MODIFICATION Teaser: I've got a Circulator mod in the works

Chuck D. Bones

Well-known member
I breadboarded the Circulator and it's a great sounding phase shifter, but there are a few opportunities for improvement. I've changed three of the pots, ditched the P/V switch, added a Throb LED and most importantly fixed the LFO so it will always start & never stall. Straightened out a couple other things too. Mod report coming up in about a week after I get the PCB version completed and tested; waiting on a couple of pots from Tayda. I think this pedal is definitely a candidate for a Boneyard Edition.

Circulator - cb mod v1.0 breadboard.jpg

Had to use 1uF tantalum caps in place of the 1uF film caps because none of the 1uF film caps I have or could buy would fit.

Circulator - cb mod v1.0.jpg
 

zgrav

Well-known member
sounds like another Boneyard learning opportunity. also a good chance to see what the fixes are on the LFO
 

p_wats

Active member
Nice! I just received a Circulator board, so I'm very curious to see what you'd improve. Any reason to ditch the P/V switch other than personal preference?
 

Chuck D. Bones

Well-known member
Just personal preference. The V position doesn't sound like much to me and the DEPTH control becomes a volume control in V mode. Had the DEPTH control been put in the dry signal path instead of the wet path, then dialing it down would have faded from P to V and we wouldn't need a P/V switch. I did that on a prototype UniVibe type pedal and I like it.
 

p_wats

Active member
Good to know! Thanks.

I like the vibe mode on my Phase 45 build and my mix knob there does act more like a dry-to-phaser-to-vibe control, which is nice.
 
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cooder

Well-known member
Any ETA on your wise words of wisdom coming our undeserving way...? Cos I have one itching to be populated and would first like to soak up your thoughts... Cheers so much.
 

Chuck D. Bones

Well-known member
Thanks to K Pedals for the beautiful acrylic pour. I used tiny clear knobs so as to not obscure any of the paint job. The white LED throbs with the LFO.

Circulator - cb mod front 02.jpg

Circulator - cb mod innards 02.jpg

Circulator cb mod v1.0 sch.png

Circulator - cb mod solder side 03.jpg

Yeah, I know, I should have cleaned the board better. Not pretty, but it works! I spent a couple of hours troubleshooting the LFO, turns out I had soldered that 22K resistor to the wrong pins. :rolleyes: (we need a dunce-cap smiley)

The Circulator (EQD Grand Orbiter) is a good phase shifter, but it has a few idiosyncrasies that are easily fixed. One thing that is not fixable is this thing is a bit noisy. Not much can be done about it; it's the nature of the LM13700. The good news is it's barely noticeable unless you follow this pedal with a high-gain pedal.

Here's what I changed and why:
1. Increased R2 & R3 to 2.2M because I like my pedals to have a 1Meg input impedance. C1 was reduced to 47nF because it could be. Totally optional.
2. Replaced C2 with a short. It doesn't do anything because the DC voltage on both sides of C2 is Vref. Totally optional.
3. Replaced C8 with 390pF. 100pF is so small it might as well not even be there. 390pF cuts the high end above 40KHz. Totally optional.
4. Inserted 10K resistors (R103-R106) in series with pins 1 & 16 on IC1 & IC2. Pins 1 & 16 program the operating current and gain of each amplifier. The way EQD connected them all in parallel, there is no guarantee that they will share current equally. Putting those resistors in forces them to share. Should make for smoother phase sweep. Optional, but highly recommended.
5. Changed the SWEEP pot to C100K, reduced R33 to 2.2K (to compensate for the 10K resistors I added in item 4) and reduced R34 to 10K. Gives the SWEEP pot more range and at the very bottom of the range, SWEEP acts like a Manual control. Leave R33 at 4.7K is you don't do item 4. Optional, but highly recommended.
6. Inserted a 1N4148 diode (D1) in series with pin 2 of the SWEEP pot. This extends the sweep range down deep into the bass frequencies. With SWEEP above 2:00, you get a nice bottom-end throb. Not for everyone, but I like it.
7. Changed the RATE pot to C500K. Some not-so-good things happen when the RATE pot is above 500K. The sweep becomes asymmetric. There is a tendency for the LFO to stall or fail to start up. The LFO will go plenty slow with a 500K pot. If 14 seconds to go all the way thru one sweep is not slow enough for you, then increase R4 to 10uF. Highly recommended.
8. Added a 3.3Meg resistor (R109) from pin 3 of the RATE pot to IC1 pin 3 (Vref). Along with item 9, ensures that the LFO will always start and never stall.
9. Added a 22K resistor (R107) from IC3 pin 8 to IC3 pin 6. Along with item 8, ensures that the LFO will always start and never stall. 8 & 9 must be done together, I consider them mandatory.
10. Changed the RES (Resonance) pot to C100K and R23 to 33K. Makes for a much smoother adjustment. Highly recommended.
11. Increased R27 to 33K. In the original design, the Circulator will break into oscillation when RES is dimed. I have no use for a pedal that screams at me. With this mod, RES has plenty of range and stops just short of oscillation. Totally optional.
12. Added a Throb LED (R108 & LED2). I think any pedal that has an LFO should also have a Throb or heartbeat LED. I prefer that LED to run all the time, not just when the pedal is engaged. R108 is soldered to LED2 and has clear shrink sleeve over it. Any color LED will work. Do not make R108 less than 10K; if your LED isn't bright enough, get a different LED. Totally optional, but if you do this you need to do items 8 & 9 to ensure that the Throb LED doesn't stop the LFO.
13. Deleted the P/V switch and jumpered pins 1 & 2. Personally, I have no used for Vibrato mode. Totally optional.
14. Subbed 1uF film with 1uF tant for C7, C9-C11. I would have preferred film, but I didn't have any that would fit. I've got some on order, but impatience won out. If you do this sub, watch the polarity. Totally optional.

