Author Topic: Switch to swap pots in op-amp feedback *pop*  (Read 554 times)

PhiloB

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Switch to swap pots in op-amp feedback *pop*
« on: August 05, 2018, 08:21:09 PM »
I have a friend that wants a simple pedal to set 2 different vol/gain levels and provide a buffer.  I thought a simple approach would be to use an op-amp and vary the feedback resistor.  I decided to lay it out as below in hopes to keep the impedance low on output.  He wants the max volume to be unity gain.  My problem is there is pop when I switch between pots.  I suspect DC build up.  I tried using "pull-down" resistors either side of the pots with no luck.  Do I need to use 2 separate sides of the op-amp and switch between op-amps?  Or can I utilize the current layout and deal with the DC?  Thanks for any help.

BuGG

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Re: Switch to swap pots in op-amp feedback *pop*
« Reply #1 on: August 06, 2018, 06:35:46 AM »
See if this helps.   If so you could use the extra side of your DPDT to switch an LED or something.   


PhiloB

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Re: Switch to swap pots in op-amp feedback *pop*
« Reply #2 on: August 06, 2018, 08:21:55 AM »
Thanks BuGG, I gave this a shot and it helped significantly.  There is still some noise when switching but I might just be hypercritical at this point.  I know mechanical switching is not silent switching.  Thanks very much for the suggestion!!

BuGG

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Re: Switch to swap pots in op-amp feedback *pop*
« Reply #3 on: August 06, 2018, 08:23:38 AM »
You might try putting a low value cap across each of the pots, 47 - 100pF.

PhiloB

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Re: Switch to swap pots in op-amp feedback *pop*
« Reply #4 on: August 06, 2018, 08:52:28 AM »
I gave the caps a try with minimal effect if any. 

Taking a step back, is there a better approach to this?  Objective: a 2 pot pedal for controlling volume with a buffer to be used at the end of the effect chain.  Max volume is unity gain.  Sounds as though he wants to use it as a “boost” pedal with no coloration and no signal boost:). So essentially he wants a “signal cut” pedal:)

Would I be better off using 2 separate sides of a dual op amp and switch between them.  Use transistor instead?  Stick the pots at the end of the circuit with your typical “volume pot” setup?


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Rockhorst

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Re: Switch to swap pots in op-amp feedback *pop*
« Reply #5 on: August 06, 2018, 09:08:27 AM »
Use the buffer for the input and follow it with a small series cap and resistor and end with two volume pots. You won't have a buffered output (but you could use the other side of the opamp for that if that is what you want). You can switch only the wipers (influences taper and impedance) or wiper and terminal 3. In any case, pin 1 of both pots is always connected to ground, sucking up the DC build up.

Aentons

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Re: Switch to swap pots in op-amp feedback *pop*
« Reply #6 on: August 06, 2018, 10:05:43 AM »
It sounds like you want the guitar volume control at your feet and then a buffer pedal. Do you want the buffer always on or switchable? And, why two pots? The second one will likely always be all the way up at unity. One pot should do it unless you want three distinct volume levels.

You could put the volume pot first and switch it in and out before the buffer circuit.
« Last Edit: August 06, 2018, 10:11:51 AM by Aentons »

gordo

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Re: Switch to swap pots in op-amp feedback *pop*
« Reply #7 on: August 06, 2018, 02:32:24 PM »
+1^  Sounds like he just needs a buffer and a switchable attenuator.

PhiloB

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Re: Switch to swap pots in op-amp feedback *pop*
« Reply #8 on: August 06, 2018, 03:04:04 PM »
yes, buffer and switchable attenuator.  I tried to talk him into one pot.  I was thinking variable resistor in parallel to fixed resistor in the negative feedback loop to set gain with the switch to add and remove the variable resistor.  He is set on 2 pots and unwavering.

Aentons

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Re: Switch to swap pots in op-amp feedback *pop*
« Reply #9 on: August 06, 2018, 03:18:26 PM »
Since you want max volume at unity... just make the volume controls and foot switching, passive and separate from (in series, before or after) the active buffer section. You could even put a mini toggle to switch in and out the buffer separately

PhiloB

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Re: Switch to swap pots in op-amp feedback *pop*
« Reply #10 on: August 08, 2018, 04:52:21 AM »
Use the buffer for the input and follow it with a small series cap and resistor and end with two volume pots. You won't have a buffered output (but you could use the other side of the opamp for that if that is what you want). You can switch only the wipers (influences taper and impedance) or wiper and terminal 3. In any case, pin 1 of both pots is always connected to ground, sucking up the DC build up.
the "small cap and resistor"...the cap to block DC and the resistor to act with the pot as a voltage divider?  or do they serve other functions?  With the cap and resistor in series I don't have to worry about the cap and potentiometer becoming a high pass filter correct?