diode clippers - OD250 vs. TS?

[dachshund]

What's the difference in placing the diode clippers in the op-amp feedback loop, as in the Tube Screamer,
http://www.generalguitargadgets.com/pdf/ggg_its8_sc.pdf
vs. the diodes to ground after the output, as in the OD250
http://www.generalguitargadgets.com/diagrams/dist_250_sc.gif

Are they accomplishing the same thing? It seems that the amplified signal is getting clipped either way. I do realize the OD250 has asymmetrical clipping, and I imagine one can do the same in the other schematic.

Just something I've wondered about, and haven't tripped over this in searches here.
Thanks in advance.

[PKV]

as I understand it from reading on this site and others (Mark Hammer has written some helpful stuff on this) diodes in the feedback loop are for soft clipping - hence the "Tube" in tube Screamer, and diodes to ground at the output are for hard clipping - OD250, Dist+ and so on.  it's a matter of what kind of tone you want.

[Mark Hammer]

The OD250 does NOT have asymmetrical clipping unless one installs it.  Just wanted to clear that up.

You need to first differentiate the manner in which gain is produced in each instance.  Both devices use a noninverting op-amp as the gain stage.  The OD250 uses the ground leg as the resistance varied to change gain.  The TS uses the feedback resistance to achieve the same goal.  Were there no caps involved, these two approaches would be generally much more equivalent.  Throw caps into the mix and things change.  The OD250 loses bottom as gain is increased, with no change in top end.  The TS loses top end as gain is increased, with no change in bottom.

That's not all that's different between the two, but it's a BIG difference in terms of why they sound the way they do.  Where the diodes go can make a difference above that, but feedback diodes vs grounded diodes will always sound different if you feed them different signals, and in this instance, each diode arrangement always gets a different sort of signal.

I'll let others outline the behaviour of diodes depending on where in the circuit they conduct, but I would be remiss in not pointing out that there's a lot of other things goiong on.  Sort of becomes a bit like saying "His parents escaped from a prisoner of war camp where they were starved while he was yet in his mother's womb.  The birth was difficult and they dropped him on his head.  The parents never fit in, subsequently becoming drug addicts and remaining poor for the remainder of their lives.  They never learned to read themselves and never taught their child either.  The kid caught encephalitis when he was 10.  I wonder why he's so screwed up?". 

It's a worthwhile question to ask about the behaviour of didoes in those positions.  Never forget that there is a whole loty f other things going on at the same time.

[dachshund]

Thanks for the information!

[johngreene]

Probably the easiest way to think about it, diode-wise, is that when the diodes are in the feedback loop, the opamp changes gain when the diodes conduct. The diodes essentially short out the feedback resistor. So the output follows the input for signals greater than +/- .6 v (for the typical diode) and applies the full gain of the circuit for signals near the zero crossings.

Diodes to ground clamp the signal to +/- .6 V, period.

So, diodes in the feedback loop allow some of the character and dynamics of the input signal to get through when it is clipping. Diodes to ground do not.

--john