Output impedance of tl072 input buffer

[mladenu]

You could always do what I did!
Build an external tone box,, and just leave the existing one alone.  8|

I plan that , because when i direct input guitar to return in, no hiss at all... thus all hiss is generated through preamp...  And external tone box will be bingo.. PhAbbTone with volume pot direct in return? And i will have clean amp..

An option;
Dave Reeves Hiwatt tone control is by far the best at mid control. :tu:

If Hiwatt control contain Fenderish character?

The mod you mentioned, the leg from Neg input of U1 with the 220n and 47k leg to ground looks good on sims. That returns all the bottom end, though the gain does go UP quite a bit so that may cause issues if circuit noise is already a problem.

Seems like Session preamp suffer noise problem because of sparkle characteristic and good response? :/ Or reason is not great topology... If that is true, another one reason for PhAbbTone..

If you want my take on Dave Reeves HiWatt tone circuit it's here somewhere,, just search for PhAbbTone Ver 3.5
I may have an updated schematic coming soon as some noise still gets in as these hi z circuits are all prone to picking up noise.

If i understand, you have greater version than 3.5? I will wait for that.. Less noise, more want..

[phatt]

Yes it will sound very Fender but that depends on your interpretation of a *Fender* sound. 8|

Bypassing preamp means you loose any OD effect but if you have some decent pedals and a tone control then you may find that more pleasing. I've done that trick a few times for local players who have struggled with there rigs, just a couple of pedals and a tone box bypassing the whole preamp.

Be aware that the Ptone does not add any distortion it's super clean but it does the classic big round bass and sparkle treble.

If you want the preamp and tone section to respond like a triode valve preamp then try *Loudthud's* design.
Here:
http://www.ssguitar.com/index.php?topic=3732.0

Just replace his tone stack with the Ptone circuit,
I've never tried his circuit but the idea looks like it will give a little half wave clipped edge to big notes emulating the classic triode rattle you get from valves.

As to noise /hiss and like issues,,
Even big name circuits can come with design flaws, just one tiny oversight in the layout of pcb can cause all sorts of excess noise to creep into the signal path.

Meantime pictures speak mountains, this screen shot is your Rockette 30 preamp with the tone section replaced with my HiWatt inspired Ptone.

The *yellow curve* is the standard Rockette tone response while the green curve is the PhAbbtone. Notice the deep mid notch. Unlike most of the other tone circuits which only tend to dip the mid with a shallow scoop this forms a deep narrow notch.

Both traces are taken with the tone controls at 90% while the mid on PhAbbTone is of course set low to show the full effect. in actual use it sounds good for me with treb around 12 o'clock, mid 9 o'clock Bass around 1 o'clock. your setup might be different as many other things alter response.
Phil.

[J M Fahey]

Fendery?

I think it goes *beyond*  Fender in chime and sparkle, so much so that it kept that sound even when heavily overdriven, go figure.

Just listen to old WHO records, even better some live ones (there's a couple YT videos too).

The 8x12" 100W Marshall was developed specifically for Pete Townshend (his Dad was a close friend and bandmate of Jim Marshall, go figure), and yet he preferred Fenders, there are many pictures showing a Fender head on top of early Marshall cabinets, so he heartily embraced the Hiwatts when they became available.

Using Fender head:


early Marshall:


Hiwatt (and anti-gravity belt or something):

including some tasty Sunn Coliseum 300W Bass heads.

[phatt]

Thanks *Jaun*,
Yes even Mark Huss notes the core difference that set Hiwatt's apart from the others was mainly due to the tone control setup.

Here you go *mladenu*,
                                 I finally got my shed computer sound card working again so a PhAbbtone demo via a proper microphone.
Dead flat with no effects just strat copy> Ptone> Laney.
The bass may not sound as deep with small computer speakers but enough to give an idea of the tone range.
This is the latest unit I built so I'm adding the schematic which you asked for, Hope it meets your approval. 8|

Play around with different values if you wish, try R1 (100k will make it insane bright)
VR3 Bass can be 100k but don't go 500k unless you; 1/ have a real LOG taper pot. 2/ need more bass.
C5 is now 1n8 which stops the mid from dragging too much treble as you turn up.
Cheers, Phil.

