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[ground-referenced]

That's a strong claim in need of proof.

You're quite right there Kothoma, but I would maintain the proof is in the simple circuit action.  All of this circuitry has been around in exactly this form for 40 years!  You are doubtless right that the Baxendall tone circuit is used by named guitar electronics manufacturers like Ampeg, that is domain knowledge which I do not possess, but in its passive mode as they usually use it it is ground referenced.  The difference is that here it utilises feedback from the collector of the third transistor.  It is not in any way in passive mode, it is an active Baxendall control setup exactly as it is used in hifi.

Check out here:  http://makearadio.com/tech/tone.htm  They confirm,

Independent adjustment of bass and treble frequencies in high fidelity  audio amplifiers is usually accomplished utilizing specially designed tone-control networks. There are versions for these tone controls based only on passive components, such as the ground-referenced  (my bold underlining) James network shown in fig. 1. Among those versions using active devices we must make mention of P.J. Baxandall´s proposal, in which the tone control was devised as a feedback amplifier. 

This is irrelevant to my claims about the DC conditions for the capacitor polarity issue however.  That is without question.  If you get a clear enough version of the circuit it even shows DC voltages posted on it.  Here is a copy for reference:



Anyone with experience of designing with transistors, (yes I go back that far  ) will be able to see what the DC voltages would be.  They aren't even close to being questionable.

The emitter of Q1 is low, it has to be. Notice Q1 DC reference is the emitter of Q2 and its emitter has to be about 0.6V lower than that.  Q2 emitter must be significantly lower than its own collector to allow it headroom to work and prevent it from bottoming out on signal swings.  So Q2's output capacitor DEFINITELY should have its +ve to C2 collector just as the schematic shows.

Q3 is set up similarly.  It's base DC point is defined by the two resistors at around 1.63V.  It's collector will then be around 5.1V.  This capacitor also has to have its +ve terminal to the collectors.

I'm sure someone else with design experience will come in to either confirm this or tell me I'm a dipstick.  (That may definitely be, but I don't think so in this case  ).

Just for the record here is what I reckon each stage should look like:

https://dl.dropboxusercontent.com/u/21235584/Diablo_Stage_Corrected.jpg

The difference is around C4/R6.  Yes, such a small change does make a big difference.

Just to put this as a CAVEAT for anyone who comes across it at a later date...

The schematic shown for the Okko Diablo above is the standard web version.  There is also an accompanying schematic for the Okko Boost.  They are both wrong in a big way!  In all of the stages the capacitors between the gate and source of the upper JFETs are attached to the wrong end of their source resistor.  They should all be connected to the drain of their lower JFET.  You cannot get your Diablo working properly with the circuit based on that schematic.

This link will make it all clear for anyone who is interested.   http://www.madbeanpedals.com/forum/index.php?topic=17962.msg172734#msg172734  There is a link in that thread to a very good site where the circuit action is described in detail.

But you ARE helping Drolo.  Between us we have fixed the problem!!!   

I've just cut the tracks of my Okko Boost clone PCB and rerouted the circuitry so the wiper of the Bass control is connected to the bottom of the resistor/drain of the JFET.  Hold your breath.  No unravelling of the space/time continuum.  It works!

Yes, we have much more gain, the controls work as you would expect, the tone is as it should, the whole thing just comes to life.  The original circuit is WRONG!  Now to revise my layouts and etch more PCBs to match for both the boost and overdrive setups.

Oh, I'll post on the Freestompboxes site to let them know.  What a partnership, eh? 

Not fully yet, I noticed the oddity and wanted to see what others thought about it first.  I have set one stage up as a Clean Boost and got about 4x gain out of it but this was less than I was anticipating.  You're right though, testing it out fully is the sensible approach from here.

What we basically have is a choice of two circuits to choose from both shown in your Geofex article.  The way it is currently wired in my post is as the JFET Muamp at the start of the article, (look at the C4 connections).   This has no drain resistor for the bottom JFET, and putting one in actually does nothing in real terms so R6 could basically be removed.  Just putting in R6 certainly does not turn it into the other alternative, the SRPP amp shown at the end, that depends on the resistor being in the source of the upper JFET and spanned by C4.

Interestingly, one of the differences between the two is that the Muamp circuit cannot drive output loads very well with its high output impedance, while the proper SRPP can as it has a much lower output impedance.  Looking at what is hung onto the outputs of each stage of the Diablo you can see relatively low impedances there, and they are all frequency variable too.  It would be essential to have a low output impedance in each stage to cope with that.

My gut really tells me that we need to shift C4 down to the drain of the other JFET!   

I found that posting in my travels too drolo.  Unfortunately it confirms the same observation I have made. I understand what the resistor does in his setup but the Diablo is not wired like that.

To do what he describes, the connection between the cap and resistor must be made at the bottom end!  Look at the pic next to that section and you will see what I mean, it is not the same as the schematic I posted above.  The Okko Diablo schematic shows its upper JFET components wired differently.

