Wet Mod on AD3208

[nosamiam]

I'm going to be building an AD 3208 and I was wanting to add a mod.  I've messed around with my friends Electra EP-150 analog delay and it has a feature that doesn't come on the stock AD3208:  It has a wet/dry pot.  That's not the name on the pot, but basically it gives you the option to have no original signal, leaving only delayed signal.  Creates a pretty cool effect.

I'm getting a really big breadboard, but since I don't have any breadboarding experience I wouldn't want this to be my first try at it.  That's a pretty big circuit.

Any, does anyone know how to add this capability to the AD3208?  Is it a simple mod, maybe done with mostly offboard wiring?

[markm]

If I'm not mistaken, the Delay Level pot is actually a "blend" control that mixes wet + dry signal.
Wow.....this is my first post after lurking here for over a year!!

[Mark Hammer]

The delay level pot is actual JUST a delay level and has no impact on the dry signal.......but keep posting anyways, markm.  Who knows, maybe the name is lucky!

The wet and dry on the AD3208 are mixed in that last op-amp stage before the output.

That op-amp stage is configured as an inverting opamp.  Normally, the gain of the signal entering such a stage is dictated by the ratio of the feedback resistor to the input resistor.  You can see that for the dry signal, which leaves point A from the input stage, and goes to point A in the output mixing stage, there is no gain added when the mixing occurs (10k divided by 10k = 1).  The wet signal, because of the gain added to it along the way, has in input resistor in that last stage of 22k.  As you can see, 10k/22k is <1, so the gain is turned down a bit to compensate for that added gain along the way (presumably coming from the filter Q plus the companding, not to mention the regen adding to signal amplitude).

One thing you could to insert the desired wet-to-dry blend function would be to use a dual 50k linear pot, wired opposite for the wet and dry signals.  Point A from the input stage then goes to a 1uf cap and to the input lug of the "dry" half of that dual pot.  Now, when the pot is centred, equal proportions of wet and dry hit the mixing stage.  Rotate the pot one way and dry increases (while wet decreases).  Rotate it the other way and the opposite happens.

The caveat to this is that you may lose a bit of level as a result.  The solution to that is to replace the 10k feedback resistor in the last stage with a 10k resistor in series with a 25k pot (wired as variable resistor).  This would allow you to kick the output level up a notch to compensate, or to even get an output level greater than bypass.

[markm]

Thanks for the welcome Mark.
You have helped me in more than one instance I can assure you!
(the search function on this site is Great!!)
I tried, I'm just a Country Tele Picker that was tired of "crappy" store bought effects and began
building my own with very good results thanks to this site!
Thanks to all,
    MM

[nosamiam]

Thanks guys! I'm starting to understand this stuff a little!  Looks like a pretty simple mod with few to no changes to the PCB.  Just what I was hoping for.

I don't get how more resistance ({10k + VR in series} > 10k) = increased gain, but I'm sure I'll figure it out some day.  I'm still new to this.

[Mark Hammer]

I don't get how more resistance ({10k + VR in series} > 10k) = increased gain, but I'm sure I'll figure it out some day.  I'm still new to this.

a) You're quite welcome.  Half the fun of this place is reaching a hand in and pulling folks out of a dark pit into the light.

b) Think of an op-amp as being this high performance sports car that has no gas pedal, only brakes.  The default, when not using the brakes is to pump that engine with gas and drive at 300mph.  Think of the feedback in an op-amp as how hard the brakes are stepped on, or how much less than max speed the car/opamp is forced to drive.  If you have absolutely nothing in the feedback path between output and input, the op-amp will produce a certain amount of gain.....a lot, actually, full tilt.  If you provide a path from the output back to the input, that signal will have a "negative" effect and reduce the gain which is normally provided by a completely open circuit.  The smaller/lower that resistance is, the more of that negative feedback occurs and the harder the "brakes" are stepped on.  So less resistance = less gain.  Conversely, the closer to an open circuit it is, the greater the gain (more resistance = taking your foot off the brakes).

For an inverting op-amp, the gain is a function of not just that feedback resistance, but the input resistance as well.  So feedback/input resistance of 100k/10k produces a gain of x10, but so does 47k/4k7 and so does 1M/100k. 

In the case of a noninverting op-amp, it's calculated differently.  Here, we think of the feedback resistor, and resistor to ground from the '-' input as behaving like a volume pot, with the '-' input acting like the wiper of that pot.  And just like a pot, it is the proportion/segment of the whole pot resistance on each side of the wiper that tends to matter.  Note that whenever you turn up any volume pot, regardless of resistance value, at max volume the resistance between wiper and ground is at max value for that pot, and at min volume the wiper is at or very near ground.  In the op-amp context, making the ground side of the "wiper" ('-' pin) a very low resistance means that most of the feedback signal coming into our "pot" is bled to ground.  In a sense we have turned the volume of the feedback signal from the output way way way down.  And as you may remember from a moment ago, that means that the "brakes" are not stepped on as hard and the op-amp starts to move in the direction of open-loop gain.  Increase that resistance to ground so that the feedback resistance is now not quite so big by comparison, the the amount of feedback making its way to that '-' pin is increased, stepping on the "brakes".

Since gain depends on two resistors in both the inverting and noninverting cases, you can alter gain by changing the value of either of those resistances.  In the MXR Dist+ and DOD250, the gain is changed by making the resistance to ground higher and lower.  In the Tube Screamer and the Rat, the gain is altered by making the feedback resistance higher and lower.  In the Mutron, the inverting input stage uses a pot as a variable input resistance on an inverting stage, keeping the feedback resistor constant.

A lot of this is nicely explained in those old issues of DEVICE that I scanned and posted on my site.: http://hammer.ampage.org  Take a look around page/screen 10 or 11 to find the links.

[nosamiam]

 

Wow Mark, my head is really spinning.  But as always seems to be the case, I'll read it a couple more times, then it'll be crystal clear.  Thanks SOOO much for the detailed explanation.  I'm learning a LOT!  Can't wait to get that breadboard!