Inductorless wah with Inductored wah response - worth pursuing or not?

[Rob Strand]

Here's another angle at it.  This implementation sort of gets at the change in Q with sweep with roughly constant amplitude and is still relatively simple to build:
http://cackleberrypines.net/transmogrifox/TransmogriNotes/skwah/index.html

That's a really nice circuit.  Good match to the response.  I suspect the response is *somewhat* more predictable than the Twin-T.  I'll have to have a closer look at it.

I haven't yet synthesized an inductorless circuit that faithfully reproduces the mathematical equivalent to the inductor wah, except for a gyrator used to emulate a grounded inductor.  With this topology you can get close, but there are other problems with the circuit practically speaking.

The gyrator implementation is too noisy to be used in a performance setting and that is where I stopped last time I messed with this...well,

I went there in early 2000's.  The simple Gyrator forms need a bit of tweaking.  And yep very noisy.
I gave-up on it.

What about a following chain: gyro sensor - > PIC uC -> two matched JFETs (one in control loop, one as a pot)? Stuff like smoothing and min/max position can be tunable, but I don't know if sensor's sensitivity is enough for such an application?

A few years ago that would be crazy but today it might actually be a nice solution.  Not sure what the cheapest gyros are.

[mac]

BTW, State Variable Wah with 2 modes and pot mechanism,

https://www.diystompboxes.com/smfforum/index.php?topic=107603

mac

[Rob Strand]

BTW, State Variable Wah with 2 modes and pot mechanism,

https://www.diystompboxes.com/smfforum/index.php?topic=107603

The mode 1 looks like a good candidate.

I'm happy to leave the Twin-T upto the point where I got it - job done  .   I might get back to it on a rainy day but it's more like solving a cryptic crossword than coming up with a good circuit.  The main issue I have with the Twin-T is it is quite sensitive to the surrounding parts.   The noise performance isn't that great either.

[mac]

I breadborded the CIW with Led/Ldr control and the mods I posted.
The control let me adjust the deepness, sweep speed, angle:
Vcc to 10kB pot (brightness) to 25kB pot (wah) //25k to LED //10kB pot (deepness) to Gnd.

I am thinking of converting to auto wah.

mac

[Rob Strand]

I breadborded the CIW with Led/Ldr control and the mods I posted.
The control let me adjust the deepness, sweep speed, angle:
Vcc to 10kB pot (brightness) to 25kB pot (wah) //25k to LED //10kB pot (deepness) to Gnd.

I am thinking of converting to auto wah.

Do you think it sounds better than the colorsound or just different?

[mac]

Do you think it sounds better than the colorsound or just different?

Same beer on different glasses.

I tamed the peaks and added variable Q control with the first mods, without the opto control.
I'd say it's a bit better, but that's my opinion.

A bit noisy when pot goes to zero, but a cap of some pf at the output helps.

mac

[Transmogrifox]

I know this is off-topic, but I like your sig:

mac@mac-pc:~$ sudo apt-get install ECC83 EL84

customer@brewery-pc$ su
**************
root@brewery-pc: /home/customer# apt-get install keg tap bartender

root@brewery-pc: /home/customer# apt-get install pintglass

root@brewery-pc: /home/customer#  cat /dev/keg/beer/stout | pintglass

[mac]

root@brewery-pc: /home/customer# apt-get install keg tap bartender

root@brewery-pc: /home/customer# apt-get install pintglass

root@brewery-pc: /home/customer#  cat /dev/keg/beer/stout | pintglass

hopefully a new kernel will be released soon:

root@brewery-pc: /home/customer#  cat /dev/keg/beer/cannabis | pintglass

mac

[Rob Strand]

hopefully a new kernel will be released soon:

Have you moved to Ubuntu 18 yet?

I've got a little frustrated with it lately.  I started on 12 and it's just get progressively less reliable over time (weird crashes etc).   I run it largely out of the box with few packages loaded, so nothing to blame.

[mac]

Have you moved to Ubuntu 18 yet?

I installed Lubuntu 18 in the hidden 10gb DELLRESTORE partition of my Dell Mini 10, and left the Windows XP MBR unchanged.
Minor issues, Back to life.
I use it mainly for browsing.

I also upgraded to Ubuntu 18 from 16 on a N455 dual core. I had to change the GRUB menu to display all the stuff Ubuntu loads at startup, if not, it always freezes at the same point. I suspect it has something to do with the new enviroment and the login screen.
Thinking of doing a fresh install on another partition.
This laptop has cheap stuff.
Wine runs fine, can do LTSpice sims. I should try if Photoshop runs but Gimp is ok for now.
I also installed a couple of guitar apps for Jack. Some latency.

mac

[Rob Strand]

I also upgraded to Ubuntu 18 from 16 on a N455 dual core. I had to change the GRUB menu to display all the stuff Ubuntu loads at startup, if not, it always freezes at the same point. I suspect it has something to do with the new enviroment and the login screen.
Thinking of doing a fresh install on another partition.
This laptop has cheap stuff.

