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Only one connection to a pot? Schematic Help
Old 1 week ago
  #1
Here for the gear
 

Only one connection to a pot? Schematic Help

Trying to get into building pedals, and I decided to do a clone of an MXR Distortion+ as my first build. The attached picture shows a portion of the schematic I drew out (drawing out stuff and annotating it helps me learn it so I copied Brian Wampler's diagram). R3 is the gain/distortion potentiometer. But looking at the schematic, it only shows one connection attached to the pot: coming from the negative end of the op-amp. After that, the pot is sent to ground, but it doesn't look like it goes back into the circuit. Is this normal? Do I only have to solder one lead or wire to the gain pot? Or am I misreading it?
Attached Thumbnails
Only one connection to a pot? Schematic Help-20200519_191514.jpg  
Old 1 week ago
  #2
Gear Nut
Quote:
Originally Posted by TwistedGlass View Post
Trying to get into building pedals, and I decided to do a clone of an MXR Distortion+ as my first build. The attached picture shows a portion of the schematic I drew out (drawing out stuff and annotating it helps me learn it so I copied Brian Wampler's diagram). R3 is the gain/distortion potentiometer. But looking at the schematic, it only shows one connection attached to the pot: coming from the negative end of the op-amp. After that, the pot is sent to ground, but it doesn't look like it goes back into the circuit. Is this normal? Do I only have to solder one lead or wire to the gain pot? Or am I misreading it?
Thats normal, just think of the two end lugs of the pot as one resistor , and the center lug, as a tap. thats all their is to it.
Old 1 week ago
  #3
Gear Nut
Quote:
Originally Posted by TwistedGlass View Post
Trying to get into building pedals, and I decided to do a clone of an MXR Distortion+ as my first build. The attached picture shows a portion of the schematic I drew out (drawing out stuff and annotating it helps me learn it so I copied Brian Wampler's diagram). R3 is the gain/distortion potentiometer. But looking at the schematic, it only shows one connection attached to the pot: coming from the negative end of the op-amp. After that, the pot is sent to ground, but it doesn't look like it goes back into the circuit. Is this normal? Do I only have to solder one lead or wire to the gain pot? Or am I misreading it?
when doing this to ground you should also solder a jumper from that lug to the pots housing. If you google images of potentiometer circuits, you'll see some like what you're dealing with and you'll understand the concept.
Old 1 week ago
  #4
Gear Nut
OK, I guess I lost ya...? Anyway you can wait for someone else, if you're unsure of my knowledge,,, One more thing as to why ground the pot housing, in case this project is star grounded in an metal housing, you wouldn't want your pots housing making an chassis ground/two ground plains. Plus it helps shield the pot from IME. Just wondering if this is an DIY Kit or if you're doing all from scratch?
Old 1 week ago
  #5
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The ground is the second connection to the potentiometer. This circuit as you describe it seems sensible enough to me. The opamp would seem to be basically set up as a noninverting amplifier, and the gain of such an amplifier is determined by the ratio of the feedback resistance (from the opamp output to its inverting input) and the shunt resistance from the inverting input to ground. Here you're directly adjusting the latter with a variable resistor, about as straightforward as you can get.
Old 1 week ago
  #6
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Quote:
Originally Posted by Rick Dalton View Post
Just wondering if this is an DIY Kit or if you're doing all from scratch?
This is all going to be from scratch. My first run is just going to be point to point without an enclosure, just to make sure I can get the basics down. Once I do that, I'll probably condense it for pedal-size, but I also want to experiment with some different parts (namely diodes and op-amps) to see how I can affect the gain
Old 1 week ago
  #7
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JohnRoberts's Avatar
 

When the wiper is tied to one end, it works like a "rheostat" or variable resistance.

