B. Blender

[seanm]

This is an outcome of my SD-1 blend thread. I decided to build a standalone blender stompbox. The blender is in the breadboard stage, but it does work. It is very useful for bass players to maintain the bottom end and note definition.

The circuit is basically four buffers. I decided to use opamps since they are easy to work with. I did not show pin numbers since you can use two dual opamps, a quad, or four singles. I used TL072s but your favorite opamp should work.

The schematic is shown below minus details such as the bypass switch and Vr.



I am not sure OPc and OPd are necessary since most effects will already have input and output buffers. OPd is nice in that this pedal then gives a constant 1M input impedence no matter what effect is attached.

Comments?

[petemoore]

Coment: Looks pretty good !!1
 Question: Can you use this to blend between two effects circuits...say two different distortions ?
 Maybe splice another one in after OPa...between the 10uf and the blend pot?

[seanm]

Coment: Looks pretty good !!1
 Question: Can you use this to blend between two effects circuits...say two different distortions ?
 Maybe splice another one in after OPa...between the 10uf and the blend pot?

Don't see why not. There is another blender here: http://www.runoffgroove.com/splitter-blend.html that does what you want. I am not saying either is better, just giving you choices.

[seanm]

Made a small change to the schematic. I was originally going to use 0.1uf input caps, but I built with 1,000,000pf = 1uf. I do not think this will make a difference, but it might....

[ultramusicman]

The schematic is shown below minus details such as the bypass switch and Vr.

What exactly is Vr?  If the +9 by the diode is +9V, then to me, it looks like a bias voltage at (9-0.7)/2 or 4.15 V.  I assume that you're powering the op amps with a single 9V battery?  So all the 10 uF caps are for blocking DC?

Cheers,

[seanm]

The schematic is shown below minus details such as the bypass switch and Vr.

What exactly is Vr?  If the +9 by the diode is +9V, then to me, it looks like a bias voltage at (9-0.7)/2 or 4.15 V.

You are correct. I am used to calling it a "reference" voltage. I really should change it to show +4.5. Or just label the two points as A or something.

I am currently not using the diode, but I thought it would be a good idea to protect the circuit.

I assume that you're powering the op amps with a single 9V battery?  So all the 10 uF caps are for blocking DC?

Yes. They block the DC level from the output. I play bass so I keep all the caps big to let all the bass through.

Note the input caps are now 0.1uf again. The 0.1uf gives 1.6Hz or so, major overkill and .1 are much cheaper than 1. Actually 0.01uf gives a 15Hz, which would also be excellent.

[ultramusicman]

Thanks for the response.

I am currently not using the diode, but I thought it would be a good idea to protect the circuit.

Well, if you're using a 9V battery and a battery clip, the clip will keep your battery's polarity correct, if that's your concern.

Cheers,

[seanm]

Well, if you're using a 9V battery and a battery clip, the clip will keep your battery's polarity correct, if that's your concern.

Cheers,

I plan on using a power supply. I even have the jack in place, just not wired in. Yes, the circuit is off the breadboard and into a box. Currently don't have the bypass switch hooked up either since I do not have the switch yet

The pedal is very handy for testing. Since it has buffers on the send and return you can build simple test circuits to go in between without worrying about impedence matching.

For those who are keeping score, I used a TL704 quad opamp.

[Mark Hammer]

Hi Sean,

We still have to get together at some point.  Drop me a note off-line.

I didn't stumble onto this thread until Ton Barmentloo noted it in another thread.

Nice circuit idea.  I note that all stages are noninverting.  This has some interesting implications.  One of them is that if the thing the send jack goes to does not preserve phase, then your blend control essential mixes an inverted and noninverted version of the original input signal.  Yes, that signal may have been altered in some way, but even the most heavily distorted fuzzes have a fundamental in there somewhere, and if the fuzz output is inverted, then blending the original end effected sound will result in some cancellation of the fundamental at least.

That is not a complaint or a drawback, from my perspective but actually a potential advantage of the circuit.  Of course, since the tendency is for pedals to be designed with phase preserved, more pedals plugged into that loop are going to be IN phase than OUT of phase.  What that suggests is that converting either the send stage or receive stage into a switch-selectable inverting/noninverting buffer would be a good idea.  I do believe RG Keen has such a schematic on his site.  If not, I am confident there is an example/illustration of how to do this with a single op-amp in the Voltage-controlled phaser project in DEVICE at my website.

[ultramusicman]

I do believe RG Keen has such a schematic on his site.  If not, I am confident there is an example/illustration of how to do this with a single op-amp in the Voltage-controlled phaser project in DEVICE at my website.

