Heavy screamer

[Le québécois]

I have put together building block scavenge here and there and want your opinions/suggestions.
Here is the schematic : http://dl.dropbox.com/u/43158392/Heavy%20Screamer%20v1.tif

The goal was to have distortion that gradually clip of part of the signal. So I started with TS/LTD type for clipping that maintain part of unaltered signal (not much finally because of GE didnot work here and it required LED). The next step is just diode to ground and diode in the signal chain (this was scavenged from the HM2 pedal). Next stage is active two band baxandall EQ adjusted to my taste. Thanks to Headwise project library. The output is the TS buffer.

I feel some part are redundant (there is 5 x 1µF NP cap!!!). Never see that in stompboxes! Buffer for the EQ and output buffer. There is also 1/2 TL072 left (The TS output was suppose to go there in a D+ type to feed the diode to ground... but finally was not attempted) 

It as plenty of output and while still on bread board is very quiet. At some setting the thing actually nail NOFX sound quite well even with my strat style guitar. The distortion could be describe as being ''edgy and hifi''. I will post a soundclip soon (hopefully). Good alternative while waiting for J201 to build Dr boogie or Spitfire.

hope it is usefull

[Le québécois]

Sound sample (30sec) with EQ set to center (no treble/bass boost or cut) here : http://dl.dropbox.com/u/43158392/HeavyScreamer%20flat%20EQ.MP3
I recorded my little tube amp with a mic so you can ear my strumming in the back.  I'm more of a funk/jazz player so my heavy metal is not that accomplish as you can ear

edit: ouff not my best recording I think I was too hurry to put a sound sample..... I will change that when I have time.

[DavenPaget]

once you throw distortion into the picture , remove hi-fi from the description .
You should really place the EQ just before the gain stage and i noticed something ,
your EQ looks really familliar

Besides , why would you use the EQ ? It has a -3db drop .
Placing the EQ before the gain stage eliminates the extra transistor that compensates the missing gain , but you really should use

[Le québécois]

once you throw distortion into the picture , remove hi-fi from the description .  note taken
You should really place the EQ just before the gain stage and i noticed something ,
your EQ looks really familliar  Indeed but I took it from Headwise project and basically play with the bass and treble cap until satisfaction sorry if they are similar
Besides , why would you use the EQ ? It has a -3db drop . Because when treble and bass are fully cut there is a mid boost-- two knobs for ±three EQ. Maybe it's also the case with the schematic you suggest?
Placing the EQ before the gain stage eliminates the extra transistor that compensates the missing gain are you talking about the opamp just before the EQ (it's a gain stage) or you suggest to place it before the TS clipping section?

thank for the comments

[CynicalMan]

Just a few suggestions:
C5 doesn't have to be NP, the negative side can point towards the diodes.
C6 doesn't have to be NP, the negative side can also point towards the diodes. In fact, it could just as well be 0.1uF.
You don't really need the output buffer, I'd just put this after U2b:


Edit:

Besides , why would you use the EQ ? It has a -3db drop . Because when treble and bass are fully cut there is a mid boost-- two knobs for ±three EQ. Maybe it's also the case with the schematic you suggest?

I think he's talking about the gain stage with U2a. With the resistor values you have there, it has a -3.5dB drop. I think a non-inverting buffer would be best there instead.

There's nothing wrong with the EQ you used. You might need to adjust the affected frequency bands, but that's just tweaking. In this application, I'd prefer it to the Bass Boost & Enhancer Plus. Your circuit can boost or cut bass and treble, the BBE+ can only boost.

[Le québécois]

@CynicalMan : This is exactly what I am looking for!!
About the 0.1µf, It will not block some of the bass? I've notice such drop when changing the 1 uf cap but maybe it was only in my head? There is not only the guitar signal but also all the harmonic coming out of the distortion stage? Well that a part that I really don't understand yet!

[CynicalMan]

It's just a RC high-pass filter, so you can calculate the cutoff frequency with . The input impedance of the next stage is 150k, so plugging in 0.1uF (0.1x10^-6) gives a cutoff frequency of 11Hz, well below the limit of human hearing at 20Hz.

