Distortion idea problems

[Dave Eason]

Hi there guys,

Just wondered if anyone could shed any light on this;

I've been "tinkering" around with a circuit of mine, to try and get a nice smooth tubescreamer esque distortion, with more gain and more top end.

I've basically taken the 808 overdrive/soft clipping stage and tone stage (as it is a fairly simple thing and you get good results from it), removed the input and output buffers and just been moving parts round, "to see what will happen" essentially.

The bottom line is, it does kind of work, when the level control is down lower than about 1 oclock, after this it turns to more of a fuzz, and progressively gets nastier, to the point where its hideously distorting the signal, and when you let it ring as the note decays it cuts of suddenly, almost gating.  I see the problem to be a bad DC offset in the signal line somewhere;  I'm not fully sure why ibanez design the TS808 like they did.  The biasing system of the TS9 is a lot simpler, and more "general" if you will.  The opamp is a 4558D.  I know its not right, and has probs with it, im more concerned about getting it to generally work, i.e the level control.  Other bugs are tweakable and its open to be played with.

I will stick a circuit diagram of what I have here, any light/help/comments/ideas would be most appreciated! Alternatively, someone please let me see a schematic of the Hotcake  which actually looks to be a similar, dual-diode softclipping/gain stage overdrive circuit

Thanks!
Dave

[slacker]

You need to bias the first opamp's + input to virtual ground. Add something like a 1 Meg resistor from the input to 4.5v. That should sort it out.

[MartyMart]

Slacker is correct, try these "tweaks" :
2m2 resistor from input to ground before the 0.02uf
1m or 470k to + pin 3 from 4.5v
Change the 51k in the feedback loop to a 22k and try a
500KC reverse audio pot for gain, should be much smoother
and smallbear has them.
The 0.22uf/4k7 off pin 2 seems a bit large, unless you want it
that way, I'd try a 0.068uf and 3k3 for less bass roll off.
For diodes in a tubescreamer type circuit, try 1 X 1N4001 one way
and 1 X 1N4148 the other, for slightly assymetrical clipping and some
more volume, but a less "compressed" overdrive, that's a nice combo.
I would also reduce the 0.22 roll off cap pre the second opamp to another
0.068 or even 0.047uf, I did this to my TS9 and it has a nice bright "sheen" to
it now, without getting any nasty "hiss".

Just my 2c's

MM.

[Dave Eason]

aha, thanks guys.  Nice ideas 

I was actually going to swap the 51K for a 22K, but never got round to it. 

[johngreene]

aha, thanks guys.  Nice ideas 

I was actually going to swap the 51K for a 22K, but never got round to it. 

--john

[Mark Hammer]

A Maxon or Ibanez predecessor to the 808 had what was essentially the same elements, but placed the tone-control stage BEFORE the clipping stage.

I've never knowingly heard one of these.  Has anybody here heard one, and what are the audible differences?

[Dave Eason]

interesting that, it'd sound totally different.  Filtering then distorting as opposed to dist then filter. Hmm. Maybye have a switch that flips the order around?!

[Dave Eason]

Here's an update:



I've changed a few things around, and it's acting perculiar indeed.  When you power the thing up, with a tone generator at the input, the sound stays for a few seconds, then peters out; much like effect turning the level up had before.  When measured, Vbias is the same as Vsupp, which indicates a shot in there somewhere to me.  I tried different power supply bypassing caps, but to no avail.  Maybe the 4558D is fried and is shorting internally? I can see no obvious problems with the circuit; it's essentially like the new Tubereamer on run off groove, with the original 808 active tone control.  It's almost circuit a power supply/bias problem.  If you short 9V and ground, it does the same thing as powering up and down; almost like somethings charging up and then making the impedance high, until it'd discharged, but I can't see to work out where.

On another note, has anyone built an Umble overdrive off run off groove?  Looks like a fantastic overdrive project.

Also: is a TL072 pin for pin layout-wise with the 4558D?

cheers
Dave

[JHS]

On the OD-855 the toncontrol and the OD-stage are reversed (buffers used too for silent FET-switching). On all later circuits (with buffers) IBZ used the TS-style configuration.The main disadvantage of the the OD-855 is the artificial, sterile sound.

The OD-2 forerunner has no buffers, nearly the same OD-circuit with the toncontrol placed on the 2nd. stage and no sterile sound at all.

JHS

[Dave Eason]

D'oh! what a silly little thing, after much studying of the circuit this was the problem:



A simple little power supply problem. I just needed to move the resistors around to make the proper divider.  The thing is sounding good now, like a much louder TS9 with more gain, but not all tinny or anything, I'm quite pleased!

But the strange voltage offset effects I was creating in my error were making interesting sounds, I may look into that more.

Dave