Weslie Leslie simulator - simulator helicopter noises

[Ksander]

Recently, I stumbled upon the Weslie simulator at GGG (https://generalguitargadgets.com/effects-projects/modulationecho/weslie-modulator/), which is based on an old magazine article. It is supposed to mimic a Leslie rotating speaker. I built it, and it is a fun modulation effect: an LFO controlled wah.

Then I thought, wouldn't it be possible to build this using just one IC - a TL072 with one op-amp as phase shift oscillator, and the other as a VCF. A 'Onesie' Weslie/Leslie simulator. I'm now experimenting with it.

For the VCF, I used this Tim Escobedo snippet (non-oscillating one):


And for the LFO, I now have this:


I use it with a 9V PSU (not a switching mode one) and have decoupled the power lines near the IC with a 100uF and a 100nF capacitor.

It works, and sounds very similar to the Weslie one. However, there is a 'whopping' helicopter-like sound that is audible also when I'm not playing the guitar. It is obviously from the LFO, as it has the same frequency as the modulation. The LFO and the VCF have their own 100k/100k Vr. I tried adding more decoupling capacitors at various places and of various sizes but it doesn't help.

Any ideas on how to get rid of the noise?
 

[Mark Hammer]

That's the same phase-shift oscillator used as an LFO on the Electronics Australia tremolo.  Moving a bandpass filter around quickly won't nail anything particularly well, but can insert some please jiggle. 

But here's what I'd like to know...

The hallmark of a decent (or at least acceptable when you don't listen too closely) rotary speaker emulator is ramping; the gradual change in speed from slow to fast and fast to slow.  Is there are way for ramping to be applied to this sort of LFO?  The only thing I can think of is RG's "LERA" circuit ( http://www.geofex.com/article_folders/lera/lera.htm ).  Is there any other method?

[idy]

Just a guess... the problem may be having both the LFO and the filter on one chip. This is one design technique you usually (always?) see: LFO and other functions separated, often even different types of opamps, sometimes lower current version for the LFO...
I understand the fantasy that started you on this path, one chip for filter and LFO, but a test would be try same circuit with two TL01s.

[ElectricDruid]

It could be that the noise is DC offset being filtered. It's possible that the DC offset actually changes when the filter cutoff moves - that happens in some filter designs. Eliminating the offset is impossible in such a situation, but you can usually do enough to minimise it and make it usable.

Alternatively, it could be LFO noise getting into the audio circuit via the power wires. If you can, use a separate power supply for the LFO to test this, or at least give each part it's own power wires all the way back to the supply (e.g. *not* the same rails on the breadboard!).

IDY is right that having both parts on a single chip is asking for problems. I know it's nice to reduce the whole thing to just one IC, but in this case, that might be one IC too few

[Ksander]

I've been away for a few days - thanks for the suggestions

The hallmark of a decent (or at least acceptable when you don't listen too closely) rotary speaker emulator is ramping; the gradual change in speed from slow to fast and fast to slow.  Is there are way for ramping to be applied to this sort of LFO?

Do you mean the Doppler effect in general, or the particular waveform of the LFO?

Just a guess... the problem may be having both the LFO and the filter on one chip.

I tested it by separately adding the Weslie transistor LFO. This reduced the whopping somewhat; using the remaining op-amp now to buffer this signal helped a bit more, but the noise is still audible.

It could be that the noise is DC offset being filtered...
Alternatively, it could be LFO noise getting into the audio circuit via the power wires...

I understand the solutions/tests you mentioned for the power hypothesis, but not really what you mean by/how to remedy DC being filtered. Is it DC offset from the Freq pot (in the Q&D filter) relative to Vr? 

[Mark Hammer]

The hallmark of a decent (or at least acceptable when you don't listen too closely) rotary speaker emulator is ramping; the gradual change in speed from slow to fast and fast to slow.  Is there are way for ramping to be applied to this sort of LFO?

Do you mean the Doppler effect in general, or the particular waveform of the LFO?

No change in the waveform or effect, just a discernible gradual increase or decrease in the speed of the LFO.

[ElectricDruid]

I understand the solutions/tests you mentioned for the power hypothesis, but not really what you mean by/how to remedy DC being filtered. Is it DC offset from the Freq pot (in the Q&D filter) relative to Vr? 

Yes, pretty much that's it. In theory, the filter's output should always have a DC level of Vr, with the audio applied on top of that. In practice, it's not uncommon for some of the controlling voltage (from the Freq pot, the CV input, or the LED switching on/off) to modify the DC level. If that happens, and the control signal is fast enough, it adds a "rapidly changing DC level" AKA "an AC signal" onto the output. The DC offset becomes something you can hear. Consequently, you're usually trying to minimise this "DC feedthrough" in a design. This generally involves tying the audio input to ground and feeding the control voltage input (*whatever* that is) with some worse-case signal like brief envelope spikes or a big square wave or something, and then trimming the circuit to limit the output as much as possible. How well that works depends on the design.