Active blend/Passive Tone control impedance mystery

[Gorgula]

I have encountered a bizarre problem when using an active blend control in combination with a passive tone control. It is somewhat of a mystery to me but I'm hoping it isn't a mystery to at least some of you in Talkbass world.

I have some generic Sire type preamps in some of my basses. I thought this would be a good platform for experimentation. I'm not entirely sure how relevant the actual preamp is to the issue I have encountered.

The stock preamp has a signal flow as follows:

Pickups > 250K M/N blend control > A250K tone control with a 22nF cap > B250K volume > Preamp.

After spending countless hours experimenting with every type of passive blend control and not finding any to my liking, I decided to try an active blend control. I have experimented with several different circuits. For this discussion, I would like to focus on the design from @RobbieK who is a memeber here on Talkbass. Here are the links for his video and schematic/docs. It doesn't appear that he is still active on the forum, unfortunately.

The blend control works really well and provides a smooth blend. Rob even describes some mods that can be made to tailor the tonal quality/resonant frequency of the pickups.

Here is the problem. No matter what pot or capacitor values I use for the blend control and volume/tone controls, the passive tone control only works when the blend control is in the center position. It's very strange. I'm assuming it is some sort of impedance issue but maybe not. I have built this 2 separate times on a breadboard to rule out any obvious/dumb mistakes on my end. I understand that if placing a passive tone control after any kind of buffer it will no longer act like a conventional passive tone control but I would like to keep it if possible. I also understand that I could place a dual tone pot before the blend control which would probably work but then the wiring gets a little ugly. Much of what I am doing here is to further my own electronics education so I can get a better understanding of how all of this works.

I could write tons more and provide many more details but I don't want to make this unreadable. If anyone has any thoughts/insights I am all ears! If you need any more info just let me know. Thanks

 

[sunbeast]

I have encountered a bizarre problem when using an active blend control in combination with a passive tone control. It is somewhat of a mystery to me but I'm hoping it isn't a mystery to at least some of you in Talkbass world.

I have some generic Sire type preamps in some of my basses. I thought this would be a good platform for experimentation. I'm not entirely sure how relevant the actual preamp is to the issue I have encountered.

The stock preamp has a signal flow as follows:

Pickups > 250K M/N blend control > A250K tone control with a 22nF cap > B250K volume > Preamp.

After spending countless hours experimenting with every type of passive blend control and not finding any to my liking, I decided to try an active blend control. I have experimented with several different circuits. For this discussion, I would like to focus on the design from @RobbieK who is a memeber here on Talkbass. Here are the links for his video and schematic/docs. It doesn't appear that he is still active on the forum, unfortunately.

The blend control works really well and provides a smooth blend. Rob even describes some mods that can be made to tailor the tonal quality/resonant frequency of the pickups.

Here is the problem. No matter what pot or capacitor values I use for the blend control and volume/tone controls, the passive tone control only works when the blend control is in the center position. It's very strange. I'm assuming it is some sort of impedance issue but maybe not. I have built this 2 separate times on a breadboard to rule out any obvious/dumb mistakes on my end. I understand that if placing a passive tone control after any kind of buffer it will no longer act like a conventional passive tone control but I would like to keep it if possible. I also understand that I could place a dual tone pot before the blend control which would probably work but then the wiring gets a little ugly. Much of what I am doing here is to further my own electronics education so I can get a better understanding of how all of this works.

I could write tons more and provide many more details but I don't want to make this unreadable. If anyone has any thoughts/insights I am all ears! If you need any more info just let me know. Thanks

Yes as you mentioned the only really functional answer to keep the action of the current passive tone is a dual-gang 250k or 500k pot placed at the inputs of the blend control with separate capacitors and pickup for each gang- that is how John East works his preamps with active blends and passive tone control and it takes up no more space than a typical tone pot (here's one option: Alpha 24MM Dual Audio Taper Pot 8MM Bushing ). The low impedance of the blend will make anything placed after it not work the same (the standard tone control placed after an active blend should still do something, it'll just be much more mild and only really have an audible change right at the end of the sweep). You can swap a 25K audio-taper pot with a 0.1uf capacitor after the active blend, consistent with the provided wiring that comes with EMG active pickups, it should work OK but it will act more subtly similar to a .022uf capacitor in my experience (the interaction of the capacitor is much more subtle than with a high-impedance passive input)

 

[Gorgula]

Yes as you mentioned the only really functional answer to keep the action of the current passive tone is a dual-gang 250k or 500k pot placed at the inputs of the blend control with separate capacitors and pickup for each gang- that is how John East works his preamps

Thanks for your reply. I didn't know East uses the dual tone pot technique. John makes nice preamps.

