Help w/ Gus' boost + mods

[AceLuby]

All right, so I'm finally getting around to my second build which at first was going to be a DOD 250 clone w/ an extra pot to control the clipping threshold.  Well I had a bad experience modding an existing pedal and wanted to slowly move up in difficulty.  Therefore I decided to make the boost again (the begginner project), but w/ a few mods for more versatility.  I've gone through the debugging page and here is what I've found:

The basic schematic:



The boost pot is a 5k linear instead of log and I'm using a 2N2222 transistor.

After the 100k resistor to ground, but before the output I've added a 50k lin pot like so:


------------------------------->out
|                |
>               >  <---
<               <      |
>               >      |
|                |-----|
|                |

( back of pot )
||     ||      ||
--     ----------

Where the connection to the signal is on the left and the middle lug and right lug are connected together and go to a switch.
The switch it is connected to is a SPDT center off switch. 

The center lug of the switch is connected to the aforementioned pot while the top and bottom lugs go to clipping sections like so with diodes being the top option and LED's being the bottom option:

                |
---------------------------
|                              |
--                             |
/\                             --
--                            /\
/\                             |
|                              |
---------------------------
               |
               |
         -----------
           -------
            -----

My thinking is this.  I'll have a regular boost w/ the switch off (center off) and the boost pot will control that.  If the switch is either up or down the boost gets 'dirtied' up a bit by the diodes/LED's, which are asymetrical.  The amount of grit added will be controlled by the 50k pot.  I also have a resistor/LED combo coming from the 9V connection to the 3pdt switch.

I'm having a few problems now:

Bypassed:  Everything works fine

Switch in center-off position: I can get a slight boost, but only very slight.  My beginner project had MUCH more boost, though it didn't have an LED and the pot was 10k, though I don't think either should make much of a difference.  I am able to control the volume w/ the boost pot.  I've also bypassed the pot before the output via an audio probe and the same is occuring, so the pot isn't sucking anything like I suspected.

Switch either up or down:  Both do the same thing, just to different sets of diodes.  I've gone through with an audio probe and I get sound before the diodes, but none after.  The pot before the diodes don't do anything to the sound.

Here are the values on my transistor:

C: 6.12
B: 2.4
E: 1.87

Any suggestions on where to start looking, possible issues or if anyone has had an issue like this would be a great help.  I've been debugging for hours now and feel like I've gone through it a hundred times and just don't have the experience to identify what is wrong.

Thanks in advance.

[AceLuby]

Some additional info:

Just scraped around areas to make sure my connections were clean, double checked my wiring and plugged it back in.

When engaged and all pots are turned all the way down the sound is the same as when bypassed.  When I turn the boost pot up it acts more like a tone control and more treble gets through.  Nothing happens when I adjust the distortion control

[AceLuby]

Ok, I've been reading and this is what I'm thinking:

Based on other peoples readouts on the beginner project it looks like my transistor values are fine.  I'm also getting response from the 'gain' pot, so it should be ok til around the output cap.

I think I should start out by removing the 100k resistor to ground before the pot to the diode switch.  This will give me more signal going to my pot/switch/diode combination.

I think the 100k to ground is shunting the signal to ground before getting to the diodes decreasing the gain of the signal once it hits the diodes.  I think this is causing the diodes to not get enough signal to start clipping, hence the same sound w/ or w/out the diodes going to ground.  (Also, my pic of the diodes above is wrong.  It is supposed to be assymetrical clipping w/ one side pointing to ground and another pointing to signal...)

Would that make sense?  I think it does except that when I went through it w/ my audio probe I don't remember a huge volume boost near the output cap.  I think I'll have to compare this side by side w/ my working booster to see how that one sounds.

[dxm1]

I think the 100k to ground is shunting the signal to ground before getting to the diodes decreasing the gain of the signal once it hits the diodes.

By placing a 50K pot in parallel with a 100K resistor, you are effectivly making a 33.3K pot.  You may be better off simply replacing the 100K resistor with a 100K pot.

As for your switched diode setup, I'm having trouble visualizing what your doing. Your ASCII diagram seems to show the pot wired with the wiper connected to the top lug, and the bottom lug going to ground.  Are you still taking the output from the top lug, and putting your clipping diodes from the wiper to ground?

