First Build - Does this make sense?

[Czyc]

Hey guys,

I've been reading a lot of info on this forum over the past couple weeks and I've been getting really interested in trying to mod some of my pedals and create my own. I've tried to design a simple tone control pedal with a true bypass switch. I just want to start with something really simple as I've never been good with electronics (Mechanical Engineer) and I haven't even touched a soldering iron in over a year. Probably the most advanced electronic work I've done is changing pickups on a guitar lol.

Anyways, here is a basic schematic of my idea, I just want to know if there are any obvious flaws. I based it off the AMZ stupidly wonderful tone control and added a bypass switch.

[GibsonGM]

Hi Cz,

Welcome!  And yes, that looks like it could work, with just a couple of caveats.  Your instincts are very good!   As an ME, you'll pick this up quick, I bet.  BUT - there are other things at play that would make that 'not the best way to go'.  You can wire it up and see what happens (in fact, you probably should!), but you'd most likely experience "tone stucking"  because the output of the tone control and its bypass are not adequately separated.  It's not "true bypass"....

Second, you'd have insertion loss...the components in there 'eat some signal' in the process of doing their jobs, thereby lowering your signal quite a bit.

To counter both issues, we use BUFFERS and GAIN STAGES.   They're made out of transistors, opamps, FETs, MOSFETs and the like, and condition the signal for the parts in the tone stack ("impedance matching"), and also will boost the signal back up to where it needs to be.  

You're in the right place, and I recommend you look up buffers and gain stages using the SEARCH function!   There are TONS of online tutorials, too!  Links at the top of this page are good places to start (as is AMZ, which I see you found ok).  

[Czyc]

Hi Cz,

Welcome!  And yes, that looks like it could work, with just a couple of caveats.  Your instincts are very good!   As an ME, you'll pick this up quick, I bet.  BUT - there are other things at play that would make that 'not the best way to go'.  You can wire it up and see what happens (in fact, you probably should!), but you'd most likely experience "tone stucking"  because the output of the tone control and its bypass are not adequately separated.  It's not "true bypass"....

Second, you'd have insertion loss...the components in there 'eat some signal' in the process of doing their jobs, thereby lowering your signal quite a bit.

To counter both issues, we use BUFFERS and GAIN STAGES.   They're made out of transistors, opamps, FETs, MOSFETs and the like, and condition the signal for the parts in the tone stack ("impedance matching"), and also will boost the signal back up to where it needs to be.  

You're in the right place, and I recommend you look up buffers and gain stages using the SEARCH function!   There are TONS of online tutorials, too!  Links at the top of this page are good places to start (as is AMZ, which I see you found ok).  

Great! Thanks so much for the detailed response. I definitely do have a lot to learn and I'm glad I've found a place that is such a great resource of knowledge.
Going to do some reading on Buffers/Gain and then back to the drawing board

Cheers!

[GibsonGM]

Great to see the enthusiasm, CZ!       Look at this page:  http://www.geofex.com/article_folders/tstech/tsxtech.htm

The very LAST schematic shows a simplified Tube Screamer, an overdrive pedal.  If you took out the 1N34A and 1N914 diodes, which clip and make the overdrive, you would have a booster with a tone control.  This is kind of an example of how something like you're talking about is done....the tone stuff is placed between two stages like this, to isolate, provide buffering and to recover the gain lost.

A more complex version of something like this, meant as an active tone control, can be seen here:  http://www.runoffgroove.com/tonemender.html


Just a couple of ways of doing this.  You could get away with a 1 transistor gain stage and just 'force' the signal thru your idea above, and it would work too, just less elegantly...

[Czyc]

That's funny that you sent me that link because I literally just printed out the technology of the tube screamer as well as the fuzz face one right before I read your reply. I took a quick glance at it and did a little reading on transistors and op amps (I've learned quite a bit about these before but well over a year ago) and things are starting to make more sense. Planning on reading both those articles this weekend for sure.

Thanks again, really appreciate it!

[GibsonGM]

YW!   Now get yourself a few transistors (2N5088 and 2N3904 are very common!) and start playing....   )

[smallbearelec]

Hi--

I wanted more or less what you are talking about with input and recovery stages using JFETs (Junction Field Effect Transistors). They are simple to design with, which appealed to me. This article

https://www.smallbearelec.com/HowTos/BreadboardBareAss/BreadboardBareAss.htm

shows what I came up with on the breadboard and may interest you. I am almost done with the perfboard design and expect to publish that in a week or so.

SD

[MaxPower]

Well, you've been recommended transistors and JFETs so I'll recommend op amps. The TL07x (TL071, TL072, TL074) op amps are good and relatively inexpensive. 

LTSpice and Tina-ti are a couple of free and capable circuit simulation programs you might want to check out.

[GibsonGM]

+1   the '07x series are in EVERYTHING...