That pretty well covers it. The pix show how I installed the mods. Make sure R109 is clear of the RES pot. I mounted my pots pretty far from the board to accommodate the height of the RANGE switch.

I used tantalums for C13 and C14 because low leakage is preferred. I suspect EQD depended on leakage in the aluminum caps to help keep the LFO running. Not good design practice. Notice that C13 and C14 connect to Vcc via the RANGE switch. EQD did that to kick-start the LFO when the power comes on. Kind of a band-aid approach and unnecessary when R109 is installed.
 
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K Pedals

Well-known member
Thanks to K Pedals for the beautiful acrylic pour. I used tiny clear knobs so as to not obscure any of the paint job. The white LED throbs with the LFO.

View attachment 7886

View attachment 7887

View attachment 7888

View attachment 7889

Yeah, I know, I should have cleaned the board better. Not pretty, but it works! I spent a couple of hours troubleshooting the LFO, turns out I had soldered that 22K resistor to the wrong pins. :rolleyes: (we need a dunce-cap smiley)

The Circulator (EQD Grand Orbiter) is a good phase shifter, but it has a few idiosyncrasies that are easily fixed. One thing that is not fixable is this thing is a bit noisy. Not much can be done about it; it's the nature of the LM13700. The good news is it's barely noticeable unless you follow this pedal with a high-gain pedal.