[AceLuby]

I think the 100k to ground is shunting the signal to ground before getting to the diodes decreasing the gain of the signal once it hits the diodes.

By placing a 50K pot in parallel with a 100K resistor, you are effectivly making a 33.3K pot.  You may be better off simply replacing the 100K resistor with a 100K pot.

As for your switched diode setup, I'm having trouble visualizing what your doing. Your ASCII diagram seems to show the pot wired with the wiper connected to the top lug, and the bottom lug going to ground.  Are you still taking the output from the top lug, and putting your clipping diodes from the wiper to ground?

Thanks for the reply, but I am obviously not explaining myself.

What I have after the output cap (10uf polarized) is the following, in order:

-A 100k resistor to ground
-A 100k pot wired to a SPDT center-off switch. 
  -I've wired it so that when looking at it installed, w/ the lugs pointing down, looking at the back of the pot, the left lug is connected to the signal path and the middle and right lugs are connected together and go to the switch. 
  -The switch is wired to be off in the center, or go to either a set of LEDs or silicon diodes.
  -Both the LED and diodes are wired for asymmetrical clipping and are attached to the ground

Basically when engaged and everything is as low as possible I get a bypassed sound.  As I turn up the 'gain' on the boost section I get more treble, but not more volume.  Turning the distortion pot or the switch does nothing audible.

My thought is that with the switch in the center off position (not grounded, not anything...) it should act as a simple boost, yet it's not even as loud as the bypassed sound.  This is more than likely the reason why my diodes won't clip (not enough gain...)

When I get home I'm going to double check the 100k resistor to ground and make sure it isn't 10k and poke around a little more w/ the audio probe to see where the loss of volume is and compare it to the other one I built.  I may also just need to start over...

 

[dxm1]

So, something like this?

[AceLuby]

So, something like this?

Precisely.  That's what it looks like after the 10uf output cap.  Thanks for the drawing.

[AceLuby]

In the above schematic what does the sw stand for, center off?

Took out the 100k resistor and it helped... A LOT... but there's still some problems w/ it.  First off, it is a boost, but it's not a very good boost.  I have to turn it up almost all the way to get more than bypass, and it's pretty comparitivly.  All of my measurements for my transistor are about .3 less than regular, but that shouldn't make a big difference.

The distortion pot still does nothing so I have a question about diode hookups.  Do I have to connect them together before I connect them to ground?  Or can I meet the two sets and ground at the same point?  Also, could I have possibly wired the pot wrong?  I don't think so because I get signal right before the diodes...

My second question is when wiring diodes for asymetrical clipping does it matter which side you put the extra diode?

If everything is ok, is there a way to get more gain out of the unit?

Sorry for the noob questions, I just feel like I'm close and I'm really excited about creating my own sounds.  Etched PCB after this... hopefull it's a little easier to work w/... 

[AceLuby]

OK, more updates (I hope this is ok aron...).

As mentioned before I got some boost out of it once I removed the resistor to ground.  This morning before work I checked it out again and compared the volumes of the circuit via an audio probe.  I noticed that my input was fine, but I wasn't getting anything after the input cap.  I took an extra lead I had around and just connected the two points where the input cap was soldered in.  INSTANT LOUD!!!  Ok, so it looks like I either put in the wrong value cap, or the cap is bad which I think is unlikely.  Anyway, the first part of the shematic now for sure works correctly, even though it adds a lot of treble, but I can fix that.  This part of the circuit isn't working still:



First off the pot doesn't do anything, even when the boost is on full, no matter where the switch is.  Using an audio probe I get signal right before the diodes if switched, but none after (which I think is ok) and none before the diodes if the switch is in the 'off' position.

Basically I have the signal going through this area, but I'm not hearing a whole lot of signal loss when grounding the signal via the diodes and I really think I should.  I guess I'll start by rewiring the diodes, but could it be possible that I am not giving the diodes enough signal to clip?  If the diodes are LED's, should they light up?  Any help would be great.

