666 hz whine in the Holy Grail

[j_flanders]

There's a faint whining noise in my EHX big box Holy Grail.
Throughout the years many other owners have complained about this issue with their HG as well.
I'd like to get to the bottom of it.
The factory schematic is linked below.

My 2 main questions:
- why does the whining noise completely disappear when there's a (buffered? Boss) pedal after it ?
- what can I do to the circuit to remove the whining noise when it's connected directly to my (tube) amp.

Additional information:

- I'm using a replacement, but sold as original, EHX power supply 9V 500mA. It supplies 15 volts DC unloaded- and 11,75 volts loaded. It drops 0.7volts over the reverse polarity diode that I added and 1 volt over the 4,7 ohm series resistor, so the circuit draws 212mA. The 7805 regulator gets 10 volts and puts out 5 volts.

- link to the psu I've bought and that I'm using: https://www.banzaimusic.com/EH-9V-DC-EU-500mA.html

- Better picture of the same PSU (but with a different plug): https://www.banzaimusic.com/images/D/50854_2.jpg Why does it say: "For I.T.E. use only" when it's clearly meant for audio???

- It's not daisy chained.

- In one pedal I replaced all 10µF and 1µF (smd) electrolytic capacitors as they had leaked and now read in the nF or pF range. This reduced the whining noise by 90% and also fixed the pedal as it had a very weak reverb signal due to a faulty C8 cap right at the input of the dsp chip.

- I have two of these pedals (rev A and rev C). Rev A still has a strong whine the first time I power it up, but after 2 minutes and 55 seconds it suddenly drops the whining noise by 90%.

- I can make the initial strong whine go away immediately by unplugging and replugging the power supply. But only when I unplug the jack from the back of the pedal, not when unplugging and replugging the wall wart. If I unplug the wall wart from wall and pedal and read the voltage it still says 15 volts. When un/re-plugging the jack from the back it's impossible to do so without hitting the sleeve(ground) with the posistive tip of the 1/8 jack.

- The frequency of the whine is 666 Hz and its harmonics: 1,33 kHz , 2 kHz, 2,66 kHz, 3,33 kHz, 4 kHz ...

- Not all harmonics are always present or as loud at every setting. I can introduce 'missing' harmonics or make some louder by touching the blend pot's casing (myself being ungrounded).

- When audio probing, the whining noise is every where, both on the ground as along the signal path. I cannot find where it starts.

- The entire pcb is microphonic. I cannot find the component (ceramic capacitor?) which causes it. Anywhere I tap, I hear the thump or click, Even when I tap the wooden table it's sitting on. I only hear this though with extremely high amplification and through headphones.

- I've tapped all the components individually, very, very softly with a wooden tooth pick to check if some produce a louder tap and it seems that the components around the first opamp (U3: c6, r5) may produce it a little louder than the rest.

- I've tried putting several buffered pedals after the circuit. The 'active' setting in my Zoom G3 works best at completely removing the whine. My Boss LS2 works very well too. My TS808 works but still leaves a fraction of the whine.

- Thinking I could never do any harm by putting a 2.2nF 630v box cap across any component in the circuit to see which or what affects the whine I found that:
   * the cap parallel to r6 (at pin7 on the output of U3 input opamp) the whine is louder
   * cap parallel to r31 (emitter resistor of the Q1 transistor before the blend pot) the whine is gone when connected to my Zoom G3 in passive mode but still there when connected to my tube amp. From what I read this was a stupid thing to do as bypassing the emitter resistor in an emitter follower config should have fried the cap or transistor. Both seem fine though.

- The whine is louder with the blend at 100% dry and slightly lower when turning towards 100% wet. At 100% wet there's so much loud white noise/hiss I can not say if there's still some whine buried beneath it.

- On the Hall setting there's a very strong high frequency roll off. From 4,66 kHz (7th harmonic of 666hz...) to 5.328 kHz (8 harmonic) it drops 12db. I don't see that ultra strong filtering on the other two settings: Spring and Bathroom/Short Spring (I replaced Flerb on both pedals) but the whine is also present on those two settings.

