Small Clone version 3207

One pedal I always wanted to build was a chorus, a lot of people don't like this effect, they think the 80's were “corny”, that we already had enouth chorus from a single decade, etc, etc... but no matter I'm a fan of this effect ... and from all the chorus I have heard no one sounded that good as the Electro-Harmonix Small Clone ... they speak much about the Boss Chorus Ensemble (the fist one) ... but I think it very boring, without that dimension and depth that Small Clone has ...

Among the obstacles to setting up a chorus there was the complexity of the circuit (one of the most difficult to build, with more components and therefore easier to commit mistakes) and also, and more important, because of the difficulty of finding the right components (BBDs) and the expensive price of these components (we are talking about Brazil...).

Then looking for one cheaper alternative I found the Old Monkey Chorus one adaptation developed by Gleiber (the 'Macaco Velho' from a brazilian diy forum). He did one circuit based in the MN3207 IC who mimicked the filters used in the CE2 and at the same time was a circuit simple, cheap and affordable (the Gleiber's article is very good and very didactic, and explains how chorus and flanger circuits work. Read this article is highly recommended!)

My PCB of the Old Monkey Chorus.

Throughout the building of this pedal I had several problems, and I redid welds several times... the pedal kind of sounds good, but because of I have done many changes on the same PCB it not gets 100% and even adjusting the trimpot he kept with one ocilation noise.

After that I researched various chorus circuits, Luix Chorus (from a Italian DIY site), Zombie chorus (interesting project of John Hollis), older Electro Hamonix chorus schematics, etc. .. my goal was to get a chorus that would sound similar to the Small Clone, but with the BBD that I had access, the MN3207 (R$7.00) ...

The problem of build a chorus have always been the expensive and rare BBDs (again I'm talking about Brazil)... The Small Clone uses only the MN3007, the CE uses the MN3007 and your partner MN3101 (in the Multicomercial store they are respectively R$ 15,00 and 13,00)

There is much discussion about which BBD is better...

Electro-Harmonix already fabricated chorus using many types of BBDs according to the market availability: (these following data were taken from their respective Data-Sheets)

SAD1024: 16pin chip, with two independent 512-stage delays, vdd +10/+15/+17 V;

MN3007: 8pin, 1024-stage, vdd -14/-15/-16 V, Total Harmonic Distortion 0.5/2.5 %, Noise Voltage 0.3 mVmrs 

MN3207: 8pin, 1024-stage, vdd +4/+5/+10 V, Total Harmonic Distortion 0.4/2.5 %, Noise Voltage 0.25 mVmrs

The SAD chip is extremely rare and virtually no longer used in chorus... the Electric Mistress flanger still uses the BBD.

Among the MN3007 and MN3207 there is a great controversy about MN3007 would have more headroom, less distortion, etc ...
But what we can see through datasheet is that they do not have major differences. And that the 3007 is made ​​for high voltage (15V) and in a usual 9V supply (common to the most of pedals), it operates below the ideal voltage and then ends up not with the performance which it was designed to work. Since the 3207 operates with 9V, it works within the voltage range which it was designed to operate ... so all this discourse about 3007 is better than 3207 ends up being a big myth... like many others myths existing in the music world ...

Looking schemes like an adaptation of Zombie Chorus to use the chip MN3207, the Old Monkey Chorus, and a version of the Small Clone SAD1024, I worked out a version of Small Clone to use MN3207 (which how we saw before is a chip more affordable and that works better at 9V).
The change for the circuit acceptance of MN3207 is just a matter of adjusting the IC connections and replace the negative input of the MN3007 by a positive value and adjust the voltage arriving at the IC into +5 V.

 

Schematic of Small Clone version MN3207

Small Clone 3207 pcb layout

(i finish this post later...)