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[M9 x 0.75mm half nuts]

Ok, so nobody knows the answer.  After a lot of asking around online I still couldn't find anyone who knows what size these should be.  Well I can clear this up myself and it may help some other poor soul who comes across the info.

With a bit of work with a micrometer I got the outside diameter, (8.8mm), and the thread pitch, (close to 2.35mm for 3 threads).  That seemed to put it into the M9 x 0.75mm category.  Looking at the thread specs that fits exactly with a M9 fine thread which is available.  I had to dig around a bit but sourced these from an ebay seller who wasn't asking for my dog and daughter's hand for 2 nuts.  Don't go for anything "Boss Replacement".  As soon as the Boss name or the idea of a pedal or guitar is attached the price rockets.  Just buy a common or garden "M9 x 0.75mm" Half Nut (or Jam Nut) from a fastenings supplier.  The half nut marks it as the slimmer type, (only 2mm thick), which we need for the Japanese pedal and guitar sockets.  These nuts are commonly used to tighten against a full nut so it is locked or "jammed" into place.

I have attached a picture of the two nuts which are originals and the new one fitted.  You can't tell the difference and the thread is a perfect fit.  I don't have the washer in place yet but it is sitting at home base waiting so the pedal will soon be back to exactly original spec as you can see.

So there it is.  Boss pedals need M9 x 0.75mm half nuts.

Hi guys.  This surely must be a known point to you pedal guys here.

I have an older Boss CH-1 Super Chorus pedal which has lost a nut from one of its jack sockets.  These are Boss' "far eastern" types so don't have our standard Imperial thread sizes.  The socket has a slightly thinner nose which a standard sized socket or potentiometer nut just drops over.

It appears to have a fine thread 8mm nose on it so I would guess it would be a M8 x 0.75mm fine thread.  The usual coarse thread for M8 is a 1.25mm pitch and this is far too fine to be that.  Then there is fine at 1.0mm pitch which again does not look right.  Finally there is the 0.75mm pitch I mentioned earlier.  You can buy the "official Boss pedal replacement nuts" which would seem from the price to be hand carved out of billet platinum but the alternatives are available as simple stainless steel standard items if you just know the correct thread.

Can anyone confirm this thread or, if this is wrong, suggest what it is?

Hi Midway.  The circuit simulated was in no way a fully finalised version.  You've pointed out some very valid points, not all of which I had picked up on, but some of them were to be addressed once I understood more fully some of the subtleties of how to control this configuration properly.  As you are aware it's nowhere near as simple a circuit as it is often assumed to be.  In fact I was surprised to find that, if you look into it, there are still ongoing arguments as to how it really works!

Incidentally, the problem of the top end fall off may have been largely solved.  I think it's mostly simple Miller capacitance in J1.  If you do the sums you find that with roughly 100k source impedance at the gate of J1 a capacitance of about 240pF is needed to rolloff at 6.6kHz and with a gain of about 10x that means in the region of 24pF Cgd which seems a bit high but in the ballpark.  That alone suggested it might be good to buffer it with a simple source follower and the sims of that are flat again.  It means a lower impedance drive control can be inserted after the follower to keep signals below clipping which seems to work.  That offers better control of input impedance as seen by the pickup, and with constant voltage biasing to half V++ we get terrific input headroom.

R9 seemed to be a good addition to me.  It is true that it reduces gain but we have that to spare, and it also increases input range dramatically.  I trimmed it so that it could cope with higher voltages without even needing the drive control.  With a drive control in place it will be possible to play with reducing or even removing it.  Input impedance of that stage was something I had not really considered yet as the other problems were more pressing but your point is well taken.  And your later suggestion regarding JFET choice is relevant here too.

Decreasing C2 was originally tried and rejected.  That value was settled on as a good compromise for flat response.  It affects the bottom end rolloff and in fact can put it well into the frequency range of choice.  That might be good to keep an overdrive pedal from muddying up but in a clean boost I wanted to start with it set as just that, to all intents and purposes flat across the board.  I was hoping to find that in reality I would have leeway to reduce it to 1uF as an easily accessible cheap non-polarised value.  What you say from your hands on knowledge suggests that is going to be on the cards.  That's a plus!

I'll try the 2N5457 in there too, I have 100 or so of them around.  I just hadn't thought of it.  You know my penchant for circuitry where the component choice is not critical?    Well that does leave me vulnerable to missing this point of course.  In my efforts to make it work fairly independent of the gain of the JFET it just didn't occur to me to try the alternative type.  I do accept JFETs are a very different beast to BJTs in this respect so the type stipulated is important.  However, once I've decided on that type it should be possible to set up a circuit which in the main does not require selecting individual suitable examples or trimming with pots.  You may have a range of gains but a drive control largely takes care of that aspect in a clean boost.  Higher gain implementation, just turn the signal down after the input buffer.  And the SRPP/mu-amp approach means you don't have those God-awful trimmers required to set the DC point to midway - absolutely shocking,         the DC point is largely set by the divider you reference your upper grid to.

