Opa Alice build noise floor

[AUDIO FREQ]

Here is mine. Hissy hiss hiss

https://cdn.imagearchive.com/groupdiy/data/audio/90/90272-9499738494cba48efaa86320e94a92c4.wav

 

[oscarhuang]

I removed his 12v Zener diode. It can be said to be very quiet now.

 

[jp8]

I removed his 12v Zener diode. It can be said to be very quiet now.

Was it noisy before removal of the zener?

I know zeners are noisy, but that doesn't mean that noise will enter the audio path thanks to the excellent PSRR of the OPA. So I fail to see a relationship between removing the zener and low(ering) noise, if that was the message you wanted to convey.

I would not remove the zener. It keeps the ground return current DC, preventing potential Common Mode signal distortions.

Jan

 

[oscarhuang]

The hizz noise can be clearly heard before removing the Zener diode. After removing the Zener diode, the noise floor is reduced a lot. Maybe increasing the filter capacitor can achieve the same job.

 

[jp8]

OK, then I'd assume there is no, or insufficient decoupling on the voltage divider, which provides the 1/2Vdd reference voltage ("virtual ground").

Jan

 

[Khron]

Maybe increasing the filter capacitor can achieve the same job.

Not if it's directly in parallel with the zener...

 

[jp8]

@oscarhuang : can you share the exact schematic of your circuit? In the original OPA Alice circuit, there are two elcaps that would effectively eliminate zener noise. The ones encircled in the picture.

Jan

 

[rogs]

.... I would not remove the zener. It keeps the ground return current DC, preventing potential Common Mode signal distortions.

I've not heard this explanation before ....
Whilst a zener is useful in the regulation of the capsule polarisation voltage in this circuit, I cannot see why it is used to regulate the DC supply to the Op-amp(s)?
The 'half rail' supply should help maintain optimum op- amp bias, and the current drawn by the 2 amplifiers (c.2mA each) will restrict the maximum supply voltage to the amplifiers to c.30v , which is well below the maximum limits.

As far as I can see, all the zener does here is draw extra current, restrict headroom and - as is being discussed here - possibly introduce extra noise?

I've not come across this 'ground return current' problem you're describing before?