MXR - Micro Chorus  [schematic]

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
User avatar
bucksears
Solder Soldier
Information
Posts: 172
Joined: 08 Sep 2007, 23:02
Has thanked: 16 times
Been thanked: 41 times

Post by bucksears »

Fender3D wrote: R17 will set the wet level, you might as well place a balance sort of lowering R16 and R17 wiper to U1 pin 6. You won't have a huge usable range though...
Otherwise you may use a pot instead of R20.
Thanks! So, just a trimmer (100K?) in place of R20 would allow to tweak the level a little up/down? I'm not looking for a lot of variation, just enough to balance it to unity if the signal is too much when engaged.

User avatar
Fender3D
Cap Cooler
Information
Posts: 573
Joined: 08 Aug 2009, 02:40
Location: Helltaly
Has thanked: 121 times
Been thanked: 153 times

Post by Fender3D »

Sorry mate, I guess I misunderstood...
I thought you were looking for a wet signal level control...
The pot instead of R20 (47KB instead of resistor) will do just that, since you have wet signal only there.
If you're looking for an overall level control, then you should edit R19 and R21 values

User avatar
bucksears
Solder Soldier
Information
Posts: 172
Joined: 08 Sep 2007, 23:02
Has thanked: 16 times
Been thanked: 41 times

Post by bucksears »

Sorry, I should have been more specific - I'm just looking to add a control so I can balance out any volume boost/cut when the chorus is engaged. 8)

User avatar
Fender3D
Cap Cooler
Information
Posts: 573
Joined: 08 Aug 2009, 02:40
Location: Helltaly
Has thanked: 121 times
Been thanked: 153 times

Post by Fender3D »

bucksears wrote:Sorry, I should have been more specific - I'm just looking to add a control so I can balance out any volume boost/cut when the chorus is engaged. 8)
oh ok
it should be unity gain, but it will depend on what (impedance) you'll connect it.

BTW
R19 and R20 will correct any boost.
Lowering R16 and R17 OR raising R15 will correct any cut.

User avatar
bucksears
Solder Soldier
Information
Posts: 172
Joined: 08 Sep 2007, 23:02
Has thanked: 16 times
Been thanked: 41 times

Post by bucksears »

Almost done with the PCB - just need to add the power/bias section (should be able to do that tonight) and it will be ready for someone to proof it.
I did have to use two jump wires to connect parts from one side of the board to another; nothing major, but necessary for this layout. It's nice and compact, roughly 2" wide x 2-1/2" long.

User avatar
bucksears
Solder Soldier
Information
Posts: 172
Joined: 08 Sep 2007, 23:02
Has thanked: 16 times
Been thanked: 41 times

Post by bucksears »

Ok, done.
Here's the link to the parts layout, using Fender3D's schematic above; the red tracings are Vcc, the blue are Vbias and the green are Vox. THIS IS NOT SCALED FOR PRINTING TO ETCH. I wanted some feedback on this layout before I call it complete and I still need to reorient the pads for the trimpot a little. I don't want to miss anything.

If everything looks fine or if there is an error, please let me know.

http://www.4thlevelmedia.com/smallpics/ ... Layout.jpg

Thanks,
Buck

User avatar
bucksears
Solder Soldier
Information
Posts: 172
Joined: 08 Sep 2007, 23:02
Has thanked: 16 times
Been thanked: 41 times

Post by bucksears »


User avatar
bucksears
Solder Soldier
Information
Posts: 172
Joined: 08 Sep 2007, 23:02
Has thanked: 16 times
Been thanked: 41 times

Post by bucksears »

bucksears wrote:If everything looks fine or if there is an error, please let me know.
One thing I already noticed: there are no ground pads - it was late last night when I was working on it and just forgot them. They will be on the left & right sides on the ground rail.

User avatar
bucksears
Solder Soldier
Information
Posts: 172
Joined: 08 Sep 2007, 23:02
Has thanked: 16 times
Been thanked: 41 times

Post by bucksears »

Resurrecting this thread.
At long last, I finished building the Micro Chorus using the PCB I posted.
It does actually work, but there is a distinct throb/woosh to the sound; when adjusting the bias to some extreme settings, I can hear the over-the-top detune sound. When I dial it back to more subtle chorusing, it's like I only hear about 75% of the chorus swirl.
Does matching the (2N5088) transistors help at all?

Thx
- Buck

User avatar
Fender3D
Cap Cooler
Information
Posts: 573
Joined: 08 Aug 2009, 02:40
Location: Helltaly
Has thanked: 121 times
Been thanked: 153 times

Post by Fender3D »

Hi Buck,
bias has only one correct setting, it isn't a "depth" control, then it shouldn't be "dial(ed) back to more subtle chorusing".
I don't know what you mean by "extreme settings", with schematic's R27 and R36 values you should have a good trimming range, feel free to adjust their values at your will of course, they're not dangerous until you remain over ~4-5K...

Matching transistors, here, are useless, almost any NPN will work, low noise Q1 and Q2 are welcome though...

Said that, check the clock signal with a scope:
it should be a square wave (measure it at MN3007's pins) for the whole clock range (slower LFO rate will help).
Long tracks and/or track capacitance will affect this.
Report if issues occour at highest or at lowest clock freq.

User avatar
SubSonics4001
Information
Posts: 5
Joined: 02 May 2014, 02:22

Post by SubSonics4001 »

Hey everyone. Ive been lurking this site for a long time and have a question today. A thread from last year yes!