I've tried an A25K tone pot but not with a .1 uF cap. I've tried 220nF and 330nF following the logic that 25K is 1/10th of 250K so I increased the cap value by 10x (from 22nF to 220nF). Both of these worked rather well, but as stated above, only when the blend pot is in the center position. I suspect .1 uF will work (although much more subtly), but again, only when the blend is in the center position. That is the real mystery here. It makes no sense, at least not to me.

 

[Passinwind]

Thanks for your reply. I didn't know East uses the dual tone pot technique. John makes nice preamps.

I've tried an A25K tone pot but not with a .1 uF cap. I've tried 220nF and 330nF following the logic that 25K is 1/10th of 250K so I increased the cap value by 10x (from 22nF to 220nF). Both of these worked rather well, but as stated above, only when the blend pot is in the center position. I suspect .1 uF will work (although much more subtly), but again, only when the blend is in the center position. That is the real mystery here. It makes no sense, at least not to me.

It might be worth trying the 25k variant at the preamp output, it certainly won’t be interactive with the blend circuit in that location.

 

[Gorgula]

It might be worth trying the 25k variant at the preamp output, it certainly won’t be interactive with the blend circuit in that location.

That's a good idea. I've known this is a possible option I could try but haven't done it yet since I've really wanted to understand why it is behaving so strangely.

I've been following your filter preamp thread BTW. Very interesting. Would you by any chance have any insight into the cause of this strange behavior? Also, would you suggest moving both the tone and volume controls to the end of the signal chain? Thanks.

 

[Passinwind]

That's a good idea. I've known this is a possible option I could try but haven't done it yet since I've really wanted to understand why it is behaving so strangely.

I've been following your filter preamp thread BTW. Very interesting. Would you by any chance have any insight into the cause of this strange behavior? Also, would you suggest moving both the tone and volume controls to the end of the signal chain? Thanks.

It does seem pretty weird at first blush. I’ll be happy to try some loading behavior models in LTspice, but I’m currently recovering from some major health setbacks and it might take a while to get to it.

And yeah, I’d try the volume control both pre and post…what value is it?

 

[sunbeast]

Thanks for your reply. I didn't know East uses the dual tone pot technique. John makes nice preamps.

I've tried an A25K tone pot but not with a .1 uF cap. I've tried 220nF and 330nF following the logic that 25K is 1/10th of 250K so I increased the cap value by 10x (from 22nF to 220nF). Both of these worked rather well, but as stated above, only when the blend pot is in the center position. I suspect .1 uF will work (although much more subtly), but again, only when the blend is in the center position. That is the real mystery here. It makes no sense, at least not to me.

Interesting- what happens when outside of center position? Following Robbie's document, what values of blend and volume pots did you use? I agree with @Passinwind that placing the active-value tone at the end of the circuit is a great approach too and should remedy the issue. Also placing the volume and tone controls after the pre would work, though you will want a low-impedance volume control in that case too (25k or so). The preamp should isolate it from any strange interactions happening with the tone control

 

[Gorgula]

It does seem pretty weird at first blush. I’ll be happy to try some loading behavior models in LTspice, but I’m currently recovering from some major health setbacks and it might take a while to get to it.

And yeah, I’d try the volume control both pre and post…what value is it?

I'm sorry to hear that. I wish you well in your recovery. If you could model this when you're feeling better, I would be VERY interested to know what you discover. I've been grappling with this for a while so I can certainly wait.

I can try any pot value you recommend, I was planning to try A25K for both tone and volume after the preamp.

 

[Gorgula]

Interesting- what happens when outside of center position?

When the blend pot is at either extreme (Neck or Bridge P/U) the tone control has absolutely zero effect on the sound. As the blend pot is turned toward the center, the tone control has gradually more effect until it reaches maximum at the center position.

Following Robbie's document, what values of blend and volume pots did you use?

I've tried quite a few combinations. B10K blend with 25K tone/volume, B10K blend with 50K tone/volume, B10K blend with 250K tone/volume, 50K blend with 250K tone/volume. All of these combinations exhibit the same behavior. Obviously, there are many more combinations I could and probably should try. I just wish I wasn't blindly stabbing in the dark. I was hoping someone would be able to look at the circuit and say "clearly the pot values need to be X to avoid this behavior". Maybe wishful thinking on my part.