- The included schematic below is rev B. The only difference with rev C is that in rev C they added a reverse polarity diode and they added 100pF caps from tip to sleeve (signal to ground) at the input and output, before the true bypass switch, so, always in circuit. Rev A is not true bypass (connects input to intersection r34 and c29 in bypass). I've converted my Rev A circuit to rev C. The only remaining difference is some changes in layout.

- Input and output jack sleeves are connected to the chassis. Sleeves of input and output jacks are also connected together with a wire. Sleeve of power jack is connected to the sleeve of the input jack. Another wire goes from there to the ground on the pcb.  Led ground comes from a spot on the pcb. During all the tests there was no connection to the chassis.


I've read countless forum posts on 'whine' issues. A big part of them are about klon/clone charge pump issues and clock whine in bbd delays, choruses etc.
I've run out of things to test or try. I've been at it for weeks now. I'm happy to hear your thoughts and suggestions.

[ThermionicScott]

I don't have any helpful advice, but you've gotta admit it's funny for a "Holy Grail" to be cursed by that particular frequency...

[r080]

Does the frequency of the noise stay the same in the different modes? How are you measuring the frequency and reduction in the noise? It is possible you are getting power supply noise, and in some modes the power supply has to run at a different frequency, making it appear to be reduced. Do you have any opportunity to use it with a really high quality power supply?

The louder microphonics around the first chip might just be because it is the first chip, and its noise level is dominating.

Does the noise go away with a really short cable to your amp, or just with a buffered pedal?

When you say you converted it to rev C, do you mean you added 100pF caps? Was the noise there before that?

[j_flanders]

Does the frequency of the noise stay the same in the different modes?

It does but the strength of the different harmonics changes a little so the overall 'tone', being the sum of the harmonics is slightly different. And if I recall correctly in one mode the 4 kHz was completely missing.

How are you measuring the frequency and reduction in the noise?

I hooked it up to a DAW (Reaper) and I'm using a real time spectrum analyzer (SPAN Voxengo). I see the spikes at 666 Hz, 1,33 kHz , 2 kHz, 2,66 kHz, 3,33 kHz, 4 kHz etc.
When I say '90%' reduction it's just a way of saying but I could get the exact db. When I say 'gone' I no longer see the spikes.

It is possible you are getting power supply noise, and in some modes the power supply has to run at a different frequency, making it appear to be reduced. Do you have any opportunity to use it with a really high quality power supply?

The whine is a well known problem described in many posts on many forums. The general agreed upon solution is to not daisychain and to only use the PSU that comes with the pedal. It makes the whine almost inaudible but it's still there.
I have no other really high quality power supply that delivers more than 200mA. I have a 1-spot switching power supply that can deliver 1700mA. There's also whine with that psu.

Does the noise go away with a really short cable to your amp, or just with a buffered pedal?

I'm using a 3m (10ft) cable. I'll try with a patch cable tonight.
When using the Zoom or Boss pedal inbetween there was a 3m cable from the HG to the Boss and a patch cable after the Boss. So, the length of the cable and its capacitance didn't seem to be the reason the whine went away.

When you say you converted it to rev C, do you mean you added 100pF caps? Was the noise there before that?

The pedal was broken when I bought it: it had a very weak reverb signal. (another well known problem)
Cause: dry signal going into the dsp chip too low or wet signal coming out too low or losing signal in the Butterworth filter just before the blend knob.
I had another working pedal so I could compare readings between the two.
I found the cause was a faulty C8 cap at the input of the dsp chip. I replaced that cap and then pedal worked again but had the whining noise. (a well known problem)
I found all electrolytic caps to be 'off' (dried out?, leaked?, capacitor plague?) so I replaced them and the whine reduced drastically. It has even less whine than my 'perfectly good' REV C pedal now.
I added the reverse polarity diode, the 100pf caps, made it true bypass and removed the circuitry that was used for the old bypass config to make it pretty much identical to my Rev C.
It's only at that point that I started testing the 'perefectly good' Rev C pedal for whine and noticed it had even more whine. I just never noticed it all those years because I had never used it without a Boss pedal after it. And also because I always used it 100% wet and mixed in the dry with a Boss LS-2 pedal.