Here's what I changed and why:
1. Increased R2 & R2 to 2.2M because I like my pedals to have a 1Meg input impedance. C1 was reduced to 47nF because it could be. Totally optional.
2. Replaced C2 with a short. It doesn't do anything because the DC voltage on both sides of C2 is Vref. Totally optional.
3. Replaced C8 with 390pF. 100pF is so small it might as well not even be there. 390pF cuts the high end above 40KHz. Totally optional.
4. Inserted 10K resistors (R103-R106) in series with pins 1 & 16 on IC1 & IC2. Pins 1 & 16 program the operating current and gain of each amplifier. The way EQD connected them all in parallel, there is no guarantee that they will share current equally. Putting those resistors in forces them to share. Should make for smoother phase sweep. Optional, but highly recommended.
5. Changed the SWEEP pot to C100K, reduced R33 to 2.2K (to compensate for the 10K resistors I added in item 4) and reduced R34 to 10K. Gives the SWEEP pot more range and at the very bottom of the range, SWEEP acts like a Manual control. Leave R33 at 4.7K is you don't do item 4. Optional, but highly recommended.
6. Inserted a 1N4148 diode (D1) in series with pin 2 of the SWEEP pot. This extends the sweep range down deep into the bass frequencies. With SWEEP above 2:00, you get a nice bottom-end throb. Not for everyone, but I like it.
7. Changed the RATE pot to C500K. Some not-so-good things happen when the RATE pot is above 500K. The sweep becomes asymmetric. There is a tendency for the LFO to stall or fail to start up. The LFO will go plenty slow with a 500K pot. If 14 seconds to go all the way thru one sweep is not slow enough for you, then increase R4 to 10uF. Highly recommended.
8. Added a 3.3Meg resistor (R109) from pin 3 of the RATE pot to IC1 pin 3 (Vref). Along with item 9, ensures that the LFO will always start and never stall.
9. Added a 22K resistor (R107) from IC3 pin 8 to IC3 pin 6. Along with item 8, ensures that the LFO will always start and never stall. 8 & 9 must be done together, I consider them mandatory.
10. Changed the RES (Resonance) pot to C100K and R23 to 33K. Makes for a much smoother adjustment. Highly recommended.
11. Increased R27 to 33K. In the original design, the Circulator will break into oscillation when RES is dimed. I have no use for a pedal that screams at me. With this mod, RES has plenty of range and stops just short of oscillation. Totally optional.
12. Added a Throb LED (R108 & LED2). I think any pedal that has an LFO should also have a Throb or heartbeat LED. I prefer that LED to run all the time, not just when the pedal is engaged. R108 is soldered to LED2 and has clear shrink sleeve over it. Any color LED will work. Do not make R108 less than 10K; if your LED isn't bright enough, get a different LED. Totally optional, but if you do this you need to do items 8 & 9 to ensure that the Throb LED doesn't stop the LFO.
13. Deleted the P/V switch and jumpered pins 1 & 2. Personally, I have no used for Vibrato mode. Totally optional.
14. Subbed 1uF film with 1uF tant for C7, C9-C11. I would have preferred film, but I didn't have any that would fit. I've got some on order, but impatience won out. If you do this sub, watch the polarity. Totally optional.

That pretty well covers it. The pix show how I installed the mods. Make sure R109 is clear of the RES pot. I mounted my pots pretty far from the board to accommodate the height of the RANGE switch.

I used tantalums for C13 and C14 because low leakage is preferred. I suspect EQD depended on leakage in the aluminum caps to help keep the LFO running. Not good design practice. Notice that C13 and C14 connect to Vcc via the RANGE switch. EQD did that to kick-start the LFO when the power comes on. Kind of a band-aid approach and unnecessary when R109 is installed.
Nice work man!¡
 

p_wats

Active member
Woah! Very interesting. Thanks for all the detail. Not sure how much of this I'll do when I make time for mine, but probably some, for sure. Thanks!
 

Chuck D. Bones

Well-known member
Woah! Very interesting. Thanks for all the detail. Not sure how much of this I'll do when I make time for mine, but probably some, for sure. Thanks!
If you're so inclined, I recommend breadboarding it to see which mods you like and which you don't.
 

cooder

Well-known member
Awesome yet again thanks for the indepth rundown and all the time taken to write it up, thanks for sharing! Certainly will implement much of those!
 

Chuck D. Bones

Well-known member
It actually helps me to write it down. I take a lot of notes along the way. What works, what doesn't. I tried a few mods, including stagger-tuning it like a UniVibe, that just didn't work, at least not to my ears.
 

RobinMallard

New member
Thanks for sharing these! I had a couple questions on implementation of mods 8 and 9.

For mod 8, instead of pin 3 of IC1, it seems like running the resistor from the leg of R12 that also connects to Vref could be an option, yes?

For mod 9, instead of going from pin 8 of IC3, could I add the resistor to ground (22k) and the throb LED (10k) to either the side R32 or R33 that connects to pin 8? Just seems easier to get the resistors in there that way.

If 8 and 9 are done, can I stick with the electrolytic caps for C13 and C14? I’d rather not deal with tantalum.

Thanks!
 

Chuck D. Bones

Well-known member
Mod 8: Yes, as long as you put 3.3M between pin 3 of the RATE pot (or pin 1 of IC3) and Vref, you're good.

Mod 9: Yes, as long as you put 22K between IC3 pin 8 and GND, you're good. I installed R108 outboard, but put it wherever you like.

Conform to the schematic and it will work as described.

If you don't have or like tantalum, aluminum electrolytics are OK. If they're leaky enough, you might not need the 3.3M. YMMV. If your issue with tantalum is how the ore is mined, then buy AVX tantalum caps. They make sure their raw tantalum comes from ethical sources.
 
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