TC electronic SCF

[POTL]

Hi all. I want to change the way the circuit works. 1) What is the advantage of using OTA13600 in Sallen Key filters, versus conventional op amps or transistors. 2) I can not fully understand the switches 4066,

[Mark Hammer]

I have an original one that an old bandmate gave me, after he hung up his bass.  It's decent sounding, but not THAT great that I would expend energy to replicate or mod it.  Its chief virtues are that it can provide 3 effects in one box and that it can be remotely switched (although recent issues use that same jack location for an external power jack, replacing the onboard transformer and power supply).  There are other flangers that sound as good or better, and same for chorus sounds, too.  Plenty of pedals provide similar "stereo" (separate wet and dry outputs).  Given how cramped it is inside, I suspect other pedals would be easier to mod.

[POTL]

Hello I don't think I will ever clone him. I just want to study it and understand how it works.

For the sound of this chorus, I have a wonderful plugin from TC.

However, it has interesting ideas that can be transferred to other devices.

However, yes, I'm still wondering why to put OTA instead of the standard Sallen Key filter configuration.

[ElectricDruid]

I'm still wondering why to put OTA instead of the standard Sallen Key filter configuration.

You use OTAs to make the filter variable rather than fixed.

In this case, it looks like U3.3 is controlled with a voltage (current really) derived from the 3007's clock. So the filter will track the flanger's clock, giving more bandwidth for shorter delays and reducing clock noise by restricting the bandwidth at longer delays.
I note they've only done it with half the chip though. The other half U3.1 has it's Iabc input hard-wired to Vcc with a 36K resistor - which makes sense if you've got half a chip hanging around, but is otherwise a complicated way to make a fixed filter.

[POTL]

Hello.
thanks for the answer. this is a smart decision, roughly speaking the chip itself decides how much filtering it needs. A similar solution is used by the old MXR delays.

It remains to understand the logic of the mode switches.

[ElectricDruid]

It remains to understand the logic of the mode switches.

You mean what each one actually *does*? There's that box on the left hand side of the schematic that tells you the logic (on/off) state of each one for each mode, so *that* part is simple.

[Rob Strand]

You use OTAs to make the filter variable rather than fixed.

In this case, it looks like U3.3 is controlled with a voltage (current really) derived from the 3007's clock. So the filter will track the flanger's clock, giving more bandwidth for shorter delays and reducing clock noise by restricting the bandwidth at longer delays.
I note they've only done it with half the chip though. The other half U3.1 has it's Iabc input hard-wired to Vcc with a 36K resistor - which makes sense if you've got half a chip hanging around, but is otherwise a complicated way to make a fixed filter.

That's certainly the motive.   I like how they implemented the variable filter with just a high-pass filter and a rectifier.

The filter does contribute to the sound of the device, especially chorus units.

I've mentioned it a few times in the past, the Boss CH-1 is pretty much a Boss CE-2 with a wide-band filter.  When the CH-1 came out many people thought it was digital and it got and it got a bad wrap.  The filters use in the CE-2 and the EHX Small Clone are closer to what people expect to hear from a chorus.

[POTL]

Actually I agree filters are very important.

But in addition to them, the overall tone is also affected by the delay time and the balance of dry and wet signals.

CH1 has the same filtering as DC2, but no one perceives DC2 as cold or bad.

The Small Clone is brighter than the CE2 and darker than the DC2, but I've seen a comparison between the DC2 and Analogman where the Analogman with minimal delay time was brighter.

TC is probably the brightest analog chorus ever made, but I haven't calculated its filters yet.

However, TC sounds like it's through a compressor, possibly due to the NE571.
At the same time, the DC2 has no compression feel, but the NE571 does.

Choruses are a strange things.

[Rob Strand]

CH1 has the same filtering as DC2, but no one perceives DC2 as cold or bad.
...
Choruses are a strange things.

Yep, it's hard to make sense of things sometimes.  I guess it comes down to the sound being due to more than one factor.   

There's also the CE1 to throw in there and different again.

[POTL]

The CE-1 is the most overrated chorus in my opinion. By itself, it sounds really interesting, but I saw many comparisons with the same DC-2, TC scf, Analogman, against which the CE-1 seemed very limited in frequency range. The CE-1 also changes its tone perfectly after adding a buffer in front of it.

[Mark Hammer]

It's a decent pedal.  Personally, I think it's most important asset is that it changes the LFO waveshape from triangular (for chorus) to near sinusoidal, for vibrato.  You can easily get vibrato/pitch-wiggle from any phaser, chorus, or flanger by simply lifting the dry signal.  However, vibrato sounds more "musical" and natural when modulated by a sine-like waveform.

I haven't looked at the SCF circuit hard enough to see whether it alters the LFO for the "pitch modulation" (i.e., vibrato) setting.

[ElectricDruid]

I haven't looked at the SCF circuit hard enough to see whether it alters the LFO for the "pitch modulation" (i.e., vibrato) setting.

It looks a bit like it might. There's C116/3u3 filtering the LFO output, but then there's also C115/3u3 which gets switched in by the CD4007, so it certainly tweaks the LFO shape between modes, at least. I just can't work out exactly when that's on.