Why my reproduction of DOD 250 does not work?

[rikk]

Hello everybody!
Guys can you help me to understand why this schematic does not work?



The sound exit from the schema without any kind of overdrive effect... As if the hard clipping did not exist 

Please help he, my mind is blowing 

[Fancy Lime]

Way more information needed. I suppose you did not plug your guitar into the printout of the schematic, did you? How did you build it? Breadboard? Perf? PCB? Pictures, please? We cannot see what the problem may be without seeing the actual circuit.

Andy

Edit: I forgot to say: Welcome to the forum!

[amptramp]

Normally, there would be a filter capacitor between ground and the junction of R1 and R2.  This is not likely to be the cause of the problem, but it should be added.  We haven't seen the layout or your soldering technique, so that limits our ability to diagnose any workmanship problems over the internet.

[rikk]

you are absolutely right.
Here some photos of the circuit. The output is connected to a powered speaker.
The opamp is a ua741.

[Mark Hammer]

The schematic "works.  The build does not.
Note that the DOD250 does not use 3 diodes in an asymmetrical configuration.  That's not "wrong", just an interesting modification from the original circuit.  It will result in greater maximum output level. 

Ideally, the Gain pot should be reverse-log (i.e., "C" taper), such that rotating the control clockwise skips through the lowest-gain settings quickly and distributes the range of higher-gain settings over more degrees of rotation.  As it is, one could use a log pot, but the high-gain settings will be squeezed into only the last few degrees of rotation, making it harder to dial in just the right setting.

Alternatively, if one wires the Gain pot in "opposite" manner, such that the pot resistance gets smaller as you rotate counterclockwise, one can achieve the desired spread, although it will mean that rotating clockwise gets you "cleaner" tone, rather than more distorted.

[duck_arse]

you have 4 capacitors on your board - the blue and the yellowish on the left, and two ceramic disc types on the right hand side. would you be so kind as to give us the number codes on each one, please?

[anotherjim]

No clipping implies that the diodes don't connect.
Is there a link between upper and lower (blue) 0v power strips?

[antonis]

The sound exit from the schema without any kind of overdrive effect... As if the hard clipping did not exist 

Overdrive (due to op-amp high gain) and hard clipping (due to diodes shunt) are different effects..
Try to isolate them and you'll find your issue(s)..

e.g. Take out of circuit clipping diodes and focus on op-amp gain stage..
You must have a loud distorted signal..
(unless your input signal is smaller than 18mV_peak ..)
If not, check VR1/R7/C3/pin2/R8/pin6 circuitry..
(also, a shorted C5 turns op-amp configuration into unity gain buffer..)

P.S.
Just realized that Stephen didn't ask for VOLTAGEs...!!!! 

[Fancy Lime]

Very difficult to see what is connected to what. But your description sounds like either the ground leg of the negative feedback loop may not have proper connection to ground or there is a short in the feedback loop. Both would cause the opamp to have unity gain.

With a circuit as small as the 250, my usual approach is to just take everything off the board and do us again from scratch. This is often faster than trying to locate the error. Another approach is that you try and trace the circuit on your board and draw a schematic of exactly the thing on your board. Then you should stumble upon an inconsistency with the original schematic at some point. This works well on complex circuits as well.

Andy

[duck_arse]

antonis, I need to leave something for posting today.

VOLTAGEs .... please.

[Electron Tornado]

Pin 7 should be 9V. Pin 4 should be connected to ground, 0V. Voltage on pin 3 should be about 4.5V, or half the supply voltage.

Do you have a signal tracer?* (If not, make one!) If so, check the signal on the IC pin 3 and pin 6. Do you get a much louder signal on pin 6? If not, rotate the gain control to each extreme and check again. If the signal at pin 6 is still not much louder than pin 3, then check your connections for C5 and R8, as well as C3, R7, and VR1.



* probably should have used the term "audio probe" here.

[rikk]

Hi everybody!
I'm designing a simple overdrive circuit with an opamp and an hard-clipping stage.
This is the schematic:



Input: my guitar
Output: a powered speaker

I have simulated it and everithing seems to work but when I try it on my breadboard I don't ear the clipping (the distortion).
In other words, the sound exits from the circuit clean and not distorted. 
Maybe it is due to the 1n4148 diode? Should I use a germanium (or Schottky) diode?

[puretube]

Your noninverting (+) input is "floating".
Should be referenced to half powersupply voltage.
Hook a 100k resistor with one end to +9v.
Hook a 100k resistor with one end to ground.
Hook the free ends together.
Where they meet, is your new 1/2 of 9V midpoint reference voltage.
From here, hook a 1M resistor to the + input of the opamp.
Also hook a capacitor (from 100nF to 10uF) from the that midpoint to ground.
Now listen again, and report here ...
Ooops: forgot to decouple the - input! - sorry.
As Merlin points out quite right, route the pot through a cap (e.g.: 10uF) to ground, instead of directly.

[merlinb]

You need another two resistors and a cap:


EDIT: Corrected input cap polarity

[antonis]

And, optionally, another small cap (100pF or so) across 100k feedback resistor..

[rikk]

Thank for your replies merlinb and puretube.
I have modified the circuit and... I obtain the same result  no clipping, no distortion...

I leave here a photo of the circuit:

[Phend]

Are input and output jacks ground lugs connected?

[puretube]

The minus (negativ pole) side of the input-cap must go towards the guitar!
(looks different in the pic ...)

[niektb]

some breadboards may split the power rails halfway, so you may need to check if yours does that as well

[puretube]

Is that grey wire that comes from the pot`s capacitor and is going to the pot (rightmost pot-connection in the pic) stuck deep enough into the breadboard? (two holes south of the turqoise jumperwire)