Thanks for the tips!    I had feeling there might be some problems changing to the new desktop.

I've been thinking of doing fresh install myself as well.   I'm trying to avoid it.
I have it on a laptop which I use more as a backup computer or to surf around when I'm tired.

BTW, I did try running XP from within Ubuntu via Virtual Box.  Seems to work fine.  You have to install two extra things: one for networking, shared storage and one for USB2 flash sticks.  I doubt something like ripping CD's world work on XP under Virtual Box.

[jatalahd]

I want to add a few words on the actual topic of inductorless wah. Basically nothing new, but something anyways...

The mathematical basis of the Crybaby style inductored wah is the theory of second order resonators combined with the loop gain of negative feedback amplifier. This theory is well explained in 6th edition of Sedra/Smith Microelectronic Circuits in chapters Feedback and Filters/Tuned amps. When a high-pass filter is placed in the feedback loop, the loop gain of the surrounding amplifier multiplies the resonance frequency w0 and quality factor Q of the second order resonator. The change in loop gain maps w0 and Q to different values. This happens only with a second order high-pass filter, if a notch filter is placed in the feedback loop, only the Q is multiplied by the change in the loop gain.

The idea of high-pass filter with gain modulation has been demonstrated nicely by Transmogrifox with his derivation of the wah-pedal transfer function and in his earlier post concerning the inductorless wah using Sallen-Key filter: https://www.diystompboxes.com/smfforum/index.php?topic=112272.0

When using the crybaby direct-coupled shunt-shunt feedback amplifier as the gain element, placing the twin-t filter in the feedback loop misses all the theoretical goodies that are the foundation of the inductored wah (Crybaby-style, there was the Kay-wah as well...). Although the notch filter turns into a band-pass filter in the feedback loop, the idea of gain-controlled resonance frequency is lost. But it is still quite amazing that it can produce quite similar frequency responses as has been now demonstrated by Rob.

I played around the crybaby circuit and simply put in a transistorized Sallen-Key high-pass filter in the place of the RCL resonator. Here is a simulation schematic of the circuit:



I was not trying to get exact copy of the inductored wah, but just to see can it get into the correct ballpark. With the component values shown on the schematic, I get the following frequency response with the loop gain modulation:



The curveforms are sensitive to values of Rin, Cin, R4, C3, C4, RE and Cf. With these components, the curves can be modified closer or further compared to the inductored wah. The downsides are that the transistor needs to have as big hfe as possible in the Sallen-Key filter buffer, and it is also nasty to need the 4.7uF cap in the feedback loop.

If op-amps are not in the list of forbidden items, then the state-variable filter used as a band-pass filter can be most likely tweaked to get close to the crybaby wah transfer curves. The ultimate question is, even if the simulations show good match between inductorless and inductored, will the sound ever be the same cause of the tiny imperfections of the inductor.

[jatalahd]

Finally got myself in a handcrafting mode and built the circuit I posted above with added input and output buffers and some minor component value tweaks. Otherwise quite nice, but as it has been noted by others already earlier, "active" inductance creates lots of hiss in the traditional wah circuit. Probably because the filter is already within a gain feedback loop and has its own gain element amplifying the already amplified noise...

But anyway, since I worked quite hard to get the proto-build ready, I made a youtube video about it. I don't have golden ears nor golden fingers, but the video demonstrates how the circuit behaves in real-life, not perfect by any means.

[dschwartz]

Sounds nice!
But the hiss is too obstructive.
You could try using a low pass filter, like a sallen key or MFB, to reduce it.

[jatalahd]

But the hiss is too obstructive.
You could try using a low pass filter, like a sallen key or MFB, to reduce it.

I tried some simple low-pass filters, but they did not really work. At the moment it seems that the hiss arises when the wah-pot is close to ground connection, but also in this case the wah-filter is at the high end. By putting a 1k resistor to prevent the wah pot to touch the ground helps, but then the sweep range is limited at high end. Also if I extend the range at high end and limit with additional resistor, then the loop gain of the circuit is not enough to cover the low end...

Probably I will still return to this if I get a good idea how to reduce the hiss.

[mac]

Opamp version,





I did not breadboard this.
If it works as expected R4,5 can be used to control peaks and Q.
Note that I added a low impedance adapter to simulate the grounded emitter.

This is an old post about an opamp version. Sadly I can't see the schematic, maybe lost.
https://www.diystompboxes.com/smfforum/index.php?topic=97440

mac