I would like to change cap and resistor values in my 1981 micro chorus to optimise it for my electric bass guitar. My pedal seems to have a nice fat eq or tiny but noticible boost which i like when i engage it, but I can hear the bass octaves cut out, about two whole octaves lower cut out of the signal. This is my ears only as i have not measured it on electronic equipment. Please help get this pedal dialed for bass! I have a lot of experience building pedals and modding them, but i dont know what to change or what values will give me best success using my vintage micro chorus for bass. :horsey:

User avatar
SubSonics4001
Information
Posts: 5
Joined: 02 May 2014, 02:22

Post by SubSonics4001 »

My pedal/what is destined to become modded for bass guitar:
Image
Image
Image
Image

User avatar
Fender3D
Cap Cooler
Information
Posts: 573
Joined: 08 Aug 2009, 02:40
Location: Helltaly
Has thanked: 121 times
Been thanked: 153 times

Post by Fender3D »

Hi SubSonic,

try raising C1, C9 and C12 values (x2 or x4) :wink:

User avatar
SubSonics4001
Information
Posts: 5
Joined: 02 May 2014, 02:22

Post by SubSonics4001 »

Thanks for the quick response! I never expected to be helped/answered overnight! I easily found the 50n caps and will "up them" to .2uF according to your suggestion. I have found that .2uF are common in onboard bass guitar tone controls so I have no hesitation about a mod to those. I'm having trouble knowing which tantalum to replace. I can guess and maybe get it right, but may I also ask, If you could virtually "point to C12" in the layout in this picture. Which one, of the three blue tantalums, Is C12? Top-right, Middle-right, or the one on the Bottom left? Thank you so much as this is a very exciting little pedal imo. Very nice to meet you. https://i.imgur.com/an3RPpl.jpg

User avatar
Fender3D
Cap Cooler
Information
Posts: 573
Joined: 08 Aug 2009, 02:40
Location: Helltaly
Has thanked: 121 times
Been thanked: 153 times

Post by Fender3D »

C12 is the uppermost tantalum :wink:
It can be 1uF or more, tantalum or not...

User avatar
SubSonics4001
Information
Posts: 5
Joined: 02 May 2014, 02:22

Post by SubSonics4001 »

Ok. Thanks for your precise help.

So, for this "bass mod" im doing:

C1: 50n Ceramic Disc - to .2uf Polyester Film
C9: 50n Ceramic Disc - to .2uf Polyester Film
C12: 0,33µ Tantalum - to 1uF Tantalum

I'll post about how it sounds when i get all fixed up. My parts are on order.

User avatar
SubSonics4001
Information
Posts: 5
Joined: 02 May 2014, 02:22

Post by SubSonics4001 »

So...the Bass mod is done! hard to fit the poly .2s into the little box because of their size. Had to put wire leaders on the cap legs and tuck the caps in down in by the jacks. Everythings buttoned up, sounds the same. haha jk. it sounds lovely and smooth and lets all the low bass through. love this pedal. :applause:

Any suggestions on setting the trimmer on the board? How do I measure whats going on with that? What is the correct setting anyway?

User avatar
Fender3D
Cap Cooler
Information
Posts: 573
Joined: 08 Aug 2009, 02:40
Location: Helltaly
Has thanked: 121 times
Been thanked: 153 times

Post by Fender3D »

The trimmer sets bias,
you should feed a ~200Hz sine signal @ input
Hook your o-scope probe to Q2's emitter, raise signal until it clips, then trim for a symmetrical clipping on o-scope.

Q2 is the transistor between SAD and TL :wink:

User avatar
sabelya
Information
Posts: 11
Joined: 19 Jun 2014, 18:26

Post by sabelya »

Hello everyone,
First, sorry for my english, I'm french and you know our reputation to learn languages.. :D

I did the Micro-Chorus MN3007 based on the Fender3D schematic (rev1.0), thanks a lot for this guys, so using the double side PCB. It works with a very lovely sound, but in addition with a strange "wind noise", don't know exactly how to call it.
I used a 300k bias pot instead of 220k but I don't think this is the problem (adjusted with an oscilloscope and a 200Hz input signal).
I ordered a TL061CP and TL062CP but I received a CN version. Could this create the problem ?
When I remove the MN3007, the circuit works even if there is no "chorus" sound, but the "wind moving noise" still there.
When I remove the TL061CN, same operation.
When I remove the TL062CN, of course there is no sound but I can hear the same noise but in a very very low level..
Don't understand what going on. Could someone help me ?

Thank you very much for your time.

PS: photo enclosed, maybe I missed something or did something bad..

Image

http://postimg.org/image/raq0em4sf

User avatar
Fender3D
Cap Cooler
Information
Posts: 573
Joined: 08 Aug 2009, 02:40
Location: Helltaly
Has thanked: 121 times
Been thanked: 153 times

Post by Fender3D »

Hi
and welcome to the forum,

300K Bias trimmer shouldn't be an issue, but I'd lower R27 and R36 just to be sure...

Could you better explain what you mean by "wind noise"?
MXR Micros have no noise reduction system, fake Dolby (pre-emphasis and de-emphasis) aside...
They were noisy, at least my '80 Micro Flanger is...

If battery is not an option, you can sub the 062 with a better (noiseless) IC, start with 072.
061 is just the LFO, the lower current it draws, the lesser the tick....

You could also try different (noiseless) NPN transistors (hfe is not so important here ;) )

Post Reply