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[MUST]

Odd looking versions! I don't like the look of the first 2 in the sense that the feedback via the 12k resistor from Q2 collector to Q1 emitter is taken from inside the output cap.  Look at the original schematic and you will see that it is connected outside so that it's setting a DC reference level in the tone area to a low value which means the coupling caps MUST be that way around. (I'm afraid it's not even "pretty sure", it's absolutely sure!) With the 12k connected straight to the collector there is nothing defining a DC level within the tone control circuitry.  It will float to anywhere the leakages of the caps dictate and the idea of which way around the caps should be connected is a shambles, it could be either way.  This is not a "design eccentricity" or a "novel approach", it is an "error".  Either the designer of the circuit made a mistake or the drafter of the schematic did but someone has goofed.

It's there to define the AC gain but it's also affecting the DC conditions within the first stage as well as any signal. That does not look wholesome at first glance. I'd strongly suggest that may be a typo in both. Notice in the third version it is outside the cap again.

I've attached the working schematics I used.  There are some slight changes.

1)  Stage 2 Q4 has components added to its grid.  These are a bias resistor to help when pots go crackly and a filter set at a high enough frequency to not affect the signal and which will help with stability which I had a problem with in a previous layout.

2)  R14 was 43k and is now 47k, a more common E12 value.  It's in series with a pot for God's sake, why choose a rarer E24 value? The extra 4k represents less than 10% of its value at the low extreme and nothing worth even calculating at the upper extreme!  The difference won't even be heard.  The control works in a balanced and full way in my system with this value.

3)  Likewise the C12 8n2 has been substituted with 10nF as it is in everyone's box.  This also makes no difference in my own unit.

It sounds great for what I want, not out and out fuzz just lovely classic blues and rock tones and every control acts in a gradual progressive and balanced way.

[So the voltage gain depends almost exclusively on the ratio of the resistors RC/RE rather than the transistor's intrinsic and unpredictable characteristics. The distortion and stability characteristics of the circuit are thus improved at the expense of a reduction in gain.]

This is from Wikipedia's entry on the "Common Emitter" transistor amplifier which is our output gain stage.  It also basically applies to the input pair topology circuit we have which is derived from that.

One common way of alleviating these issues is with the use of negative feedback, which is usually implemented with emitter degeneration. Emitter degeneration refers to the addition of a small resistor (or any impedance) between the emitter and the common signal source (e.g., the ground reference or a power supply rail). This impedance Re reduces the overall transconductance Gm = gm of the circuit by a factor of gm(Re+1) which makes the voltage gain

    Av = vout/vin = -gm RC/gm*(Re+1) = -RC/Re  (where gm*Re >> 1 and RC is the collector load resistor).

So the voltage gain depends almost exclusively on the ratio of the resistors RC/RE rather than the transistor's intrinsic and unpredictable characteristics. The distortion and stability characteristics of the circuit are thus improved at the expense of a reduction in gain.

That's a lot of maths to a non technical guy but the gist is that this circuit is designed to remove transistor gain from its workings.  It is set up so that the gain that is left after the feedback is applied is under our control with the resistors in the circuit, and is exactly what we need to give the effect we want.  It must be true, Wikipedia says so! 

It does that, hence the instability problems and my inclusion of "in an audio circuit", i.e. not at higher frequencies.   

    

That's the danger of fixating on things that you just "know" or that "my tech mate" has told me MUST be the problem.  It stops you looking at the issue logically.  This type of circuitry is so well established, understood  and documented in electronic terms it's a yawn.  It's so predictable yet people want to elevate it into a magic black art.  And there are those who have a vested interest in doing so.

In the country of the blind the one eyed man is king.

Cortexturizer has got it right in his post when he says:

It is very unusual that a lot of people are struggling with this build, myself included.

and it's telling when he points out:

I made three attempts on this before making it right, all of those on vero before having it done on madbean pcb.

That's not unusual.  So does building it on Veroboard cause strange effects in the circuit action?  Of course not!  The only problem you should be likely to get with a Vero build in an audio circuit is instability - unless you have used an incorrect component, fitted one incorrectly, or your layout is wrong.  And none of those are the fault of the Vero or the circuit.  Vero itself cannot produce a fault like "sometimes the bass control wouldn't work".  Us putting components into the Vero has produced the problem.  And then he got it right and lo and behold the circuit is fine.

