NPN Based Overdrive Circuit Check, will it work?

[Hallmar]

I just drew up a schematic for an NPN based overdrive(or fuzz?) and i don't know how it will act or if it will work.

I know the basics of an amplifier using Common Emitter connections since i'm on my second year of electronic engineering(don't know if that's the correct translation).

I'm a beginner at this, i just started doing DIY Effects about 6 months ago on my own.




Moderator's note: Please use more descriptive subject titles, its helps everyone including yourself.

[Hallmar]

ALSO i forgot to mention that the transistors are either 2N3904 or BC550.

[amptramp]

Neither transistor has a connection from the positive supply through a resistor to the base, so neither transistor will turn on.

The connections to S1 are wrong.  You want the connection on the bottom of the diagram to be connected directly to the input.  You want the connection from the bottom of S1 to go to ground.

You should try to design around standard values - you will not find a 139K, 8.6K or an 86K resistor as a standard value.

The transistors are not identified, so I cannot comment on the current levels in each stage, but the input impedance looks to be a bit low for running directly from a guitar output.

[Hallmar]

Neither transistor has a connection from the positive supply through a resistor to the base, so neither transistor will turn on.

The connections to S1 are wrong.  You want the connection on the bottom of the diagram to be connected directly to the input.  You want the connection from the bottom of S1 to go to ground.

You should try to design around standard values - you will not find a 139K, 8.6K or an 86K resistor as a standard value.

The transistors are not identified, so I cannot comment on the current levels in each stage, but the input impedance looks to be a bit low for running directly from a guitar output.

The transistors arer BC550's.

I just guessed on what resistor values to use, when i was sifting through the resistor values i just picked up the standards since it's not all in my head wich is standard and wich is not.


I should've seen that connection with S1! My bad 

So, connect the bases to 9v and how do i fix the input impedance?

Make R3 or R2 bigger?

Worst part is, the only teachers that i have don't know shit about this, they're all Electricians but the only teacher that knows something about this is on leave until after this summer.

[amptramp]

R3 gives you feedback that affects the gain and reduces input impedance at high frequencies.  For both transistors, you need to add a bias resistor from the base to the positive voltage supply.  The input impedance of this design is very low for a buffer driven directly by a guitar.  Your input impedance is going to be R1 in parallel with R2 in parallel with whatever resistor you use to pull the base up in parallel with Hfe times a resistance made up of the source resistor in series with a resistance of 26 ohms divided by the number of milliamps carried by the emitter.  Both stages need a bias resistor going to the positive supply.  You don't really need C4 and R3 - they reduce the impedance at high frequencies, which kills the treble.

One thing I didn't catch before - you have a DC path on the output through R9/R10/P2.  You might want to eliminate R10 or you will get the grandaddy of all switch pops.

The interconnect circuitry between Q1 and Q2 is a bit strange - you do not need to bias the drive control P1 and you can eliminate R6.  P1 should have one end connected to C3, the other end grounded and the slider going to C5.  Putting current through a pot adds to the noise, so R6 is working against you and reduces gain as well.

[Tony Forestiere]

R3 gives you feedback that affects the gain and reduces input impedance at high frequencies.  For both transistors, you need to add a bias resistor from the base to the positive voltage supply.  The input impedance of this design is very low for a buffer driven directly by a guitar.  Your input impedance is going to be R1 in parallel with R2 in parallel with whatever resistor you use to pull the base up in parallel with Hfe times a resistance made up of the source resistor in series with a resistance of 26 ohms divided by the number of milliamps carried by the emitter.  Both stages need a bias resistor going to the positive supply.  You don't really need C4 and R3 - they reduce the impedance at high frequencies, which kills the treble.

One thing I didn't catch before - you have a DC path on the output through R9/R10/P2.  You might want to eliminate R10 or you will get the grandaddy of all switch pops.

The interconnect circuitry between Q1 and Q2 is a bit strange - you do not need to bias the drive control P1 and you can eliminate R6.  P1 should have one end connected to C3, the other end grounded and the slider going to C5.  Putting current through a pot adds to the noise, so R6 is working against you and reduces gain as well.

