In-circuit voltages (Mysterioso / OP-amp basics)?

[Vallhagen]

Hi folks.

I am playing around (breadboard, simulation...) with the 4049UBE circuit, i have a Red Lama ready to box, and i guess i will build something like the Mysterioso.

As i read the Mysterioso schematic i have a question that i guess is fairly basic, but i dont understand why:

The "9V" input meets "the usual" reversed diod and cap to gnd, and is divided in half for "VB". Also, "9V" connects to a serie resistor to "VR".

Question 1: To me it makes sence/is obvious that the 4049 IC is powerred by "VR". BUT, why is the TL072 IC not fed with the same "VR"? It is powerred by "9V". I dont get the point, even though i guess there is one.

Question 2: The unused half of TL072 (pin 5, 6, 7) has VB connected to its positive input, and then a negative feedback loop. Does this design help to stabilize VB? Or is it just there to not leave the pins unconnected?

Question 3: The unused inputs of the 4049 IC (pin 9, 11, 14) is connected to "VR", and their associated outputs (pin 10, 12, 15) are left unconnected. This is the same design that we find in Red Llama. Is there a reason for this? To me it would be more logical to connect unused inputs to ground, but maybe the circuit feels better if it can "rest in high position" as the (inverted) output will be low?

***

I would really appreciate if someone has the time to answer this!

Cheers in advance
/Bengt

[mgwhit]

I don't really know the answers to all of your questions, but here's an interesting article that probably explains Question 2:

http://www.maxim-ic.com/app-notes/index.mvp/id/1957

Question 3 (the 4049) is probably the same kind of thing, but I don't know enough about inverters and how they relate to (inverting) op amps to effectively answer it.  I just assumed that it's a stabilizing thing.  I breadboarded a Llama once and didn't hook up the unused inputs like that and the distortion sounded pretty splatty.  Same chip sounded awesome on the Snarkdoodle board.

Regarding Question 1, that might just be a quirk of the original Blackstone circuit or maybe it had to do with how the board was being laid out.  I've always assumed that those low value resistors (in this case the 120R R18) were a current-draw failsafe in case there was a short on the board.  I can't imagine there's any benefit from powering IC1 and Q1 directly off of 9V instead of VR, but I'd love to hear from anyone who can elaborate.  If I were designing from scratch I would power all three from VR.

[madbean]

1) Separate supply rails is a pretty common way to reduce noise in a circuit. Given that this is a high gain overdrive, my guess is the inverter is supplied this way for that reason. Note that the voltage drop is very very small across a tiny resistor, so it's not so much a matter of different voltages, but unique supply lines, I think.

2) There is probably no difference between tying the unused pins to ground or virtual ground. It is not buffering the VB supply in this case.

3) From what I remember there is little difference in tying those un-used pins between ground and VR. Check FSB for a thread on the Red Llama re-issue which discusses this very topic.

[Vallhagen]

Thank you gentlemen for some clearifying answers!

I don't really know the answers to all of your questions, but here's an interesting article that probably explains Question 2:

http://www.maxim-ic.com/app-notes/index.mvp/id/1957

Perfect link, explained it all! May i passs it forward as a brief summary: To avoid noise from an unused stage of an OP-amp, its in- and outputs have to be correctly terminated. There are some different ways to do this, some "right", some "wrong". On the mentioned circuit - Mysterioso - its done the right way:)

I've always assumed that those low value resistors (in this case the 120R R18) were a current-draw failsafe in case there was a short on the board.

If i have read my books right (correct me if i am wrong here!): Failsafe yes, but not for board-shorts in the first place, rather for protecting the circuit for ESD-damage.  Anyway, it is a "security" resistor, which still makes it a little confusing that Blackstone design a circuit and let a fairly sensitive device as TL072 not take advantage of it... Maybe Madbean just gave us the answer though;

1) Separate supply rails is a pretty common way to reduce noise in a circuit.

Sounds reasonable.

1) Separate supply rails is a pretty common way to reduce noise in a circuit. Given that this is a high gain overdrive, my guess is the inverter is supplied this way for that reason. Note that the voltage drop is very very small across a tiny resistor, so it's not so much a matter of different voltages, but unique supply lines, I think.

2) There is probably no difference between tying the unused pins to ground or virtual ground. It is not buffering the VB supply in this case.

3) Check FSB for a thread on the Red Llama re-issue which discusses this very topic.

Hmm, ive done my search, and im not sure i find the right thread? Do you mind linking me right?
... it seems a bit like common sence though, that "nothing" shall be left floating, and when it comes to inverters they shall "rest" with a high input. It's their natural passive state.

Cheers and thanx again!

[culturejam]

1) Separate supply rails is a pretty common way to reduce noise in a circuit. Given that this is a high gain overdrive, my guess is the inverter is supplied this way for that reason. Note that the voltage drop is very very small across a tiny resistor, so it's not so much a matter of different voltages, but unique supply lines, I think.

Yep, it's a noise surpression thing. It would be better (but probably overkil) to also add a large-ish cap to ground after that resistor (22µF or larger).

[Vallhagen]

Thank you Culturjam.

If you and MadBean are right - which i guess you are - then my earlier assumption about ESD-safety was wrong. I was pretty sure i have read it somewhere (Craig Andertons DIY for guitarists) but i guess i mixed it up with series resistor in the signal path (input). Learnin stuff is a good thing.

Cheers