You mention that the circuit is oddly noisy.  I would suggest you either have a grounding problem or you have instability due to the layout.  This happens to all of us and more easily with Veroboard.  I have just built a 3 stage JFET SRPP type overdrive circuit of my own and suffered from this to start with.  It was my own PCB design too and it had to be made with off-board pots, 5 of them, and the flying leads to these were a problem.  It oscillated.  There were slight squeaks and high noise at high gain settings.  Fortunately I have a scope to see this, I would assume you don't have that luxury.  Once you acknowledge and accept the problem you can come up with a way to solve it.  Just poking the circuit without having any idea of why you are doing what you are doing is like trying to fix your car backfiring by starting at the starter motor and......

It's not the problem that's the problem it's the way you see the problem that's the problem.

My point is that, if you have tried different transistors and they increased the volume that does not show that higher gain transistors are the solution.  It shows that, knowing what we know about that circuit design and knowing that others have built it with any old transistors and it still worked perfectly, then either your original transistors were ridiculously bad quality out of spec possibly damaged, or far more likely there is something wrong elsewhere in your build to allow them to affect the volume.  As I said:

For all modern transistor types like the ones you  listed it works.

and I meant:

it - just - works!

Half decent transistors cannot make that difference in an otherwise working model of this circuit.  It is basically immune to transistor gains within any normally accepted band, it is designed to be so.  Over the years I've designed dozens of circuits with that exact same input gain block without any of them having an issue of the type you have.  This one is bog standard.  As a tool it's on a par with a spanner in a workshop.

Please tell us:

What transistor types are you using?

Are you building with Veroboard?  If so can you post a decent clear closeup picture of both sides of your board?

If you can we can have a look and try to spot any other issues.  My gut and experience tell me there must be at least one.

I posted in another thread:-

"A warning.  Different transistors will make no difference to this circuit, (they rarely do), don't fixate on that!

"The circuit uses very basic standard elements to perform each task.  The first two transistor stage has been around since Adam and the final stage is a classic single transistor amplifier.  They both employ local negative feedback, (it's a GOOD thing), within them.  One of the jobs negative feedback is used to do is to make the circuit immune to differences in the gain of the transistors.  For all modern transistor types like the ones you  listed it works.  'Nuff said?"

That applies here also.  It's one of the reasons why designers use negative feedback!  As long as you have modern normal gain transistor types in working order their hFE will make no difference at all to this circuit.

Apologies if I give you loads of tech info and explanations you don't want but there are others out there who would be interested to understand what is going on, not just get their pedal moving.  This one will work but what about the next and the one after that?  Almost inevitably nowadays, info based on good sound engineering fact often spoils what we want to think is the case or points to a solution we don't want to follow.  Mostly people are going to do what they are going to do anyway and are really only looking to be told that is the best option.

Anyway, I'm sorry, no more long descriptions from me.

Hi Midwayfair.  I think you may have missed the point of what I was trying to say with my example figures.  I was trying to put a little perspective into the situation with an example of some basic theory.  Unless you know the people who are posting, you can never tell how much they understand of what is going on and it is better that they become aware of a little of what they are doing rather than just plug in values at random.  I always take the line that too much info is better than too little with apologies to the already informed.  With too little data the wrong issues get addressed and the myths begin.

The numbers I used are certainly exaggerating the effect it will have in this case but the theory is correct and it was only the theory I was trying to illustrate.  Multiply the capacitor by a factor of 10 and you drop the cutoff frequency by the same factor.  I'm sure we don't disagree on that.  I chose 400Hz and 40Hz for my example simply because it would be clear to see that it is a serious issue and needs consideration.  It wasn't meant to relate to the real problem that the poster was having.

Without knowing the input impedance of each stage, ('the' reusable stage in effect), we can't guess at the current 3dB point.  It is confused by having to take into account the shunt feedback applied via the 470k resistor which drops the value of that resistor considerably from its 470k.  You have missed the effect of that in your 470k//100k suggestion, it is not just a case of paralleling the two values!  As the feedback resistor has the small input voltage at one end but a much larger voltage on the other output end it appears to the input as though it is a very much smaller resistor than its value, (sort of bootstrapping in reverse)!  Then there is also the paralleled effect of the low non linear impedance of the feedback diodes as they turn on and off which we have ignored.  I would suggest that that input impedance may even be generally below 40k and could even drop lower than 10k under some conditions!  All of this pushes our frequency up way above what you seem to be suggesting.  If it slips down to even 40k, (a definite in this situation I would suggest with an open loop gain before feedback of 100x for each stage), the frequency has become: 1/2*Pi*40k*0.047uF which is 84Hz.  The lower it goes, the higher the frequency goes and we are already conservatively within the range of the guitar.