Please provide links!

Cheers,

[Mark Hammer]

Go to http://www.geofex.com and scroll down through the list of articles until you reach the point where the links change from big letters to small ones (mid-2001).  Just below that transition is a link to an article about a "polarity reverser".  That will explain how to have in/out-of-phase from  the same op-amp.

Again, this can go either on the send/output or receive/input portion of Sean's circuit.  One of the things to take into consideration is whether any padding or gain is needed for patching things into the loop.  For instance, would it be helpful to add some juice to the send signal to use it with devices expecting something closer to a line level?  Alternatively, is there a need to have a way to reduce the level of the return signal or jack it up a notch?

I ask these questions because the feedback resistor in the circuit/schem shown can be altered (and even replaced with a fixed/variable resistor combo) to provide different signal levels.

As well, the visual informativeness of the mix pot, in addition to its "nailability" relies on having essentially equal signals arriving at each side of the mix pot.  Should the device patched into the loop have a much higher or much lower level than the signal coming in from the other side of the mix pot, than can seriously restrict the number of degrees of rotation you have to work with to dial in your preferred blend.  Being able to produce equal signal levels arriving at each side of the pot will mean that the *visual* middle of a linear pot *will* represent a 50/50 balance.

What might be of great assistance to some folks working in some contexts - though it would obviously hike up the complexity of the circuit a notch - would be some LED-based level indication.  For instance, when the two LEDs glow about the same brightness, that means the two signals are in equal proportion and the visual middle of the blend pot's rotation can be treated AS the 50/50 split.  Again, however, that is largely unnecessary for a great many people, and also increases the current requirements (two LEDs glowing bright enough that you can spot differences in their relative brightness?  you bet).

[snap]

like in an FM radio: phase-discriminator with 2-color LED display

[seanm]

Go to http://www.geofex.com and scroll down through the list of articles until you reach the point where the links change from big letters to small ones (mid-2001).  Just below that transition is a link to an article about a "polarity reverser".  That will explain how to have in/out-of-phase from  the same op-amp

Again, this can go either on the send/output or receive/input portion of Sean's circuit.

Way ahead of you there. I already whipped up a schematic with R.G. Keen's circuit, but I put the schematic in the wrong place (doh!). You can find it here: ftp://ftp.seanm.ca/stomp/bblender-mod.gif. Note: I have not tested it. Try 10k or 100k for the resistors. I put it on the send since I want to handle a partial "effect" that does phase inversion. The final effect would have a second buffer to invert it back.

One of the things to take into consideration is whether any padding or gain is needed for patching things into the loop.  For instance, would it be helpful to add some juice to the send signal to use it with devices expecting something closer to a line level?  Alternatively, is there a need to have a way to reduce the level of the return signal or jack it up a notch?

I ask these questions because the feedback resistor in the circuit/schem shown can be altered (and even replaced with a fixed/variable resistor combo) to provide different signal levels.

Good points. It would be easy to add pots to all the opamps for maximum control. I wanted to keep it simple to start with

As well, the visual informativeness of the mix pot, in addition to its "nailability" relies on having essentially equal signals arriving at each side of the mix pot.  Should the device patched into the loop have a much higher or much lower level than the signal coming in from the other side of the mix pot, than can seriously restrict the number of degrees of rotation you have to work with to dial in your preferred blend.  Being able to produce equal signal levels arriving at each side of the pot will mean that the *visual* middle of a linear pot *will* represent a 50/50 balance.

What might be of great assistance to some folks working in some contexts - though it would obviously hike up the complexity of the circuit a notch - would be some LED-based level indication.  For instance, when the two LEDs glow about the same brightness, that means the two signals are in equal proportion and the visual middle of the blend pot's rotation can be treated AS the 50/50 split.  Again, however, that is largely unnecessary for a great many people, and also increases the current requirements (two LEDs glowing bright enough that you can spot differences in their relative brightness?  you bet).

This is a cool idea. Wish I had thought of it.....

[Mark Hammer]

Looks good.  Bit by bit, you will have created an indispensible tool.  For my money, that is generally a more lasting contribution than just about ANY fuzz. :wink:

[puretube]

forgot where/when, but there was talk about a variable "bipolar coefficient..." circuit with continuous variation from gain=+1 thru 0 to -1...
I use it often to save one pot, in various feedback/blend designs.
:wink:
In your case: if you`re sure that one of the paths never has to get louder than the other, you can stick that in there...
(combined blend/invert control)