Since my last post I also wired it for star ground by the way. Power ground, output jack ground, input jack ground all going to the same spot on the pcb. Only the LED ground I haven't touched but it's connected to the same big ground plane on the pcb.

The next thing I should try is adding a bigger e-cap parallel to those 10µF filter caps. They read 10µF (brand new caps) on the quieter RevA and 7-ish (older/original) on the slight whinier Rev C. None of that explains to me why a Boss pedal makes the whine go away though.

Here's a photo of the populated side of the board.
You can see the extra cap I soldered parallel to the faulty C8 cap. Just to see if that would fix the 'missing reverb' problem. Afterwards I removed the faulty smd cap and did the same for all the e-caps.

[r080]

You do have a different power supply, anyway. Is the frequency of the noise the same with the 1Spot?

I see you mentioned noise on the ground. You may already be familiar with RG's "sewer ground" idea. I can't say I understand it very well myself, but is there any chance the changes you made to the grounding could have polluted your signal ground with sewer ground?

[Rob Strand]

A 220 ohm dummy load is a good place to start for switching power supply noise.
Play with the value.  The fact the whine shifts with load is a good indication it is the power supply.

A lot of switchmodes produce noise if they don't have enough load.
They might "work" with light loads but they are noisy.

[j_flanders]

The next thing I should try is adding a bigger e-cap parallel to those 10µF filter caps.

That didn't help one bit.

Since my last post I also wired it for star ground by the way. Power ground, output jack ground, input jack ground all going to the same spot on the pcb. Only the LED ground I haven't touched but it's connected to the same big ground plane on the pcb.

That seems to have made things worse. Now the noise reduction by using a Boss pedal after the Holy grail is less than before.
I moved the power ground/return back to the ground pin of the 7805. That seems to have made the most difference. Boss noise reduction is working again.
Moving the ground from the output jack sleeve back to the input jack sleeve doesn't seem to make a lot of difference.

You do have a different power supply, anyway. Is the frequency of the noise the same with the 1Spot?

I checked and the frequency is the same. Good idea though.

I see you mentioned noise on the ground. You may already be familiar with RG's "sewer ground" idea. I can't say I understand it very well myself, but is there any chance the changes you made to the grounding could have polluted your signal ground with sewer ground?

My rev C is still completely stock and has now (slightly) more whining noise than my currently modded rev A.
Also, a lot of people have this problem with their HG and I'm pretty sure very few have attempted to mod it (DSP, smd etc.)

[j_flanders]

A 220 ohm dummy load is a good place to start for switching power supply noise.

Do you mean the original PSU is of the switching type?
To me it looked too big and heavy to be of that type. Is there any way I can tell for sure?

[Rob Strand]

Do you mean the original PSU is of the switching type?
To me it looked too big and heavy to be of that type. Is there any way I can tell for sure?

That type is unlikely to be a switchmode.
The weight is a tell tale sign that it is a transformer not a switchmode.

[Rob Strand]

Actually, my apologies for not going through all your details.  I just saw some posts about the power supply.
Switchmodes are common source for that type of noise but it's not the case here.

You have done a good job documenting a lot of details.   I need to read over it in detail to see where the
cracks are.

This part is interesting,

- why does the whining noise completely disappear when there's a (buffered? Boss) pedal after it ?

When you do that, is the Boss pedal powered from the same power supply as the EHX unit?

A couple of things,

It seems the EHX unit follows the CS4811 datasheet quite closely.   I'm not sure how clean the buffered CMOUT on the
EHX since they didn't use any CMOUT filters (the filters in the datasheet can cause trouble with some opamps).

Also the filter cap C10 on the EHX looks pretty small.  Not enough to filter out 600Hz signals.

They are just some initial observations I wouldn't read too much into it.

[Rob Strand]

I can make the initial strong whine go away immediately by unplugging and replugging the power supply. But only when I unplug the jack from the back of the pedal, not when unplugging and replugging the wall wart. If I unplug the wall wart from wall and pedal and read the voltage it still says 15 volts. When un/re-plugging the jack from the back it's impossible to do so without hitting the sleeve(ground) with the posistive tip of the 1/8 jack.

