madbeanpedals::forum

Projects => General Questions => Topic started by: Aleph Null on February 23, 2018, 05:09:04 PM

Title: Pork Barrel Blend
Post by: Aleph Null on February 23, 2018, 05:09:04 PM
I'm looking to add a blend control to the Pork Barrel. It looks like the wet and dry signals are mixed before hitting IC1_B. Could I omit R9 and R26 and use a b100K pot in their place?

RedLug 1
GreenLug 2
BlueLug 3

Would this be enough resistance to go 100% dry to 100% wet?
Title: Re: Pork Barrel Blend
Post by: blearyeyes on February 23, 2018, 07:07:19 PM
You could add an external blend


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Title: Re: Pork Barrel Blend
Post by: madbean on February 23, 2018, 08:10:33 PM
You will need to use those 47k resistors rather than omit them otherwise the output will distort badly as you turn it to the highest and lowest settings on the pot. Instead, attach wires to the blue and red spots, a 47k on the other end of each wire and then the other end of the 47k to lugs 1 and 3 resp. And, still attach the wiper to green.

Blue -----wire-----47k-----lug3
lug2-----wire----- Green
Red -----wire----47k-----lug1

Not sure if 100k pot is enough - might have to use 250k.
Title: Re: Pork Barrel Blend
Post by: Aleph Null on February 23, 2018, 08:15:22 PM
You will need to use those 47k resistors rather than omit them otherwise the output will distort badly as you turn it to the highest and lowest settings on the pot. Instead, attach wires to the blue and red spots, a 47k on the other end of each wire and then the other end of the 47k to lugs 1 and 3 resp. And, still attach the wiper to green.

Blue -----wire-----47k-----lug3
lug2-----wire----- Green
Red -----wire----47k-----lug1

Not sure if 100k pot is enough - might have to use 250k.

So we need the resistors to avoid clipping the IC input. Makes sense. Thanks!

Would you expect the additional resistance from the pot to reduce the overall output? Can we gain up the IC with a value change somewhere?
Title: Re: Pork Barrel Blend
Post by: madbean on February 23, 2018, 08:28:49 PM
You will need to use those 47k resistors rather than omit them otherwise the output will distort badly as you turn it to the highest and lowest settings on the pot. Instead, attach wires to the blue and red spots, a 47k on the other end of each wire and then the other end of the 47k to lugs 1 and 3 resp. And, still attach the wiper to green.

Blue -----wire-----47k-----lug3
lug2-----wire----- Green
Red -----wire----47k-----lug1

Not sure if 100k pot is enough - might have to use 250k.

So we need the resistors to avoid clipping the IC input. Makes sense. Thanks!

Would you expect the additional resistance from the pot to reduce the overall output? Can we gain up the IC with a value change somewhere?

Well that's a good point. Yeah, overall the output is going to be reduced depending on where the blend knob is set. You can offset that by changing values with R11, R10, C5 and C6. What exact values would be best I don't know - but you would want to scale them up equally otherwise you are going to change the overall frequency response. Another thing I though of - R26 is DC coupled by C17 but R9 is not DC coupled. So, you'd also probably have to a 100n cap before R9 to avoid scratchiness on the pot.

Whether or not all this is worth the effort is up to you. It's possible to do a better blend knob but it requires more parts and a ton of off-board wiring to do it. An alternative to all this would be to simply leave R9 in place and put a 100k or 250k pot in series with R26. This would allow you to reduce the chorus level. But, it won't let you change the dry level for vibrato type effect. To do that you would have to lift R9 with a switch. In fact, the very early versions of the Pork Barrel had both of these mods but I ditched them when I went down to a 1590B layout.
Title: Re: Pork Barrel Blend
Post by: madbean on February 23, 2018, 08:54:23 PM
This kind of setup would work better because it operates by sending either the dry signal or wet signal to (virtual) ground and doesn't require changing anything in the output stage.

When the Blend pot is in the middle you have equal amounts of dry/wet
Turning it clockwise grounds the dry path so you get vibrato
Turning it counter clockwise grounds the wet so you get straight dry signal
Title: Re: Pork Barrel Blend
Post by: Aleph Null on February 23, 2018, 11:27:45 PM
This kind of setup would work better because it operates by sending either the dry signal or wet signal to (virtual) ground and doesn't require changing anything in the output stage.

When the Blend pot is in the middle you have equal amounts of dry/wet
Turning it clockwise grounds the dry path so you get vibrato
Turning it counter clockwise grounds the wet so you get straight dry signal

This looks do-able (in a 125B). Stand two 22k resistors in the R26 holes and another two in the R9 holes. Twist the other leads together and connect them to lug 1 and 3 respectively. Then connect lug 2 to right side of R27 maybe?

Thank you! I'm learning a lot here and getting more excited about this build.