Dumbloid

Post by **The G** » 07 Mar 2014, 00:05

juansolo wrote:No urge to build one... Does anyone actually like the sound of op-amp clipping?

I do. Op amp clipping is far more common than you think. Even if it's followed by diodes or whatever different clipping stages, op amp clipping still contributes a lot to the final sound. Some examples are Proco Rat, Marshall Guv'nor, Boss Metal Zone and other Boss distortions, Barber Electronics Dirty Bomb, G2D Morpheus, MI Audio Crunch Box and Tube Zone, Suhr Riot, and even the Klon Centaur at max gain. Then there are Sansmap Classic and GT2, Crowther Hotcake and Okko Dominator, which rely solely on op amp clipping.
What I don't like at op amp clipping is the ugly jitter when the voltage borders the clipping threshold and the sudden transition from clipped to non-clipped and vice versa.

Cannibal wrote:The biasing resistor of the first buffer is only 150k, this thing has a pretty low input impedence. Do you think that's a deliberate choice or just a mistake by the "designer"?

Could be deliberate, to shave some highs if you guitar has passive pickups.

MoonWatcher · Post by **MoonWatcher** » 07 Mar 2014, 01:32

ggedamed wrote:Op amp clipping is far more common than you think.

What is typically less common are deliberate attempts to shape or harness it.

Choices of clipping diodes and how they are configured either oftentimes appears to be arbitrary or chosen by some empirical method. Or, it's the standard germanium at one end of the spectrum, "silicons in the middle," and LED's at the other end.

If the characteristics of diodes are not well understood by many, it would make sense that attempting to shape the clipping with the op amp itself would probably be a bit of a luck of the draw.

Lots of pedal tweakers don't even know how one chip may differ in circuit construction versus another, and why something with a stupidly low slew rate might be preferable, or what the semi-unique characteristics to the OP275 are (both design-wise and performance-wise).

I think that op amps are also typically chosen for certain purposes because of performance factors that minimize unwanted distortion, provide good input and output impedances, a low noise floor, and generally allow for a reduced parts count compared to using discrete amplifier designs. So from a scratch-building method, they are probably typically considered for use as input and output buffers, or for active EQ circuits. They obviously are great for booster designs as well, so even in a pedal made to distort, it's not uncommon to see them used as the initial boost stage, but less common to see them as a subsequent stage driven into distortion (sans clipping diodes).

If you look at the BJFE/Mad Professor designs and how negative feedback is used to drop the clipping for particular frequencies, it might be possible to push the idea outwards to a design that would not require diodes. I know that the Hotcake was mentioned as being such a pedal, but it could go a bit further to someone interested in it.

But the Dumbloid just doesn't seem intentional at all with its lack of diodes (except for one), beyond probably just random experimentation.

andregarcia57 · Post by **andregarcia57** » 07 Mar 2014, 01:34

I'll ride on a PCB the tube screamer tonepad (dumbloid mods)
the schematic is correct?
thanks

Whalestone · Post by **Whalestone** » 07 Mar 2014, 09:40

Cannibal wrote:The biasing resistor of the first buffer is only 150k, this thing has a pretty low input impedence. Do you think that's a deliberate choice or just a mistake by the "designer"?

Note that the Dumbloid manual lists input impedance as 1MΩ.

IvIark · Post by **IvIark** » 07 Mar 2014, 09:56

He may think the pulldown resistor sets input impedance. Or maybe he measured it by throwing a dart at a page of random numbers.

IvIark · Post by **IvIark** » 07 Mar 2014, 09:59

andregarcia57 wrote:I'll ride on a PCB the tube screamer tonepad (dumbloid mods)
the schematic is correct?
thanks

Looks right to me. Except the value of the Accent pot.

andregarcia57 · Post by **andregarcia57** » 07 Mar 2014, 14:22

IvIark wrote:
andregarcia57 wrote:I'll ride on a PCB the tube screamer tonepad (dumbloid mods)
the schematic is correct?
thanks
Looks right to me. Except the value of the Accent pot.

ok, is 25k! correct.
now my doubt is only going to know where jazz-rock? R16 or R19?

amplifiednation · Post by **amplifiednation** » 07 Mar 2014, 17:44

The R/J switch is a .47uf cap switched to ground (when in Jazz) that is in the second position (off the 100r) on your TS mod schem.

