Opamp's invulnerability to noise??

[zencafe]

Hello Everyone!

I've trying to figure out why Op amp based circuits (Sansamps for example) are less subtle to some kinds of noise (mostly power supply) compared to discrete or CMOS designs...
My idea would is to find a simple discrete circuit that could increase the PSRR, as current mirrors in op amps (I'm not actually sure if current mirrors are there for that reason, correct me if I'm wrong )

Maybe a current source?

Thanks in advance!

Greetings

Douglas

[R.G.]

I've trying to figure out why Op amp based circuits (Sansamps for example) are less subtle to some kinds of noise (mostly power supply) compared to discrete or CMOS designs...
My idea would is to find a simple discrete circuit that could increase the PSRR, as current mirrors in op amps (I'm not actually sure if current mirrors are there for that reason, correct me if I'm wrong. Maybe a current source?

Opamps are less susceptible to noise by design, and by design over along period of time using many techniques.

The problem with trying to retrofit these techniques into discrete circuits is that you soon have something as complex as the opamp, but without its advantages.

For instance: the input of all opamps is one or more differential stages. These work best if there is a big impedance in the connected emitters (or sources, if FET), and so a current source emitter supply gets used. This has the side effect of the emitter power supply being made immune to ripple and hum on the power supply if the current source is well done. It wasn't put there for the noise reduction, but once you have the current source, noise reduction is easy. On the collector (or drain) side of the diffamp, noise is injected if you use a simple resistor load. But using a current source load makes the gain of the input stage higher, and at the same time makes the collectors more immune to noise on the power supply. A current mirror load is even better for both purposes, and is in all modern opamps. So now we have a minimum of three transistors to make the two transistors in the differential amplifier work better, and also reject noise. There are similar side effects in other stages.

The simplest thing you can do in discrete circuits as an add-on to reduce noise is to regulate the power supply. This reduces supply-borne noise at about 40-60db at one swoop, and does not change the discrete circuit at all. Doing a good, careful job of routing the grounds so that sewer grounds do not contaminate reference grounds helps even more.

[zencafe]

Thanks RG for the quick and enlightening answer!
The issue is that in my circuit, is that I used a regulated power supply. I'm quite sure the issue is in the 4049 section. I added a 220r in seried with the VDD pin to limit current and reduce headroom in order to get more saturation/compression in the overdrive (the VDD voltage drops from 9V, to 7.5V)
The main issue happens when I allow more (maybe too much) bass before the 4049, I added a 3 psition SPDT switch to select between 3 coupling capacitors to change the amount of low end going into the clipping (4049) stage.
With the biggest capacitor, at max gain, the hum is quite noticeable, but with a 9V battery of course there's no hum.
I opened this thread because I have a love/hate relationship with 4049 distortions... I love the way they sound, but they too susceptible to, both hum and white noise. So I was thinking that with some help from you guys I could come up with some circuit that could help us all with this problem.
One solution that comes to my head is by a 8, or 5V regulator, as I've seen in many of Gez's snippets. Probably this double regulating will reduce any hum, but If there's any way to reduce any other kind of noise, I'd be glad to try it out.
I hope a hint of any type of circuit to experiment with is not too much to ask .

Thanks again!

D.

PD RG do you have any thumb rules for a good ground routing in PCBs?