Les Modifications De Pédales

placing a high-current-delivery/low-input-impedance op-amp in parallel with a lower-current-delivery/high-input-impedance opamp should give the best of both worlds.

opamp stacking also reduces the noise floor.

Recommendé par barber electronics, (pas n'importe quoi donc), doit etre fait avec un fer pas trop chaud ( 700-750° ) avec une panne avec un dimatre de 0,32.

700/750, t'es sur de ton coup?
A ces températures, il n'y'a plus besoin d'étain!!!!

Jack Orman (www.muzique.com), another highly respected and knowledgeable source about such matter, has the following to say about stacking:

…what you get is increased current drive. Typically, a small value resistor, 10-50 ohms is inserted in the output of each opamp before they are combined to the signal output. This ensures that the opamps share the drive more equally. Using a pair of opamps in parallel will actually decrease the noise by about 3db due to random cancellations. Four opamps in parallel would reduce it even further, but that’s about the practical limit.”

So stacking sounds like an interesting approach to use in the ScreamerLab project. But it has one drawback—basically you are getting a composite of the tone of each opamp summed together. I.e. you are not getting the ability to “switch” between opamps. So how do we get the benefits of stacking and the benefits of switching? This piece of the puzzle was provided by Keith of BuildYourOwnClone.com in one his forum posts:

“Pin 1 and pin 7 are the outputs of each op amp. Wire both so they share all the other connections, but use a switch to connect or disconnect these 2 sets of pins. It could be done with just a DPDT.”

Bingo! Now we have the ability to do both stacking and switching. In essence, the ScreamerLab will have three opamps stacked, but pins 1 and 7 of each opamp will remain open. We will then use a 4PDT switch to change between the inputs and outputs. The fourth position of the switch will connect pins 1 and 7 into the overall opamp stack.
Opamp Design

The ScreamerLab includes three opamps: the JRC4558D, the RC4558P, and the LM833 in a stacked, switched configuration. The JRC part was chosen because it is the most common stock Tube Screamer chip, the RC4558P because it is widely used in the boutique world, and the LM833 because it is a more modern, low-noise part. The low-noise characteristic is important when used in conjunction with the Distortion Boost Mod (discussed later in this document). The DBM introduces noise, so the LM833 can be used to mitigate the increased noise.

One TLO72 and one Op275 the TLO72 is very clean with the whole guitar frequency spectrum. The OP275 is even more cleaner for guitar applications but I have found that it focus the bass more and there seems to be more clarity.

In conclusion you get a tone that is scooped with awesome distorion, the lows are great and the highs but the mid is lowered.

If you do this with 2 4558 you will get more focused bass but I think that there is less high contend therefore, I did not like this mod.

Robert Keeley
Boss Stacked SD-1 with Diode Switch

Description
New as of August 2007--we're introducing a new feature now included in our SD1. We're now stacking two TI4558P chips. These two TI chips work in parallel, mounted on top of each other and soldered together. Many hi-fi stereo enthusiasts enjoy this technique for noise reduction (read The Art of Electronics for some really technical data!) and it’s been done in a couple of other boutique effects too. The same Keeley SD1 tone is still there, now with noticeably BETTER DISTORTION, LESS NOISE, ARTICULATION and TRANSPARENCY! This tone is so good we've decided to stop including the Burr Brown chip. (Don’t feel like you’ve missed out; this new version and technique is truly better sounding in the SD-1.)

Then try "stacking" them - my personal favourite is an OPA2134/LM1458/LM833 stack.

En mettant 2 fois la même?