Ripdivot · Post by **Ripdivot** » 07 Mar 2014, 18:51

amplifiednation wrote:The R/J switch is a .47uf cap switched to ground (when in Jazz) that is in the second position (off the 100r) on your TS mod schem.

Is the R/J cap on the i/p or o/p side of the 100 ohm resistor?

andregarcia57 · Post by **andregarcia57** » 07 Mar 2014, 19:57

amplifiednation wrote:The R/J switch is a .47uf cap switched to ground (when in Jazz) that is in the second position (off the 100r) on your TS mod schem.

is now correct J / R switched ?

amplifiednation · Post by **amplifiednation** » 07 Mar 2014, 21:09

Yeah you got it

IvIark · Post by **IvIark** » 07 Mar 2014, 21:33

Thanks for confirming AN

mmolteratx · Post by **mmolteratx** » 07 Mar 2014, 22:55

Did up a PCB since a buddy wants one. Once it's verified, I could do a small run. Price would be $5 plus shipping.

pappasmurfsharem · Post by **pappasmurfsharem** » 07 Mar 2014, 23:49

mmolteratx wrote:Did up a PCB since a buddy wants one. Once it's verified, I could do a small run. Price would be $5 plus shipping.

Those switch holes big enough? Or is the expectation to use PCB mount SPDT?

mmolteratx · Post by **mmolteratx** » 08 Mar 2014, 01:03

pappasmurfsharem wrote:
mmolteratx wrote:Did up a PCB since a buddy wants one. Once it's verified, I could do a small run. Price would be $5 plus shipping.
Those switch holes big enough? Or is the expectation to use PCB mount SPDT?

PC mount. And a specific type at that. http://www.mouser.com/ProductDetail/CK- ... kyBbrr0%3d

ppluis0 · Post by **ppluis0** » 08 Mar 2014, 01:07

ggedamed wrote:Op amp clipping is far more common than you think...

Hey folks, perhaps this unit employs an opamp similar to TLC2262 that can be driven rail to rail and still sound like any Cmos inverter ??

Post by **pamaz** » 08 Mar 2014, 07:35

I would be in for 3 pcb if they are going to be made.
Paolo

astrobass · Post by **astrobass** » 08 Mar 2014, 08:41

ppluis0 wrote:
ggedamed wrote:Op amp clipping is far more common than you think...
Hey folks, perhaps this unit employs an opamp similar to TLC2262 that can be driven rail to rail and still sound like any Cmos inverter ??

It's not. If it's the OPA2134 per the schem, that IC's datasheet indicates that its output voltage doesn't go all the way to rails.

johnk · Post by **johnk** » 08 Mar 2014, 19:44

I built one using IvIark's rev2 layout. I didn't care for the OPA2134's fizzy decay , especially noticeable at the brighter settings, so I tried 30 different dual opamps in it and the MC33172N sounded great in it so I went with that.
I also added the Jazz/Rock switch to it so it's now a 'special'.
here's a couple of pics of mine in 1590B:

andregarcia57 · Post by **andregarcia57** » 08 Mar 2014, 19:59

johnk wrote:I built one using IvIark's rev2 layout. I didn't care for the OPA2134's fizzy decay , especially noticeable at the brighter settings, so I tried 30 different dual opamps in it and the MC33172N sounded great in it so I went with that.
I also added the Jazz/Rock switch to it so it's now a 'special'.
here's a couple of pics of mine in 1590B:

[ Image ]

[ Image ]

very lovely pedal!

I find where to buy this model led?

Dumbloid [traced]