Guitar pedal FX blender 2.0 schematic / idea check **NEW SCHEMO ADDED**

[jwhmca]

Hi Guys,

I started playing around with a simple guitar pedal FX SND / RTN "blender" circuit pedal.

Original thread is here http://groupdiy.com/index.php?topic=61356.0

This schemo is part two.

Overview of ideas:

1. Standard INPUT feeds circuit and BYPASS switch
2. Signal is duplicated to feed MASTER DRY BLEND and FX INPUT.
3. Signal is introduced to OUTPUT feedback to fold back in some feedback. (**I'm thinking this isn't going to work right the way I have it.**)
4. Signal is sent to a MICPRE transformer. This has a very pleasing effect on some guitars that can handle the level drop.
5. Option to BYPASS the TRX and option to assign it to the MAIN INPUT stream or the FX1 / FX2 output.
6. FX1 / FX2 series order switch to assign which FX comes first.
7. FX1 / FX2 SND / RTN circuits with the ability to BLEND the local FX with a local DRY signal.
8. FX1 / FX2 local BYPASS.
9. LPF / HPF (ESP)
10. MASTER WET / DRY BLEND
11. OUTPUT.

** It seems like the feedback circuit/idea would need to be a "MIXER" rather than a "BLENDER"? What happens when the control is all the way to the the OUTPUT FEEDBACK? It doesn't seems like I would get anything then?**

Thanks in advance for the input and help! I'm still very raw!

 

[midwayfair]

Few things I see:

1. The feedback mix pot can't fully isolate the feedback from the "main dry" signal.  In fact, the isolation is likely to be pretty poor given the extremely high impedance of pin10. Even at the extremes that 10K from the pot will look trivial.

It's a couple extra components, but it's possible to wire a single pot so that it can fully kill the feedback signal. There are a couple ways to do it, but RG's method is the best: http://www.geofex.com/Article_Folders/panner.pdf. Actually, the output of the Echoplex is almost the same thing. You just make up the gain loss in the next op amp stage.

2. You don't need both R30 and R31 as currently wired. Just put the resistor at the pin itself. It's possible that after redoing the feedback blend that you can just DC couple the op amp stages and you won't need either of these resistors or C19 or C20 (which you presently don't need anyway). If you use RG's blender, it ties the pot to your op amp's bias voltage (ground in your case).

3. Ditto for all the blends in the circuit. In my opinion the simple panning for a blend circuit is really only useful when you really can't handle any signal loss at all or to tack onto existing circuits. If you're building the circuit from the ground up, you can get perfect make-up gain (read the Geofex article) so there's no loss of signal strength or dynamic range, and you can get complete wet-dry isolation in any percentage. Given the size of the circuit, there's not even a reason to fret about the small amount of extra PCB space for a handful of extra passive components.

 

[jwhmca]

3. Ditto for all the blends in the circuit. In my opinion the simple panning for a blend circuit is really only useful when you really can't handle any signal loss at all or to tack onto existing circuits. If you're building the circuit from the ground up, you can get perfect make-up gain (read the Geofex article) so there's no loss of signal strength or dynamic range, and you can get complete wet-dry isolation in any percentage. Given the size of the circuit, there's not even a reason to fret about the small amount of extra PCB space for a handful of extra passive components.

"panning" or do you mean buffering?

Thanks for everything else, some good info

 

[Monte McGuire]

I haven't exhaustively checked, but all of the op amp stages will not work since the inputs are DC coupled and tacked to ground, but the negative supply of the op amp is 0V. So, the input DC voltage is outside of the allowed common mode range for the op amp, which would be -12V with a -15V negative supply. You need to use coupling caps and a rail splitter to bias the op amp inputs to 4.5V so that they will operate properly.

There are some op amps which can handle inputs at or slightly below the negative rail, but why bother when you can just address the bias issue directly and choose an op amp for better reasons.

 

[Bo Deadly]

[quote author=jwhmca]
Does it makes sense to have ground lifts on the input and output, or would just one ground lift suffice?
[/quote]
Actually a ground lift on the return might help with reducing ground loop hum in this particular circuit. Consider that if you have both the send and return jacks connected to ground that likely makes a ground loop with the device(s) inserted. In this case, it is very likely that only one end needs to be grounded. Meaning the shield of the return cable would be connected to ground at the send jack but not at the return jack and thus no loop. If cables are short a ground lift probably would not help that much. But in theory it might.

Ground loop noise is most obvious when you connect two pieces of wall powered gear. The ground loop is created by the ground of the cable connecting the gear, though the gear and then the neutral wire in the wall (possibly running all the way back to some junction box) and then back out of the wall into another piece of gear. But in this case the only really good way to eliminate ground loop hum is to use balanced I/O such as with a transformer.

 

[jwhmca]

I haven't exhaustively checked, but all of the op amp stages will not work since the inputs are DC coupled and tacked to ground, but the negative supply of the op amp is 0V. So, the input DC voltage is outside of the allowed common mode range for the op amp, which would be -12V with a -15V negative supply. You need to use coupling caps and a rail splitter to bias the op amp inputs to 4.5V so that they will operate properly.

There are some op amps which can handle inputs at or slightly below the negative rail, but why bother when you can just address the bias issue directly and choose an op amp for better reasons.

Ahh... yes... that's a GREAT point!  ;D First time thinking about opamps with a single rail... makes perfect sense, though.

 

[jwhmca]

http://www8.plala.or.jp/KandR/cir_bassthru.html

Seems to use a similar circuit but with a 100k pot to achieve the blend.

Problems with this?

 

[midwayfair]

http://www8.plala.or.jp/KandR/cir_bassthru.html

Seems to use a similar circuit but with a 100k pot to achieve the blend.

Problems with this?

Work it out. It's a voltage divider. The 1M is after the pot (whereas in yours it was before the pot). 100K divided against 1M is how much attenuation at the extremes of the pot? Is that enough in your opinion?

 

[jwhmca]

On the feedback idea,

What if I just made a simple active or passive mixer and varied the gain of the feedback buffer opamp?

 

[jwhmca]

Okay,

version number #3a is included in first post.