[Czyc]

Alright guys, I've done quite a bit of reading on transistors/op-amps used as buffers and gain stages.

I've done a quick redesign on my original circuit (pretty much complete rebuild) based on this. How does this look? I know there are probably quite a few errors, but I want to learn from my mistakes and learn why what I did was wrong.

Also of the transistors and op Amps you mentioned would there be a best selection of which ones to use here or would the difference be negligible. I understand that certain transistors and op-amps have different gain and input/output impedance but I'm not sure what properties are advantageous for certain applications.

Thanks!

[Czyc]

Here is the new desgin..

[GibsonGM]

Nice work, Czyc!  Great for your first time out!!    I whipped this up fast, hope it makes sense....

Ok - question - why are you using the "-" side of the power supply?  I'd flip that, make 9V + going to the transistor collector.    The buffer looks ok to me.
NOTE:  My POWER PINS ARE REVERSED in this (I flipped the opamp from an inverting to non-inverting setup) so make sure in real life YOURS are connected right!  

Check this image out, I threw together a basic gain stage for you.  If you change R1 to a pot wired as a variable resistor, you can vary the gain.  Some just put a 100k or even 1M pot there for testing purposes, can measure it and put a resistor in later.  Gain is set by the formula there, 1+R1/R2.  In this case, about 11.       Up in your first post, you showed what you want to do.    Use the output from the buffer as "IN" and "OUT" would be where your original OUT was (minus the volume pot) where I indicated on the pic.

I wouldn't worry about the output buffer now, as the opamp will have a low enough output Z to cover that.    You can always put one in later if you want )

Do you have a breadboard?  I'd advise playing with this on one, rather than just building it.  That way you can tailor components to your needs!!   Might want to make C1 & C3 = 1u or 10u, etc....it's from an existing thing I had lying around...same with the 1K resistor at output, that's not doing anything.

BTW - I'd use a TL071 for this, one of the most common opamps.   The TL072 is a DUAL opamp (2 on 1 chip, you could use it for your buffer instead of a transistor....), and so on.    

Note the power supply for the gain stage I drew - you use a voltage divider to give you 1/2 the supply voltage, and feed it to the unused input (in this config, there are others).    The 4.5 volts is called a BIAS VOLTAGE, which sets the chip to operate as if 4.5V was your Zero (as the AC signal crosses zero...) - then it can swing UP and DOWN almost 4.5V.      Lots of reading on the net! Look up "opamp single supply" FMI.....
Anyway, Vr goes to R1, the first gain resistor.     Don't forget to power your chip from the 9V !

This'll get you 3/4 of the way there....
ENJOY!

*************IMAGE EDITED TO CORRECT ERROR NOTED IN POST BELOW - THIS IS CORRECT ****

[GibsonGM]

Crap, just noticed your buffer isn't biased either, LOL!!!  I happens.   Read this, copy the buffer 5th one down, that will be good:  http://www.muzique.com/lab/buffers.htm

Or, alternatively, use the next one, 6th down, that has a resistor to Vr, since you'll have a bias available there anyway!   

[MaxPower]

GibsonGM, I think you have your R1 and R2 backwards. As drawn on your schematic, the gain equation is: Gain = 1 + R2/R1, giving a gain of 1.1. Unless my textbook has been lying to me. Wouldn't be the first time.

[Czyc]

Thanks much for the detailed response GibsonGM, I really appreciate you taking the time to help me out with all this.

I don't know why I put the power supply backwards haha, I always messed that up in my electrical circuit/electronics classes in University as well :p
And yes, I do have a breadboard and was planning on setting up my circuits on there before actually soldering them to a CB for the exact reasons you mentioned since I'm a bit clueless about which values should be used for each circuit element. I'm trying to understand why certain resistors and capacitors are used, I know that capacitors are used to block DC signals and can be used to modify the frequency and that resistors (and their respective ratios with each other) are used to determine input/output Z as well as gain from Op Amps and Transistors., but some of the placements and values are still pretty confusing to me.

For example, in the circuit you drew, what is the purpose of the capacitor in parallel with R5? I understand the voltage divider concept (Vr = (V3*R5)/(R4+R5)), however I'm still unclear on why you want a bias of 4.5V and what parts of the circuit would go to the bias and which parts would go to ground?

I just saw your post a few minutes ago so I haven't had a chance to read up on what you mentioned, but I am going to right now.

Thanks a lot!

