I need help with tweaking low pass filtering on a chorus

[Rob Strand]

The hiss seems to be removed in distortion mode because of the extra filtering in the distortion circuit.  (1K, 47n, 6k8, 10n near the TL4558P IC)

After looking a bit closer the distortion is before the chorus so the drop in hiss cannot be attributed to the distortion filter.

So perhaps that's hint.   It's pretty strange for the thing connecting to the input of the chorus to change
the noise.    From what I can see so far only the 2SC732 transistor at the top left corner of your (reversed) overlay is in the signal path.  It is possible for that transistor to be damaged and go noisy.

If that's the case then the circuit details in and out of the chorus need to be traced.    Maybe pre-emphasis and de-emphasis can be shoe-horned in.  However,  a different tact is to modify the BBD filters I traced earlier.  The aim is to build in some pre-emphasis into the input filter and some de-emphasis into the output filter.  Such a redesign needs some care and the filter design goes outside of the textbook type filter designs.  It might be enough to save it.

I'm fairly certain that there is no pre-emphasis/de-emphasis.  So that's going to make the BBD noisy.   There doesn't seem an easy way to incorporated pre-emphasis - other than the tricky filter mod I mentioned earlier.

[tokenofdevotion]

Thanks for that markup Rob, I funnily enough increased C2's rated voltage to 25v as per ElectricDruid's suggestion to no avail.

I'm fairly certain that there is no pre-emphasis/de-emphasis.  So that's going to make the BBD noisy.   There doesn't seem an easy way to incorporated pre-emphasis - other than the tricky filter mod I mentioned earlier.

I tried increasing C4 to 100uF and there is no noticeable change. I'm almost certain now that you mentioned there is no pre/de-emphasis that this is most likely the cause of loud hiss when chorus is toggled on. I'm going to try the filter mod, what factor would you recommend to start with and what would the values look like after increasing them? I haven't done this before

[Rob Strand]

I tried increasing C4 to 100uF and there is no noticeable change. I'm almost certain now that you mentioned there is no pre/de-emphasis that this is most likely the cause of loud hiss when chorus is toggled on. I'm going to try the filter mod, what factor would you recommend to start with and what would the values look like after increasing them? I haven't done this before

It's very tricky to mod the filter to get the desired result.

I have some values already.   Give me an hour or so and I'll see what can be done.

[Rob Strand]

OK here's about as good as I could get by only tweaking the filters.

Just for the record it's clear there is an improvement but it's not as good as a proper de-emphasis circuit (and was never expected to be).

Schematic of filters original and modded:
(R1 is a short and C1 is removed.   I've got tiny values on the schematic)



Response of filters.  Before and after.   I've also plotted how much the BBD noise is reduced with the mod and how much it is reduced with the Boss CE-2 de-emphasis.    Certainly better than nothing.

[Rob Strand]

Here's a very rough skeletal schematic to give you an overview of the structure:

[tokenofdevotion]

I just finished replacing all the parts and the hiss was lowered by a lot even with the increased vco cap value. And just like that, this pedal is now a studio-friendly double effect little box!

Another thing I noticed a while ago when I compared it to my Aria ACH-1 is that the Guyatone was unnecessarily much more brighter. And since this is a distortion pedal as well it just wasn't a good decision to preserve the highs as much since it can sound gnarly the more distorted the signal is so whoever called the design ready was either hearing impaired or rude. Now the delay line is much darker and the modulation is not that obvious and pairs really well with the gain circuit. Even if it's only a -3db difference, it's obviously perceived as way more.

Thanks for all the help and information Rob! Also thanks to Mark Hammer and ElectricDruid for all the suggestions that I still need to incorporate properly. This was a fun project and I'm satisfied that more people will definitely find this mod useful!

[Rob Strand]

I just finished replacing all the parts and the hiss was lowered by a lot even with the increased vco cap value. And just like that, this pedal is now a studio-friendly double effect little box!

Very cool.   So clearly the lack of de-emphasis was the cause of the hiss.  Perhaps not a good design decision to remove the pre-emphasis/de-emphasis. 

The PCB overlay you did greatly helped find that missing link otherwise it would have been very hard to spot.

Another thing I noticed a while ago when I compared it to my Aria ACH-1 is that the Guyatone was unnecessarily much more brighter. And since this is a distortion pedal as well it just wasn't a good decision to preserve the highs as much since it can sound gnarly the more distorted the signal is so whoever called the design ready was either hearing impaired or rude. Now the delay line is much darker and the modulation is not that obvious and pairs really well with the gain circuit. Even if it's only a -3db difference, it's obviously perceived as way more.

The distortion has a quite a bit of gain from what I can see.

I'm surprised you can hear the difference, especially an obvious difference.   I only knocked the -3dB point down by 10% which is normally just perceivable.  The new filter has a steeper slope so it is possible that is also contributing to what you are hearing.  I checked the simulations and they should be close to the real thing for a filter.  (There is an assumption the BBD output impedance is low.)  Maybe check your part values?

Thanks for all the help and information Rob! Also thanks to Mark Hammer and ElectricDruid for all the suggestions that I still need to incorporate properly. This was a fun project and I'm satisfied that more people will definitely find this mod useful!

No problem.   It was a tricky problem to find and solve.   Who would ever think no de-emphasis especially when their other chorus models do have it.

[tokenofdevotion]

Very cool.   So clearly the lack of de-emphasis was the cause of the hiss.  Perhaps not a good design decision to remove the pre-emphasis/de-emphasis. 

The PCB overlay you did greatly helped find that missing link otherwise it would have been very hard to spot.

