What is this oscillator?

[samgraysound]

What is this oscillator circuit? How does it work? It is for an LFO for the tremolo section of a soviet Effekt 1 Fuzz-wah-tremolo-autowah pedal.

It looks like a twin t oscillator but without the low-pass filter and transistors instead of an inverting op amp? I think the 1st two transistors are the oscillator and the 3rd is just an amplifier. The lead coming off the left of the circuit goes to a variable resistor to ground.

Sam

 

[JohnRoberts]

I need more data... like more of the schematic.

JR

 

[samgraysound]

Here's the full schematic. https://www.offsetguitars.com/personal/jay/baybay_schem.jpg

It's a mess of a schematic. The oscillator is in the top left corner. I'm thinking it's a bridged T now.

 

[JohnRoberts]

may be a phase shift oscillator but not tonight, maybe tomorrow when I'm sober...

[edit] being sober doesn't help... schematic is hard to read. Good luck maybe look for bad solder or faulty component /edit]

JR

 

[samgraysound]

Oh it works. I just want to understand it.

 

[JohnRoberts]

Perhaps consider redrawing the schematic more clearly.

A phase shift oscillator involves stacking several RCs in series so the cumulative phase shift supports oscillation. But still unclear from that drawing... there should be some overall feedback connection from the end of the several RC poles back to the input.

JR

 

[abbey road d enfer]

I've never seen a phase-shift oscillator working with only two RC's. Very odd.

 

[JohnRoberts]

I've never seen a phase-shift oscillator working with only two RC's. Very odd.

tag you're it....

JR

 

[groovyloop]

I've never seen a phase-shift oscillator working with only two RC's. Very odd.

Any amp become an acsilator only with 1 capacitor between input and output. So they just want to sell more capacitors)))

 

[JohnRoberts]

I've never seen a phase-shift oscillator working with only two RC's. Very odd.

The last sine wave oscillator I designed (in the 80s for the Loftech TS-1) used 2 poles in a typical SVF (state variable filter) configuration.

JR

 

[abbey road d enfer]

Any amp become an acsilator only with 1 capacitor between input and output. So they just want to sell more capacitors)))

Not with an inverter. This type of circuit usually use three RC's.

 

[abbey road d enfer]

The last sine wave oscillator I designed (in the 80s for the Loftech TS-1) used 2 poles in a typical SVF (state variable filter) configuration.

Indeed. But the SVF filter has non-inverting open-loop gain. Here I don't see how the single inverter stage can oscillate.

 

[groovyloop]

Any amp circuit has R (input and output resistance) and C (input and output capacitance) and any amp has a phase shift on some frequency, so only 1 right value capacitor needed to turn any amp into oscilator. This is just a matter of gain. sometimes just resistor between output and intput works well, because capasitor needed only for phase shift, but any amp already not idealy phase linear in some frequency range, so even resistor between in and out do the same. Oscilation is just the matter of amp gain in this case. Extra capacitor needed only for moving this phase shifting to desired frequency range.

 

[JohnRoberts]

Here I can't read that schematic which is why I tapped out...

there are several topologies for single stage oscillators (I am pretty sure you know that).

JR

 

[groovyloop]

Not with an inverter. This type of circuit usually use three RC's.

For MUSIC produsing sound effects using ideal sin LFO gives not so deep effect, but primitive non linear LFO makes more notisable sound effect. Ideal linearity - is not good in sound FX's. Some "humanizing"needed)) 3 capacitors give sin wave, but 1 gives distorted sin wave, that might be more useful for MUSICIANS, not for ingeneers.

 

[abbey road d enfer]

Any amp circuit has R (input and output resistance) and C (input and output capacitance) and any amp has a phase shift on some frequency, so only 1 right value capacitor needed to turn any amp into oscilator. This is just a matter of gain. sometimes just resistor between output and intput works well, because capasitor needed only for phase shift, but any amp already not idealy phase linear in some frequency range, so even resistor between in and out do the same. Oscilation is just the matter of amp gain in this case. Extra capacitor needed only for moving this phase shifting to desired frequency range.

After running an approximative simulation, it appears it would oscillate correctly for the task.
Here we learn everyday...

 

[Bo Deadly]

For an LFO you know it has to have a big cap slowly charging and discharging somewhere. The one on the gain stage is just bypassing the emitter resistor so that's probably not it. The only other big cap is the one is on the right connected to the variable resistor to ground.

Post the full schem and preferably better resolution.

 

[groovyloop]

The one on the gain stage is just bypassing the emitter resistor so that's probably not it.

Just for understanding the schematics. As I see this schematics is from cold war ara. The reality of USSR that era was that they use in non-military circuits components that should be used not for the better sound, but just because they got a lot of them. The designers was pressed to spent components the factory do not need any more. This is important! So this schematics used germanium transistors MP41A with low input impedance - so designers should used T1 as emiter follower to increase input inpedance (and spent one more transistor). So actually this is single-transistor oscilator on T5 with 2 caps C2 and C3 and and variable resistor to ground.

 

[JohnRoberts]

When I was designing LFO for studio efx (like chorus and flangers) I used a triangle wave, but it could also depend some on your clock generator (I used a voltage to period convertor and it liked triangle waves. YMMV

JR

 

[groovyloop]

When I was designing LFO for studio efx (like chorus and flangers) I used a triangle wave,

yes, I found that sin wave is not the best solution for FX LFO