[mladenu]

Phill, you are marvelous! To mention J M Fahey too.. I saw PhAbbTone V4 schematic just after you attached, and via Line out jack from my Laptop through Return in to Session Rockette 30 amp heard awesome tone shaping.. All of this drive me to open some free tool and for the every first time go and try to design PCB layout for ypur PhAbbTone circuit... I know this is just a try, and sure contain many error, but if you have patience and desire, help me to made this acceptable... Well, please, spot the errors, and advice me what to change (sure, if i know how)..

Thanks Phill again for this updated circuit...

I have meny question especialy for you, but it will be time for that..

Cheers, Mladen

[phatt]

I'll assume you don't have a lot of experience at board design so this might help.
The pic attached is the actual PCB layout I just made.

Note how the output is well away from input.

Keep the track from Treble wiper to C6 and + input short as this is a High Z circuit and will be prone to noise issues if not careful.

As you will likely be doing this board at home then a big mistake with most pcb software is the pad and track sizes are set for a motherboard and although tiny tracks might look professional it's going to come to grief when you try to drill pads and even worse problems await when trying to solder. Don't make tracks and parts layout too close together as it's also asking for trouble.

I use KiCad to do this but just as easy with the free version of Eagle, just make sure you set Global parameters for tracks and pads.

My board is using 16mm metric pots and the spacing is 23mm which is about as close as you can go with the knobs that are available to me, any closer and you will first need to source those tiny narrow knobs.

BTW how are you going to make/print/drill the PCB?

I use a Laser printer and then a hot iron to melt the print onto the copper, great for home builders like me.

Also a picture of the box I used to make this one, it's an unused foot switch which saves me a lot of fussing to make the box which is the hardest part.
As it's on all the time I don't need the expensive 3 pole stomp switch and that also saves cost.

Have fun, Phil.


how do you print screen


free upload image

[mladenu]

Phill, i have zero experience on pcb designing! As this "pcb layout" by me clearly shows it. It's supposed to be my first pcb production tryout.. So it would obviously wait since my layout is bad... :/ Or Phill, if you were kind, and if i don`t ask to much, you may pleased me with pcb layout, because i suspect i`ll make it in resonable time... :/

Thanks anyway...

I noticed that the schematic you first attached is different from PCB layout, and these changes i recorded quickly and half glimpsed.. Red one values are from PCB layout..

Regards, Mladen.

[phatt]

You are doing well just keep asking until it makes sense :dbtu:
Don't expect perfection on the first build in fact if the first build does not work when you throw the switch it's the best way to learn as once you find and understand those small mistakes it stays with  you and unlikely you will make that mistake again.

Years back I wanted to build a killer guitar amp,, foolishly kidding myself that it was not that hard as the instructions were clear enough. What could go wrong,, I can solder? :duh
Ha ha ,, It took me another 10 years before I finally built something worthwhile. All that was before I could even spell cOmputa let alone the amazing internet we have today so with the web and voluntary help from some seriously talented technicians who grace these forums you will likely get ahead way faster than myself.  :dbtu: :dbtu:

As to the different values;
Don't worry most values are not critical, I'll try to explain.
R10 can be any low value from say 10 to 50 Ohms it just helps isolate the supply from other pedals and can help noise.
Not all of those pedal power supplies are the same, some can bleed noise into the circuit. If you have a super clean supply you can delete it if you wish. C10, similar to above, DC filtering often any value from 10 to 100uF.

R5/6 is a voltage division for bias, again any value from 10k to 100k can be used as long as they are the same value to give the half rail (bias) supply.
The general rule of thumb is 1k Ohm per volt, so if you have 10VDC supply then 2x 4k7 resistors would work, lower values give a stiffer bias but pull more battery.

C12 filters the node of DC bias voltage,, can be any value from 10 to 100uF.
You will eventually come to understand which parts of a circuit require close tolerance and which ones are not so important.
I'm not sure about C11 but often used in split supplies systems sometimes right at the supply pins. As I'm trying make it as low noise as possible it can't hurt to add it.

Note:
     Always remember when trying to read schematics it's TWO circuits in one drawing as there is both DC and AC happening.
That is how amplifiers works, AC floating on DC.
You setup the DC so as to pass the best possible signal and if the electrons like your magic smoke design, with luck amplification might just happen. <3)

With discrete circuits (transistors) of old you had to do a lot of DC work before you got it right. Now with opamps even dummies like me can make cool stuff even without a slide rule. :lmao:
Oh R11 (2k2) does not go to ground that is to power the LED light. Choose R11 value to suit the LED you use as some are very bright and you may want a larger value to dull it down. I find those bright blue ones quite annoying on stage.

