Rockman Chorus Pedal

[Peetem]

So, I've adapted the Rockman X100 chorus circuit and modified it for a foot pedal design.  Would love to hear any feedback on the circuit - criticism, concerns, modifications, etc.

I didn't try to recreate the 1/2 rack version yet, so this design doesn't have a "depth" control (e.g., Scholz called it "Long Chorus") or the stereo mix options (Wide, Stereo, etc).  The output would be the "wide" setting using a TRS plug (left side is the chorus/detune and right side is the unaltered guitar) or a 1/2 mix of both using a mono 1/4" plug.  However, if this circuit works out I will modify it for /- 9V with those options (e.g., wide and long). 

The schematic as shown says a bipolar 9VDC is supplied, but as it stands now, everything is optimized for bipolar 6v.  I didn't want to have to change the FET bias (2n4339) and other values for this protocol circuit.

Lastly, I have a PCB layout created and will getting 5 boards made.  If it works without issue, I'll post that as well (Gerber files).

Let me know your thoughts and thanks in advance for the feedback!

[Peetem]

I should add that if there is any interest, I can creat a layout using DIY Layout Creator using a perf board.....

Just let me know....

[antonis]

Hi & Welcome..

Is there any particular reason for biasing U2A one diode forward voltage drop above GND..??

[Peetem]

Hi & Welcome..

Is there any particular reason for biasing U2A one diode forward voltage drop above GND..??

To adjust the gain of the filter before it goes into the BBD - it depends on the gain coming out of the previous stage.  It can be jumped if needed.

Am I thinking about this the wrong way?

[antonis]

Am I thinking about this the wrong way?

I'm trying to figure out the influence of bias level into gain setting..

[Peetem]

See circuit and diagram below. The high impedance input of the op-amp is connected similar in the chorus circuit as a voltage divider. By connecting to the high-Z input, the only current that flows through the resistor will be the input bias current to the op-amp.

In addition, the Re terms on the top and bottom of the gain equation cancel out leaving just a function of N. This removes the impact of end-to-end resistance from the gain equation altogether.
The gain adjustment is non-linear with respect to N, which we want as we're using it as a low-pass filter before the BBD.  However, the diode further reduces the bias to lowering the gain.

The circuit is useful as an audio control or brightness control (if a Vr is used as in the circuit), but in the case of the Chorus, R is not variable and therefore, keeps the gain down as fixed.

[antonis]

Sorry but I can't follow you.. 

The high impedance input of the op-amp is connected similar in the chorus circuit as a voltage divider. By connecting to the high-Z input, the only current that flows through the resistor will be the input bias current to the op-amp.

Through which resistor..??

[Peetem]

Sorry but I can't follow you.. 

The high impedance input of the op-amp is connected similar in the chorus circuit as a voltage divider. By connecting to the high-Z input, the only current that flows through the resistor will be the input bias current to the op-amp.

Through which resistor..??

Assume its not there (its been jumped as shown in the schematic).

[caspercody]

Hello

Did you get this pedal to work?

[caspercody]

I got the Rockman Chorus bread boarded right now. I am using a +9/-9 volt supply. I got the MN3007 from Small Bear. Schematic is basically the same as posted in the first post on this thread.

Using a wire from the output jack as a audio probe, I get tone up to pin 3 on the MN3007, and I get a tik, tik sound on pins 2, and 6. But no out put on pins 7, or 8. Voltage reading on the MN30007:

1 - +9.05v
2 - low/high (voltage starts low, then another reading of higher voltage, keeps in beat with the tik, tik sound. The voltage changes with every tik going up).
3 - +.60v
4 - -7.06v
5 - -8.56v
6 - same as pin 2 readings
7 - 5.88 then 5.80
8 - 5.6 then 5.9

Any thoughts on why no sound? If I put my audio probe on either pin 7, or 8 should I hear something?

Thanks
Rob

[Vivek]

Great thread !!

Did you get it to work ? If yes, please ignore all comments below:

I have been spending few hours a day for the last few months, staring at the X100 schematics and it's LTspice simulation.

Eventually, i redesigned the Rockman Chorus to use MN3207 which are more easily available. I also removed 555 LFO and used a PLL instead, then adjusted all parameters to Rockman standards)

I see that you have No summing mixer !!!!