I agree with @Passinwind that placing the active-value tone at the end of the circuit is a great approach too and should remedy the issue. Also placing the volume and tone controls after the pre would work, though you will want a low-impedance volume control in that case too (25k or so). The preamp should isolate it from any strange interactions happening with the tone control

You're probably right, that is the next thing to try. If you look at p. 5 of Rob's documentation, I've tried wiring it both ways he suggests, for both stand alone operation and for feeding a preamp. On the last page it is clear that Rob would expect the volume/tone to be at the end of the preamp. I am going to give this a shot and see what happens. I'll report my findings.

 

[Passinwind]

I'm sorry to hear that. I wish you well in your recovery. If you could model this when you're feeling better, I would be VERY interested to know what you discover. I've been grappling with this for a while so I can certainly wait.

I can try any pot value you recommend, I was planning to try A25K for both tone and volume after the preamp.

I'd start with 25kA for both, but not be surprised if 50k works better in one or both cases.

I took a better look at Rob's circuit last night, he's always had a lot of interesting takes and was quite helpful when I was working up the first versions of my open source resonant LPF project.

 

[sunbeast]

I'd start with 25kA for both, but not be surprised if 50k works better in one or both cases.

I took a better look at Rob's circuit last night, he's always had a lot of interesting takes and was quite helpful when I was working up the first versions of my open source resonant LPF project.

Yeah Rob’s Youtube and Talkbass presence has always been a huge gift! I hope he comes back to Talkbass!

 

[sunbeast]

When the blend pot is at either extreme (Neck or Bridge P/U) the tone control has absolutely zero effect on the sound. As the blend pot is turned toward the center, the tone control has gradually more effect until it reaches maximum at the center position.

I've tried quite a few combinations. B10K blend with 25K tone/volume, B10K blend with 50K tone/volume, B10K blend with 250K tone/volume, 50K blend with 250K tone/volume. All of these combinations exhibit the same behavior. Obviously, there are many more combinations I could and probably should try. I just wish I wasn't blindly stabbing in the dark. I was hoping someone would be able to look at the circuit and say "clearly the pot values need to be X to avoid this behavior". Maybe wishful thinking on my part.

You're probably right, that is the next thing to try. If you look at p. 5 of Rob's documentation, I've tried wiring it both ways he suggests, for both stand alone operation and for feeding a preamp. On the last page it is clear that Rob would expect the volume/tone to be at the end of the preamp. I am going to give this a shot and see what happens. I'll report my findings.

I wish I were more versed in the “why” of things side, I mostly just know the things to try but not why! A shame if Rob is no longer visiting TB as he is an invaluable resource!

 

[Gorgula]

Yeah Rob’s Youtube and Talkbass presence has always been a huge gift! I hope he comes back to Talkbass!

I couldn't agree more. In addition to his TB contributions, he has so much great material on his youtube channel and his own website. I wish I wasn't so late to the party!

I wish I were more versed in the “why” of things side, I mostly just know the things to try but not why! A shame if Rob is no longer visiting TB as he is an invaluable resource!

Same here. I wish I understood much more of the underlying theory. In a lot of ways this feels very similar to when I first started playing music and had zero idea of how any of it worked. From the studying I've done, it's looking like music theory is vastly simpler than electronic theory!

It would be great if this thread miraculously got Rob's attention. I'm fairly sure he would know exactly why things are behaving the way they are.

 

[Passinwind]

I couldn't agree more. In addition to his TB contributions, he has so much great material on his youtube channel and his own website. I wish I wasn't so late to the party!

Same here. I wish I understood much more of the underlying theory. In a lot of ways this feels very similar to when I first started playing music and had zero idea of how any of it worked. From the studying I've done, it's looking like music theory is vastly simpler than electronic theory!

It would be great if this thread miraculously got Rob's attention. I'm fairly sure he would know exactly why things are behaving the way they are.

See R6 in the stadalone buffer blend variant? I think there’s a good chance that adding that before the tone control (right after the blend pot wiper connection) will change your symptom, if not solve it. If you go much above 1k in value you may not like the associated volume cut, but it’s probably worth trying anything up tp 10k or so, just for learning’s sake.

 

[Gorgula]

See R6 in the stadalone buffer blend variant? I think there’s a good chance that adding that before the tone control (right after the blend pot wiper connection) will change your symptom, if not solve it. If you go much above 1k in value you may not like the associated volume cut, but it’s probably worth trying anything up tp 10k or so, just for learning’s sake.