Thanks, Ron 
That is one "Hellacious Fishing Lesson". It will take a few reads for me to comprehend it all, but your description of why DC voltage is leaking into the Volume pot made sense to me right away. Cap only coupling.

@Halmar: You've got the ball. Welcome to the Forum. We're watching.

[Hallmar]

Sometimes it's hard for me to comprehend all this because what i'm learning in school is in Icelandic.

But i get the gist of it.

I have the formulas for the Common Emitter connections and i'll go through the calculations tomorrow.


I got rid of R10 right away after i started to improve the schematic, i raised the values of R3 and R12 or should i raise the values of R2 and R7?

Should i just decide one input impedance and work my way through there? By calculating what resistors i need.

I added C7 and C8 to increase the Voltage amplification("AV" as said in the formula).

With C7 ;  AV= P1/ r'e
r'e= 1/ 40*Ie
Without C7; AV= P1/ r'e+R5

Thus getting more amplification with the Capacitor.

It's logical to seek to get more amplification right?
Since a distortion/overdrive pedal basically is just an amplifier that cuts the signal and makes have more of a "distorted" effect to it?


You've been a great help!

I'd hug you if i wasn't 8 hours(or more) away from you, that is to say if you live in the States.

[amptramp]

The first stage looks better - the input impedance is still low, but it works, until you get to the output at C3 which you have taken to ground.  Disconnect the gain control from the positive supply.  connect C3 to the top of the gain control and connect the bottom of the gain control to ground.  Put C5 in series with the slider and the other side of C5 goes to the junction of R12, R7 and the base of T2.

You have about 3 volts on the T1 base which will give you 2.3 to 2.4 volts on the emitter.  Your first stage is biased to carry 15 mA because Vbe/R5 is your emitter current.The collector current is about 99% of the emitter current, but with a 2K collector resistor, you need 32 volts.  Since you don't have that, you have to either reduce the collector resistor or raise the emitter resistor.  You could do with less current, so I would go with a higher emitter resistor first.

P1 is too low a value - it should be about 10K to 20K or it will be too much of a load on the first stage.

In the second stage, R7 is way too low, assuming you cennect R12 to the transistor base.  39K would be a better value, giving you about 2.5 volts on the base.  Allowing for a Vbe drop of 0.7 volts, this puts 1.8 volts on the emitter and R8 sets the current to 12 mA, still quite large.  Leaving R9 at 8.9K with 12 mA going through it would give you 106.8 volts on the supply rail - not what you want.  Bring R8 to 1.5K and cut R9 down to 3.9K and you will be in the linear range.

You have an input stage with 3 volts on the base and an output stage with 2.5 volts on the base - a bit high for a system running on 9 volts but understandable if you want a clean boost.  But for an overdrive, you can afford to go lower.

I have shown you the progression for designing a common-emitter stage:

1. Set the base voltage where you want it using bias resistors.  Size the resistors so they carry more current than the base but not too much or they will drop the input resistance too low.
2. The emitter will be one diode drop lower voltage.
3. Set the emitter resistor to a value that sets the emitter current where you want it.
4. With this current flowing through the collector (actually slightly less - the base takes a small amount of current) set the value of the collector resistor so the collector voltage is halfway between the emitter voltage and the positive rail.  This gives you the most headroom.  Of course for an overdrive, you may want to have less headroom.

[Hallmar]

Alrighty, thanks!

I can't thank you enough  

Just out of curiosity, for how long have you been doing this?

[amptramp]

I have been interested in electronics since I was eleven, but I graduated as an EE in 1970.  There are a number of us on here with varying levels of experience as can be seen in this thread:

http://www.diystompboxes.com/smfforum/index.php?topic=100489.0

and you will find some people on here with more experience in stompbox issues than I am.

[duck_arse]

sw1a is backwards. you are switching the signal to ground instead of switching the c1/c2 junction to ground.