Millenium Bypass 2 Question

[tmcdonagh]

Hi all,

I've been working on a mostly 3D printed guitar that has two built in guitar pedals activated by push pull knobs. Since it seems like push pull knobs only come in DPDT versions the Millenium Bypass has been an amazing thing to come across so I can have LEDs that show which effect is activated. The problem I'm running into is that although the first Millenium Bypass 2 works exactly as expected, the LED for the second one comes on when the whole thing gets power, stays on for about a second, and flickers quick until turning off. This is pretty minor in the grand scheme of things as everything works as expected after that little bit of flickering but I was wondering if there is an easy fix or something I'm doing wrong.

Millenium Bypass circuit I'm using:



taken from #5 of this geofex article: http://www.geofex.com/Article_Folders/Millenium/The%20Next%20Millenium.pdf

I admit I've only tried that circuit and not the CMOS logic based one as I didn't have any of those chips on hand but I can try that if it seems that might solve my issue.

Part of my circuit in question:



SW6 in the diagram is always switched to 9V. I added that switch as a test to see if my tremolo rate LED signal could switch the Millenium Bypass LED but it's really dim when I've tried. Also R12 and R14 are 10K and it's the second Millenium Bypass shown that's having the issue.

Picture of guitar:


Thanks,
Thomas

[Rob Strand]

The problem I'm running into is that although the first Millenium Bypass 2 works exactly as expected, the LED for the second one comes on when the whole thing gets power, stays on for about a second, and flickers quick until turning off.

The way I understand your problem: You turn on LED D8 via switch SW5A.    The LED comes on but then it goes out again (or flickers) and you expect the LED to stay on.

I'm not sure what your Freq line is for or where it is going but it needs to be a constant 9V.

Apart from that issue, the circuit is trying to balance leakages but the leakages  aren't well define.  RG's found a good recipe from experiment and combines that with judicious part choices to help it work.   If the leakage of diode D7 or transistor Q5 are at the extreme of leakage tolerances then it might push the circuit into non working region.    The simple answer is to replace D7 and if that doesn't work replace Q5.  The idea here is it is unlikely to get another diode/transistor pair which is near the extremes of tolerance - parts from a different batch even better.    (FYI  leakage tolerances aren't like resistors with 5% tolerance.  You can get a situation where a particular diode has 10 times less than normal leakage and a particular transistor has 10 times more than normal leakage.  What is normally a reasonable safety margin has now be reduced by a factor of 100!).

Anyway that's a good place to start.

If that doesn't work maybe the MOSFET has been zapped and is leaking!

[anotherjim]

Try removing the BJT. Its leakage might be factor.
The BJT is used as a gate protection Zener. Is it actually possible for it to get a damaging voltage in your circuit? Same goes for the series resistor in the control - that's only to limit a fault current when the Zener conducts.
Have the FX got a sufficiently low DC path to ground on their outputs?
Do the fx outputs actually go silent when bypassed?

[Rob Strand]

Try removing the BJT. Its leakage might be factor.
The BJT is used as a gate protection Zener. Is it actually possible for it to get a damaging voltage in your circuit? Same goes for the series resistor in the control - that's only to limit a fault current when the Zener conducts.

The protection is using BC junction as a diode clamp not as a zener.

It's similar to the CMOS gate protection,



As for the Millenium:

The top diode provides the dual function of clamping voltage and current to pull-up the gate and turn the MOSFET on.   The transistor is chosen for low leakage so it doesn't interfere with the diode's pull-up current.

The more I think about the problem it's is either:
- MOSFET damaged and leaking
- Transistor damaged and leaking
- Transistor in the wrong way (for example C and E swapped so the BE junction is used instead of the BC junction)
- Low on the list, there noise in the circuit.   Normally I'd expect the MOSFET turn on more than off.

[duck_arse]

in my opinion, D7 needs to go hard to supply, whether the indicator is on or not. that will establish the starting point for the mosfet. as for whether the oscillator will drive the led/clr or not depends on the oscillator circuit.

[tmcdonagh]

The problem I'm running into is that although the first Millenium Bypass 2 works exactly as expected, the LED for the second one comes on when the whole thing gets power, stays on for about a second, and flickers quick until turning off.

The way I understand your problem: You turn on LED D8 via switch SW5A.    The LED comes on but then it goes out again (or flickers) and you expect the LED to stay on.

I'm not sure what your Freq line is for or where it is going but it needs to be a constant 9V.