[Le québécois]

It's just a RC high-pass filter, so you can calculate the cutoff frequency with . The input impedance of the next stage is 150k, so plugging in 0.1uF (0.1x10^-6) gives a cutoff frequency of 11Hz, well below the limit of human hearing at 20Hz.

after C5, there is 10k, can I use the same calculation?

[CynicalMan]

Short answer:
Not quite. It's around 20k if the diodes are conducting, and practically infinite if they aren't.

Long and confusing answer:
For an RC HPF, you're considering the series capacitance and the resistance to ground.  With C5, you need to consider the two paths* to ground: through the diodes to ground and through C6 and R7. If the diodes are all conducting, then you can treat them as shorts, and the resistance to ground is 20k. If only the series diodes are conducting, then the resistance is 10k+10k+150k (the impedance of C6 is relatively small). If none of the diodes are conducting, then the resistance is practically infinite. So the resistance to ground is 20k or more. The cutoff frequency equation is then:


*We usually consider the inverting input of an op amp gain stage to be AC ground. The voltage on it should have no AC unless the op amp is clipping. That's why I used 150k for the C6 calculation.

In fact, the basic RC filter equation doesn't really apply to C6 either. This is because the series resistance that precedes C6 acts as part of the resistance to ground. If one of the diodes to ground is conducting, this resistance is around 0 and the basic equation applies. If not, then the series resistance is the resistance of the series diodes, plus the 10k resistors, plus the impedance of C5. The minimum value of R is 150k. So really, the equation that fits is this:

But we don't need to be too worried about that kind of accuracy, because the cutoff frequency will always be well below 20Hz.

An easier way to think of it is that the capacitors act as frequency-dependent resistors. At 0Hz (DC), they are at infinite reactance. At frequencies of over 0Hz, their reactance (basically AC resistance) decreases following this formula:

So at any given frequency, you can treat the capacitors as resistors. Then, you can calculate AC gains like you would in a DC circuit.

[Le québécois]

Edit:I think he's talking about the gain stage with U2a. With the resistor values you have there, it has a -3.5dB drop. I think a non-inverting buffer would be best there instead.

In fact I increase the value from 100 k to 150k to decrease the gain. I have unity gain with the volume pot at 2 or 3. I can easily afford some DB drop.

At 0Hz (DC), they are at infinite resistance.

DC is just 0Hz AC !!!!! So obvious once you read it.  But I never realized it before.  I will have to integrate and digest this

[ashcat_lt]

That's usually what we mean when we talk about DC, yes.

Not to confuse things too much, but somebody fairly recently told me that DC is actually when the current only flows one way.  In AC, one side is more positive than the other for a while, then it crosses zero (both at the same potential) and then they switch so that the first is more negative than the other. 

DC doesn't do that.  The one wire stays more positive than the other all the time.  The voltage difference between the two can oscillate all it wants, but it's still technically DC.

For example, a half- or full-wave rectified AC signal still wiggles up and down, but since it doesn't cross zero it's still considered DC.

Or, at least, that's what sumbody told me recently.

For our purposes, though, we usually want our DC sources to wiggle as little as possible, and try to get them to be 0Hz.

[Le québécois]

You should really place the EQ just before the gain stage

A. What are the pro and con of such move? For noise , or frequency??
B. If I go for it (EQ before the gain stage), can I just replace one of the resistor of U2b by a pot and turn it into an output volume control?

[Le québécois]

I've made some change according to some of your suggestions and put a gain stage before the clipping stage (from GuvNor). The previous version gave sudden distortion drop when the gain knob pass below the clipping treshold. This new version is better in that way.
This pedal is modern sounding with a lot of gain. I was expecting some TS sound at low gain setting but that's not working without removing diodes to ground and in line with the signal. I gave up for that aspect.
Again any comment will be appreciated.
For me the next step is PCB!

http://dl.dropbox.com/u/43158392/Heavy%20Screamer%20V2.gif
Sound clip of about 2min 30 : http://dl.dropbox.com/u/43158392/NE4V1_03.MP3