Issues with TS variant (pulsing, feedback). Fresh eyes requested

[FingerBlisters]

Schematic:



As you can see it's a pretty basic TS variant with a FAT switch for different bass responses and a CLIP switch in the feedback loop (on/off/on for 2x red LEDS, 3x 4148s, or none).

The tone control is one of AMZ's ts variants which you can see here: https://www.muzique.com/lab/tstone.htm

Problems:

Pulsing/tremolo sound when FAT switch uses the 150n cap. This goes away when gain is rolled back and I'd like it to be available at max gain.

Feedback at extreme treble end of TONE control.

FINDINGS:

I probed around with my DMM. When the pulsing happens the VREF ratings go bananas and swing all over the place from 1v to 8v to everywhere in between. I don't understand why that is but suspect either that 150n cap is dead, or the switch is crappy. When not pulsing the ratings are all stable.

I've tried 4 different dual opamps and same results.

Any ideas where to investigate? I feel I'm missing something very obvious. Thanks guys.

[ElectricDruid]

It sounds like something is oscillating. Have you tried increasing the value of the 33pF cap in the first stage, or adding back in the 220n to ground that the original uses after the 1K on the way to the tone control?

Of course, it might well be a simple build error. Are you *sure* your built circuit matches this schematic? On the face of it, it's a well tried and tested circuit and there shouldn't be any issues. So the fact you're having problems suggests you might be looking at something that is subtly different!

[FingerBlisters]

It sounds like something is oscillating. Have you tried increasing the value of the 33pF cap in the first stage, or adding back in the 220n to ground that the original uses after the 1K on the way to the tone control?

Of course, it might well be a simple build error. Are you *sure* your built circuit matches this schematic? On the face of it, it's a well tried and tested circuit and there shouldn't be any issues. So the fact you're having problems suggests you might be looking at something that is subtly different!

Hello Mr Druid. It's an honour.

I clipped the 10k to vref in the second opamp and that cleaned everything up. Why? I don't know. But it works and that's good enough for me.

Now to wrangle a tone stack that doesn't suck quite as much, this one has very little range... But that could be the B taper and not the W taper you see so often in these things. However I'm thinking a subtle big muff style stack with a mid hump might increase the tonal variation available, and then the 2nd opamp could be used for recovery. 

I appreciate the help. Damn that 10k.

[m4268588]

In the asymmetric clip, this 10kΩ must not exist.

[FingerBlisters]

In the asymmetric clip, this 10kΩ must not exist.

Well there we go. I appreciate the input, thanks for that. I just removed it and there's seemingly zero negatives to doing so.

Now onto the tone pot. Presumably a w20k provides a much better and more usable sweep than the b25k.

[ElectricDruid]

Now onto the tone pot. Presumably a w20k provides a much better and more usable sweep than the b25k.

Yes, it will do. This problem with the TS tone control (or other similar "graphic EQ" style circuits) comes up repeatedly. It's not that a linear pot sacrifices range (you still get +/-10dB or whatever irrespective of what pot type you use) it's just that almost nothing happens until you're right at the extremes of the pot (0-10%, 90%-100%), and then you suddenly get the whole lot. Most recently, Baran Ismen has been struggling with it in a commercial EQ pedal he has:

   https://www.diystompboxes.com/smfforum/index.php?topic=131587.0

I designed an equivalent-but-more-even baxandall EQ for the TS as a replacement:

   https://electricdruid.net/designing-a-classic-overdrive/

This substitute circuit *can* use a linear pot without everything getting bunched up in the middle. I'd recommend it as an alternative to getting the weird taper pot. As a bonus, you can easily add a Bass control too if you wish, like the Xotic BB Preamp and others.

HTH

[GibsonGM]

You know, I never messed around w/your mods, Tom - I think I will, looking at this again  

[FingerBlisters]

Now onto the tone pot. Presumably a w20k provides a much better and more usable sweep than the b25k.

Yes, it will do. This problem with the TS tone control (or other similar "graphic EQ" style circuits) comes up repeatedly. It's not that a linear pot sacrifices range (you still get +/-10dB or whatever irrespective of what pot type you use) it's just that almost nothing happens until you're right at the extremes of the pot (0-10%, 90%-100%), and then you suddenly get the whole lot. Most recently, Baran Ismen has been struggling with it in a commercial EQ pedal he has:

   https://www.diystompboxes.com/smfforum/index.php?topic=131587.0

I designed an equivalent-but-more-even baxandall EQ for the TS as a replacement:

   https://electricdruid.net/designing-a-classic-overdrive/

This substitute circuit *can* use a linear pot without everything getting bunched up in the middle. I'd recommend it as an alternative to getting the weird taper pot. As a bonus, you can easily add a Bass control too if you wish, like the Xotic BB Preamp and others.

HTH

Great work as always. It's a weird design for sure in the TS. I'm leaning towards a very simple big muff style control to maintain the mid hump and make it easier to live with (and understand), and no need for weird pots which is always a bonus.

 

[ElectricDruid]

You know, I never messed around w/your mods, Tom - I think I will, looking at this again  

Thanks.
I'd like to think of them as "improvements" more than "mods". I wasn't aiming to change the sound, just to make it easier/simpler to build. The frequency response is the same, it's just that a different circuit topology avoids the problem with the pot taper. I don't really understand why Maxon/Ibanez did it like that in the first place, if I'm honest. Seems like a odd design choice. But that's what they did, so that's where we are.

