Demystifying the MojoVibe

madbean · Post by **madbean** » 07 Sep 2010, 19:13

I've got about 80% of this traced out. No big surprises so far, other than the use of a Vactrol for the speed control (http://www.geofex.com/circuits/ldrlfo.htm).

There is a switchable jfet input stage (modern/classic). The preamp and output stages are pretty typical except for a few value changes. Q1 is 2N5088, and most every other transistor is C945. The phase stages are standard, but I haven't gotten to the last stage or the speed control yet. There is also a charge pump. Haven't seen any regulator on the board, so the biasing voltage is maybe around 17v.

More later

mrkaploca · Post by **mrkaploca** » 07 Sep 2010, 19:30

it's not the circuit itself that makes the whole mojo but the LDR's...

lolbou · Post by **lolbou** » 07 Sep 2010, 19:31

madbean wrote:There is a switchable jfet input stage (modern/classic).

I'm still working on the - harassing - Dunlop UV-1. It's really above my skills, but the challenge is my engine. There is also a vintage/normal switch behind with JFETs. And also the optocoupler for Speed control.

I'm really looking forward to see your work as an inspiration to help me being done with mine (though there's also a bloody lot of CMOS circuitry that

very much)...

Hang on for these 20 percent left!!

madbean · Post by **madbean** » 07 Sep 2010, 20:59

mrkaploca wrote:it's not the circuit itself that makes the whole mojo but the LDR's...

I kind of doubt that. Actually, the lamp looks custom made, but the LDRs look pretty much like everything else I've seen. I think between using an "offset" and "gain" mod for a generic lamp, it will get pretty close. We'll see.

It does look like the whole thing is running off 17v, except for the last stage of the phase sequence (before the output mix) which is powered by 9v, contrary to the traditional Univibe.

There are some small variations with how the lamp driver is set up, too.

madbean · Post by **madbean** » 07 Sep 2010, 21:03

lolbou wrote:
I'm still working on the - harassing - Dunlop UV-1. It's really above my skills, but the challenge is my engine. There is also a vintage/normal switch behind with JFETs. And also the optocoupler for Speed control.

In the Mojo, he uses a 3 pos dip switch to go between the two input stages. It's kind of clever, but it could be done with just a regular SPDT, I think, at the cost of an extra resistor or two.

MWichni · Post by **MWichni** » 07 Sep 2010, 21:26

Very interesting, can't wait to see more. Thank you madbean!

Regards,
Mariusz.

pf_fan · Post by **pf_fan** » 08 Sep 2010, 09:22

Here's what I found on the 'other' forum...
https://www.diystompboxes.com/smfforum/ ... #msg398192

mrkaploca · Post by **mrkaploca** » 08 Sep 2010, 15:03

madbean wrote: I kind of doubt that. Actually, the lamp looks custom made, but the LDRs look pretty much like everything else I've seen.

Dynamic characteristics of all LDR's must match.Every ldr needs to be tested to pair them right.Opto system is all the mojo.

madbean · Post by **madbean** » 08 Sep 2010, 15:41

mrkaploca wrote:
madbean wrote: I kind of doubt that. Actually, the lamp looks custom made, but the LDRs look pretty much like everything else I've seen.
Dynamic characteristics of all LDR's must match.Every ldr needs to be tested to pair them right.Opto system is all the mojo.

In what way would they be tested to be matched? This has got me very curious!

culturejam · Post by **culturejam** » 08 Sep 2010, 15:50

madbean wrote:
mrkaploca wrote:
madbean wrote: I kind of doubt that. Actually, the lamp looks custom made, but the LDRs look pretty much like everything else I've seen.
Dynamic characteristics of all LDR's must match.Every ldr needs to be tested to pair them right.Opto system is all the mojo.
In what way would they be tested to be matched? This has got me very curious!

Me, too.

Would you test only for max/min resistance? How about reaction/recovery speeds?

ppluis0 · Post by **ppluis0** » 09 Sep 2010, 02:12

Would you test only for max/min resistance? How about reaction/recovery speeds?

Hi Culturejam, hi Madbean,

I think that some sort of test circuit can be made to put a LED in front of the LDR under test, apply square waves to the LED with different amplitudes (currents), duty cycles, and see what happen with each photorresistor.

I don't know if this is enough to match groups of ldr's to be used in a uni vibe, tube vibe, tube wah, or whatever application involve several ldr's working togheter, so I'm also curious about this matter...

Best Regards,
Jose

madbean · Post by **madbean** » 09 Sep 2010, 05:14

Okey doke, here's the first pass. I've left off the expression jack wiring to simplify things, but I will be adding it in later. I'll also be posting some more details about the LDRs and light shield...maybe a video of the bulb in action.

Please let me know if anyone spots mistakes

MojoVibe_ver1_rev0.gif

phibes · Post by **phibes** » 09 Sep 2010, 05:22

Great work as always Madbean!

MWichni · Post by **MWichni** » 09 Sep 2010, 05:35

Hi and thanks madbean.
Could you post some gut shots?

Regards,
Mariusz.

Sweetalk · Post by **Sweetalk** » 09 Sep 2010, 12:37

lolbou wrote:
madbean wrote:There is a switchable jfet input stage (modern/classic).
I'm still working on the - harassing - Dunlop UV-1. It's really above my skills, but the challenge is my engine. There is also a vintage/normal switch behind with JFETs. And also the optocoupler for Speed control.

I'm really looking forward to see your work as an inspiration to help me being done with mine (though there's also a bloody lot of CMOS circuitry that very much)...

Hang on for these 20 percent left!!

The UV-1 it's a pain in the a**!!!, I got one that it's not working, so many CMOS for what?!. Do you have the schematic?

gilmour_pugliese · Post by **gilmour_pugliese** » 13 Sep 2010, 12:51

hi guys....

here: viewtopic.php?f=10&t=9334&hilit=neovibe

mictester says:
There's no way you're ever going to run a Neovibe from a MAX1044 - the Neovibe draws much too much current.

how it's possible that the mojovibe is powered with a 1044?

madbean · Post by **madbean** » 13 Sep 2010, 16:54

That's a good question, because it most undoubtedly does

What's the typical draw on a Univibe?

gilmour_pugliese · Post by **gilmour_pugliese** » 13 Sep 2010, 22:23

madbean wrote:That's a good question, because it most undoubtedly does What's the typical draw on a Univibe?

It depends by the bulb, but I don't know the exact draw... more or less than 100mA?!?

outtahear · Post by **outtahear** » 14 Sep 2010, 17:00

No parts numbers, but here's a set O' LDRs and incandescent lamp courtesy of Uncle Bob hisself.
Same ones he used on his vibes, got 'em 12 or so years ago.

telecaster · Post by **telecaster** » 14 Sep 2010, 17:13

The ldr looks like the ones I got from smallbear.
edit: madnifying glass reveals their not the same.

Demystifying the MojoVibe [traced]