So this one is interesting as well.

The input supply filter C1 on the EHX is very small.   There will be a a big cap inside the wall-wart.   When you pull the wall-wart at the mains power is still supplied by that big cap.   However when you pull the DC jack at the pedal the big cap is removed and the power will drop quicker.

Any low frequency supply currents will not kept local to the PCB because of the small C1 value.

I'm not sure but it looks like the DC jack is not isolated from the chassis that means any low frequency currents have the potential to pass through the signal grounds.

Those points and your symptoms are strong evidence ground currents are the cause.

Some ways this can be reduced,
- increase C1 to prevent the low frequency current pulses on the PSU from going between the PCB and wall-wart via the DC jack
- Isolate the DC input jack and make sure the wiring from the jack connects close to C1's ground.  (That might not be *the* best place as it depends on the ground layout on the PCB.)

As a test you could unscrew the DC jack so it is not grounded.

You probably need 100uF to 1000uF for C1.  Use a big value as a test.

Not sure if the power is switched by the (6.5mm) input socket.    The schem doesn't show it is but that assumes the schematic is correct.


EDIT:
I just noticed that board is smd.   If the 4.7R ohm input resistor is a tiny SMD part you might not be able use 1000uF.  What happens is the resistor can fry during power up.

[j_flanders]

So this one is interesting as well.

The input supply filter C1 on the EHX is very small.   There will be a a big cap inside the wall-wart.   When you pull the wall-wart at the mains power is still supplied by that big cap.   However when you pull the DC jack at the pedal the big cap is removed and the power will drop quicker.

Any low frequency supply currents will not kept local to the PCB because of the small C1 value.

I'm not sure but it looks like the DC jack is not isolated from the chassis that means any low frequency currents have the potential to pass through the signal grounds.

Ok, thanks, that explains.
The DC jack is not isolated from the chassis, but during all these tests there's nothing connected to the chassis but the 3PDT switch. It's the only way I have access to the populated side of the board.

Those points and your symptoms are strong evidence ground currents are the cause.

Some ways this can be reduced,
- increase C1 to prevent the low frequency current pulses on the PSU from going between the PCB and wall-wart via the DC jack
- Isolate the DC input jack and make sure the wiring from the jack connects close to C1's ground.  (That might not be *the* best place as it depends on the ground layout on the PCB.)

The original C1 electrolytic cap was bad, as were all the other e-caps.
I replaced it with a through-hole version and soldered it directly across the pins of the 7805 regulator.
The power ground/return I moved to the ground pin of the 7805 since the cap's ground is also there now.
In rev A the power ground/return was originally soldered to the sleeve of the input jack.
In rev C they did it as you suggest: smd e-cap on the pcb and power ground/return was brought to the pcb at the ground leg of that cap.

As a test you could unscrew the DC jack so it is not grounded.

see above (circuit and connections being out of the box)

You probably need 100uF to 1000uF for C1.  Use a big value as a test.
EDIT:
I just noticed that board is smd.   If the 4.7R ohm input resistor is a tiny SMD part you might not be able use 1000uF.  What happens is the resistor can fry during power up.

It's funny you mention that as the number 1 problem with these pedals is a fried R1.
Rev A has a 4r7 smd resistor but it's bigger(1W smd?) than the rest of the 'regular' resistors.
Rev A had a 100µF C1. It was bad and read in the pF or nF range. I replaced it with a 10µF cap during the conversion to rev C.

Rev C has a big 1 or 2 W through-hole resistor for R1 and a 10µF C1.

Not sure if the power is switched by the (6.5mm) input socket.    The schem doesn't show it is but that assumes the schematic is correct.

