Boost and Buff A/B Box using TL072

[GarryGirthOak]

Hey guys,

Trying to see if this circuit would work before building it.

It's a A/B box which also grounds  the unused output when switched, also has a indicator LED for each output. My friend also wanted a slight Boost, but i only had a dual op-amp so i decided to make the other half of the op-amp a buffer as well.

[Keppy]

You don't need to ground the outputs because they're already grounded through the volume pots R7/R8. Grounding both ends of the pot doesn't really accomplish anything, so you can eliminate S2 completely and use a DPDT switch instead. It won't do any harm how you have it, though, if you don't want to change it.

You have a buffer (U2) and a booster (U1) in parallel, and when R6 is turned up they'll fight for supremacy since both opamps are trying to achieve specific voltages at the combined output. This will probably result in audible distortion and is the one thing in your schematic that won't work as designed. I can't really tell what you're trying to do with U2, but you'd be better off without it IMO, especially since an opamp gain stage really doesn't require a buffer before or after.

I'm not sure what you're trying to do with R6 either, but looks like it was meant as a volume control (if so, it's redundant with R7/R8). R6 is useful as a pulldown resistor between the output cap and the switch to prevent switch pops, but it doesn't need to be a pot.

If you decide not to use U2, you can just remove C4. There are other ways to properly terminate the opamp, but that's the easiest from what you have here.

Your input, gain stage, and switching all look fine to me.

[GarryGirthOak]

Thankyou for your response, i really appreciate it

It would be probably better in this case just to use a single OP-AMP, like a TL071 and disregard U2. I only have a TL072 and im trying to find a use for the other half rather than just terminating it, adding no function to the circuit. Perhaps if you wouldn't suggesting something that would be sweet

R6 was meant to act as a "gain" pot, however i could just omit R6 and make R7/R8 gain pots. This still maintains the function of R7/R8 being adjustable volume controls for each output. However would there be any popping or anything i would need to worry about when i switch between outputs?

S1,S2 and S3 are the one switch, being a 3DPDT switch, but i can omit S2 and make it a DPDT meaning it would be a cheaper build....plus i have DPDT switches laying around.

[GarryGirthOak]

Edit. Forgot to add a ground port to the op-amp. just pretend it's there

Here is a updated schematic.

R1 is a pull down resistor, thinking of making that 1meg.

Would you recommend making R2 and R3 2M2 or something else?

I've made C2 and C3 100uf, is 100uf overboard?. I'm making C1 4.7uf to let in more bass, can C1 be polarized/ electrolytic?

R4 will be 100k and R5 will be  470k. R6 im not sure, perhaps 1meg?

R7 and R8 will be both 10k linear pots.

R9 and R10 have been taken care of.

[Keppy]

C1 doesn't need to be so large. If R2 and R3 are 2M2, then an input cap of .01uF will pass bass down below 20 Hz. Film caps generally have better signal properties, so I'd advise a value below 1uF, which will allow the use of a film cap.

100uF for C2 is overkill. Even 10uF is high, but not absurdly high. 10uF will pass full bass onto equipment with a 1k input impedance, which is 1000x lower than a guitar amp, and at least 10x lower than any studio gear you're likely to use.

If R4 is 100k, then C3 only needs to be .1uF to pass frequencies down below 20 Hz. 100uF is way overkill.

I would add a pulldown resistor from the C2/R6 junction to ground. This may help prevent switch pops. I suggest 100k. Other than this addition, I think the design looks good. Good luck!

[GarryGirthOak]

Awesome. I can make C2/C3 only as low as 470nF or .47uf, My local shop doesn't sell electrolytic caps in a lower value than but i think it should be okay.

Cheers for your help Keepy, i'll post a link to audio when i get it working  

[mth5044]

Hmm wouldn't a 1Meg resistor on R6 be a very large resistance to pass signal through? Perhaps you meant to put one end to ground, with the other end connecting the C6/Sw2?

[GarryGirthOak]

That's what keepy was describing. After C6 you have a T junction where R6 connects to C2. The signal then connects the switch after R6 while the other end of R6 connects to ground, I dun goofed in the schematic above, still learning! haha.

Here is the corrected schematic.

[GarryGirthOak]

  Sooo the circuit is functioning, a little buzz but it has plenty of gain on tap.

I am really suprised how much bass has been attenuated, i suspect changing some cap values will solve this. What do you guys think i could changed to fix this?

The chain is like so. Guitar > booster > line in on interface, no DI HI-Z disengaged

I will post another example with a DI in the chain, but i suspect the result to be similar.

https://soundcloud.com/asher-gregory/booster-comparsion

[PBE6]

C1-R2||R3

This is the most likely culprit. R2 and R3 will act in parallel here, removing much more bass than you expect based on R3 alone. Instead of connecting the opamp directly to the R2/R3 junction, use a 1M resistor to make the connection.

[GarryGirthOak]

Could you possibly explain, cause i am little naive and i would like to learn stuff  , why putting a 1meg resistor between the op-amp and the junction would allow more bass to pass out of the circuit? 

[GarryGirthOak]

Currently i am using a 2m2 meg resistor for R2/R3. Would changing both R2/R3 to 1meg help as well?  . Also my other problem could be that i used 1 meg on R6. I'll change that to at 470k, then go down to 100k when i obtain more 100k resistors. Thoughts?

[PBE6]

You'd think that C1 and R3 would form the only high-pass filter at the input since the basic topology is always shown with the resistor going to ground. However, the 18V supply also presents a low-impedance path to ground as far as AC signal is concerned because of low internal resistance in the battery/power supply. Therefore both biasing resistors are effectively in parallel, giving you a stronger filter and less bass.

Inserting a 1M resistor between the signal path and the R2/R3 junction introduces a series resistance, so instead of a resistance of R2*R3/(R2+R3) you now have 1M + R2*R3/(R2+R3). This moves the cutoff frequency much lower, giving you a weaker filter and more bass.

[GarryGirthOak]

Thanks PBE6! That makes sense. I'll try it out and see if that works!

[PBE6]

I did not read some of the previous posts for component values, all the cap and resistor values are plenty high enough to provide full bass. Double check to make sure you haven't made any improper connections.

[GarryGirthOak]

I did a test today, this time comparing the guitar with the HiZ switch engaged, as well ass modifying the circuit as PBE6 said too. The two samples are a allot more similar, i think the only difference is the booster 'opens' up the sound ie. adding a little more brightness. I am happier with the circuit now , but Perhaps soldering the circuit onto vero as apposed to breadboarding will change thing as well.

https://soundcloud.com/asher-gregory/booster-comparison-v21

I'll post a drawing of how i am going about populating the breadboard with components. Perhaps you guys could find a error in the layout of parts.

[GarryGirthOak]

Here it is

[GarryGirthOak]

Here again is the schematic.