Basically my problem is I turn on power with all switches in default position (except SW6 that is switched to 9V) and D8 flickers on for a second, then off. When I switch SW2A and SW3A it turns on D6 as expected. When I switch SW4A and SW5A it turns on D8 as expected. Switching either of them back to the default position turns them off as expected. Really the circuit works exactly as expected after the power is turned on for a little over a second. I'm just curious why I get a little on, flash, then off from D8 when the circuit first gets power.

As for the Freq line, that isn't being used. I have a tremolo built that is represented by FXInB and FXOutB that flashes an LED based on the frequency of the tremolo. I wanted to see if I could use the LED both as a rate LED and a status LED but it's a lot dimmer than I was hoping. I probably should have modified the schematic before posting to make that more clear as SW6 is always set to 9V and not the Freq line.

I'll try doing some breadboard tests with different diodes, transistors, and transistor orientation.

Thanks,
Thomas

[Rob Strand]

Basically my problem is I turn on power with all switches in default position (except SW6 that is switched to 9V) and D8 flickers on for a second, then off. When I switch SW2A and SW3A it turns on D6 as expected. When I switch SW4A and SW5A it turns on D8 as expected. Switching either of them back to the default position turns them off as expected. Really the circuit works exactly as expected after the power is turned on for a little over a second. I'm just curious why I get a little on, flash, then off from D8 when the circuit first gets power.

Ah, I get it.

I don't think it's the parts or leakage at all.  In fact those parts probably won't be able to change the behaviour.

The key thing about the Millenium circuit is it needs a DC path to ground on all the FX outs.   

For example, R33 at the output socket of this circuit,

https://www.hobby-hour.com/electronics/s/boss-bf2-flanger.php

Which is also shown explicitly on RG's pics,

http://www.geofex.com/Article_Folders/Millenium/millen.htm

In general, the smaller you make R33 the less switching pop.  IIRC with 100k the pop should be very very low.
[I'm talking about switching pop due to the Millenium circuit here, not the power-on thump below.]

Some possibilities for your problem:

- you are missing that resistor to ground on some of your outputs.
   Without the resistor the Millenium circuit will misbehave.

- your audio outputs have a turn-on thump on power-up, where the audio outs have
  a positive going transient on power up.   This raises the voltage at input to the
   Millenium circuit and the LED goes on.  Effectively the Millenium circuit "hears" the thump.
  Normally for the LED to go on the input to the Millenium is left open and the diode
  pulls the MOSFET gate up to a positive voltage.   However the turn on thump *forceably*
  makes the MOSFET gate go positive - the diode/transistor cannot overcome or impact
  that effect.
 
If your audio outs have large resistors to ground and/or large output caps it will promote a long positive pulse at the output.  However, the root cause is actually the thump from the output before the output cap.   With smaller output resistors/cap you can stop some of it getting out, especially if you can slow down the rate of rise of the thump.   The rate of rise of the thump can be set by the size of the cap on the Vref (4.5V bias voltage).   You might be able to play with some of these parts to reduce the thump that gets out.

Why you see the thump on one output and not the other depends on the details of your circuit.

[tmcdonagh]

Basically my problem is I turn on power with all switches in default position (except SW6 that is switched to 9V) and D8 flickers on for a second, then off. When I switch SW2A and SW3A it turns on D6 as expected. When I switch SW4A and SW5A it turns on D8 as expected. Switching either of them back to the default position turns them off as expected. Really the circuit works exactly as expected after the power is turned on for a little over a second. I'm just curious why I get a little on, flash, then off from D8 when the circuit first gets power.

Ah, I get it.

I don't think it's the parts or leakage at all.  In fact those parts probably won't be able to change the behaviour.

The key thing about the Millenium circuit is it needs a DC path to ground on all the FX outs.   

For example, R33 at the output socket of this circuit,

https://www.hobby-hour.com/electronics/s/boss-bf2-flanger.php

Which is also shown explicitly on RG's pics,

http://www.geofex.com/Article_Folders/Millenium/millen.htm

In general, the smaller you make R33 the less switching pop.  IIRC with 100k the pop should be very very low.
[I'm talking about switching pop due to the Millenium circuit here, not the power-on thump below.]

Some possibilities for your problem:

- you are missing that resistor to ground on some of your outputs.
   Without the resistor the Millenium circuit will misbehave.