I'll also look at values of R8.  I chose it basically because it is the value usually seen in other implementations of the SRPP/mu-amp.  I just hadn't thought to go above this level, but I'll give it a whirl and see what happens when I do.

I really do appreciate your time and thoughts on this, I've got a few more things to catch up on now.  More sleepless nights at the keyboard and bench!  Many thanks for that.

Hi guys.

I'm simulating the following SRPP stage in LTSpice with the schematic below.  The idea is to use it as a clean boost stage so driving it into clipping is not the aim. It must accept a range of input signals up to about 1V which explains the additional R9. I added this to lift the DC levels of J1 and, I would think, offer greater headroom at the input. Yes it cuts down on headroom at its drain but that I can spare. For bigger signals I can switch the rail voltage to a generated 16.5V and more than regain it. Thankfully all that seems to be what I'm seeing.



Originally I set it up without R7 and R10 in place with the input from V1 connected directly to R11. The frequency response top end was flat to way beyond the audio range. I didn't think there was anything odd in adding a divider (simulated drive control) to the input which may have allowed me to remove R9 but when I did I got the response plot below.



The top end rolloff now displays a 3dB point of 6.6kHz. Investigating a little further shows that any series resistance in that input line, adding a series parasitic resistance to V1 for example, affects the upper rolloff.

Now I can't really see any mechanism which would account for this but then I'm much more a BJT than a JFET man. Maybe the additional R9 in the source line is having an effect I did not anticipate. Can anyone explain what I am seeing here?


And if anyone can suggest why I'm having to post the thread before I can get to the link location for the images which then have to be "modified" into place I would appreciate it.  Using Firefox, is it an "eccentricity"?

Edit: Nevermind, it's not worth it.

My sincere apologies guys, I'm really not trying to shake any trees or push my own brilliance.  I only want to learn and you do that by putting your own current understanding forward to those who have knowledge, seeing what case can be made to show it's wrong, then taking on board a more accurate version if that can be demonstrated.  That way your own position becomes more informed.  However that only works if it's a two way street.

I've obviously touched on an issue which there is no interest in discussing in this way.  I think we can say that with my background I'm not the right type for here so I won't push myself or my ideas onto you any further.  Once again my apologies.

Hi Rob.  I see what you mean about the swapping of R13/R14, it was the word swap which confused me.  'Adjust' was no doubt what was intended.

And more importantly, of course you are right about the bypassing effect too.  There is no feedback via this path with it being attached to the virtual earth of IC1B.  It would be unable to feed DC back as I stated anyway with series caps in the output path, Duhhh!  I take your point there, it is bypassing the distortion area for low frequencies and feeding clean low end signal forward to IC1B.  The 1k5/1uF is still setting the break point to 106Hz which is about right for just the very bottom end. I wouldn't have thought you would want too much more in this area, it could get muddy very quickly.  But I should point out that the only thing you can change by adjusting these components is the rolloff point, without introducing additional components to increase the order of the filter the slope will remain at 6dB/oct.

Thanks for the corrections, it's difficult to be totally accurate with just a quick glance in a circuit you are unfamiliar with.  I should also apologise for starting on the other aspect, component types.  I noticed Vallhagen was very tactful with his comments regarding this and I've no doubt he has seen this topic flair up before.  Sound engineering principles and practice should be what is offered as advice, not 'common knowledge' and rumour, and that means what can be proven to be true.  The trouble is it isn't really what most amateur pedal builders want to hear.  'Nuff said!

C10 is indeed a filter but not the sort it looks like most people are imagining, it is not there to alter the signal in any way.  There are a number of feedback paths around IC1D and this is only one of them.  R14 and C10 make a low pass filter but it's corner frequency is 10Hz!  In the other direction R13 and C10 would give 100Hz.  If you swap R13 and R14 the feedback corner frequency becomes 100Hz, NOT a good thing!  (Doesn't anyone bother to work anything out before advising?)  I think it's basically removing all the signal and only passing the DC level back from IC1B to IC1D to stabilise the setup and perhaps passing some signal forward past the distorting mechanism.  If anyone has a different slant on its task I would be interested to hear it, I may of course be missing something.

Changing the value of this capacitor will have NO good impact on the sound at all unless you go for wildly different values which will affect its intended role.  The lowest note on a guitar is 82Hz so you would need to drop it's value to about 100nF before there is any real change (10Hz -> 100Hz).  Increasing it will only drop the corner frequency lower than 10Hz so it again will make no difference to sound.  Don't even play with it, it's there for a different purpose!