JR
Old 1 week ago
  #8
Here for the gear
 

Just one more question: R5 is used to set the bias at 4.5v for the op-amp. How would I solder this?
Attached Thumbnails
Only one connection to a pot? Schematic Help-20200520_135816-2.jpg  
Old 1 week ago
  #9
Gear Nut
Quote:
Originally Posted by TwistedGlass View Post
Just one more question: R5 is used to set the bias at 4.5v for the op-amp. How would I solder this?
that would be an 1 mega ohm trim pot or variable resistor
but your symbol is not correct. It should look like this or similar. Look up variable resistor images, with this you solder one end lug and the center lug and leave one end lug floating/unconnected,,, that's it.

also for future reference, If it is a variable resistor, it should have a symbol similar to the one I uploaded, plus the schematics should also have VR5 , but they don't always.
Attached Thumbnails
Only one connection to a pot? Schematic Help-untitled.jpg   Only one connection to a pot? Schematic Help-untitled.jpg  

Last edited by Rick Dalton; 1 week ago at 07:55 PM.. Reason: added
Old 1 week ago
  #10
Here for the gear
 

Quote:
Originally Posted by Rick Dalton View Post
that would be an 1 mega ohm trim pot or variable resistor
but your symbol is not correct. It should look like this or similar. Look up variable resistor images, with this you solder one end lug and the center lug and leave one end lug floating/unconnected,,, thats it.
Thanks, I copied Wampler's schematic and he wrote it out as a regular resistor so I couldn't tell if it was variable or not. You can tell I'm new to this
Old 1 week ago
  #11
Here for the gear
 

Quote:
Originally Posted by Rick Dalton View Post
that would be an 1 mega ohm trim pot or variable resistor
but your symbol is not correct. It should look like this or similar. Look up variable resistor images, with this you solder one end lug and the center lug and leave one end lug floating/unconnected,,, that's it.
So one lug is coming from the rest of the circuit, but where does the other lug get soldered to? Are they soldered to each other, or does it go to something else?
Old 1 week ago
  #12
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I don't see any reason why R5 would be a variable resistor in this case. The input is AC coupled to the non-inverting input of the opamp (through C2 and R1), and having a variable resistor for R5 would just alter the gain (unnecessarily) at this input rather than adjusting the biasing in any meaningful way.

The one terminal of R5 is connected to the opamp, as shown. The other terminal goes to whatever the 4.5V supply looks like, presumably shown elsewhere on the schematic and identified as 4.5V. One good possibility for this supply is merely a pair of equal-valued resistors in series between +9V and ground, with the 4.5V taken from the midpoint of the voltage divider (or between the two resistors). It's hard to say, of course, with just seeing a little detail of the schematic.
Old 1 week ago
  #13
Gear Nut
Quote:
Originally Posted by TwistedGlass View Post
So one lug is coming from the rest of the circuit, but where does the other lug get soldered to? Are they soldered to each other, or does it go to something else?
See if your schematics has this power section. You may wont to s-can that VR5?

Did you just sketch that schem,,,? You do know how to use Print Screen button on your keyboard, and paste it in MS Paint or in a picture folder...? Anyway as
DrewE pointed out, that 4.5volt goes to the power section.
Attached Thumbnails
Only one connection to a pot? Schematic Help-untitled.jpg  
Old 1 week ago
  #14
Gear Nut
Quote:
Originally Posted by DrewE View Post
I don't see any reason why R5 would be a variable resistor in this case. The input is AC coupled to the non-inverting input of the opamp (through C2 and R1), and having a variable resistor for R5 would just alter the gain (unnecessarily) at this input rather than adjusting the biasing in any meaningful way.

The one terminal of R5 is connected to the opamp, as shown. The other terminal goes to whatever the 4.5V supply looks like, presumably shown elsewhere on the schematic and identified as 4.5V. One good possibility for this supply is merely a pair of equal-valued resistors in series between +9V and ground, with the 4.5V taken from the midpoint of the voltage divider (or between the two resistors). It's hard to say, of course, with just seeing a little detail of the schematic.
DrewE, Thanks for helping out,,, I'm half brain dead from undoing a bunch of crap on an Onan Generator, that someone really screwed up. lol! So my head is still in there.
Old 1 week ago
  #15
Gear Nut
Quote:
Originally Posted by TwistedGlass View Post
So one lug is coming from the rest of the circuit, but where does the other lug get soldered to? Are they soldered to each other, or does it go to something else?
Looks like others ran into the same thing as you.
https://www.diystompboxes.com/smffor...topic=114348.0
Old 1 week ago
  #16
Gear Nut
Quote:
Originally Posted by TwistedGlass View Post
Thanks, I copied Wampler's schematic and he wrote it out as a regular resistor so I couldn't tell if it was variable or not. You can tell I'm new to this
R5 is a regular resistor.
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