I'm not an engineer, but it's interesting they removed the de-emphasis while not modifying the filter to compensate for the hiss. Specially since this new filter you set up is 2 parts less and the thing is now silent. Very weird decision or maybe I'm lacking CEO brain.

The distortion has a quite a bit of gain from what I can see.

Probably meant for another post, but I'm trying to figure out a simple way of having a more broad gain control since the circuit goes from no distortion to full on distortion the moment you touch the knob. But, I really really like the way it saturates the signal before reaching that point and unfortunately gain has to be barely at 8oclock in order to achieve that. I was also thinking making the chorus only mode to be a chorus+distortion parallel mode would be a nice addition.

I'm surprised you can hear the difference, especially an obvious difference.   I only knocked the -3dB point down by 10% which is normally just perceivable.  The new filter has a steeper slope so it is possible that is also contributing to what you are hearing.  I checked the simulations and they should be close to the real thing for a filter.  (There is an assumption the BBD output impedance is low.)  Maybe check your part values?

This is my guitar going into the guyatone into the amp, straight into a cab simulator on my daw:

As you can hear the noise is obscenely loud (vco value increased to 69pF) on the first recorded track and on the second one which is after I modified the filter it almost sounds like it's being masked by the usual guitar noise amplified by the distortion circuit + amp which works perfectly for recording and it's even quieter than having the pedal in stock form.

I did use 20k resistors for R4, R5 and R6 bc I didn't have 18k ones. All the other values are matched.

No problem.   It was a tricky problem to find and solve.   Who would ever think no de-emphasis especially when their other chorus models do have it.

I think your mod is the definitve solution. It's not worth including a de-emphasis since the noise floor is now so low it would make a 1% difference while yours was more like 10%. Still wondering what they were thinking when they sent this board to factory

[Rob Strand]

'm not an engineer, but it's interesting they removed the de-emphasis while not modifying the filter to compensate for the hiss. Specially since this new filter you set up is 2 parts less and the thing is now silent. Very weird decision or maybe I'm lacking CEO brain.

The decision for no pre-emphasis/de-emphasis is the real problem.  The filter redesign is a solution way off the text books.  It's not common knowledge you have to the option do it.  Also, there's no recipe how to design the filter.  That said the I'm pretty sure the Boss BF-2 Flanger has this idea in the filter, and it uses pre-emphasis/de-emphasis as well!

Probably meant for another post, but I'm trying to figure out a simple way of having a more broad gain control since the circuit goes from no distortion to full on distortion the moment you touch the knob. But, I really really like the way it saturates the signal before reaching that point and unfortunately gain has to be barely at 8oclock in order to achieve that. I was also thinking making the chorus only mode to be a chorus+distortion parallel mode would be a nice addition.

You can tweak the distortion easily with these two parts.



Raise the value of that resistor.   I don't know the pot values in that unit but I'm guessing start at 470 ohm.   I suspect you can raise that resistor quite a bit without affecting the maxed out tone yet having more control in the middle settings.    You could even choose to take out some of the maxed-out gain, upto you (max out the distortion then increase the resistor for the maximum distortion you like).   You can also tune that capacitor.   Once you have an idea of a better resistor value try choosing the cap such that C = 1/(2*pi*700*R) where R is the resistor value, so for R=470 ohm you get C=470nF.   The smaller cap will clean-up the low end but by all means play with the value (large = more lows).

As you can hear the noise is obscenely loud (vco value increased to 69pF) on the first recorded track and on the second one which is after I modified the filter it almost sounds like it's being masked by the usual guitar noise amplified by the distortion circuit + amp which works perfectly for recording and it's even quieter than having the pedal in stock form.

Yes a significant improvement.

I did use 20k resistors for R4, R5 and R6 bc I didn't have 18k ones. All the other values are matched.

That might make it a tad darker.   Nonetheless it helps remove more noise.  If I wasn't happy with the 18k results I'd be suggesting using 20k or 22k to shave off a bit more noise.  So what you have is good.  It's very fine balance between removing noise and not messing up the tone.

I think your mod is the definitve solution. It's not worth including a de-emphasis since the noise floor is now so low it would make a 1% difference while yours was more like 10%. Still wondering what they were thinking when they sent this board to factory

I suspect you could hear an improvement with Pre-emphasis/De-emphasis.   It would take out more of the mid-range noise/whine.   The filter mod definitely reduced the hiss.

[tokenofdevotion]

Raise the value of that resistor.   I don't know the pot values in that unit but I'm guessing start at 470 ohm.   I suspect you can raise that resistor quite a bit without affecting the maxed out tone yet having more control in the middle settings.    You could even choose to take out some of the maxed-out gain, upto you (max out the distortion then increase the resistor for the maximum distortion you like).   You can also tune that capacitor.   Once you have an idea of a better resistor value try choosing the cap such that C = 1/(2*pi*700*R) where R is the resistor value, so for R=470 ohm you get C=470nF.   The smaller cap will clean-up the low end but by all means play with the value (large = more lows).

I tried changing the values from both parts. lowering the cap does change how soon max distortion is achieved, but volume drops with lower values than stock. Increasing the resistor value cuts the highs. How can I lower that cap value without losing volume?

[Rob Strand]

I tried changing the values from both parts. lowering the cap does change how soon max distortion is achieved, but volume drops with lower values than stock. Increasing the resistor value cuts the highs. How can I lower that cap value without losing volume?

Changing the cap or the resistor will drop the level to some degree.   It depends on how over the top the gain is in the pedal, that depends on the gain pot value.

You can get some signal back by changing 47k to 220k and 120k to 47k; see my rough schematic R3 and R4. That tries to work with the input stage and JFET switching at the input.



When you play with the cap and resistor you are sort of redesigning the distortion to suit your tastes.   Basically playing around with values like you did.