In this position, if you keep things as they stand, you would be best advised to use a non-polarised type if you have it.  You will get away with a tantalum or an electrolytic but running them without a reasonable bias voltage is not best for them.  They get noisy and don't maintain their insulation layer as they should.  (Running them reverse biased is disastrous!)

By all means try to connect C10 -ve to -9V but don't be surprised if it introduces noise or sensitivity to pops and clicks.  The -9V line is only a synthesised line which will carry elements of IC2 switching frequency, (around 45kHz with pin 1 to +9V).  It has limited current capability, (higher impedance than normal), so may also carry relatively high levels of signal related noise too.

One way you could get around this is to create a 'false ground' with 2 unequal resistors across the +9V/-9V lines and a large electrolytic cap from their junction to true ground, (off the top of my head 22k/10k/100uF would be fine).  Let's say you use these values, then the voltage between them will sit at about -3.4V and 10k/100uF has a corner frequency of 0.16Hz so it will be a clean DC voltage.  You can now connect your C10 -ve to this point and it will have a 3.4V DC bias across it but still be effectively attached to ground as far as the signals are concerned by the 100uF capacitor.

The choice of capacitor type is only to do with the DC conditions, you will NOT hear any difference between types at audio frequencies in this application.  You are trying to introduce huge levels of distortion to the signal then saying you can hear an additional <0.1% introduced by a capacitor?  It's a myth!  For the same reason, as long as you take care to not introduce IC clipping you will hear no difference in your choice of IC1, as long as the pinout is the same, (virtually guaranteed), opamps like a TL074 would do exactly the same job and be indistinguishable.  Remember, the opamp is only giving clean gain which is introduced to the distorting mechanism.  Differences between any good ICs are to do with other parameters than audio frequency distortion in this application.  Blind listening tests show that the whole issue of component types is largely nonsense.

Driving off at a tangent...

If your PCB is home made I would drill your wiring holes, (not your component holes), a tiny bit bigger.  A little makes a lot of difference.  That's pretty standard and a set of PCB drills is cheap.  A slightly loose fit is not a problem as long as you follow Lincolnic's advice and improve your soldering technique.  If they are bought ready made then get your hands on a cheap little round rat's tail file, couple of pounds/dollars for a complete set on fleabay.  That way you can gently relieve the holes by manually using it like a little drill.  The wire will slip in easily then.

Like I said, a tangent.  As a self-confessed pedantic old fart, during your builds it's really helpful to try to get a handle on a little bit of theory.  Keep on questioning everything and do your best to understand the answers you get.  There are too many ridiculous myths out there started by people who know a little and think its a lot.  Don't become one of them.

Good luck!

Doohhh! - - - Last time I promise.  If they're in working order and general purpose or better, in this type of circuit there is no best, they're all the same!  It's a myth staqrted by those who don't know enough to understand how the circuits work!!!

R24 - Changes the amount of high frequency available at the top end of the tone control.  When it's dialled down to 0 it increases the tone control cap C10 from 6n8 to 39n8.  Heap BIG change!

R16 - Do you mean R18?  Dropping the value 9% will change the high frequency end of the tone control slightly.  Probably not even enough to notice.  33k is common value, 30k not so common.  If you don't have one solder a 330k in parallel with the 33k which will give you 30k.  Then see if it makes a difference.

2N5088, 2N5089, BC550 - already answered to death.

[lower]

now im confused what i should use :/ guess im stucking with the 47n (read in this thread there may be a bass cutoff when using bigger caps)

No Luks, it's the opposite.  You really need to go back and read the posts again a little slower.  The issue is: "higher value caps will lower the cutoff point and give us more bass but does it need that doing in the first place?  You could say "let's just do it anyway, even if it doesn't make any difference to the sound it won't matter will it?" but that's not true.  You don't want to take the cutoff points lower at the bottom or higher at the top end further than they need to be because it doesn't alter your sound, it just opens the door to muck you don't want.  50Hz is less than an octave below a guitar bottom string and 60Hz is even closer!  Do you want to increase the amount of hum and low frequency noise?  Do you want slight power supply pops to become big thumps?  That's clearly spelled out in a number of posts in this thread.

If you have a bigger value or even values available try the following.  Change a single cap to 100nF, that's dropping the cutoff point of one filter by a whole octave.  If it doesn't change what does that mean?  Does it mean you haven't made it big enough?  Does it mean they must all need changing?  No!  If they are well below the value they should be and therefore need increasing there will be a change with the first replacement.  So it makes sense to continue to see if there is more to be gained, if it doesn't have any effect there will be no point in going any further so change it back.