The point here is that, as you and I have both pointed out, if the cutoff frequency sits down close to or below 82Hz there is nothing to be gained by changing all of the coupling caps to 470nF at all, and lots to be lost!  So I suggested changing the caps one at a time and by values a lot less than 10X and playing by ear as to when to stop.  No change?  Then change back!

And of course, 6dB/octave and 20dB/decade are interchangeable to all intents and purposes, it is just that often one is clearer to envisage than the other depending on the circumstances, and 47nF to 470nF is also 10x.

A warning.  Different transistors will make no difference to this circuit, (they rarely do), don't fixate on that!

The circuit uses very basic standard elements to perform each task.  The first two transistor stage has been around since Adam and the final stage is a classic single transistor amplifier.  They both employ local negative feedback, (it's a GOOD thing), within them.  One of the jobs negative feedback is used to do is to make the circuit immune to differences in the gain of the transistors.  For all modern transistor types like the ones you  listed it works.  'Nuff said?

Don't know if this has been touched on elsewhere but if I were you I would change these one at a time and listen after each individual change.

If you are changing from 47nF to 470nF you are effectively changing the frequency of the cutoff of the filter by a factor of 10x.  i.e. as an example, a filter originally set to break at 400Hz will break at 40Hz.  For this filter the frequencies from 400Hz down to 40Hz will be lifted by an increasing amount up to 20dB, that's 10x !  Each filter you change will have that effect separately and they all add up.  If you change 3 filters by that factor of 10 you have lifted the response of overlapping frequencies in their ranges by an extremely large amount.  That may be too much in one step.

In real life it won't be quite as vicious as that but there is another factor here.  With filters which go down very low you are going to allow a lot more hum and low frequency noise through.  This is not a hifi application where we want extended bass, the best strategy is to actually set the filters to act a little below the open bottom string on your guitar, which is just above 82Hz.  For 82Hz, a 47nF resistor would need to see 43k.  This is certainly within the right ball park of the stage design used so not much change to capacitors is required to lift your bass response noticeably.

The schematic I have of the Pharoah shows exactly the same gain stage for each of three stages.  Hence the filter effect of each of the coupling caps is set at almost exactly the same point.  That's a big potential lift!  If you have capacitors available you could try just increasing one of the caps by doubling it at each change and gauge the difference in sound each time.  You can make sure to only go as far as you prefer then.  Each doubling of the cap value halves the frequency of that filter.  You will tailor the sound exactly to your own requirements this way.

It is entirely possible to change the filters to set them at different break points to achieve different degrees of lift across that frequency range.  In fact this is often a good thing as it improves transient response and stability in some circuits, (though probably not of interest here).

Also, do you have the unit input resistor 39k/390k High/Low option in your clone?  If not what is the value of your input resistor and first coupling cap?

You haven't reused electrolytics which you have had connected the wrong way around and just reversed them have you?  That would be a "bad thing".  Once used with the wrong polarity they are severely stressed and compromised.  At a few pence each they should be binned!

If it's in a case, is the case grounded at one point and one point only?

Check what the noise sounds like when you play with the tone controls.  Does the character of the noise change in tune with the tone control setting?  If so then the noise source is likely (not definitely) in front of them in the input stage.  Also how does the gain control affect the noise level and character?

I will have to start by saying that I owe you an apology.  I have been interested enough to take a second more serious look at this circuit and there are a couple of points I missed in my first quick scan which I should correct.  The PDF is too big to post so I have attached a small JPG of the original schematic taken from it.  If anyone wants the full document it is posted here:  http://www.madbeanpedals.com/projects/Sunking/docs/Sunking_ver.3.pdf

I criticised it for leaving the distortion generators in place when the pedal is deselected and connecting the output of that stage back to the input introducing distortion into the deeper works.  This does not happen quite like I said!  It isn't shown on the first schematic in the PDF but the selector switch shorts out the distortion diodes when not in use and that prevents any distortion from being generated in the second stage.  However it is still far from a good idea to leave that shaped feedback in place in the loop of the buffer amp which is used in all cases.

Something I did not mention but which I should, is that the output of both signal paths, clean and distorted, is actually blended at the output.  Each path terminates in a 68k resistor and the selector switch simply shorts out the resistor in the desired path bringing its signal level up.  The other path is then left in place but reduced by running through its 68k resistor before being summed at the output.