Without spanning the resistor with the capacitor it does nothing!  The whole point is to give the resistor the same AC signal voltage at each end, then it effectively acts as a constant current source and looks like a very high impedance load to the JFET below.  The top JFET is just a source follower which gives the same voltage at its source as it sees at its gate, and of course that is the signal fed to it by C4 from the drain of the lower JFET.  So the same voltage appears at each end of R6.  No voltage change means no current change, hence a constant current.  And constant current sources have very high impedances.  The lower JFET likes that and can give high gain because of it.

I think we may need someone with an original Diablo to chase this part of the circuit through and see which is right.  Any takers out there?

Hi guys.  I've been working with what is usually considered to be the Okko Diablo schematic to come up with my own take on it.  I have found this same version in a number of places on the web.  The circuit is pretty standard stuff that has been tweaked fairly well.  2 x Mu-amp stages and a source follower.  It should offer good performance for classic overdrive rather than metal stuff, at least that's what I'm hoping.

Understanding the action of the SRPP stages is not too hard but the schematic doesn't seem to have things as I expected.  I've attached a pic of one stage of what normally passes as the circuit for reference.

Usually the bottom end of C4 would be expected to be connected to the lower end of R6 not the upper end as it is here.  Surely R6 is having no influence in this circuit?  The intention is for C4 and the upper JFET follower to bootstrap R6, (put the same AC signal at both ends so there is only a constant DC current through it with no changing AC current).  This makes it appear to the lower JFET as an extremely high impedance load.

Can anyone else who knows a bit about this circuit shed any light?  I do wonder if the original schematic has been drawn out incorrectly.  This same oddity occurs in both gain stages of the Diablo and in the Boost portion too.  If there is anyone out there who has one of these pedals and can check this out I would appreciate it.

Woah!  Your 2 caps on the MAX1044 are 10uF, both of mine show as 47uF.  Now that is a significant difference as it affects the amount of charge that can be dumped into the reservoir capacitor on each cycle.  The MAX1044 datasheet application notes show capacitors as 10uF for most of the other applications but for the voltage doubler circuit they are not given a value.  As long as the chip can handle it bigger is better here.  It passes more charge each cycle and gives more storage capacity to offer higher currents for the same ripple.  And we have to remember the ripple will be at much higher frequencies than a standard 50Hz PSU.

One little thing I've noticed is that your power line shows no smoothing right at the input, only the reverse polarity protection diode.  All of your 200uF smoothing is after the 9/18V selector switch.  This puts effectively 250uF across the MAX1044 setup on 18V selection.  It won't strain the chip but I'm not sure of the ability of the preceding 47uF cap to top up the 250uF in the same way.  Instinct tells me it should make no difference as the overall current draw from it is the same as the original.  It will simply take a little longer to come up to 18V and should smooth the line better.  My original schematic shows 100uF and 100nF across the input and 100uF after the switch.

That's great Teknoman, thanks for that.  It clearly shows that the additional cap is not there.  That's good confirmation that what we are saying is correct and the cap is a mistake and not needed.

This circuit uses a standard Baxandall tone control circuit.  I don't know how it would sound in this application, (I wouldn't have picked it myself), but it was designed for hifi amps as it gives a gradual slope to the range with both cut and boost.  It's hardly ever seen in tone controls in guitar based circuits which it does not suit, they generally sound better with the tone stack concept which works with cut.

As far as DC levels are concerned, the ringed cap is between the collector of Q3 and the emitter of Q1.  Follow the DC path through the resistors and ignore any paths which meet any capacitors which block DC of course.  Q3 collector would be expected to sit just above half the supply voltage to give the best output swing and the emitter of Q1 would sit very low at around 0.5V.  I think the picture has values on there but they are not distinguishable at its resolution I am afraid.  The cap MUST be placed with it's positive end to Q3.  Incidentally the cap from Q2 collector must be the same way round for the same reason.  The diagram you have posted is absolutely correct with its orientations.

Turn it the other way around and you will reverse bias the cap by about 4V.  It's wrong and it won't like it!  And reverse biasing electrolytics does not give you a better sound!

Thanks Madbean.  Logic told me it was not needed but there is always the chance that I missed something that an experienced used of this chip would know.  If there is a genuine improvement in performance for the cost of one cap it's worthwhile.  Thanks for the confirmation, C20 is out!

Hi guys.  I'm building a Diablo Clone and putting in the 18V rail option.  The MAX1044 circuitry is simple enough but it has a confusing aspect.  I've tried to attach it below, I hope it comes through ok.

I have looked into the suggested applications in the Maxim datasheet which is very clear and helpful with a voltage doubler mode straight out of the Diablo schematic.  The 1044 works by alternatively applying the rail voltage and the ground to pin 2 the base of C18 and top of C20.  This allows C18 to charge to almost V++ (-Vdiode) before lifting it onto the rail voltage to dump its charge into C19 bringing that to around 2xV++.  C20 is irrelevant to all of this and while it is all going on, it's bottom end is alternatively linked to the ground and pin 5 which is not used.  Why are the two systems being used in the Diablo circuit?  For voltage doubler mode surely C20 is redundant.

Is there anyone who has experience with the MAX1044 or the Diablo who can point me at something I've missed?