It's not. With a 500mA PSU and a circuit drawing over 200mA I guess it's not feasible or sensible to run it from a 9v battery


Since audio probing didn't tell me where the whine comes from (it was everywhere, signal and ground) I not only tried to reduce the whine but also tried to increase the whine.
When 'bypassing' R6 (150 Ohm resistor right after U3) I get more whine. So, I guess I'm having less resistance for higher frequencies going to the DSP chip.
Could that mean that the whine comes from U3 at the input?
That's also where it seems to be slightly more microphonic.
I once tried adding a cap parallel to the 39pF C6 to limit high end gain. But I was just holding it there (not soldered) and I got so much buzzzz (probably because of the interference coming from my body) that I didn't hear any difference.
Maybe I should try that again but solder the cap this time. I'm just a bit reluctant to experiment solder/unsolder too many times to those smd parts.

[Rob Strand]

The original C1 electrolytic cap was bad, as were all the other e-caps.

A larger cap is rarely worse but in the light of the R1 issue a compromise might be required.

It's funny you mention that as the number 1 problem with these pedals is a fried R1.
Rev A has a 4r7 smd resistor but it's bigger(1W smd?) than the rest of the 'regular' resistors.
Rev A had a 100µF C1. It was bad and read in the pF or nF range. I replaced it with a 10µF cap during the conversion to rev C.

Rev C has a big 1 or 2 W through-hole resistor for R1 and a 10µF C1.

There seems to be two contributing factors.  R1 dissipates quite a bit of power while running, 2W would be wise just for that.  Then there's the turn-on issue.   I doubt a 2W through-hole will blow.  Clearly they have had problems in that area with all the mods!

When 'bypassing' R6 (150 Ohm resistor right after U3) I get more whine. So, I guess I'm having less resistance for higher frequencies going to the DSP chip.
Could that mean that the whine comes from U3 at the input?
That's also where it seems to be slightly more microphonic.
I once tried adding a cap parallel to the 39pF C6 to limit high end gain. But I was just holding it there (not soldered) and I got so much buzzzz (probably because of the interference coming from my body) that I didn't hear any difference.

Some good debugging work there.

The R6 bypass thing seems very significant.  I can see some angles.

The first is the capacitive load on U3 is making it oscillate.    That would happen if R6 is too low.     R35 has the same value so maybe the capacitive load of the cable is causing that to oscillate.   Adding the Boss pedal would buffer the HG from the cable capacitance.   However, I would call this one a very weak theory since you are seeing a clean 666Hz whine and if the oscillation was from two places I doubt the frequency would be consistent.

High on the list is small values of C10 and C11.  For debugging I would be trying some larger cap values in parallel maybe 10uF up.  It's easy to do and would cut off a lot of crazy thoughts for debugging.

Something worth noticing is this device is digital.  If you have strong high frequency components these can get frequency shifted by the sampling process.   By bypassing R6 you would be letting more out of band highs and that would make such signals stronger.   However, you have to ask whey are they there in the first place.   The high frequencies no doubt originate from the digital circuits or power and they could get into analog signals from the power or the grounding on the PCB.    The way they would get in on the power would be via the small C10 and C11 values.
[Of course, the 666Hz could be getting in directly via the power as well.  Perhaps something to do with the software.  It seems odd the 666Hz is stronger on power-up, like you mentioned before.  That would seem more like a software related frequency.]

A way to get more filtering is to increase C9 or R6.  The R6 value is show in the CS4811 datasheet.   I'm not sure how much you could increase that.

As mentioned before R6 and R35  are on the low side anyway.

So the crack in all this thinking is why/how a buffered pedal after the HG also removes the whine.  It's possible high frequencies are getting out of the unit perhaps because of the small C10 value but the fact the whine is complete removed by a post buffer seems like it's something else.   The points to maybe R35 too small.  Unfortunately the links between cause and effect aren't clear yet.

As for the microphonics I'd be thinking C5.   If you temporarily replaced it with a through-hole poly cap it would soon answer the question.

[j_flanders]

This pedal refused to whine any longer, making further investigations impossible.
On a rare occasion it still does the whine on startup, but after a minute or so it also magically disappears.
So, there was nothing I could try and add to this thread.

I still wanted to get to the bottom of this and managed to find and buy another big box Holy Grail that had some whine.

The R6 bypass thing seems very significant.  I can see some angles.