That might be fine for the on state where you can go from mild to hard distortion by blending the two but it's far from ideal in clean bypass mode.  It still leaves potential paths for unwanted signals to be fed into the output signal via IC2A and IC2B.  The test of turning each of the pedal Volume and Gain fully up and down in turn while playing in bypass mode should tell if this is the problem.  The Volume would solve the problem completely as it cuts out any contribution when fully down, (though it can't be used like that in practice), while the Gain may only change the character of the noise if the path through its circuitry and on through the IC2 opamps is a problem.  If the background fuzz is there when they are turned full up but gone when they are fully down then this is the culprit.

The real answer if the noise is a showstopper for you is to follow that true bypass procedure described in the documentation.  If this is an old pedal circuit, at the time that it came out TPDT switches may well have been expensive and rare so it needed economy measures to be able to use a DPDT switch.  Then the noise problem would have been noticed and diagnosed which could be why the true bypass mod has been added to the PDF.

Get it in true bypass mode asap!

I don't know the Sunking unit intimately but I have the build PDF.  There's nothing at all unusual about the schematic but I notice there are specific instructions as to how to "make the Sunking true bypass" and these are a bit unique.  At first sight it looked a bit strange with only a DPDT stomp switch, I would have expected a TPDT type here.  Then I looked a bit deeper and - yuk!

The input is passed through the first opamp which is intended to act as a clean buffer even when not in use.  Having a line buffer in the signal path when not on is not a nono for me, I know there are those who are retentive about this!  I've never found it a problem.  Tone suckout?  Hmm!  The second opamp input, the diode drive unit, is left connected to the output of this buffer and there is feedback from the distorted output applied back to it.  In the textbook world of perfect opamps this should make no difference, in the real world it will!  Does the background noise change character in OFF mode when you alter the gain pot?  The distorted signal is fed into the VB+ DC reference voltage circuit via the gain control in a very poor way.  Each of the opamps uses this as its V+/2 middle line!  Again in a real world situation the limitations of the "perfect" voltage supply setup will get in the way.

If you really like this circuit and want to stick with it I would really recommend you set this up for true bypass with a 3 pole switch instead of the 2 pole.  There are instructions in the build PDF to do this.  That way the input and output are completely disconnected from the circuitry at both ends and effects like this cannot happen, (you just have a different set of problems ).

[ANY]

Hi Cooder.  Great work on the schematic.  Basically you have everything I have spoken about in place.  The following is a bit cold and pernickety but it should make it clear and easy to check:

1. You have corrected the connection of C9 to R9.
2. You have corrected the connection of C13 to R13.
3. You have made sure the redundant 47uF capacitor from pin 4 to 5 of IC1 is removed.  (This one only seen in some versions of the schematic).

I also made a couple of additions.

1. As I experienced some slight oscillation in the second stage at the highest settings of the gain control with my original layout, when I designed a new PCB I added additional components between the wiper of the Gain pot and the gate of Q3 as a precaution.
   a) I added a 2M2 resistor between the wiper and ground.  This simply means the JFET still has bias as the pot is used and maybe loses contact with the track occasionally as it ages.  Crackly pot protection!  Notice this has already been done to the first stage Q1 and the source follower Q5.  I wasn't sure if the values used for R6 and R18 were meant to affect the controls in any way so I chose 3M3 across the Gain setup to try to prevent that.  (Overkill I think but it costs nothing.)
   b) A series resistor between the Gain wiper and Q3 gate and a capacitor from the gate to ground.  This acts as a low pass filter removing any high frequency signals from the second stage input.  The input filter at Q1 is 47k/470p which has a 3dB point at 7.2kHz.  This is lower than I was expecting and must be part of the overall tone shaping design which is flawless to my ears.  I chose a higher frequency of 18.5kHz and used 39k and 220pF for the filter at Q3 in order to not affect the sound in any way.

2. I run it along with my own Boost (Okkoish) pedal and a Behringer EQ pedal from a shared power supply block supplied with the Behringer. It gives me a reasonable steady nominal 9V, actually 8.75V.  This showed an internal supply line of about 8.4V and a generated supply line of around 15.5V.  However, looking at the supply line when the unit was in use and the LED was on it dropped to 7.4V and 13.4V, too low for my liking.  This was actually due to the extra 10mA LED current passing through R1 on the input to the power supply.  And this was without the main Pedal ON/OFF LED in place adding another 10mA.  R1 is there to work with D1 and protect the unit against connecting reverse polarity power.  If that happens, R1 originally set the current through the 1N4001 to around 80mA.  I decided to drop the value of R1 to 27R.  Fault current would then become about 320mA.  Remember this is only through the protection diode which can handle 1A, no current passes through any other part of the circuit.  In this way the voltage drop across R1 when in use is divided by 4.

