Here is a melon scratcher! RE: CD40106 LFO Ticking Clock

[john_h]

hey there!

I have used a somewhat modified version of the Ken Stone Psycho LFO to modulate a PT2399 delay.

An interesting thing I have found with it is that you can faintly hear the clock ticking only with a guitar source. With synths and line inputs it is dead silent, but as soon as a guitar is plugged in, it will start ticking. What is even more strange is that when you roll off the volume of the guitar, the ticking will stop.

I have tried the usual remedies that I have read about (decoupling the +9v from the audio path with caps and resistors, tying unused gates, over grounding, bigger caps, shielded wire etc) but they dont seem to fix this.

I am lead to believe there is something happening with the grounding when a guitar is plugged in that doesnt happen with line inputs or synths, but for the life of me I cant figure out what is going on!

It really isnt a huge nuisance as the sound is very faint, but I could imagine that it would be heard in a recording situation which could lead to problems.

Anyways, any insight into this would be greatly apreciated!

[R.G.]

First guess: source impedance. Guitar pickups are inductive, so their impedance goes up with frequency. Rolling off the guitar volume introduces some series resistance, so that might be a clue. Other audio sources are generally both higher voltage and non-inductive.

Are you using some kind of buffer in front of the delay circuit's audio? If so, that points more to a possible grounding issue.

[john_h]

it is buffered by a simple TL072 Op Amp. I will look further into this! Thanks for the help.

[john_h]

I have since noticed that if you put a buffered effect before this delay, the LFO ticking stops, but a buffered bypass pedal after the delay will still tick. Ill buffer the input and see how it works out

[john_h]

buffer on the input fixed it! 

I guess the delay circuit is buffered, but the pedal itself was true bypass, so some of the square wave current draw from the lfo was interfering with the unaffected signal? Anyways, having a bypass buffer solved it and doesnt seem to have any audible effect on the bypassed tone.

[anotherjim]

That hasn't really solved it.
It has proved RG's suspicion that the pedals own input is over-sensitive to noise when in a high impedance condition.  The noise will be there even with a buffered pedal feeding it, but that pedals low output impedance is effectively loading down the noise level enough to make it inaudible. Note that the pedal itself can make the noise and it can be picked up inside the pedal via radiation.

Where the input of a pedal is a non-inverting opamp, many will fit a series resistor of around 1k into the +input of the chip and add from there a small capacitor to 0v. The cap is often a 47pf to 100pF ceramic disc.  This makes a low-pass filter to help reject any high frequency noise coming in, but the cap will also load out noise from the pedal itself.

Where the input opamp also has gain, another small cap of similar size is added across the feedback resistance making another low-pass filter. This won't stop noise getting in, but it will stop it being amplified.

If you use screened cable in the input - maybe in two parts with the bypass switch in between, the cable screen must not be grounded at multiple points. The total screen only needs grounding at one end and connect the input jack sleeve to the circuit 0v by an ordinary wire.

[john_h]

Thank a lot for this. I will give it a shot! I am certainly new and curious to all of this. Ive been learning so much through the dilemma and your help is very much appreciated.

To be clear, I didnt add a buffered pedal before this delay, but I made a simple JFET buffer (2N5458) that I injected between the input jack and footswitch and took power straight from the DC input. Perhaps Ill put a toggle in to switch between buffered and true bypass after making these adjustments to the input op-amp, but honestly I have noticed a lot of benefits to this buffer beyond now making everything over all quieter.

Is it recommended that I do not do this other than true-bypass snobs telling me it ruins fuzz-faces?

[john_h]

The original circuit called for a 1M resistor in series with the TL072 after a 47nF input cap, so I tried your suggestion and replaced that 1M with a 1K and tied a 47pF ceramic cap to ground from the + input of the op amp. Problem solved! Kind of...now there is a significant volume drop when the effect is engaged. I tried to find the sweet spot in lowering the noise while avoiding the volume drop (using resistors rated 1k up to 10k) and anything higher than 8.2k brings back the clock ticking, but anything under 10k sees a volume drop as well as a slight cut in low frequency. I really prefer this approach to the buffer as it seems like much less of a bandaid approach, but I am wondering if at this point the input buffer is my only option. Could it be that the 47pF input capacitor would need adjustment depending on the series resistor I use? Ill keep mucking around with this anyways...

[anotherjim]

1M series into an op-amp input is a bit mad. Have you got schematic of that to show?

[ElectricDruid]

+1 for a schematic, please.

47pF is a pretty "unusual" value for an input cap, so we need to see what you've done.

[anotherjim]

I'm wondering if there's a misunderstanding. 1M is typical bias resistor from a reference voltage to the +in of the opamp. If you change that for 1k it certainly will make it quiet! That resistor acts as load on the input signal and is *in parallel*, which is why we have a high value like 1M there. 47nF would be right for the DC blocking capacitor that will be *in series* with the input -  the signal has to go through it.
The 47pF was what I suggested as an rf filter.

In this opamp input, there is R1, a 10k in series. This is a fault protection resistor in case a stupid high voltage gets into the input, but I can turn it into an rf filter by fitting a small cap from the junction of R1-C1 to ground. Frankly, if you go to the trouble of fitting a series protection resistor, you may as well fit an Rf filter capacitor too. The input of a circuit is nearly always the best place to do it.

[john_h]

Yes, this was absolutely my mistake! Youre totally right. Here is the schematic for the input and for the record, this is from scott swartz' pt80 schematic:




I was just replacing that 1M bias resistor with a 10k...oof...

so, to be clear, what youre suggesting is putting a 10k *in series* (again, my amateur mistake) directly on the input and fitting a small cap to ground between that and the 0.047uF cap on the schematic to create an rf filter.

I really truly appreciate your help with this

[anotherjim]

This!

This works against radio frequency noise. For guitar use the cap can be as large as 1nF which cuts above 7Khz. Not much comes out of a guitar over 5Khz so you can cut a lot of hf down without making a guitar sound dull.