 times higher impedance,
need only 20 times smaller Capacitor,
to get same roll off point.
It is good if the roll off point is 20 Hz or lower
for headphones.

So in this case both C1 and C2 can effect the low roll off point.
One at the input, and the other at the output.

Roll off point, also called f: Where curve(volt) is at 70% of the max

f=1/(2 x 3.14 x RC)
RC=1/(2 x 3.14 x f)

[Bricolo]

Ok

I've just realized Mr Szekeres made his one for 32Ohms Grados
->1/(2*Pi*32*470.10-6)=10Hz

with my 150Ohms HD565, it will be good with 100µF
maybe 200 will be nicer


always bypassed with a 1µF?

[Bricolo]

some pspice simulations later... (very great tool!)


C1 would be optimal with 4.7µF, if you want better <10Hz response (I'm not sure it's usefull...)

This R1 is still annoying me, I find 4.7K is too much, with 1K my 10KHz->30kHz is pratically flat
with 4.7K, output voltage decreases from 10kHz to 30kHz


AN IMPORTANT POINT
I set R2 and R3 to 15k and 22k, as Mr Pass told me once

when I redraw my shematic in pspice, with default values (150k and 220k), output voltage went from 1.25V to 1.8V!!
Nice gain
Don't forget that R2 and R3 are a voltage divider for the gate, but R3 and R1 are a voltage divider for the input voltage going in the gate

the greater R3 will be, the lower the voltage drop will be (setting R2 and R3 to 1.5M and 2.2M gives an 1.9V voltage) Not bad for 2V input

[Bricolo]

Hi all


I've mounted the whole thing on a test board this afternoon
it doesn't work at first attempt, too bad :/
Only the right channel works, I certainly made a bad wiring



Can someone tell me where my mistake is?