Thanks for the suggestion. Let me make sure I understand you correctly. You said "before the tone control (right after the blend pot wiper connection)". This is right where C5 sits. Do you mean inserting R6 after C5 and before the tone control? Or do you mean insert R6 right after the blend pot wiper, omitting C5 going into the tone control?

 

[Passinwind]

Thanks for the suggestion. Let me make sure I understand you correctly. You said "before the tone control (right after the blend pot wiper connection)". This is right where C5 sits. Do you mean inserting R6 after C5 and before the tone control? Or do you mean insert R6 right after the blend pot wiper, omitting C5 going into the tone control?

Between the wiper and C5. Another possibility is two resistors at 100ohms to 1k, each between the opamp outputs and the blend pot. We’re trying to mitigate the loading offect of a very low impedance at the opamp output, which kills the effect of the tone pot. This could be why the tone pot only works with the blend centered.

I’m just spitballing, and my meds are kicking my butt, hence all the typos. But I have experienced similar issues when blending per-pickup LPFs, and I think it’s worth a try to see if we learn anything.

 

[Gorgula]

Between the wiper and C5. Another possibility is two resistors at 100ohms to 1k, each between the opamp outputs and the blend pot. We’re trying to mitigate the loading offect of a very low impedance at the opamp output, which kills the effect of the tone pot. This could be why the tone pot only works with the blend centered.

I’m just spitballing, and my meds are kicking my butt, hence all the typos. But I have experienced similar issues when blending per-pickup LPFs, and I think it’s worth a try to see if we learn anything.

Thanks for clarifying. I'm going to try these options today. I'm also going to try placing volume/tone at the end of the preamp and see what happens. I'll report back.

 

[Passinwind]

Thanks for clarifying. I'm going to try these options today. I'm also going to try placing volume/tone at the end of the preamp and see what happens. I'll report back.

Cool. My bet is that going post-preamp is going to be the best play.

 

[Gorgula]

Cool. My bet is that going post-preamp is going to be the best play.

I'm happy to report that progress has been made! Unfortunately, I wasn't able to test all three options tonight, only your first recommendation, inserting R6 (1K) between the blend pot wiper and the + leg of C5. Now the tone control works even when the blend pot is in the extreme CW and CCW positions! This is great news. It is noticeable that the tone control has more of an effect when the blend is in the center but it is certainly usable in all positions now. I need to experiment with different resistor values up to 10K as you mentioned to see if this behavior changes at all. This experiment was done with 25K tone/volume controls. I wonder what effect changing these values would have.

I also need to try separate resistors after pins 1 and 7 of the opamp (TL072, FYI) in addition to placing the tone/volume controls at the end.

Thanks again for your input. I'll report back after I have tried the other options.

 

[Gorgula]

I have some interesting findings to report.

It appears that placing 1 resistor after each of the opamp outputs has the same, maybe the exact same, effect as adding 1 resistor of the same value after the blend wiper.

For example, placing a 1K resistor after pin 1 and pin 7 of the opamp has the same effect as adding a 1K resistor after the blend wiper. Thus far I have only tested this with my ears but I can't hear a difference between the 2 different resistor locations.

I initially tried placing individual resistors (1k, 5k & 10K) on the bread board and conducted the tests. 5K seemed to be a good value.

Then I tried placing a 10K linear pot after the blend wiper to find the ideal resistance value. Somewhere between 3K and 5K seems to be the sweet spot though I haven't settled on the perfect value yet.

With a resistance value between 3K-5K, the tone control varies from no effect to dramatic in all blend pot positions, though it is somewhat more pronounced in the center. Since the tone sweep is so smooth, the amount of cutoff can be easily be adjusted to taste in any blend position. The tone control is now very usable. It may not be functioning exactly as a tone control connected directly to the pickups but I'm not really hearing how it is different. It sounds like I would expect a tone control to sound.

One other tidbit. This did not work well with 25K tone & volume pots which was to be expected based on Rob's recommendation of a 50K minimum (5x blend pot value). The A25k tone cap was still bleeding high frequencies even with the tone control wide open. Switching to 50k pots seemed to solve this problem.

It would be interesting to test and plot this. I've used REW in the past, maybe this would be a good use for that program. Does anyone have any suggestions on how best to accurately measure/plot this?

The next experiment is to try placing the volume & tone at the end of the preamp.