Now the AC, the signal you hear;

As the input of U1 is very high due to R4 being very large then even if C6 was 2nF it would still pass reasonable bass response. If R4 is a low value then you have to make C6 larger.
This is all to do with impedance and R/C maths. As long as R4 is at least 1 Meg then 10nF ~ 50nF will be fine.

To get the best from these high Z (High impedance) tone circuits the output has to be looking into a hi Z input otherwise the tone control will be severely degraded and will not be able to give the same performance.
**By far this is the most critical point of the whole circuit if R4 was 100k the tone would be dull and lifeless and hardly working.**

As with most passive tone circuits there is no actual GAIN or Boost, they can only subtract or cut but they suit guitar quite well. A 1 Volt signal swing going in might come out as only 300mVolts but that loss is the price you pay for good tone control. Being hi z and lossy in nature there is a signal noise penalty but care with layout and shielding it's fine. I'm loosing count now but I've built about 20 of these units for local muso's.

R5 (1k) works in combination with VR4 to set the gain, another maths equation you likely don't really need to know for this to work. Just know that larger values of VR4 will increase gain and smaller values of R5 will also increase gain. (if you want to understand more, then google opamp design rules as they are not hard to find)
C7 sets the bass roll-off point if the main amp has massive bass you can make that value smaller.

C8, rolls off top end. again this is going to depend on your gear, your pickups, your ears and even your taste in music more that what dictates a perfect circuit.
In My Humble Experience, too much high frequency is a major reason for amplifier hiss and with hi gain now a standard part of most commercial guitar amps it can lead to a lot of hassles. as you have already heard the PhAbbtone it has more treble than you would ever need. The lower the value of C8 the more hi freq content will get through and the more hiss you will hear when distortion and hi gain pedals are added to the signal path.
So up to you? At least make it 100pF.

R6 (100 Ohms) is a safety backup for the output of the opamp, anywhere from 50 Ohms to 1k Ohm. RL is the load resistor. This little sucker helps to drain off capacitor buildup. It pays to have a load there to drain off any Dc when you switch the circuit, the dreaded pop when you stomp on a bypass switch. :grr

Just keep asking questions and even if I don't know the answer there are greater minds here that can step in and help you and also correct any stuffups I might make.

BTW, How are you going to make the PCB?
Phil.

[mladenu]

Phill, you gave me a remarkable hour of electronics. Much of it is now clear, and many answers i got. I used your screenshot and practically crossed out PCB layout...  I would love very much if you pointing me to possible errors... Also, i plan to use METALFILM resistors, due to a lower tolerance, and for pF value ceramic capacitors, while for nF value Wima MKS2.. Is this acceptable or there are better solutions? One more thing, the only unknowns to me about layout is potentiometers dimensions like Lug size, Diameter and Spacing..  :loco I would ask you to write me a most suitable, and also their labels.. Believe that`ll be a no problem to find them by the label...

Thank you for everything so far and advance for your answers Phill!

EDIT: Oh, sorry Phill, i forgot to write that i`ll use your Printer-Iron method, as is simple enough..

Regards, Mladen.

[Roly]

Nice thick track widths.  :dbtu:   Better for home etching, and generally there is no point in skimping on track widths where you don't have to.  The more copper you leave, the longer your etchant will last.

I'd suggest moving your jumper to the right, over to between C6 and C11 and nudge C6 left a bit, then you won't have such a squeeze under C6.

Try to get poly-(anything) rather than ceramic's if you can.  It's no big deal but they are better suited.

[mladenu]

Thanks for suggestion Roly! Something like this? Very hard to find Poly Film, or any Poly- with pF values.. But i can try... What proper to choose for potentiometers? Which brand, diameter, Piher`s.. ?

[phatt]

Hey I'm impressed, good effort. :dbtu:
Q/ What software are you using to make the layout?
If I know that I might be able to give better advice.

Yes Roly is on the mark with wide tracks, especially around pads and even more so around pot pins as these are the only thing holding the whole PCB. Ideally you would use pots with an extra support bracket to support the pcb but I'm in no position to track down those for a home build which will never be abused but be careful once the pots are soldered as the pins don't like being bent much so get your case sorted and holes all drilled ready to mount.
 