3. I added a little bit of circuitry to control a LED according to the 9V/18V selection for the power line.  This is simply a couple of transistors used as saturated switches.  I designed a couple of cleverer designs like Schmitt triggers and comparators but they were more complicated than I needed.  This is simple and works well as there is no switching speed issue here.
   a) It simply turns off the first transistor when the voltage rises to 18V and a second transistor senses this and turns on passing current through the LED.  PNPs make the level sensing easier to switch.
   b) I was a little worried about the amount of current available from the generated 18V line and tried to be careful to not add to the load on it in any major way.
   c) The LED will draw about 10mA so the control circuit must be powered from the 9V input line.
   d) ANY general purpose PNP transistors will do!  Take my advice, there are NO magic sweet sounding types.  In 99% of reported cases that issue is a myth and down to pure placebo effect.  Any small signal diode will do for D40 too, it is only shifting the DC voltage level of Q41 to make the action more secure.  1N914, 1N4148 are both common types for this and dirt cheap.  Use anything you have lying around for this whole area.

At some time I will put up a full schematic for this but mine is in scribbled bits and very messy at the moment as the project is not completed.

Good to hear someone else is interested in it, on first listening to my prototype it seems to be a really well set up box and deserves more exposure.  Far better than the "diode clipping" approach which is never done properly for a crunch pedal rather than all out fuzz.  The controls are great, useful range without being overdone.  The crunch is well tamed and very gradual and allows loads of feel to your playing.  All in all one of the best overdrives I've heard.  Mind you if you play death metal look elsewhere, this pedal is more suited to the classic blues/rock player than the thrasher.

I built my own into the standard Hammond 1590BB enclosure and I got a bit creative with the layout.  I put the 5 pots and their associated passive components, and the 9V socket onto one board which mounted independently.  Each separate stage came out to pads on the edge of the board.  Then PCB mounting input and output jack sockets, the voltage doubler circuit and 9V/18V switch, and each active stage went on a second board.  I then wired up the connections between them.  It meant a very easy mount and breakdown procedure.  It oscillated!  I've played around with moving a few of the components from board to board and cured this but the pedal is so good I'm going to start again and do it all on one board in a more conventional manner.

If you come up with a working PCB I would be interested to see it.  I should also say that I've built the Okko Boost stage as a separate clean pedal again with voltage doubling and that works really well too.  While I'm sure there are improvements to be made by trimming values, (not myths like capacitor and resistor types and swapping semiconductors), it's a very usable setup as is.  Once you correct the schematic error of course! 

Absolutely with you there Kothoma, if you try it and prefer it then use it.  Personally I have always used the tonestack approach only because it does what I want with little fuss.  Consequently I have little experience of using a Baxandall setup in this area.  I have used them many times in other non-guitar related audio equipment where they are a standard and do a great job.  I have heard it said that they sound too smooth and "sterile" for guitar work, possibly because of their reliance on negative feedback, but that would not account for using them in passive mode as you have pointed out.  You have more experience than I do as to whether that is true as you have used them.  I should have made it clear that I was speaking mainly from the point of what I had been told regarding that matter.  My apologies.

Let's not forget that the original point I was really wanting to address was whether the caps were the right way around and my comment on the Baxandall was only intended as a throw away remark.  (Teach me to keep my mouth shut  ).  The cap polarity is a technical matter and incontrovertible, and in this area I do have 40 years of designing complex electronic equipment to fall back on.  The caps are shown in the schematic originally posted and the higher res version which I posted THE CORRECT WAY AROUND.  Using them the other way will not give any possible improvement in sound and will destroy the capacitors and possibly other components as well.

Why do people think that electrolytic capacitors come with a clearly marked -ve if their polarity is somehow optional?  They can explode if they are used with reverse polarity!!!!  Using them the other way around is not an option to try out just to see if there is any improvement, it is a fault.  Any circuit of the above, including the mysterious Cherrybomb, which shows the caps the other way around is WRONG, it will fault, and it is potentially dangerous!

Incidentally I can't find any reference to the Cherrybomb at Fuzz Centrasl and a Google search returns nothing.