If that improves things a little change the second to 100nF.  If it does increase the first to 220nF.  (I'm doubling the value each time to give you a whole octave of improvement.  You will hear any genuine improvement over that range.)  See if that improves things more.  And so on.  Stop when there is no further improvement.  That way you will get the best sound you want in the shortest time and introduce no more problems than you need to.

Guessing whether it's alive and poking it with a stick to see if it moves is not going to give you the best results you can get and I'm sure that is what you would want.  This doesn't take experience and knowledge, it just takes a little patience and the ability to ignore your signature when you will really benefit from it.

(I take it you don't agree with Newton and his "If I have seen further it is by standing on the shoulders of Giants" comment?)

C16 - Bleedthrough capacitor for tonal adjustments.  Adds (bleeds) more high frequency into the signal as Sustain control is turned down.  As the Sustain is turned up it has less and less effect.  Try without it and with it see which you prefer.  Remember it acts only as the Sustain control is turned down.  It's often seen in preamp circuits and even in certain Strats to emulate Teles.

R28 - Oddity.  Marked as 560pF.  Should it be a small cap?  Can't see a reason for it, pot ensures DC level stays at 0V when disconnected.

Diodes - Diodes are right, not really sure what you mean?  If you mean the capacitors C6 and C7 compared to C6 and C9, then it makes absolutely no difference, they are in series.  If you mean they should be after their transistors not around them, there are 2 ways of using them.

• You can produce a large signal with a basic amplifier and slap it across the diodes which are connected to ground and just let them cut the top and bottom off passively.
• You can put them in a feedback loop and let them do the same thing as part of an active approach.  That's what is happening here.  They are within the feedback loop of the transistors they are across.

I'm with you on the board mounted pots.  I can't easily get the vertical with extended legs type here.

My first attempt was with separate pcbs, one for the gain stages and one for the pots along with their surrounding signal processing components.  It carried the 5 controls positioned above the board on stiff 1.5mm copper wires to slot straight into the case.  It fitted a treat and looked great.  The biggest problem was that the gain board had to go right to left and the pot board had to go left to right.

Installation and removal were a gem.  I came up with a system where the board was off centre with respect to the pcb jack sockets which were fitted to just protrude through the case sides. This meant that it could slide into one jack hole far enough to allow the other to slip into place on the opposite side, then it was moved back across and an insulating wedge slid down the side of the board locking it in place.  However the extra inter board wiring was a bit of a nightmare.  Predictably it oscillated!

I could have tried to fix it but I decided to cut my losses and came up with the single PCB you see and loose flying pots.  I think with top mounted jacks you could come up with a better pcb design.  Make sure you let us see it when you get it finished.

Haha!  That bit is all my own. I wanted a way to show whether the 18V option (16.5ish on mine) was selected.  At first I considered just putting a LED and current limit resistor across the selected power line and the 9V line.  When the 9V line was selected there would be no voltage across the LED and it stays off, when the 18V line is selected it has 18V at the anode end and 9V at the cathode end and would turn on.  Then I figured that as LEDs draw significant current it might be a bad thing for the state of the MAX1044 generated 18V line which is only for light current use.  So I drew up that circuit.  It takes its power from the 9V line where there is more than enough and it is switched on or off by the selected voltage.

It uses more than it may have needed to achieve that but it is only a couple of general purpose cheapest in your box PNPs and a few resistors.  Unless of course you have to go for the Russian Super High Gain Germanium NOS types sourced from Middle Earth in the time of the Elvish Lords.  That would make it sound much sweeter! 

I have a PCB layout which you are welcome to look at based on fitting in a Hammond 1590BB enclosure.  I have attached a rough print of it.  In reality all of the copper regions which show hollow are filled, it's an eccentricity of the PCB design program I use to display them just hatched in.  It uses PCB mounting 9V power socket, voltage selector switch and jack sockets and it is self supporting in the box via these but the fit in the case therefore has to be via slots for the jacks and switch not just round holes.  It's not as complicated as it sounds.  I'll try to post a couple of pics of the install when I get time.

EDIT:  I attached a better version of the PCB so some of the comments above don't apply.

EDIT EDIT:  I should have pointed out that I used a smd version of the MAX1044.  It's easy with a decent iron and tip.  I would have like to do the whole thing in smd but I only had through hole J201s.  Maybe next time. 

One last attempt.  There are versions of this circuit out there using MPSA12, MPSA18, 2SC1815, 2N5089 and doubtless more transistors.  All of these versions work, possibly to different tastes but consider the following:

MPSA12 - min hFE 20,000  (it's a darlington and effectively 2 transistors connected)
MPSA18 - min hFE 500
2SC1815 - min hFE 70
2N5089 - min hFE 400

that's a range of 70 - 20,000.  In the light of that, do you think it matters?

(I'm suffering from Cassandra Syndrome.)