Re pot pin-outs;
I'm using standard 16mm Alpha pots, often suppliers will have pdf files with all the dimensions you will need. Be warned, get the parts before you make a pcb and make sure they are going to fit.
Print out a test pcb on plain paper to reference the front edge of the pcb as they have to line up with the collar at base of thread. Never assume the model on the software to be exact.
Even though those pots all look similar there are many versions which are all slightly different.
Of course if you don't need to use pcb mounted pots then it matters little.

These are the Alpha pots I have used;
http://www.tube-town.net/ttstore/Potentiometer/Alpha-Potentiometer-16-mm-PCB/Potentiometer-Alpha-16-PCB-220k-log::4413.html?MODsid=galpd1drup2pglocpijk17pgh2

Regards to capacitors it will work with ceramic if that is all you can get.
C1 and C7 can be ceramic the rest can be green caps. but yes if you can get hold of poly film for C1 they are technically a better type for this situation,, though you won't hear much difference but may help with noise as ceramics tend to be micro-phonic. C7 not an issue just use a Ceramic.

FWIW, I scrounge a lot of junked electronic equipment for parts, a treasure trove of stuff can be found in almost any old El junk these days. For ceramics, greencaps and resistors Electronic junk is free parts supply store. I've had no need to purchase the fine shielded cable for years due. 8|

Take time with the software layout, I often spend a week or 2 just re-arranging the layout and it's amazing how often you stare at something for hours on end and after a good sleep it hits you that there is a better way to layout the tracks.

Yes Metal resistors are less noise prone than carbon so worth the extra cost.  :tu:

Regards to printing PCB.
Assign Full Density of ink, software should have a drop down menu for advanced setting somewhere. Make sure board is super clean.

I use those thin glossy pages from my wife's house and garden magazines.
the print on the page is not plastic and won't bother the plastic ink of laser print.
(though don't print on heavy colured pages, just normal word print pages is not a problem)
The reason is the thin paper only takes a few miutes to roll off after a dunk in Hot water.
When I first tried this trick I used photo paper and it took hours to remove the damned paper.  :grr
(Disregard all that if you already know)
   
Phil.

[Roly]

Something like this?

Yep.   :dbtu:


I've done a lot of boards so I can get pretty close first time, but I still might spend hours tinkering with exact positions of components and tracks to share the available space around.  For a one-off home etch there is also no virtue in cramming everything into the smallest possible space - laminate ain't that expensive and you just make life hard for yourself with layout/build/debug.

There are a few tiny things I'd tinker, but meh, on the whole that's a pretty good layout  :dbtu:  (we just hope that all the tracks go where they should and nowhere else - I haven't checked it against Phil's circuit).

[phatt]

Ta Roly :tu:, Yep I checked the layout and mladenu you are lookin good. :dbtu:

Just note right before you etch add a big fat ink pen along the back like shown as you need an extra 2 common/ground points, One for the LED return and one for input common.

I like to add a fat copper pad as there is always something you forget usually a ground wire. I also at this point ink pen a bit extra around the pot pins (I've shown 2) and anything else that looks a bit suspect.
Don't over do it and don't use a water base pen.

I use a 1mm drill bit for holes (0.8mm is ideal but they break so easy)
You will also need 1.5mm drill for the pot pins.

Assuming a hand battery drill or similar is used;
Drill holes with pcb laid flat on a heavy block of clean flat wood and tape it down so it can't move.
Oh yeah should add,, lightly centre pop before you drill will help keep your bit from wandering off course.

Drill bit should be deep in the chuck with only 5mm or so exposed as drilling with a long shaft is asking for the drill to break.
Drill speed should be fast, a Dremel tool or similar is ideal for this as battery drills and like are rather slow and that actually will prematurely blunt the drill bit.

Believe it or not,, tiny drills like these need to go quite fast to cut properly.
Metal workshops often have a chart with all the drill sizes and the ideal speeds to ensure you don't spend half the day resharpening drill bits.
   
Phil.

[Roly]

An important point that needs to be stressed; just before you transfer give the copper a really good scrub with steel wool or similar so it is nice and bright, and don't touch the surface after cleaning (or you might get a lovely etched thumbprint).