LFO signal leaking into power rails

[Buran1997]

I'm building a tremolo unit based on the Electronics Australia tremolo. The schematic is here. I've breadboarded the circuit, and it's working, but the LFO signal is leaking into the signal path, probably through the power rails; it's audible as static which pulses at a rate controlled by the tremolo's rate knob. Does anyone have an idea how to get rid of this noise? Thanks.

[GibsonGM]

Hi Buran, this is a common issue with LFOs in general.  They can create a current spike when they draw on the power supply.  Static? Or 'popping' / 'ticking'?  The traditional remedy is often 2-fold (at least):

- make sure your pot wires and jumpers (if any) are routed away from the LFO section to lessen coupling.  If from a designed board, component placement SHOULDN'T be an issue, but if it's DIY layout, it could be...

- add a 100R series resistor followed by a BFC (big f.... cap, up to 1000u, tho 470u is often a good compromise for space savings) to ground where power enters the board. This creates a filter - really, a reservoir that discharges when the LFO demands power, hopefully taking the 'strain' off the power supply.    Post a pic of your build if you require more info.


HTH! It's fixed many of my own builds.

[antonis]

Just make sure for PLF suggested by Sir MIke above is placed as close as possible to LFO supply..
(e.g. right on R8 upper leg..)

[ElectricDruid]

+1 what they said.

Using a lower power chip for the LFO is another trick you often see. Dunno if it's enough on it's own, but everything helps a bit with stuff like this. TL022 or TL062 are common candidates.

[merlinb]

Isn't this a transistor circuit??

[antonis]

The above mentioned "ticking" issue is more noticeable in versions with rate indicator LED (in series with 15k Q3 Collector resistor) and needs it's own power supply LPF..

[antonis]

Interesting solution for a transistor circuit

Druides and Transistors squabble all over the time..

[duck_arse]

me? I'd put the line filter RC in between the audio section and the lfo. let the bugger swing in the wind, too hard to clean the supply for the pso. imo.

[anotherjim]

If built to the scheme, it might be useful to try it powered by a battery. Otherwise, check the components around the LFO output and depth control as I suppose it could click if Q2 is cutting in and out sharply.

[Buran1997]

Hi Buran, this is a common issue with LFOs in general.  They can create a current spike when they draw on the power supply.  Static? Or 'popping' / 'ticking'?  The traditional remedy is often 2-fold (at least):

- make sure your pot wires and jumpers (if any) are routed away from the LFO section to lessen coupling.  If from a designed board, component placement SHOULDN'T be an issue, but if it's DIY layout, it could be...

- add a 100R series resistor followed by a BFC (big f.... cap, up to 1000u, tho 470u is often a good compromise for space savings) to ground where power enters the board. This creates a filter - really, a reservoir that discharges when the LFO demands power, hopefully taking the 'strain' off the power supply.    Post a pic of your build if you require more info.


HTH! It's fixed many of my own builds.

This helped a bit, but the issue is still present. It is static, not ticking or popping; it sounds like hissing which fades in and out rapidly. I suspect that it's the actual sine wave being generated by the LFO.

Using a lower power chip for the LFO is another trick you often see. Dunno if it's enough on it's own, but everything helps a bit with stuff like this. TL022 or TL062 are common candidates.

Interesting solution for a transistor circuit

As it turns out, I decided to change the basic amplifier circuit used in the tremolo. I wasn't getting good results with the BJT used in the original (the thing just wouldn't bias and was quite noisy), so I decided to use a TL072 instead with the JFET configured as a variable resistor in the feedback loop. That part works perfectly; the noise from the LFO is the only issue.

Perhaps I can use the other half of the TL072 as a current buffer for the LFO? Alternatively, does anybody know a simple, non-noisy LFO which generates a sine wave using a single op amp?

[ElectricDruid]

Interesting solution for a transistor circuit

Druides and Transistors squabble all over the time..

Doh! Should have checked the schematic! 

[GibsonGM]

TL062 and lead dress (??)  Should be doing that if you have adequate decoupling (R-C filter described above, right at the IC if possible)

That said, my Easyvibe was noisy enough that I build the LFO on a daughter board and placed it literally 4" from the LDR part of the circuit. Quiet as a mouse As with relationships, sometimes a little separation is a good thing, ha ha

[antonis]

I decided to use a TL072 instead with the JFET configured as a variable resistor in the feedback loop.

IMHO, it should be better to place JFET in place of (or in series with) gain resistor (the one set in denominator of gain formula) due to its relatively low value in ohmic region..

[Buran1997]

I decided to use a TL072 instead with the JFET configured as a variable resistor in the feedback loop.

IMHO, it should be better to place JFET in place of (or in series with) gain resistor (the one set in denominator of gain formula) due to its relatively low value in ohmic region..

I should have clarified: I've configured the op amp to be non-inverting, so I placed the JFET in the feedback loop to avoid gain dropping below 1. The JFET isn't the only component controlling feedback; it's in parallel with a 33K resistor, the idea being that rather than gain being controlled directly by the JFET, as the JFET turns on and off the feedback loop alternates between the 33K and a 4K7 in series with the JFET. The circuit doesn't seem to have any issues other than the LFO noise, so I think I'm going to leave this section as-is except for tweaking the value of the feedback resistors.

[antonis]

R4 can be omitted without less than unity gain issue..

P.S.
Despite RC supply filtering, place a 10μF and a 10 - 100nF (ceramic) as close as possible to IC pin 8..

[Rob Strand]

R4 can be omitted without less than unity gain issue..

Yes, it's actually a gain of 1.92 (2 in round figures) with the 4k7 present.

[Buran1997]

Well this is embarrassing.

It was not, in fact, the LFO that was causing the noise. My circuit was working properly, amplifying the incoming signal in time with the LFO... including the ambient 60 Hz hum, which was what I was hearing. Simply unplugging the JFET caused the noise to become constant; shorting the op amp's feedback loop caused the noise to drop back down. Using a battery didn't make a difference, probably just because the breadboard itself picks up so much noise that a clean power source makes no difference. If this were a distortion circuit, I probably would have dismissed the noise as just part of the breadboarding process. It was only the oscillation that made it noticeable.

Thank you all for your help and advice; I'll keep it in mind when designing my layout and implementing power filtering.

By the way: Does anyone have any neat ideas for what do to with that second op amp, other than use it as an input buffer?

[antonis]

Does anyone have any neat ideas for what do to with that second op amp, other than use it as an input buffer?

Vref stabilizer or LFO substitution..

[Buran1997]

Does anyone have any neat ideas for what do to with that second op amp, other than use it as an input buffer?

Vref stabilizer or LFO substitution..

I've been looking around for sine wave LFOs which can be implemented with a single op amp. In your experience, which design has proved the easiest?

[antonis]

All-time-classic Wien-bridge sinewave oscillator..
Easily modified for single supply..
ZD₁ + ZD₂ can be replaced by a pair of anti-parallel diodes (strongly recommended for +9V supply) for op-amp output max peak-to-peak amplitude of double the forward diode voltage drop..
R3 could be a 10k pot for presice gain (output amplitude) setting..



Further reading: https://nvhrbiblio.nl/biblio/boek/Marston%20-%20110%20Operational%20amplifier%20projects.pdf Chapter 4.