Blendable feedback loop

[ragingben]

Hey everyone.

I'm trying to build a feedback looper like this one

http://www.beavisaudio.com/techpages/PedalHacker/LooperWithFeedbackLoop.gif, but with an active blender. I choose the Blendimum

http://www.aronnelson.com/gallery/main.php/v/BarcodeVeroLayouts/Vero/BlendimumVero.gif.html

as I have the components, and it is simple. It's all good, but I can't for the life of me work out how to make it so that I can blend the feedbacked signal with the dry, as I want to ensure that the signal can't travel back down to the dry input of the blender circuit as it is connected to the send jack. Do you see what I mean? I want to be able to blend in the feedback loop with the dry to any degree possible, from 100% dry/0% wet, to 0% wet/100% dry

If I exclude the feedback I assume it will work fine. I foolishly thought a diode in series between the dry input and fx send would solve the problem, but obviously it didn't! Do I need to add a buffer between the dry signal and the send jack?

Any help is 100% wholeheartedly appreciated!

[earthtonesaudio]

If you want to do 100% wet 0% dry at the same time you with feedback, you need two mixers.  An input mixer takes the input signal and the feedback as inputs, and the output is the "send".
And then an output mixer takes the dry and "return" as inputs, and the output is split to the overall circuit output as well as the feedback path.

That's the minimum anyway.  Op-amps make better mixer circuits than transistors so that's what I would recommend.


Something like this (ignore component values):

link

[a soBer Newt]

If you want to do 100% wet 0% dry at the same time you with feedback, you need two mixers.  An input mixer takes the input signal and the feedback as inputs, and the output is the "send".
And then an output mixer takes the dry and "return" as inputs, and the output is split to the overall circuit output as well as the feedback path.

That's the minimum anyway.  Op-amps make better mixer circuits than transistors so that's what I would recommend.


Something like this (ignore component values):

link

I really like that design I am going to have to build it!

[earthtonesaudio]

If you want to build it with a 9V supply, it requires some changes: link

Added bias voltage for single supply operation, as well as coupling caps so the inputs and outputs are still ground referenced.  Also changed some component values around.  The input impedance is a little low for connecting directly to a passive guitar without a buffer, but it will work.

[ragingben]

earthtonesaudio - thank you for a fantastic reply!

I think I understand now. Which opamps would you recommend? Would a TL072 be a good choice?

So if I have this right the dry signal would be totally cut out when the input mix was at 100% wet and the output was at 100% wet, and you would just hear the feedback signal? That's a cool feature. In my original plan I was only concerned about the dry signal, the wet signal (feedback or no feedback) and stopping the feedback getting back into the dry mix. Would this be achievable by replacing the dry mix pot with a resistor (10k?) so that the dry mix was only ever letting through the dry signal and not the feedbacked signal? I'm guessing this would cut out the feedback altogether.

Great little program as well by the way, I'll have to remember that one!

[ragingben]

I have been having a good old think, and following your advice I can see how to do it.

Using the circuit you provided, could I cut out the "feedback mix" pot (would this set the mix as 50/50?). As the opamps blend the signal it won't allow any of the feedbacked signal to 'bleed' back into the dry signal going to the output mix, which would solve my problem.

And then with the feedback mix pot, when feedback is off the pot will be able to control the level of the effect in the loop (so probably want it all the way to the left, 100% dry) which to all intents and purposes work as a fx volume level, and then when feedback is on wherever I want it to be, as this will control how much of the feedbacked signal and the dry signal is getting sent to the effect in the loop.

I have attached a link to a very crude diagram I drew (in paint!) of how I think the logic would look - does this logic look correct to you? I just want to make sure I understand the 2 blender part of the schematic!



I really, really appreciate your help as this has been doing my head in trying to work out! I'm really new to electronics, and have built a few really, really simple pedals, but this has pretty much bent my mind back on itself!

EDIT: Is the feedback pot needed?  Wouldn't the feedback be controlled by the feedback blender circuit, in which case this would work just as well?



If that is the case, unless I have really got in a muddle with this I could use the second side of my DPDT toggle controlling the feedback function to bypass the feedback mix section altogether when feedback is off, as in non feedback mode I would want the blender to use the effect in the loop at full tilt at all times.