[GibsonGM]

MaxPower, thanks for catching that. I have to fix my LT Spice.  I've managed to bone up the working opamp, and if you flip it from inverting (which I use most often) to noninverting, everything goes haywire (you can't test it to see if what you did works).   That was a very newbie error, corrected now, ha ha  

Czyc:  No problem, everyone here likes helping people!  you might want to look at/save this, it is where I got the majority of info on opamps. It's like a holy grail:  http://www.eng.yale.edu/ee-labs/morse/compo/sloa058.pdf


Cap to ground, you mean in the power supply....that cap (10u to 100u, kind of up to you!) filters the incoming voltage.  It provides a buffer of sorts for when a circuit may call for a lot of current for a brief time...can smooth out the fast pulse that will drop voltage a little bit.    That is a filter capacitor, sometimes called a reservoir.   Remember, a cap will stay charged to that 9V unless something comes along to lower the voltage on the other side; at that time, the cap starts to discharge into the circuit, and the power supply has much less 'interruption' due to a spike, if you will.    It is another R-C function, similar to your noticing the use of caps for frequency shaping.

If we didn't set the input (let's pretend it's -.5v to +.5V, an AC signal) to ride on that 1/2 supply bias, your opamp would amplify the 0 to +.5, but the 0 to -.5 would be cut off, resulting in nasty noise (half wave rectification).      That 4.5V DC tells the opamp to raise the signal up to 1/2 the supply, so it can amplify both above AND below the zero!     The other way to do that is to use a +9 and -9 volt supply (dual supply).   That is cumbersome, so we use this trick.   No magic in the 9V, it could be 18V, 5V, etc. but then you get into *other* issues like headroom, so 9 is the magic number.  

The input and output caps block DC from getting IN to your circuit, but really they are there to block your BIAS voltage from flowing OUT of your circuit!   So in between those caps, you will have 4.5VDC  (in addition to whatever you input to amplify).   The DC can't flow past your caps, so all you get out is AC in a state of whatever you did to it (amplified, etc).  Interesting, huh?

Your power supply is, of course, grounded.  For all practical purposes, that is the "-" battery terminal.   The volume pot has one end grounded so it too can be a voltage divider.   Your input signal has one end grounded, as does the opamp itself.     The R1 gain resistor goes to Vr, in order to provide that bias we talked about (aka VIRTUAL GROUND).    That's the "pivot point" around which the opamp will amplify, rather than zero volts.   An arbitrary reference point, to make the opamp "understand" where to place the zero part of the AC signal.

The process of adding DC via the virtual ground, to make an AC signal 'ride' on it, looks like this.  Maybe this will help you understand better:  



You're hitting on many great and exciting things about this hobby, best taken one step at a time, LOL....good to see the enthusiasm!  

[Czyc]

So I read that article on buffers and did a bit of googling and I am really thinking that I'm starting to learn what they are all about as well as why bias voltages are important for transistors and op-amps.

I redrew up my circuit with the modifications and broke it up into stages. (I know constantly re-drawing the circuit and making mistakes is probably not the best way to learn this but I find I learn best with this trial and error method, especially when I'm learning something like this).



I didn't assign values to the circuit elements on the diagram but I wanted to make up this table and see if I'm on the right track of understanding the purpose of each component.




Let me know if I'm getting closer, I ordered a ton of parts yesterday that I am missing. I probably won't get them till next week so lots of time to learn more and improve on my design before I actually try to build it. I've learned so much over the last couple days but I feel like I have about 100x more to learn. Very interesting stuff to me.

[duck_arse]

in your "power supply" section, you have a wire shorting Vcc to ground. this is less than ideal.

redraw the circuit as many times as you like. develop your drawing style until you find a way you can follow, from front to back. then stick with that style. usually, if you get it to make sense to you, it will make sense to everybody. the first thing I do is redraw someone else's circuit, that's why I have so many pages covered in pencil scribble.

[GibsonGM]

+1, D.A.    I redraw other circuits, and then leave errors in them so others get confused, ha ha  (just kidding).   Takes time to develop a 'language' with schematics that follow conventions and helps others understand, and you're doing great.

You are TOTALLY on the right track, C.  One thing, R8, that gain pot?   Make it a variable resistor, like this: http://www.electronicspoint.com/attachments/untitled-jpg.2514/      Don't want it going to ground!    This way it will take a 100k pot and make it vary from 1 ohm up to its full value...variable....by shorting the wiper (middle) to one side.     

I would go ahead and start to breadboard some of this.  You have a few blocks now that you can experiment with, measure the voltage outputs and so on.   The buffer won't 'do much' on signal, but the gain stage will!    Note D.A.'s comment about the wire short; look at how my V1 is in the schematic above, with + up and - down; that's the battery.

[Czyc]

Thanks a lot guys, the guidance and quality of advice here is actually incredible.

Yeah, looking at it now it seems obvious that I would be shorting the battery to ground, another dumb mistake :p
As for the variable gain resistor, I was sort of unsure of what to do there, only thing I could think of was to connect the third lead to ground but I understand why that would cause problems.

I'm really glad that I'm on the right track and I feel way more confident in my understanding of circuits and circuit components. I can't wait to start making some of these!