What am I doing wrong? (Clean blend and hiss)

[Boba7]

Hey guys,

So I was working on a generic schematic for a clean blend that I'd use with an effect that reverses the phase (overdrive, compressor, distortion).
I picked up some blocks in a few different schematics and came up with that (see below) and it works well, but there is some hissing going on, even with the clean pot at maximum (clean sound) compared to bypass. And I have no idea where the noise comes from!

So what am I doing wrong? I looked at the Florist schematic and it is pretty similar (but with a Jfet buffer) and I'm sure it isn't noisy!

If anyone had any idea, it'd be greatly appreciated!

Cheers!

[Zerro]

If pot CLEAN lets Clean signal out, and FX loop is at minimum, does it hiss? Or if FX loop goes out, and clean is at minimum? I ask, to find the direction/source of noise coming. And what opamps do you use?

[Boba7]

If pot CLEAN lets Clean signal out, and FX loop is at minimum, does it hiss? Or if FX loop goes out, and clean is at minimum? I ask, to find the direction/source of noise coming. And what opamps do you use?

Thanks for the answer!
Full clean : hiss (even with volume of fx loop at zero)
No clean : hiss (even with volume of fx loop at zero)
Basically hiss is everywhere, in a little yet noticeable amount.

Im using 2n5088 for the input buffer and tried tl072 and ne5532 for the opamp. Im stumped!

[Zerro]

At first try to hard connect both points I asigned red with ground - see picture. Thus we will know, if noise is coming from bad quality pot or input section. If hiss is there on, problem is at second opamp, or feedback pot - gain. Maybe caps or resistors too, but this we can see after this first test.

[Boba7]

At first try to hard connect both points I asigned red with ground - see picture. Thus we will know, if noise is coming from bad quality pot or input section. If hiss is there on, problem is at second opamp, or feedback pot - gain. Maybe caps or resistors too, but this we can see after this first test.

Thanks a lot! I'll do it in a couple hours and will let you know

[Boba7]

At first try to hard connect both points I asigned red with ground - see picture. Thus we will know, if noise is coming from bad quality pot or input section. If hiss is there on, problem is at second opamp, or feedback pot - gain. Maybe caps or resistors too, but this we can see after this first test.

Yep, still hissing when I ground both points.
I tried several opamps so I doubt it comes from there. And there's only a couple caps and resistors left! I really don't get it.

[Zerro]

Now rise value of C21, at 820pF. It is only freq compensation on HF. And, block VR power with 100nF cap. There is only this way to compensate noise, if all details are ok. Otherwise I must say, that some cap or resistor at the end is wrong.

[Boba7]

Now rise value of C21, at 820pF. It is only freq compensation on HF. And, block VR power with 100nF cap. There is only this way to compensate noise, if all details are ok. Otherwise I must say, that some cap or resistor at the end is wrong.

Yes, raising C21 helps, but the white noise is still present, and the dry sound sounds a tiny bit more muffled with 820p.
Im gonna make myself a new audio probe, see if I can figure out if the output cap or resistor are bad, which sound quite unlikely, but who knows...
Thanks again for your help, I'll let you know how it goes!!

[Zerro]

Yet you can ground left end of C20, so at IC2 is not any signal from anywhere. And controll again hiss. We will find it. BTW allways do you compare it with bypass branche?

[Boba7]

Yet you can ground left end of C20, so at IC2 is not any signal from anywhere. And controll again hiss. We will find it. BTW allways do you compare it with bypass branche?

When I ground the left end of C20 I get an even louder hiss! Like way louder.
I have it in a true bypass loop at the moment, so I can compare my dry sound and the clean sound on the pedal.

[Zerro]

Of course, gain of output opamp is ratio of input and feedback resistor - so, if input resistor is null, only wire at ground, gain is teoretically 50k/0,001 = endless. It was bad trial. But we know, that problem is output amp set. I guess that problem will be in details around. If you changed opamps and nothing changed... BTW, serially with 50k pot set there minimally 1k resistor, otherwise when pot is at minimum, opamp lose meaning - signal is going out around it.

Here is another question - if ANY opamp will deliver such a low noise output, you want to get. It will allways add some noise. So, what noise is normal for that conception, and what is too high level?

[Boba7]

Yes, I didn't think about adding that minimum resistor in series with the pot!

I think I'll have to redesign the whole thing, it doesn't work well enough as is... It works, but the noise level is a little bit much to my taste...

Thanks a lot for your patience and help!

[Zerro]

That concept is good. Only here is source of noise - maybe opamp is not able to deliver noise level you desire (in this case none concept will work), maybe still try to block VR at noninverting (+) input with 100n cap or so, against to ground - maybe noise is going from power supply! Thats all I can offer now.

[Boba7]

That concept is good. Only here is source of noise - maybe opamp is not able to deliver noise level you desire (in this case none concept will work), maybe still try to block VR at noninverting (+) input with 100n cap or so, against to ground - maybe noise is going from power supply! Thats all I can offer now.

Yes sorry I forgot to say I tried it! Didn't make a difference...

[Zerro]

And you did it at pcb? If you have only prototype at breadboard, then I must warn - breadboard always add big noise to signal!