[earthtonesaudio]

earthtonesaudio - thank you for a fantastic reply!

I think I understand now. Which opamps would you recommend? Would a TL072 be a good choice?

So if I have this right the dry signal would be totally cut out when the input mix was at 100% wet and the output was at 100% wet, and you would just hear the feedback signal? That's a cool feature. In my original plan I was only concerned about the dry signal, the wet signal (feedback or no feedback) and stopping the feedback getting back into the dry mix. Would this be achievable by replacing the dry mix pot with a resistor (10k?) so that the dry mix was only ever letting through the dry signal and not the feedbacked signal? I'm guessing this would cut out the feedback altogether.

Great little program as well by the way, I'll have to remember that one!

My names for the different part of the circuit are different from yours, but I think that's right.  If you consider the left op-amp to be the "input mix" and the right one to be the "output mix" then yes.  But to be more specific the left op-amp is a mixer that mixes the dry signal with the feedback signal, and the right side op-amp is a mixer that mixes the wet signal with the output of the left op-amp.  If you want a dry mix without feedback that's a simple matter of adding the dry (before the left op-amp mixer) through another pot+resistor to the output op-amp.  But this resistor load is in parallel with the signal source, which may end up causing the input impedance to be a bit too low.  Simple solution again, add a buffer op-amp to the input.  Example here.

This configuration made the original dry mix pot redundant and so it was removed.  It's good to note that this circuit behaves slightly differently.  Before, you could have the dry mix fed back into itself (with or without the wet mix).  Now that is not possible (which is fine, really, because it's not that useful) and now the feedback is for the wet mix only.

I have not looked closely at your second reply with the images; I will respond to that one later.  Have to get some coffee now.  Priorities, you know.  

[ragingben]

Ha ha yeah, sounds top priority to me!

I did think about a buffer when you said about impedance's but I didn't want to complicate things for myself. Would a TL072 do for that too?

With the diagrams, the third one is really what I want I guess, assuming that the input mix will control the feedback. I just noticed though, if it was at 100% feedback/0% dry there would be nothing to get the feedback going!

But I think that now the buffer is added and that pot removed, I don't need to worry about toggling the opamp out of the circuit as I was when feedback was off on diagram 3.  

What I don't understand about the new circuit you posted is whey there is a pure dry mix pot and a wet pot? I guess it is so the pure dry (with no feedback or effect or whatever is coming from the loop) is blended as part of the dry signal. Is the wet mix pot acting as a volume for the wet signal? Could I just have the pure dry going to the output mix opamp, the wet going to the output mix opamp and then one pot to control the output mix? That would leave me with a pot for feedback (probably 500k linear), and a pot for blending the output mix (50k linear?) as well as a toggle to control if the feedback is on or not, which would be between the 100k resistor and the 100nf cap after the output mix opamp? One other thing I noticed is in the circuit you were using the mixed output to route back to the send rather than using the signal direct from the return, is there a reason for this? I would have assumed the unmixed signal from the return should have been used, but the only reason I have for this is because it could be at a lower level than the mixed output signal, depending on the mix settings, and this may affect the feedback character.

I guess this (again, very crude) diagram sums up how I see it



Note: the output mix square represents the opamp, capacitors, resistors etc that make up that section of the circuit and the mix pot etc. Also I have used the mixed output to route back to the send as per the circuit.

After rereading your posts I think I should just clarify exactly what I want to build to make sure I don't waste your time: I'm building a blender pedal that has an input, output, send and receive jacks. By flipping a toggle switch the blender can be used as a feedback looper. I want to use 2 pots - feedback which controls the amount of feedback going back into the looped pedal when in feedback mode, and blend, from full acw (dry only) to 12 o'clock (50% dry/50% wet) to full cw (wet only).

Again, I can't thank you enough for the time you are spending helping me here!