- your audio outputs have a turn-on thump on power-up, where the audio outs have
  a positive going transient on power up.   This raises the voltage at input to the
   Millenium circuit and the LED goes on.  Effectively the Millenium circuit "hears" the thump.
  Normally for the LED to go on the input to the Millenium is left open and the diode
  pulls the MOSFET gate up to a positive voltage.   However the turn on thump *forceably*
  makes the MOSFET gate go positive - the diode/transistor cannot overcome or impact
  that effect.
 
If your audio outs have large resistors to ground and/or large output caps it will promote a long positive pulse at the output.  However, the root cause is actually the thump from the output before the output cap.   With smaller output resistors/cap you can stop some of it getting out, especially if you can slow down the rate of rise of the thump.   The rate of rise of the thump can be set by the size of the cap on the Vref (4.5V bias voltage).   You might be able to play with some of these parts to reduce the thump that gets out.

Why you see the thump on one output and not the other depends on the details of your circuit.

Thanks I really appreciate the detail in your response. I do have resistors going to ground at the output like the RG example (assuming an output potentiometer counts?) so I'm thinking I'll probably have to mess around with capacitor values. Here are the schematics of the effects themselves:

Distortion (FXA)


Tremolo (FXB)


I probably won't get around to testing until tomorrow but I'll reply to this post with my results.

Thanks,
Thomas

[Rob Strand]

Distortion (FXA)

Might need some care to be sure you don't affect the sound.

Tremolo (FXB)

The C10/R25 values are enormous. That could be the issue.   Very long time constant.

Definitely recommend dropping R25 to 100k even for the sake of reducing switching pops in the Millenium ckt.

C10 could also be reduced to say 1uF but I have a feeling R25 at 100k might fix everything.

[tmcdonagh]

Distortion (FXA)

Might need some care to be sure you don't affect the sound.

Tremolo (FXB)

The C10/R25 values are enormous. That could be the issue.   Very long time constant.

Definitely recommend dropping R25 to 100k even for the sake of reducing switching pops in the Millenium ckt.

C10 could also be reduced to say 1uF but I have a feeling R25 at 100k might fix everything.

Thanks I'll give it a try.

Thomas

[anotherjim]

Additionally, the Distortion FX may not be silent in bypass with the input left disconnected like that. It could be passing noise to the indicator.

[tmcdonagh]

Distortion (FXA)

Might need some care to be sure you don't affect the sound.

Tremolo (FXB)

The C10/R25 values are enormous. That could be the issue.   Very long time constant.

Definitely recommend dropping R25 to 100k even for the sake of reducing switching pops in the Millenium ckt.

C10 could also be reduced to say 1uF but I have a feeling R25 at 100k might fix everything.

This worked perfect. Thank you.

Changing R25 to 100K reduced the time the LED was on at bootup but didn't completely solve the issue. Changing C10 to 1uF along with the change to R25 completely solved the issue. I just spent a little over 3 hours soldering another board together as a test so I didn't have to rip out the board in the guitar and it was an enormous relief for that work to not be for nothing.

I might post a video or pictures when I get the new version of the guitar put together and working if anyone's interested. I just finished the code for the built in tuner I have planned so I'm pretty excited that everything is finally coming together.

Test board (couldn't embed without it being super zoomed in even with compression for some reason):
https://i.imgur.com/XYX69mk.jpg

Prototype guitar tuner demo:
https://www.youtube.com/watch?v=Y2LPyC0RhXk

Thanks again,
Thomas

[Rob Strand]

This worked perfect. Thank you.

That's good news.

Changing R25 to 100K reduced the time the LED was on at bootup but didn't completely solve the issue. Changing C10 to 1uF along with the change to R25 completely solved the issue.

That puts a nice threshold on the workable values.  100k & 4.7uF is a 470mS time constant, 100k & 1u is a 100mS time constant.  Notice the supply ramp up time constant is set by the 2k2 and 22uF cap, which is a time constant of 48mS.    So by having the RC filter time constant less than (in this case not much more) the supply ramp-up time the power-up thump is kept to a low level.

I might post a video or pictures when I get the new version of the guitar put together and working if anyone's interested. I just finished the code for the built in tuner I have planned so I'm pretty excited that everything is finally coming together.

Test board (couldn't embed without it being super zoomed in even with compression for some reason):
https://i.imgur.com/XYX69mk.jpg

Prototype guitar tuner demo:
https://www.youtube.com/watch?v=Y2LPyC0RhXk

It's a really nice project.   I like your idea of keeping it all in the one box and the tuner is a nice addition to follow that theme.