The first is the capacitive load on U3 is making it oscillate.    That would happen if R6 is too low.     
R35 has the same value so maybe the capacitive load of the cable is causing that to oscillate.   
Adding the Boss pedal would buffer the HG from the cable capacitance.   
However, I would call this one a very weak theory since you are seeing a clean 666Hz whine and if the oscillation was from two places I doubt the frequency would be consistent.

[...]

As mentioned before R6 and R35  are on the low side anyway.

So the crack in all this thinking is why/how a buffered pedal after the HG also removes the whine. 
It's possible high frequencies are getting out of the unit perhaps because of the small C10 value
but the fact the whine is complete removed by a post buffer seems like it's something else.   
The points to maybe R35 too small.  Unfortunately the links between cause and effect aren't clear yet.

Thanks for all your help and suggestions Rob! The above was crucial in solving the whine in my last acquired HG.

I audio-probed the circuit again and found in this pedal that the whine occurred just after the output buffer (U5).
There's no whine at the middle lug of the blend pot which goes to the input of U5, but there's quite a lot of audible whine at the output of that opamp.
Similar to the other HG, the whine disappears when it is connected to my Zoom G3, but only when the input of that unit is in 'active mode' instead of 'passive mode'.

I went over all your suggestions and hints and googled what those 150 Ohm resistors are for, which lead me to this article:
https://www.analog.com/en/analog-dialogue/articles/techniques-to-avoid-instability-capacitive-loading.html

For now I tried the easiest solution: adding additional resistance in series with that 150 Ohm resistor at the output.
An additional 1k to 2k makes the whine inaudible. 
I haven't checked on a scope if it's still there, but I can no longer hear it. Not through headphones, not through my amp with all controls on 10.
The Zoom's 'active input' as well as some of my Boss pedals must have some series resistance at the input.

[mozz]

Monitor the 5v line and see what changes once the oscillation starts.

[idy]

I still wanted to get to the bottom of this and managed to find and buy another big box Holy Grail that had some whine.

Wow. Just wow. One station closer to Stomp Box Sainthood. To so love trouble-shooting pedals that, when a problem goes away, you seek out another broken specimen.

[j_flanders]

Monitor the 5v line and see what changes once the oscillation starts.

Ok, will do. Can you be a little more specific. Where, how, what? There are two 5V lines (analog and digital)

I still wanted to get to the bottom of this and managed to find and buy another big box Holy Grail that had some whine.

Wow. Just wow. One station closer to Stomp Box Sainthood. To so love trouble-shooting pedals that, when a problem goes away, you seek out another broken specimen.

I know. Just don't tell my wife.

[Rob Strand]

I went over all your suggestions and hints and googled what those 150 Ohm resistors are for, which lead me to this article:
https://www.analog.com/en/analog-dialogue/articles/techniques-to-avoid-instability-capacitive-loading.html

For now I tried the easiest solution: adding additional resistance in series with that 150 Ohm resistor at the output.
An additional 1k to 2k makes the whine inaudible. 
I haven't checked on a scope if it's still there, but I can no longer hear it. Not through headphones, not through my amp with all controls on 10.

That's a good sign you are on the right track. 

The Zoom's 'active input' as well as some of my Boss pedals must have some series resistance at the input.

The series resistance at the input of the next pedal might only be helping a small amount.    The root cause is the opamp with the low output resistor driving the capacitance of the cable.    The input resistor on the next stage pedal is after the capacitance of the cable so the opamp still sees the cable capacitance.   The resistor at the opamp output is *between* the opamp and the cable capacitance, which effectively makes the opamp see less capacitive load.

The way I normally check this type of thing is to have no cable at the output  of the device, that makes the capacitive load as low as possible, then add different value capacitors to ground on the output jack, say 10p, 100p, 470p, 1n, 10n, 100n.     If there is a zone where it kicks into oscillation I might use finer grain steps.  Especially in the sub 1n region as this would represent different lengths of cable.   The idea is to choose an output resistor high enough where it never oscillates.

I'd say the low value of R6 is if equal concern.  If you raised R6 to 1k it's probably going to have no effect on performance with the existing C9 value.

