Sonic Distortion Build Report #2!!!!

[RDV]

This is just plain weird. My SD-9 clone comes on like a tube amp. I've tested it over & over, but after power is applied it takes about 5 seconds to come on, after that you can turn it on & off with the footswitch just fine. I have no explanation what-so-ever. I'm flabbergasted, I'm discombobulated, I'm confused, the room is starting to spin, I'm seeing spots! Just kidding. :wink:

Has this happened to anyone else??

RDV

[aron]

I had something similar but I have no idea what it was. It never happened again.... I wish I knew although I always hear about reversed polarity caps, but I swear I looked for those.

[Peter Snowberg]

If you have a partially blown current limiting resistor on the power feed that sits in front of the reservoir cap you could see that kind of effect. If the circuit draws very little and the cap is large enough it will take a few seconds to charge and the voltage will slowly rise in the process. the cap with keep the power supplied to the pedal and it gets "trickle charged" by through the resistor. I can understand it happening consistently, but only once? :shock: Must be Elvis.

Some commercial pedals have a low ohm resistor between the power jack and the reverse protection diode. If your pedal has one, check that with a meter and see what it says. It might be been over-stressed by a reverse battery or adapter at one point.

Take care,
-Peter

[RDV]

I've found out why the pedal was taking so long to come on. I had the 22uF/470 ohm combo in the feedback loop going to ground instead of Vr.

That said, now I don't like the sound nearly as much(with it corrected) as I did before. The Eric Johnson sound comment no longer applies, though it now has a bit more range to the tone control.

I now want to know what I was doing to the circuit when I ran the cap/resistor combo to ground instead of Vr. Did it make a filtering change? It sure did sound good! I'm going to make it switchable I think. If no one responds in this thread, I'm gonna start a new one, cause this seems a bit profound.

RDV

[Doug H]

Weird... Theoretically it shouldn't make a difference. the 4.5V rail is still AC ground as far as the signal is concerned. Some have mentioned that there is a slight tone difference but I wouldn't expect anything this dramatic. I thought the reason they tied it to 4.5V was for convenience. Have you tried flipping the polarity of the electrolytic cap? How about the filtering on the 4.5V rail? Do you have a big cap (47uf or so) from the rail to ground? That will be necessary for establishing a good AC ground.

Something tells me there must be something else going on.

Doug

[RDV]

I always assume that I've made a mistake. I just checked my ground rail's integrity(& all grounded points) which was good. I also checked again that I had my electros polarized correctly. I definitely have a 47uF going from Vr to ground.

It's not that the unit is malfunctioning, but that there is such a different feel when the 22uF/470 is going to ground rather than Vr. Maybe something is out of whack somewhere, but I sure can't seem to find it. I still don't know why it was taking 5 seconds or so for the circuit to power up when I had the 22uF/470 to ground raher than Vr.

My only deviation from the schem @ GGG was a 1meg pot for gain, but since it's wired as a variable resistor it shouldn't make any difference, right?

Oh Well :roll:

RDV

[Paul Perry (Frostwave)]

In my experience, whenever something takes time to come on, it is a bias problem, with a leakage from somewhere charging up a cap to allow the circuit to operate.

[Peter Snowberg]

I wonder if this has something to do with the impedance of the Vr connection? This might be one of those places where a TI rail splitter chip or a 386 rail splitter (R.G.'s trick) would improve the tone.

I would try an active Vr generator or even buffering the passive Vr generator with a nice low impedance opamp like a 5534/5532 to see what that does. Another trick you could try would be to lower the resistor values in the Vr generator and up the size of the stabilizing cap.

Interesting.

Take care,
-Peter

[RDV]

I'm going to reheat the joints around the Vr splitter....damn perf.

RDV

[RDV]

With my DMM hooked to Vr and I plug the power in, the v immediately comes up to where it's supposed to be. I also used a 2N4401 instead of the 2N2222a that I had in there, no difference. I'll try another OA instead of the TL072 I suppose.

The only thing that makes it power up instantly is having the 22uF/470 going to Vr instead of ground, so far. GGRRRR!

RDV

[RDV]

I tried it live tonight with the FB loop still taken to ground. I didn't like it, wasn't really able to use it, the output was very low, very, very, low. The EQ is so hollow that it practically disappears. I think what made me like it so much in the house(big bottom and top with no mids) through a JRC386 made it lousy with the Marsnall(broken h). I'm going to put the 22uF/470 back to Vr & try it again tomorrow night. If that doesn't work I'm going to make a gain stage out of the unused OA side.

ARIVERDIRTY

RDV

[RDV]

Today I made some major mods to the circuit.

1. I put the FB loop cap back to Vr(or Vbb as GGG calls it)
2. I changed the cap in the tone circuit from .027uF to .033uF
3. I used the unused half of the OA to create a volume recovery stage with a gain of 3, inserted between the output of the tone control(pin 2) & the input of the volume control(pin 3). I used .68uF caps on the in/out of the stage & a 1uF to ground in the FB loop.
4. I removed the 1k to ground on the volume pot(pin 1) so I can go to absolute 0

The pedal is now very usable. I used it tonight at the gig with very few troubles. I get unity gain with the volume at 12:00, whereas before I don't think it ever really got there(had a very thin sound anyway). The tone control change seemed to help also, where before it went from all bass to squeaky treble, there are now some mids there(though I think I'm going to try a .047 there for even less high end. It also cleans up with the guitar volume 100% better than before as it used to just kill all the highs when you backed it off. Now it just cleans up almost like a good FF and retains the highs really well.

Another place I'm going to do something different is the gain pot. I put a 1Meg there cause it was sort of what I had at the time. This is WAY too large. All the good stuff is with it all the way off to about 10% open. I'm going to try a 100k, and if that doesn't give enough gain, I'll order a 250k as per GGG's schem.

I see why this pedal was discontinued, it sounds very good, but can be frustrating in it's stock configuration. With an added gain stage it's happening!

Stay Tuned

RDV

[RDV]

One more "late night mod". I changed the .1uF cap to ground in the tone circuit to a .047uF to see if I could tighten up the bass response. It seems to have worked pretty well, but then I've only tried it through the JRC386 amp.

Modded schematic:



YAWN

RDV

[Dan N]

I've found out why the pedal was taking so long to come on. I had the 22uF/470 ohm combo in the feedback loop going to ground instead of Vr.

Those DO go to ground on the real units.

[RDV]

I had the 22uF/470 ohm combo in the feedback loop going to ground instead of Vr.

Those DO go to ground on the real units.

I read your post on this a while back, which was why I tried it to ground instead. It didn't work too well, though I'm certain that you're right about your particular unit.

Regards

RDV

[Dan N]

I just double checked. The cap is a tantalum marked R22. Doesn't that mean 0.22?

Anyway, sounds like you are having fun tweaking!