[m4268588]

I'd like to think of them as "improvements" more than "mods". I wasn't aiming to change the sound, just to make it easier/simpler to build. The frequency response is the same, it's just that a different circuit topology avoids the problem with the pot taper.

That's not right, right?
And I'm sure you know that.
https://www.diystompboxes.com/smfforum/index.php?topic=129573#msg1252618

[ElectricDruid]

I'd like to think of them as "improvements" more than "mods". I wasn't aiming to change the sound, just to make it easier/simpler to build. The frequency response is the same, it's just that a different circuit topology avoids the problem with the pot taper.

That's not right, right?
And I'm sure you know that.
https://www.diystompboxes.com/smfforum/index.php?topic=129573#msg1252618

Compare the graphs.

TS-808 original tone control:


Baxandall modified tone control:


That's basically the same, right? Or at least "the same" in the sense of "covers the same ground", which is what I was after. Only without the strange taper.

[GibsonGM]

Yes, 'improvement' is a better term. I've always parked the tone control in one place and tried never to move it off its sweet spot (like the distortion control on a Dist +).  I like the idea of some more spread, and especially a BASS control!  Fancy!  Add a diode lift switch and it probably does it all.

I'd imagine they didn't do something similar cuz....parts count.... 

[Ben N]

I don't really understand why Maxon/Ibanez did it like that in the first place, if I'm honest. Seems like a odd design choice.

Maybe it was to stop cloners.        

[m4268588]

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This is not correct as a characteristic simulation only for Tubescreamer's tone control.

TSTC reduces the midrange in counterclockwise. Not a high frequency shelf.


I don't think the TS's design is good, but there are differences anyway.

[Rob Strand]

This is not correct as a characteristic simulation only for Tubescreamer's tone control.

TSTC reduces the midrange in counterclockwise. Not a high frequency shelf.

I don't think the TS's design is good, but there are differences anyway.

I've looked at this problem a number of times over the last 20+ years.  Some using Baxandall and some using other variants.

I can summarize the problem at a higher level:
- You want to emulate the TS9 control behaviour but you want to use a linear taped pot which doesn't cramp-up in the mid boost settings - basically behaving like a TS9 with a G- taper pot.
- It is very easy to match a Baxandall to the TS-9 control when the control is set to full.
- What happens though, when you set the TS-9 control to a mid position and set the Baxandall to the same amount
  of boost/cut  the Baxandall is tuned to a much higher frequency.
- It doesn't matter what Baxandall circuit variant you use they all do the same thing.
- It is possible make the Baxandall circuits match better at the mid boost setting -  yes you can!!
  However, guess what happens? 
  The closer you make the the Baxandall circuit match the TS-9 at mid boost settings the more it starts
  to behave like a TS-9 with a linear pot!!!  ie. the cramping with a linear pot comes back.
- As it turns out pretty much all tone control circuits have this problem.
- So what you have to do is make a direct trade off between matching the TS-9 response and
  cramping.  It actually doesn't matter what circuit you use!!!
  In fact you can even modify the TS-9 control as is to be less cramped with a linear control.
  What you need to do is increase the 1k's to something larger then tweak the other caps
  and resistors to get the same amount of boost/cut and frequencies.
  (I'm sure I posted one like this 20 years ago.)

Conclusion:
You can't win. 
When using linear pots you can only make a trade-off between matching the TS-9 behaviour and reduced the cramping

I can put up a heap of circuits and they will all have the same trade-off to varying degrees.

Try the last case I mentioned of modifying the TS-9 control itself and see what happens.
Here's a simplified comparison which shows the fundamental problem,



At full all the responses match.

At mid boost settings the twerked circuit with a linear pot has (roughly) the same amount of boost as the TS-9 with a G-taper pot.
However the frequency is shifted up.

At mid boost settings a linear pot on the TS-9 circuit is very ineffective and has a cramped behavior.

Here's a further trade-off which makes the frequency matching better overall,



Instead of making the mid boost settings match between the linear and G-taper pots we can make them not match a bit.  Sort of like the linear pot is 1/2 a control point off in rotation from the G-pot.  Because pots have tolerance some small amount of position mismatch will not be noticed.  What that does is allow for slightly better frequency matching because the part values are closer to the original TS-9 values.

The circuit also has the cap tweak for better mid-boost setting match.  An overall close match.

[ElectricDruid]

Yes, this is basically the same as the graphs I have in my article.

The first graph shows the "bunching" you get from using a linear control. The second graph shows how the baxandall tone control spreads out the curves and gives you much better control with a linear pot. No, they're not absolutely identical, but there's nothing magic about the TS's curves in the first place, and I know which of the two sets I'd prefer to use if I only had a linear pot handy!

This is not correct as a characteristic simulation only for Tubescreamer's tone control.

TSTC reduces the midrange in counterclockwise. Not a high frequency shelf.

You've included the RC ahead of the tone control, which is why we're getting different results. My graph is for *just* the EQ-style tone control circuit. That's the part that's causing the problem, and it's easier to see what's going on without the previous passive lowpass included. And without the phase curves on the graph too

I don't think the TS's design is good, but there are differences anyway.

Yes, like Rob said, it's a compromise. You choose what's important to you.