RV2 and RV3 look odd to me

You are running the MN3007 at the absolute max total supply voltage of -18V which might make it more prone to failures. Recommended total supply is -15V.

[Fender3D]

X100 had +6V and -6V power supply not +9V and -9V...
Read stock schematic.

Furthermore, in your schematic there's no BBD's bias regulator, then preceding ICs offset and bias become really important.
Without a proper bias no BBD works...

[caspercody]

Thanks for the replies

Vivek, can you please share your schematic

I have both a +/- 6, and 9 volt supply. I tried both to see if any difference.

I am not sure if the full Rockman schematic has a BBD? I do have the full schematic and built my board off of that. Just the chorus portion. But since the OP had an image on the thread that is almost the same, I just referenced it in my post.

Any help is appreciated,

[caspercody]

Here is the original schematic. Again, I am just trying to recreate the Chorus portion.

[Vivek]

Thanks for the replies

Vivek, can you please share your schematic

I have both a +/- 6, and 9 volt supply. I tried both to see if any difference.

There are very few ways to use these BBD in a chorus schematic. Most pedals just use the examples in the Manufacturer's sheet with minor modifications.

So you can refer to the schematics of

LICH CHORUS
ZOMBIE CHORUS
PURPLE CHORUS
HYBRID CHORUS
any other MN3007 or MN3207 chorus

The schematic on my breadboard is really a very minor variation of those. Any of those can work in place of the Rockman Chorus to deliver essentially the same effect (Single delay line chorus effect) . There is no real magic hidden in the Rockman Chorus.



According to my LTSPICE analysis, the power supply voltage makes the most difference in DIST mode, and almost no difference in other modes.

[caspercody]

I have built the Walrus Audio Julia chorus, but some have talked about how this chorus sounds different and there are no others on the market that sound like this one.

I also have the TC Electronic Corona.

[caspercody]

Has anyone made this chorus schematic and got it to work as is?

[ElectricDruid]

some have talked about how this chorus sounds different and there are no others on the market that sound like this one.

I'd say that would be down to the rest of the Rockman circuit around it - the compression, the filtering, and the cab sim. Tom Scholz' tone shaping was pretty detailed and that gives the whole circuit its sound, but there's nothing remarkable in the chorus itself, like Vivek says.

[caspercody]

I do have the compressor, distortion, clean portion, and filters created and working in a pedal form.

I just hate giving up when I read others have built it and got it to work. I believe I either have a bad MN3007 (but voltages indicate it works), or bad 4013 chip. I swapped out the 7555 for a 555 chip

[ElectricDruid]

I do have the compressor, distortion, clean portion, and filters created and working in a pedal form.

Well, that should give you the Rockman sound!

I just hate giving up when I read others have built it and got it to work. I believe I either have a bad MN3007 (but voltages indicate it works), or bad 4013 chip. I swapped out the 7555 for a 555 chip

If you've got good clock signals and good biasing, and you still don't get any output, it's possible the MN3007 is bad. But faults in either the clock or the biasing are much more likely. I recently exchanged emails with a chap who could get his Flangelicious to flange - he thought the BBD chip must be dead. I suggested checking the bias and it turned out he'd inadvertently used a 1K instead of a 10K for one of the bias resistors.
Do you have any way to see or measure the clock signals? One way in a pinch is to increase the size of the clock timing cap so the frequencies come down into audio range, and then you can hear if it's working with an audio probe - and can hear if the LFO is modulating it correctly too.
CD4013 isn't too bad to get running, but you need to make sure all the pins are tied to the correct levels, either high or low. That's at least simple to check, going around with a continuity test.
That brings us to the Rockman's unusual BBD input biasing. This would be my chief suspect. Pin 3 needs to be at the right level or no signal will go through the BBD. Often there's a trimmer to adjust this, although it's possible to do it with fixed resistors instead. This schematic basically uses the 0V level as the bias, but shows an (apparently optional) single diode and a 10K resistor to push the bias away from ground by 0.6V. This is the only adjustment you get. I'd be inclined to take those components out and add a trimmer and a cap so the bias is adjustable just to see if that didn't help get it running.
The MN3007 datasheet suggests a pair of 100K resistors and a 3u3 cap to make a midpoint bias supply, but suggests that the lower resistor should be "adjusted for minimum disortion". On a proper bipolar supply, the situation is slightly different, but you can still create a virtual ground for this bias point alone.