Deletalab RD1 rock distortion tweak, a little confused

[screamersusa]

For 20 bucks its handy enough to leave in the backup bin.
I modded one a couple of years ago with a minor tone tweak and dropped the gain a tad.
I forgot what I did but I was flying blind anyway..

Anyone got a schematic for this thing cuz I'm confused. I'm trying to
drop the gain to a more usable range somewhere between a TC electronics line driver distortion and
a Rat at lower gain. I dropped a 150k resistor across R5 (158k) which helped a bunch.
I'm confused as to what that diode pack is doing.  The input seems to split to the diode pack and
a unity gain buffer, then they both feed the next stage as if in a feedback loop as it loops back
to the unity buffer?? Huh?? The gain pot is in the feedback loop of the second stage.

[Mark Hammer]

Those Deltalab things that GC sells seem to be boilerplate pedals (hence the price), so I'm guessing it is a clone of something else out there there sells tens of thousands of units.  Any clues as to what we can peg it to?

[screamersusa]

Guess I'll have to keep tracing it.  Doesn't look like a ts or a timmy configuration at first glance.
Not even sure if the diode pack is for distortion or not.

[screamersusa]

This thing is wacky... I need a scanner...
It appears to be a big feedback loop. Vbias enters at the tone control.
I've never seen anything with both +and- opamp feedback loops.
Im lost!!!!
tl062 op amp, 4th input is for vbias like the riot, 
#1 is a unity gain buffer, but its bypasses before the input with the feedback loop from #3
#2 gets input from #3 on + and from #1 as - and i guess mixes the clean out of phase
with the distortion. 
there is a 2.8k resistor from output 1 to the -input of #2. shorting this causes the whole thing
to oscillate so I assume it's a fine balance. Looks more like an amp.
Opamp feedback based clipper?
Couldnt find anything similar schematic wise so far.

[Ice-9]

Post some pictures of both sides of the PCB someone may recognise the circuit.

[screamersusa]

Ive gotta get Image hosting and find a simple schematic proggie.

Looking at it again, it appears to have two signal paths. Input-buffer-differential output (-), and
input to diodes, tone/bias section, buffer, then into the + side of the differential output.
So the input appears to be subtracted from the effected input. 

[Ice-9]

Ive gotta get Image hosting and find a simple schematic proggie.

Looking at it again, it appears to have two signal paths. Input-buffer-differential output (-), and
input to diodes, tone/bias section, buffer, then into the + side of the differential output.
So the input appears to be subtracted from the effected input. 

Easy to use schematic program - use diptrace free
host ya pictures at photobucket also free.

[screamersusa]

Pardon the Fred Briggs type drawing, I'm trying to figure out diptrace.
Here's a basic idea of whats going on in this thing.
Lowering the input level externally has very little effect on the distortion level when
it's at its lowest setting.  Since it's smd I'm not sure if I want to mess with the diodes without
guidance.  I like the timbre of this thing and I'd like to simply lower the minimum distortion level.


UPDATE:  Deltalab does not have a schematic of this thing per Deltalab service.....Lol.
http://imageshack.us/photo/my-images/209/wn5e.jpg/

[/URL]

[screamersusa]

Thanks for the photobucket tip.
Here is what I've been able to trace.
It almost seems like the diode pack may have to be changed to 4 4148's as the lowest setting
is already clipped.  I think teh vias is throwing me off.

[Mark Hammer]

What you've shown so far does not seem to correspond to anything I've ever seen.  It also doesn't make enormous sense...so far.  Having attempted to hand-trace circuits in past, I can vouch for how easily one can pay rapt attention to where this and that trace seem to go to (flipping the board over and back, repeatedly), while completely losing track of whether the presumed connections make sense in the larger picture.  I might note that the same logic applies when checking over PCB etches - if you look only for where there appear to be breaks/gaps in a PCB, you may tend to overlook unintended bridges. They can look normal until you stop and think "Well why would they join those two points together there when they could have easily connected that same path over there?  Those pads probably asren't supposed to be connected."

But back to the matter at hand.  The back-to-back diode pair you show, probably don't go where you show them going.  Whether they are in the feedback loop of something, or going to ground, the threshold at which they begin to clip the signal can be moved upwards for less overall clipping, or downwards for more constantly occurring clipping.

As I've droned on and on about before, "gain" is often treated as identical to distortion by many, but it is NOT the same thing.  One can get wads of gain without any distortion whatsoever (e.g., voice mic preamps), or distortion without any gain at all (e.g., the "Black Ice" passive clipper module).  Having said that, "gain" and distortion" ARE related in that, if you have a circuit that produces deliberate distortion, increasing the gain applied to the signal prior to that stage will result in more distortion.

If one changes the pair of diodes you show, to a second pair that don't start to conduct until a higher signal level is reached, then you will reduce the amount of distortion produced, but will have done absolutely NOTHING to the gain.  Likewise, if you switch them for some diodes that conduct at a lower voltage, you will produce more distortion, again without changing the gain at all.  Paradoxically, use of diodes with a lower clipping threshold will result in more distortion, but lower volume output, because the ceiling on signal level has been lowered without any gain added afterwards to compensate for that drop.

The voltage at which conduction occurs can be the sum total of the diodes used.  So if a single 1N4148 conducts at 612mv and a Schottky added to it conducts at 237mv,  their combination will conduct at 849mv, which is a "higher" clipping threshold.  All of which is to say that you can fish around for some diodes of the same type as are already in the pedal, but at the high end of their tolerance range, OR you can consider adding some other diode types with much lower forward voltage in series with what's already there.  Either of these will reduce the amount of clipping produced, as well as increase the overall maximum output level of the pedal.

[screamersusa]

The circuit baffled me as well. It appears as if the diodes are across the one circuit.
I checked it over and over again expecting to find a familiar pattern. 
I'm going to take the plunge and lift the diodes later to see what happens. I suspect they actually go to vbias
as a ground reference along with the tone control.
It seemed wierd to have the diodes there in parallel with the input of the first op-amp but then, the input seems to
loop over to the negative input of the output buffer while the distortion path enters the positive.
I'll recheck it a 4th time.
Thanks

[screamersusa]

Its making its distortion at the opamps NOT the diodes.
The diodes affect the tone circuit and add a little distortion and compression.
It looks like the diodes are more for limiting than tone.
Different diodes do yield some nicer effects but minimum distortion is
coming from the opamps.
My riot does the same thing.  hmmmm.
With the diodes removed the unit still distorts at minimum gain,
and gets quite crunchy at higher gains.
Only thing I can think of is dropping both feedback resistors on the last stage in half, or
somehow adjusting the vbias?

[screamersusa]

Remove diode pack.
Put 75k across r5, this drops the opamp distortion at min gain.
Change diode pack to something with 4 diodes.
I wound up with a pair of mosfets, a 4148 and a In270.
I'm going to add a switch tonight to put the 75k and a different c4 across r5,
the other side of the switch will switch the original c4 (or a slightly different one) across
r5 (due to the diode change).
That seems to do it while adding a high/low switch as well.
It now goes from an overdrive up through a lightly compressed med rock chord friendly drive in low mode.
It has a less compressed high mode letting your hands do some work.
In addition the output of the pedal is increased which is a complaint I've seen alot.
It should also be more stack friendly in low mode.