[ragingben]

I have had a bit of a play with that applet, and I think I have modified the circuit you did me to show what I'm thinking. Does it look OK? I'm not sure about the + and - with the opamp between the return and the blend pot? A guy at work helped me a bit and we think it is getting there, although there appears to be -15v on the output?

http://www.falstad.com/circuit/#%24+4+5.0E-6+1.6308177459886661+1+5.0+43%0Aa+288+352+368+352+0+15.0+-15.0+1000000.0%0Ag+288+480+288+496+0%0Aw+288+336+288+288+0%0Ar+288+288+368+288+0+100000.0%0Aw+368+288+368+352+0%0Ar+288+288+224+288+0+100000.0%0Aw+368+288+368+272+0%0Aw+368+112+416+112+0%0Aw+480+112+528+112+0%0Ar+288+224+288+48+0+150000.0%0Aw+288+224+288+288+0%0Aw+352+16+768+16+0%0Ax+-13+277+6+283+0+24+in%0Ax+876+358+910+364+0+24+out%0Ax+244+-11+342+-5+0+24+feedback%0Ax+342+85+395+91+0+24+send%0Ax+509+86+573+92+0+24+return%0Ac+96+272+32+272+0+1.0E-7+9.999999268415216E-4%0Ar+288+400+288+480+0+10000.0%0Ar+96+400+16+400+0+10000.0%0Av+16+480+16+400+0+0+40.0+9.0+0.0+0.0+0.5%0Aw+224+480+16+480+0%0A174+352+16+224+48+0+10000.0+0.5+Resistance%0Aw+288+400+224+400+0%0Aw+128+16+128+400+0%0Ac+224+400+224+480+0+1.0E-5+3.800886142936646%0Aw+224+480+288+480+0%0Aw+224+16+128+16+0%0Aw+128+400+224+400+0%0Ac+528+112+528+224+0+1.0E-5+-0.7474367634342056%0Ac+368+112+368+192+0+1.0E-5+-1.02190068926478%0Ac+768+352+832+352+0+1.0E-5+4.34008014797738%0Ar+832+352+832+480+0+100000.0%0Aw+832+480+288+480+0%0Ar+368+192+368+272+0+100.0%0Ar+416+112+416+176+0+100000.0%0Ar+480+112+480+176+0+1000000.0%0Ag+480+176+480+208+0%0Ag+416+176+416+208+0%0Aw+832+352+864+352+0%0Aw+368+112+368+96+0%0Aw+528+112+528+96+0%0Aa+144+288+224+288+1+15.0+-15.0+1000000.0%0Aw+144+304+144+320+0%0Aw+144+320+192+320+0%0Aw+224+320+224+288+0%0Aw+96+272+144+272+0%0Ar+96+272+96+400+0+1000000.0%0Aw+96+400+128+400+0%0Aw+192+320+224+320+0%0Aw+768+224+528+224+0%0Aw+528+384+528+464+0%0Aw+192+320+192+464+0%0Ax+348+445+509+451+0+24+pure+dry+signal%0Aw+288+400+448+400+0%0Aw+528+224+528+288+0%0A174+528+288+560+384+0+1000.0+0.5+Resistance%0Aa+560+352+704+352+1+15.0+-15.0+1000000.0%0Aw+704+352+768+352+0%0AS+800+224+800+16+0+1+false+0%0Aw+768+224+800+224+0%0Aw+768+16+784+16+0%0Aw+288+368+288+400+0%0Ax+541+302+602+308+0+24+blend%0Aw+560+368+560+400+0%0Aw+560+400+624+400+0%0Aw+704+352+704+400+0%0Ar+624+400+704+400+0+100000.0%0Aw+448+400+448+368+0%0Aw+448+368+560+368+0%0Aw+192+464+336+464+0%0Ac+336+464+416+464+0+1.0E-5+0.008990667221327353%0Aw+416+464+528+464+0%0A]http://www.falstad.com/circuit/#%24+4+5.0E-6+1.6308177459886661+1+5.0+43%0Aa+288+352+368+352+0+15.0+-15.0+1000000.0%0Ag+288+480+288+496+0%0Aw+288+336+288+288+0%0Ar+288+288+368+288+0+100000.0%0Aw+368+288+368+352+0%0Ar+288+288+224+288