Yet another OD idea

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Yet another OD idea

Postby phatt » 13 Dec 2018, 14:17

With time to spare I've been poking a few resistors into bread board holes,, as you do :roll:

The Aim being to see if I could get a little more rattle out of an all SS rig.
Most dirt pedal demos are done into mega expensive tube amps which is not a real example of how well the pedal will sound into a clean SS amp hence most demos are of little help. I'm not chasing big fat distortion just a gentle ODriven sound enhancement.

I recall seeing a circuit somewhere in my travels that had something similar to this so thought I'd try the idea.
Nothing special here except the 4 back to back diodes are tapped at the center junction and a big cap to ground.
(See Schematic C7)
With Cap removed it performs much like most others with the standard diode fizz. (not what I want) Add the Cap and you get a big Clean jump in volume, It still distorts but the pot has to be dialed up a lot higher.

I figure I've either got something good or maybe I've just invented another barbed wire canoe and it's all been done before. :mrgreen:
So this brings up a Q? Anyone care to fill me in on what's going on, how it works? :scratch:

A sim reveals harder clipping (square edge) with the cap added but it's a cleaner OD if that makes sense.
Anyway I know there are many here who like to tinker so thought I'd share the idea and some may find even better results. :thumbsup

notes;
Diodes are Germanium,, **Silicon does not work**.
I'm using 1N34a and OA92 (I think?)
You can delete C5,C6 & R9 but it adds a bit of extra mid.
VR3 is just a top edge pot adding presence.
Fiddle around with interstage caps for more or less bottom end.
Smaller values of C7 will wipe off more bass. Try ~47uF for some country twang. 8)
R13 & R14 are bleed off caps when switched.

I may end up just building this and see if it floats better than all the other barbed wire canoes I've built. :lol:
Phil.
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Re: Yet another OD idea

Postby bajaman » 13 Dec 2018, 20:20

Hi Phil
Interesting
I think what you have done with the big cap is essentially remove the diodes from the op amp feedback loop :wink:
To the audio signal the 470uF is a short circuit low impedance path to ground, so essentially you now have a back to back pair of diodes from the inverting input to ground, and a back to back pair of diodes from the output of the op amp to ground. I suspect that this latter pair will be the dominant clipping mechanism in this circuit. :wink:
cheers
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Re: Yet another OD idea

Postby grrrunge » 13 Dec 2018, 20:56

My thought exactly Baja!
Whichever pair of diodes is clipping the most would depend on the gain setting of U3, as i see it.
Looks cool though Phil! How does it sound?
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Re: Yet another OD idea

Postby Manfred » 14 Dec 2018, 12:17

Hi Phil,
I agree with Bajaman and Grrrunge.

The diodes D1 and D3 does not affect the signal because the path between the positive and negative Opamp input
forms the so-called "virtual ground", therefore that diodes can be omitted.
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Re: Yet another OD idea

Postby phatt » 14 Dec 2018, 12:24

Thanks *Bajaman*,,
Yep makes sense,, and just confirmed it by removing the diodes, first output side then input side and made little difference so all I've achieved is opamp distortion.
Oh well,,Another Barbwire canoe sinks. [smilie=lol_xtreme.gif]
Now if I can just track down some reverse wound diodes it just might float. :mrgreen:

Hi Sir *Grrunge*,
As far as sound, think of the song *Start me up* by the Stones, the circuit is just starting to break up so you get that classic triode rattle.
I've found it works alright with VR1 (Drive pot) at 3 O'clock and VR2 (OD pot) at low setting but with VR2 past half it's just much the same as a lot of other dirt /OD circuits.
Sounds ok on single notes but too harsh for chord work.

Maybe I should just be asking if anyone knows of a pedal circuit that does that triode rattle into a clean SS amp rig.

Mictesters simple compressor circuit does help as it tends to distort a bit as it compresses but I need just a little more hair.
I have a Boss OS2 but you have to wind the gain almost off and it;s still a bit harsh. My guess is it's not just a tone shaping it's those diodes switching on/off that makes it so harsh.
Thanks for the input chaps,, Phil.
ED1,,, Yes *Manfred*, I had a hunch it was something like that but I'm a little lost with some things.
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Re: Yet another OD idea

Postby phatt » 16 Dec 2018, 10:11

Well I refuse to go down without a fight,, :blackeye so I came up with this. 8)

I had way too much gain in front of the diode section.
I think my sims are lying as they now show that u4 is clipping before the diodes clip at u3.
But back on the BBoard Removing the diodes from circuit proves they are indeed clipping way before u4 clips.
Silicon diodes allow a much bigger signal to pass and with fiddling might work but the feel of the Germanium seems to suit my needs.
No matter it's close to what I wanted to achieve. :thumbsup
I figured if I make R10 A pot then I would have 3 pots doing similar jobs.
I found that raising the value of R8 and bypassing it with a pot (VR2) was more progressive than having a pot at R10.
So working between the Drive and Hair pot seem to give me a fair range of rattle.
Oh yeah,, be aware that I had to use LM833 for u3&u4 as the dreaded hiss was quite evident with generic 4558.
Of course a real Valve amp might be better but it's close enough. :lol:
Phil.
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karul (16 Dec 2018, 18:11), Manfred (16 Dec 2018, 13:37)
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Re: Yet another OD idea

Postby phatt » 20 Dec 2018, 12:59

Well it's on the bench so why not keep poking ideas at this and see what happens.
This time driving a telephone line transformer. These little suckers are designed with limited bandwidth and I'm sure it's been done before.