[dxm1]

Well, let's consider what's going on with the diodes as shunts.  Before the diode will clip, the signal
must exceed the diode's threshold voltage.  For small signal silicon diodes, this is around 0.7 volts.
Two silicon diodes in series will be twice this, around 1.4 volts.  Assuming that you've wired them as
in the drawing, the negative half of the signal will clip at 1.4v, while the positive half at 0.7v.

LEDs usually have a much higher threshold voltage, depending on the type you use.  Most "standard" (not
low current or superbright) will be around 1.2 to 1.4 volts _each_.  So, your signal will have to be 2.8
volts to begin clipping on the dual LED string.

Here's something you can try - set you meter for AC volts, and measure the signal level at the top of the
100K resistor.  Your DMM is not likely to give you a real accurate measurement, but it should give you a
good idea of what's going on.  If your booster isn't cranking out at least 2 volts here, you are not going
to hear much clipping.

Or, you can pull out your trusty ol' HP 331 and get the true RMS voltages...

[AceLuby]

Ok, well here are a few questions about diodes:

1)  If the diodes are not getting enough voltage to clip do they act as a ground point, or does it need to have enough voltage to clip before the signal is grounded?

2)  What would happen if instead of the side w/ two diodes were pointing towards ground instead of away from it in the pic, will it make a difference?

3)  If the diode pairs are wired to gether and soldered to ground at one point instead of the schematic where the diodes are wired together at one point and connect to groun at another will it make a difference?

I think I will have a MUCH better understanding on how diodes work w/ the above explained.  Thanks!

[AceLuby]

Ok went through and found out a bunch of stuff and my pedal is finally working.  Just so there is an answer if someone uses the search, here was what happened (also, it was really nice when people followed up on what they found so for the searchers, here ya go...).

First off I found that in the above hard clipping scheme that the signal bleeds to the ground through the diodes only if above the clipping threshold.

I know from the above that the diodes I'm using have .7-1.2 clipping thresholds.

Measuring my voltage at the collector it is around 7.5, then after the output cap it drops dramatically to .3.  This wasn't nearly enough to clip my diodes which is why I was getting the same sound whether the diodes were engaged or not.

So now instead I have decided to add a small tone circuit and an overall volume so I can dial in that little bit of grit better.  The volume is installed and the tone goes in tonight.  I need to tame the highs a little better, but that should be fixed somewhat w/ a tone control   I'm getting a breadboard tonight as well and am going to start playing around w/ a two transistor fuzz schematic to see what I like.  Thanks for the help and basically a place to gather my thoughts.

[dxm1]

Ok, well here are a few questions about diodes:

1)  If the diodes are not getting enough voltage to clip do they act as a ground point, or does it need to have enough voltage to clip before the signal is grounded?

The diodes will not pass any voltage until the threshold is reached.  The threshold voltage is required for the diodes to conduct (turn on).

2)  What would happen if instead of the side w/ two diodes were pointing towards ground instead of away from it in the pic, will it make a difference?

Sure - it will change which half of your signal is clipped.  If the cathodes are grounded, you will be clipping the positive half of your signal.  Ground the anodes, and clip the negative half.

3)  If the diode pairs are wired to gether and soldered to ground at one point instead of the schematic where the diodes are wired together at one point and connect to groun at another will it make a difference?

No, no difference. Ground is ground no matter how you get there.

[dxm1]

Ok went through and found out a bunch of stuff and my pedal is finally working.

Glad to hear it's working.  Now it's time to dink around on the breadboard and get it sounding just the way you want it.

You've just passed part of 'Diodes 101' in the DIYStompboxes correspondence course...

[AceLuby]

Yeah, first pedal was a big learning experience.  Learned a few debug techniques, learned how to read a schem.  This second one I learned how the transistor actually works, how diodes work, and started to really open up to the idea of tweaking schematics to my own tastes.  Gonna get an enclosure painted and ready to go and throw the bad boy in there.  Then I'm gonna breadboard up a two transistor fuzz and start tweaking w/ that...  might throw in some PCB etching as well because of how easy it is compared to bending leads together and soldering to get the connection.

Also have to put the 100k resistor back in.  I'm getting popping from my pedal now when I disengage especially and a really slow volume rise when engaging.