Schematic critique, please - multi distortion/boost pedal

[duck_arse]

cake. looks like mud cake to me. what are we talking about?

[antonis]

what are we talking about?

Definitely not for OP's pedal..

P.S.
Moderation duties, plz...

[duck_arse]

what are we talking about?

Definitely not for OP's pedal..

P.S.
Moderation duties, plz...

I torte you deserved to be punished, so it'll have to stay.

[patricks]

A generous amount of Sachertorte could help..

Ooh, yes! Aus = Australia for me but honestly I'm just excited about the idea of cake 

With a pedal input, there are parallel resistances that reduce noise generation.  If the guitar has a volume control and a tone control, these are in parallel with the input resistance of the pedal to the extent allowed by the coupling capacitors and of course, the pickup has a series resistance. 

Interesting! I recently finished a 12-string build and put a G&L PTB (passive treble and bass) circuit in it, but as master fine controls after the volume pots and pickup selector switch. The bass control didn't work properly until I soldered a 220k resistor across the output jack so that the bass pot could "see" the equivalent of a maxed out volume pot.

My understanding from the reading I've done so far is that this might raise the clipping threshold?

Definitely not..!!

What raises clipping threshold (for a given diode V_FWD) is the addition (somehow) of a voltage source in series with diodes..
(e.g. a resistor in series with diode pair raises threshold voltage up to V_FWD plus current through resistor times resistor value..)

For a hard clipping configuration (diodes in shunt with signal axis), both diode pair edges should be lie on the same DC potential..
(leaving space only for AC addition/subtraction..)

So, in case of op-amp output DC blocking capacitor absence (like U2.2), diodes MUST be connected to Vref..
(I let you guess the reason why.. )

Okay, I've read through this a few times and I think I'm starting to get it. With the absence of blocking capacitor after U2.2 the "baseline" voltage is Vref that biases the op amp, hence connection of the diodes to Vref.
That then brings us back to adding a blocking capacitor after U2.2 and SW2, which means U2.3 needs a bias resistor - as you said in a previous post (and hence not needing all of the other capacitors connected to Vref).

So to add a voltage source in series with the clipping diodes i could theoretically either
1. Add a series resistor as you said
2. Add a different voltage source in series; either create another voltage at, say, 4.5 volts, or connect then to the low voltage pin of U1 (the 1044)?
Of which option 1 seems simplest, as you suggested.

Hope that's not too simple of me, spelling it out back to you like this, but repeating it helps me understand the concepts, and hopefully helps the next person reading it with a similar problem, too.

Follow-up question then: if putting a resistor in series with clipping diodes to raise the clipping threshold is a simple option, why isn't it seen in other circuits (that I can remember - forgive my ignorance if it is)?
Any good starting points for what the value of this resistor would be, or is it easiest to just pop a resistor in there and measure the vintage across it, and go from there?

[antonis]

Any good starting points for what the value of this resistor would be, or is it easiest to just pop a resistor in there and measure the vintage across it, and go from there?

https://www.electronics-tutorials.ws/diode/diode-clipping-circuits.html
http://www.muzique.com/lab/sat2.htm
https://www.diystompboxes.com/smfforum/index.php?topic=46415.0

[patricks]

Wonderful, thank you! 

[patricks]

Okay, updated schematic (in this post only, the timer has run out on the original post and I can't edit it again, sorry):


All of the capacitors connected to the inverting inputs of the op-amps (high pass filters) and all of the clipping diodes now reference to GND, there's a blocking cap before and after the clipping diodes following U2.2, a bias voltage to U2.3 with a shunt capacitor (per the original Rat schematic/first stage of this circuit), and all bias voltages (to op-amps and jfets) now point up rather than some up and some down.

I've added a pot in series with the clipping diode network following U2.2, with a resistor and capacitor in parallel, combining a couple of ideas on this page from Jack Ormon: http://www.muzique.com/lab/sat.htm, with the aim of providing more control over the amount of clipping and some shunting of high frequencies to stop it getting too "flubby".
This is something I'll probably test on the breadboard first to see whether it's worth it (Jack notes in one of the pages on the "warp" control that it might not be commonly implemented because the effect is subtle for the added complexity http://www.muzique.com/lab/warp.htm)

Thanks again all for the input!

[antonis]

 

Although a SPST (instead of SPDT) SW2 wouldn't make any cost difference here, for more than 2 diode clipping pairs (e.g. Ge, Si & LED) the pair of the highest forward voltage drop could be permanently connected on POTB lug 1, thus saving a throw posistion..

[patricks]

Oh cool!
The zener diode idea in one of the previous links did interest me... 

[duck_arse]

R14 and R15 - replace them with a single 2k2. is C15 needed?
C36 means you don't need C35.

[patricks]

Oh, good point, thanks.
I thought C15 was necessary because of the bias voltage on U2.3, is that right? Also should R33 go after C39 and before R13?
For C35/C36, keeping C36 and deleting C35 will allow more bass through, correct?

[antonis]

Under the risk of POT2 crackling (and C16/R18 HPF omission), you may delete C15, C16 and R18..

Theoretically, R13 should go after R33 (no signal attenuation due to R13/R33 voltage divider) but I think U2.3 can live with an incoming signal of 0.9985 instead of unity..

P.S.
Is there any particular reason for Q3 (and relevant circuitry) existence..??

[duck_arse]

C35 is for a heavy loading. C36 is valued for a 1M load. doesn't matter what C35 allows, C36 will decide. caps in series and all that.

also, R14/R15/POT2/C15 is a series string, and as we all know, the golden rule with a series string is the order of the parts doesn't matter.

[patricks]

Great, thanks both!

P.S.
Is there any particular reason for Q3 (and relevant circuitry) existence..??

Not really, except for it being there because I read in a circuit analysis the rat output buffer is a part of its sound. But that's probably more to do with it separating the tone and volume pots and providing a low impedance output, yes?
I had just copied over the circuit as a "building block", but I guess it's not necessary because there's no tone control, and the output from U2.3 will be low impedance anyway, won't it?

[antonis]

 

[patricks]

Look at me, leaning things!

[patricks]

Resurrecting the thread with an update, I've got a prototype PCB on the way to build and test the circuit

After all the fantastic help here I went further down the rabbit hole with Diptrace and tried clicking "convert to PCB". Many hours of tutorials and YouTube videos later, on placement, routing, planes and layers, crosstalk, and copper fill (vs no copper fill), I'm giving it a shot.
I figured it was not that much harder designing a PCB than working up a stripboard layout, and the parts count is high, so a stripboard wouldn't fit in a 1590BB, whereas if the PCB works I've got a fancy new pedal.
I sent the Gerber files to jlcpcb last night, and now there are 5 boards on their way to me soon for less than $15.50 AUD including shipping! Crazy!

[ElectricDruid]

Good luck with the boards!

I find that Rev.1 is almost never completely right, but can usually be tweaked to work. By the time I'm onto a Rev.3 PCB, I've got something I'm happy with. You may be better at it than me!

[patricks]

Thank you! 

I know, I'm trying not to hold out too much hope that it'll solder up and work straight away. With cascading gain stages I'm expecting some squealing or motorboating at first 
I'm sort of on revision 1.1 straight out of the gate, I almost got to the point of sending it away and then decided to redesign and move certain components closer to shorten the power traces.

Updates and fun noises to follow! 

Also the boards I ordered are purple 

[patricks]

Hi all, reviving the thread - will R22 and C26 turn into a low pass filter with Pot7 at full counterclockwise and affect the high frequency content of the distortion?
I get a cutoff frequency of 159 Hz if I've calculated it correctly, will that make the distortion sound more muffled?