Interesting, looking at the datasheet you would expect it handle quite a bit of capacitance, figs 19 thru 21

https://www.ti.com/lit/gpn/lmv358a-q1

[j_flanders]

The root cause is the opamp with the low output resistor driving the capacitance of the cable.    The input resistor on the next stage pedal is after the capacitance of the cable so the opamp still sees the cable capacitance.   The resistor at the opamp output is *between* the opamp and the cable capacitance, which effectively makes the opamp see less capacitive load.

I get your point.
I'm testing with a patch cable though, which has between 20pF and 80pF capacitance. The 20pF is 10cm, the 80pF 40cm long.
I don't know the capacitance at the input of the next device.

The way I normally check this type of thing is to have no cable at the output  of the device, that makes the capacitive load as low as possible, then add different value capacitors to ground on the output jack, say 10p, 100p, 470p, 1n, 10n, 100n.     If there is a zone where it kicks into oscillation I might use finer grain steps.  Especially in the sub 1n region as this would represent different lengths of cable.   The idea is to choose an output resistor high enough where it never oscillates.

How do I test without a cable?

I used a 5k pot to find a resistor where the oscillation is no longer audible.
However, it's not a case of 'kicking in'. The larger the resistance, the lower the whine in volume.
At around 1,5k I could no longer hear the whine over the white noise this pedal has.
I don't seem to hear any difference in treble for dry and wet signal though.

Another thing I noticed is that when I touch the tip out the output jack in the pedal, the whine disappears as well.

I'd say the low value of R6 is if equal concern.  If you raised R6 to 1k it's probably going to have no effect on performance with the existing C9 value.

It's harder to test as I have to add series resistance in the middle of a circuit. But it's definitely something to keep in mind.
For now, since I don't hear any whine at both sides of the blend pot, I'm not going to unsolder any of the smd parts in that area.

[Rob Strand]

I'm testing with a patch cable though, which has between 20pF and 80pF capacitance. The 20pF is 10cm, the 80pF 40cm long.
I don't know the capacitance at the input of the next device.

How do I test without a cable?

10cm is pretty short and it's sure possible to kick some circuits off with small capacitance values like that.   The next device might be around 5pF to 10pF, not much.

With your short lead you probably won't gain much by making the set-up less convenient.   I use two methods clip parts onto the board directly or use a dummy plug that I can clip or solder parts to.  Unless you have strong need to test very low capacitance just do whatever is convenient and it should be OK.

I used a 5k pot to find a resistor where the oscillation is no longer audible.
However, it's not a case of 'kicking in'. The larger the resistance, the lower the whine in volume.
At around 1,5k I could no longer hear the whine over the white noise this pedal has.
I don't seem to hear any difference in treble for dry and wet signal though.

That method is perfectly fine.  The only thing worth doing is add a little bit more capacitance to see if you can get it to oscillate.  Then use a resistor a little higher than that value.   When you are right on the critical resistance value you might be able to control the level of the whine a bit.  However when you set it to the slightly larger "safe" value it should not oscillate all.   That's where you can dial up you amp to 10 and you cannot hear any oscillation.   That's what you want - no oscillation at all regardless of what cap value you load the output jack with.

Another thing I noticed is that when I touch the tip out the output jack in the pedal, the whine disappears as well.

That could mean only small capacitance values are affecting it, or, it could mean your resistance is damping the oscillation.     These things can be very fussy.   I often stick my fingers on the back of boards to kill oscillations - after that try to work out what that's doing.

It's harder to test as I have to add series resistance in the middle of a circuit. But it's definitely something to keep in mind.
For now, since I don't hear any whine at both sides of the blend pot, I'm not going to unsolder any of the smd parts in that area.

Yes, it can be a pain.  Since you have identified a problem at the output you know there's a possible issue using 150 ohm.  A slightly less optimal way would be to load the output jack with 2.2nF then check out what R35 value stops the oscillation.  That would give you an idea what value should be in R6.    If you don't want to solder a new R6 that's fine.   If you get problems down the track you know where to look and you know what value to change R6 to.   You can just keep that up your sleeve for a rainy day.