Tube screamer... but dual supply positive and negative

[m4268588]

If you think so, do it in your own way.
There is no such this issue with dual supply, so I'm not going to discuss.

[fryingpan]

If you think so, do it in your own way.
There is no such this issue with dual supply, so I'm not going to discuss.

Of course with a dual voltage supply and referencing to ground it's all moot. I'm just trying to understand the designer's reasoning behind the inclusion of such a resistor to Vref. You're right, the first opamp's output will take precedence (1k impedance vs 10k). And the 1k resistor is to set the cutoff frequency for the LPF (of course, I just had a quick look at the schematic on my phone between beers 🍻 although it will also prevent opamp oscillation due to the large capacitance to ground, but this is a side effect). Honestly I would rework values a bit, maybe, doubling the resistance and halving the capacitor perhaps? Like 2.2k + 100n. (IIRC the original cap is 220n). Less of a load on the opamp for a very slight increase in noise, although I doubt it makes any difference.

If you swap opamps for a lower noise version, remember to add in 0.1n ceramic bypass capacitors from both V+ and V- to ground. The original 4558 probably doesn't strictly need them, but faster opamps will.

[ElectricDruid]

I'm just trying to understand the designer's reasoning behind the inclusion of such a resistor to Vref.

This is the key point, and the most difficult thing to work out, since we're trying to get inside someone's head. That's not a electronic engineering question!

The TS-808 includes this 10K to Vref. The Boss SD-1 from two years later doesn't bother with it. The history here is interesting. Boss produced the OD-1 in 1977, which is the earliest "overdrive" pedal I can find, and which established the "dioes in feedback loop" style we all know. Ibanez then added a tone control circuit in 1979 and called it the TS-808. Their version was more of a success than Boss's earlier one, so then Boss added a tone control to theirs and called it the SD-1: 

https://electricdruid.net/images/overdrive/IbanezTS-808Tubescreamer.jpg
https://electricdruid.net/images/overdrive/BossSD-1SuperOverdrive.gif

Notice also Boss scaled the resistor up and the cap down, as suggested by fryingpan. There's a small change in cutoff, but it's not much - 732Hz for Ibanez, 884Hz for Boss (three-and-a-bit semitones). They could easily have gone for 22n and kept it the same, so it looks to me like a change for change's sake.

[antonis]

Maybe I shouldn't mention R9 at all..

@andreaagnole: U2 output directly bias U1 input at +4.5V - period..!!
If you like to delete R9, it's fine.. If not, it's also fine.. 

P.S.
@fryingpan: Previous stage doesn't amplify any DC offset due to C3 existence..

[fryingpan]

Maybe I shouldn't mention R9 at all..

@andreaagnole: U2 output directly bias U1 input at +4.5V - period..!!
If you like to delete R9, it's fine.. If not, it's also fine.. 

P.S.
@fryingpan: Previous stage doesn't amplify any DC offset due to C3 existence..

Yes, I know, that was the beers' fault.

[mzy12]

everything that I already built is based on a dual power supply system positive and negative +/-15v

Off topic, but do you have a dedicated power supply for this (I think you do based on your post but I'm not sure)? What DC connector are you using to interface with your pedals? (I'm interested in having a similar system for my bass pedals and am just curious how you're setting it up)