Pearl OC7 octaver clone and Meridian : trimmer questions

[eh la bas ma]

Hello,

I just finished building this octaver :

https://www.delykpcb.com/product/up-down-daddy-pcb/?attachment_id=1089&download_file=k0sve9jvlkveb

There's a trimmer.

Nothing in instructions about its purpose, I tried to turn it, but I can't notice any change in the sound.



Can you please tell me what is it ?

Similar question about the Meridian :

https://aionfx.com/app/files/docs/meridian_documentation.pdf

I guess the trimmer is supposed to bias the LM13700 pin 3, but how would you do that, I mean what value should I set ?

[antonis]

Can you please tell me what is it ?

IC6D is a half-wave rectifier (by itself) which is turned into a full-wave rectifier by the action of IC6A (a summing amp..)
TRIM100k serves for precisely setting the gain x1 [R15/(R14+TRIM)]..

Further reading: https://circuitdigest.com/electronic-circuits/half-wave-and-full-wave-precision-rectifier-circuit-using-op-amp

[anotherjim]

Balancing the rectifier is almost a lost cause with musical signals, you need a regular and symmetrical waveform to get to the "ideal". A sine wave gives the best chance of doing it. Even then, there's little chance of getting a result that sounds at all "sweet". The problem is the sharp transitions in the narrow pointy bits of the rectified waveform are harsh and not really harmonic which can't very well be filtered out. It's another flavour of nasty fuzz but you can find sweetspots if you're prepared to find its strengths.

[eh la bas ma]

Thanks for your replies !

About the OC-7 octaver's trimmrer, I understand i will need an other kind of signal to hear its action. I will try with an electric keyboard, maybe there are some sine waves in my cheap Casio.

Thanks for the link about rectifier's circuits, I can't understand everything, but I'll keep trying...

From what I can understand, rectifiers are used to convert AC to DC. AC is the guitar's signal IN and OUT. DC is inside the box, in the circuit, allowing to modify the signal, because it's easier and more practical than AC current...right ?

About the Meridian compressor's trimmer, I am guessing that it is controling the "offset biasing" of the LM13700. Probably to avoid background noises, or unwanted distorsion ?

My build isn't noisy, so I guess i can just leave it at 12'...

[antonis]

From what I can understand, rectifiers are used to convert AC to DC. AC is the guitar's signal IN and OUT. DC is inside the box, in the circuit, allowing to modify the signal, because it's easier and more practical than AC current...right ?

Wrong (quite..) 

Full-wave rectification (without smoothing capacitor or inductor) results into an also AC signal of double the frequency of the original signal (predominately case for octaves in pedals..)
(negative part of signal waveform is inverted so we have 2 positive peaks in one period..)

By off-setting 0V axis (via a Vref, say) we have an octave-up of original signal..

[ElectricDruid]

Balancing the rectifier is almost a lost cause with musical signals, you need a regular and symmetrical waveform to get to the "ideal". A sine wave gives the best chance of doing it. Even then, there's little chance of getting a result that sounds at all "sweet". The problem is the sharp transitions in the narrow pointy bits of the rectified waveform are harsh and not really harmonic which can't very well be filtered out. It's another flavour of nasty fuzz but you can find sweetspots if you're prepared to find its strengths.

I've been wondering about this, and whether adding some cross-over distortion ahead of the FWR would help. If there was a flattish area around zero, once the FWR does its job, you've got more-rounded lumps at the bottom of the waveform, instead of the very harsh sharp points you get with a typical signal. Essentially, the problem is that typical signals have a very steep slope close to the zero crossing, so that makes very sharp spikes when rectified. If we can remove that very steep slope, we can prevent the spikes.

Haven't tried it yet. It's bubbling away on my mental back-burner!