My reasoning;
Phone line Tx's are flat from 200hZ to about 3khZ which happens to cover most of the main frequencies for guitar. :hmmm:

From my understanding Diodes seem to have a horrible habit of adding fizz (High Freq Hash) to any Odrive circuit you wish to build but these transformers can't pass really hi freq,, (Same goes for output Tx's of some famous Valve Amps :secret: ). So will it help remove the fizz without loosing too much treble? Well it certainly seems to help in this case. Although the low frequency of the transformer is rolled off I don't notice any bass loss,,My SS Amp has a big Bottom end so that likely helps. I'll plug it into a Valve Amp with less bottom end response tomorrow and see what happens. There is voltage loss with the transformer but still way louder than in bypass and when you consider the advantage of removing the fizz I don't mind a little signal loss. 8)

By accident I found that the Value of C11 (470nF) interacts with the transformer. Obviously an L/C thing happening in combination with the output
cap and Tx. Raise that to ~2uF for a more normal resonance. 470nF gives a real resonant hump, gives an old school boxy sound some may like.
Adding a pic of the bench, probably not much use but may inspire someone though the schematic is likely more useful.

Notes for the BB pic;
The pots on the front are not used,, only the 4 pots are on the BB.
Pots Left to right; Drive level, Intensity, Edge, Master out.
The Tx is on the floating PCB. Which was a small unit to tap into a phone line. The Tx on that psb measures 47/47-Ohm.
But other 600-Ohm phone Tx's may work even better if you want to experiment further :thumbsup
Have fun,,Phil.
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Re: Yet another OD idea

Postby deltafred » 20 Dec 2018, 16:28

phatt wrote:By accident I found that the Value of C11 (470nF) interacts with the transformer. Obviously an L/C thing happening in combination with the output

Yes, and R15 also affects the Q of the circuit. See https://www.electronics-tutorials.ws/ac ... nance.html

Interesting things happen at resonance (when xC = xL) because the current though them is 180 degrees out of phase so cancels out which means the output impedance drops to the value of the resistor.
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Re: Yet another OD idea

Postby phatt » 20 Dec 2018, 18:36

Thanks Mr Fred, cool page, explains it well. :thumbsup
I get a little lost with all those equations but I know enough to get the idea. :oops:
For me, I just want to find a way to remove that dreaded diode fizz. :evil:
I've seen a lot of extremely complex ideas on how to emulate triodes but diodes work reasonably well but they tend to be harsh even with small amounts of gain.
I figure there has to be a way to remove that fizz,, it may never be perfect but I feel I've improved it a little.
Of course I'm testing at fairly low volume most of the time but I'll get a chance to crank it soon. What might sound ok at bedroom level can end up sounding like crud at gig levels. Been down that road before :slap:

Electric guitar through clean SS circuits sound very lifeless and just by adding a little distortion you get much more dynamics. Of course even a basic valve guitar amp does add a little breakup by design.
What I'm chasing is that little edge/rattle. A lot of younger folks would likely think *Hank Marvin's* guitar sound was super clean but if you listen close there is quite a bit of breakup on the edge of those notes which gives it the dynamics that the otherwise sterile electric guitar would have.
That is all I need for my sound,, I have plenty of other stuff to do harder rock but it's that clean edgey rattle I can't get.
Phil
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Re: Yet another OD idea

Postby deltafred » 21 Dec 2018, 00:27

phatt wrote:I get a little lost with all those equations

So do I and I am supposed to have studied it at degree level. :oops:

I think the last time I used j notation was in the final exam and that was a very long time ago.
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Re: Yet another OD idea

Postby Electric Druid » 15 May 2019, 22:29

You could have a look at a differential transistor pair for generating the subtle overdrive you're after. I've been using something similar to the following circuit with an electric organ to give a bit of "growl". Since you can hold down a lot of notes on an organ, you don't really want huge levels of distortion - it just gets mushy. It works very well, and I've been thinking of adapting it for a pedal design.

http://electricdruid.net/wp-content/uploads/2017/06/SineSubOsc.jpg

This circuit is actually a "sine waveshaper" circuit from a synth, to turn a triangle waveform into a "Sine" waveform by rounding the peaks. So for overdrive purposes, we can ignore all the upper-row stuff. And you could adjust the input amp to taste - gain there alters the drive amount, although you can also do that by altering R5. The Symmetry trim is interesting too.

Tom

PS: Is there a way to post a img tag and get it to resize the image? I tried what I found on this page https://www.bbcode.org/examples/?id=10 but it didn't seem to work.
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Re: Yet another OD idea

Postby phatt » 22 May 2019, 14:01

Hi Tom sorry for delay,, been busy,, life gets in the way of hobbies.
Thanks for the idea,, looks interesting.
When time permits I'll BBoard it and see how it sounds.

I never finished this circuit mainly because I broke the tiny transformer (long story) :oops: I tried others but could not source another with the same specs. Seems the effect was specific to this Tx which measured 45R/45R DC. others I found were much higher,, like 60R or higher.

So ended up scrapping that and built this;
GeODrive.png


No cigar but it does the triode rattle quite well.
I ended up just adding a pot to the diode string allowing some control over how hard the diodes clamp.
Buffer always on.

Phil.
ps,
I use this to resize pics; https://www.irfanview.com/ It's free and does a whole stack of tricks. :thumbsup
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