OCD Question

[Frank_NH]

Hello all,

As the quest for the ultimate Marshall style overdrive continues, I am currently building/testing a vero OCD v3 overdrive.  For those who are familiar with this circuit, I have a question...

My build appears to work fine - gives me lots of gain, so much so that I'm using 500 K pot for the gain control rather than 1 Meg.  My question concerns the MOSFET clipping.  I am using sockets for the 2n7000s so I can try out different MOSFETs.  By accident, I discovered that I can remove the 2n7000s and the circuit still works!  After looking at the schematic, this is not surprising since the clipping doesn't interfere with the main signal path between op amp stages, and so what I am hearing is op amp distortion.  However, I don't hear much (if any) difference in the sound if I have the fets in there or not.  Is this normal?  Perhaps the clipping is subtle, and/or occurs only at lower gains.  The voltages around the clippers and the IC seem normal (in the 4.3 - 4.5 V range).  I am also aware that the clipping occurs relative to Vr = 4.5 V rather than ground (like you have on the MXR Distortion +).  Does this make any difference?  Anyways, if anyone has some insights, let me know.  I'll post my findings after I've had a chance to work with it a bit.

If this circuit doesn't work out, I'll move on to the Super OCD...  

[IvIark]

Yes the circuit will work fine without them.  The mosfets are just used like shunt diodes to ground in the Rat and so when you remove them it doesn't break or affect the signal path at all.  Without them you're getting all opamp gain/clipping, and adding them should certainly make a difference, although the difference may be less significant with the lower gain pot when the signal from that opamp stage is reduced, and so there is less for the mosfets to limit and clip.

[Frank_NH]

Thanks Mark.  I'll do some tests with the 1 Meg gain pot to see if I can detect any difference with/without the clippers.  I would have thought that the MOSFET clipping would be much more noticeable.  In contrast, the Crunch Box gets most of it's personality from its LED clippers, and so far it is a superior circuit than the OCD for Marshall amp-like tones.

I may also try some different ICs.  Right now I'm using a LF353N, but I have some TL072s and other dual op amps, so maybe that will make a difference.

[Frank_NH]

I did some testing tonight.  Since I had socketed the 2n7000s, I was able to put in some 3mm yellow LEDs.  I hit a chord and sure enough, the LEDs lit up, so there is in fact some clipping going on!  Actually, I decided I **liked** the LEDs better than the MOSFETs so they'll stay!  I'm also getting some oscillation at high gain, but that can be cured easily by increasing the 220 pF cap at the first gain stage.  Finally, I'm tweaking the tone to preserve some treble at higher gains, as the circuit can get bassy and wooly - and there's plenty of gain to be had, even with a 500 K gain pot.

Tomorrow, I'll try out some different ICs...

[IvIark]

Yes the Crunch Box was much better for me, going higher and more saturated gain if you wanted, and I found low gain was more pleasant too.  I ended up selling the OCD (V4) because I just could never see myself using it.

[Frank_NH]

Well, I'm putting the OCD aside for a while.  The LED clippers are fine, but the tone is too muddy unless the tone control is up full.  It needs a SWTC but I'd rather try something different, since the Crunch Box is much superior and has the same Marshally vibe, and a bit more gain.

I may try a CD4049UBE circuit next as I have over 20(!) of those ICs (impulse eBay purchase ).

[IvIark]

Miro was telling me the other day how much he likes the Hot Tubes so that may be a good one to try.
And a Great Destroyer and 18 Red Llama's
I know what you mean about impulse buys though, I'm exactly the same.  I can't seem to buy 1 or 2 any more, I buy random ICs in bulk.  Check out this seller for a spending spree

[rocket88]

haha. mark that's where i've been ordering all my IC's from. and yes bulk is way to tempting, especially at those prices. like i'll need 1 of a specific chip and next think i know i have 50. i got an order from there where i spent like $30-40 and it showed up with 500 IC's. i didn't even remember placing the order and had to go back into all the layouts and figure out why i bought them

[Frank_NH]

Well - here's my 4049 story...

I was building the Runoffgroove UBE Screamer and had purchased one 4049 online for the project.  I actually used my own vero layout (first time!) so I new the circuit like the back of my hand.  Well, it didn't work!  So I went over every connection with a fine-toothed comb.  I then used my voltmeter to trace the signal, and for some reason the output of one of the inverters was way too low.  I thought it may be the chip...so...I went on eBay and saw a Buy It Now for 25(!) RCA 4049s for a great price.  I thought what the heck, maybe one of those will work.  Sure enough, after popping one of the new 4049s into the circuit, it worked like a charm!  

So now I have 24 left over chips, but there are a lot of great circuits, including the ROG 22/7, Double D, Three Legged Dog, Mr. EQ.  Also, as you mentioned the EHX Hot Tubes and WH Red Llama would be fun.   Still, I have more 4049s than I'll ever need.

(Maybe we need to have a swap meet...)