+0+100000.0%0Aw+368+288+368+272+0%0Aw+368+112+416+112+0%0Aw+480+112+528+112+0%0Ar+288+224+288+48+0+150000.0%0Aw+288+224+288+288+0%0Aw+352+16+768+16+0%0Ax+-13+277+6+283+0+24+in%0Ax+876+358+910+364+0+24+out%0Ax+244+-11+342+-5+0+24+feedback%0Ax+342+85+395+91+0+24+send%0Ax+509+86+573+92+0+24+return%0Ac+96+272+32+272+0+1.0E-7+9.999999268415216E-4%0Ar+288+400+288+480+0+10000.0%0Ar+96+400+16+400+0+10000.0%0Av+16+480+16+400+0+0+40.0+9.0+0.0+0.0+0.5%0Aw+224+480+16+480+0%0A174+352+16+224+48+0+10000.0+0.5+Resistance%0Aw+288+400+224+400+0%0Aw+128+16+128+400+0%0Ac+224+400+224+480+0+1.0E-5+3.800886142936646%0Aw+224+480+288+480+0%0Aw+224+16+128+16+0%0Aw+128+400+224+400+0%0Ac+528+112+528+224+0+1.0E-5+-0.7474367634342056%0Ac+368+112+368+192+0+1.0E-5+-1.02190068926478%0Ac+768+352+832+352+0+1.0E-5+4.34008014797738%0Ar+832+352+832+480+0+100000.0%0Aw+832+480+288+480+0%0Ar+368+192+368+272+0+100.0%0Ar+416+112+416+176+0+100000.0%0Ar+480+112+480+176+0+1000000.0%0Ag+480+176+480+208+0%0Ag+416+176+416+208+0%0Aw+832+352+864+352+0%0Aw+368+112+368+96+0%0Aw+528+112+528+96+0%0Aa+144+288+224+288+1+15.0+-15.0+1000000.0%0Aw+144+304+144+320+0%0Aw+144+320+192+320+0%0Aw+224+320+224+288+0%0Aw+96+272+144+272+0%0Ar+96+272+96+400+0+1000000.0%0Aw+96+400+128+400+0%0Aw+192+320+224+320+0%0Aw+768+224+528+224+0%0Aw+528+384+528+464+0%0Aw+192+320+192+464+0%0Ax+348+445+509+451+0+24+pure+dry+signal%0Aw+288+400+448+400+0%0Aw+528+224+528+288+0%0A174+528+288+560+384+0+1000.0+0.5+Resistance%0Aa+560+352+704+352+1+15.0+-15.0+1000000.0%0Aw+704+352+768+352+0%0AS+800+224+800+16+0+1+false+0%0Aw+768+224+800+224+0%0Aw+768+16+784+16+0%0Aw+288+368+288+400+0%0Ax+541+302+602+308+0+24+blend%0Aw+560+368+560+400+0%0Aw+560+400+624+400+0%0Aw+704+352+704+400+0%0Ar+624+400+704+400+0+100000.0%0Aw+448+400+448+368+0%0Aw+448+368+560+368+0%0Aw+192+464+336+464+0%0Ac+336+464+416+464+0+1.0E-5+0.008990667221327353%0Aw+416+464+528+464+0%0A]http://www.falstad.com/circuit/#%24+4+5.0E-6+1.6308177459886661+1+5.0+43%0Aa+288+352+368+352+0+15.0+-15.0+1000000.0%0Ag+288+480+288+496+0%0Aw+288+336+288+288+0%0Ar+288+288+368+288+0+100000.0%0Aw+368+288+368+352+0%0Ar+288+288+224+288+0+100000.0%0Aw+368+288+368+272+0%0Aw+368+112+416+112+0%0Aw+480+112+528+112+0%0Ar+288+224+288+48+0+150000.0%0Aw+288+224+288+288+0%0Aw+352+16+768+16+0%0Ax+-13+277+6+283+0+24+in%0Ax+876+358+910+364+0+24+out%0Ax+244+-11+342+-5+0+24+feedback%0Ax+342+85+395+91+0+24+send%0Ax+509+86+573+92+0+24+return%0Ac+96+272+32+272+0+1.0E-7+9.999999268415216E-4%0Ar+288+400+288+480+0+10000.0%0Ar+96+400+16+400+0+10000.0%0Av+16+480+16+400+0+0+40.0+9.0+0.0+0.0+0.5%0Aw+224+480+16+480+0%0A174+352+16+224+48+0+10000.0+0.5+Resistance%0Aw+288+400+224+4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