Blending coils in series

[JohnH]

This thread is about different options for blending two coils or pickups in series. I worked this out when designing the new wiring for my HSS Strat

Here are three arrangements, in which the upper coil is at full and the lower coil is being blended in using a pot. Similar arrangements can apply with both coils being controlled by pots.



Type A has one coil bypassed by a variable resistor, such as in common ‘spin-a-split’ wiring.

Type B has the pot wired as a voltage divider, like a volume control.

Type C is as B, with treble bleed components to maintain the in-between tones

All three have the same performance at max and min, but what is interesting is the tonal journey in between. Also, Type A has the added possibility that the pot can be of a ‘no-load’ type, to completely remove it at maximum.

Here are plots of the results, with the pot changing in 10% increments. They were calculated using 5Spice using models for a typical pair of single coils, and 250k pots. As usual, only electrical response can be modelled, not that due to coil position etc.



Type A
As the pot is turned up from 0 (single coil), the tone effect is that the peak response diminishes slightly, and the low and mid frequencies fill in, up to about 15%, which is about 5 on a log pot. Then above 15%, or 5 to 10 on a log pot, the resonant peak for the two coils develops, at a lower frequency to that of the single coil, and overall about 6db louder.

To the left, is a trace to show how if the pot can go to ‘no-load’, an extra bit of brightness (about 1.5db) is restored to the full dual-coil sound. Nice to have, though in practice, I can’t actually hear this extra difference.

Type B
Turning up from 0, change is slower than for A, and more characterised by a diminishing of the single coil treble peak rather than building up the lower frequencies. The dual-coil peak starts to establish above about 50%, or about 7 on a log pot. I have not built this arrangement, the expected in-between dull single coil sound does not appeal.

Type C
Type C adds treble bleed to B, and this results in a very smooth transition from single to dual coil tone, with no more muted tone in between. I use this arrangement in designs where two pickups are being combined with two volume controls, and it works well.

Conclusion
Types A or C arrangements work quite well, and are confirmed in practice. The tonal variations in between are different however. For my recent HSS Strat build, using tone pots to do this series blending, I used type A to get a bit more difference in between (since there is no conventional tone pot) and also to use no-load pots.


cheers
John

[Ro_S]

JohnH - 

The subject of your topic/post is of  direct relevance to me as I am planning to use a spin-a-split or similar arrangement in one of my mod projects. ( I want to add an overwound single coil in varying amounts in series with a vintage spec Strat type single coil.)


So, the pot used as the series blender pot does have a loading effect on the circuit, on the two pickups when combined, is that correct?   Have I understood correctly?

If 'yes', then I have the following two additional questions.  In order to mitigate the loading effect of the series blender pot, is it therefore best to:
(a) use a high value pot for the series blender pot?   such as 500k or 1meg?
and,
(b) not have any tone pot (or instead use a no-load tone pot) affecting the single coil that is being blended in?


Also, two further questions: 

1. when the blender pot is at zero setting, does that pots still have any loading effect on the sound of the single coil alone?

2.  what type of taper is best for the series blender pot in order to get the most control over the varying amounts of series blend?  linear taper?

thanks 
 

[JohnH]

I really like the Type A design best. Its very smooth and clear. I use a 250k log no load pot and it has no tonal or loading effect at either end if its travel.

On my Strat, I have this, and I probably dont need it as no-load, since theres no audible change as it goes 'off' but YMMV.

[Ro_S]

Nov 25, 2018 17:48:34 GMT -5 JohnH said:

I really like the Type A design best. Its very smooth and clear. I use a 250k log no load pot and it has no tonal or loading effect at either end if its travel.

On my Strat, I have this, and I probably dont need it as no-load, since theres no audible change as it goes 'off' but YMMV.

JohnH - 

A few questions, please:

1.  Would a linear taper for the spin-a-split series blender pot provide better control over the variance of the series blend amount, compared with an audio taper? 

2. Would a 500k pot for the spin-a-split series blender - if you not using a no-load pot - help mitigate against the loading effect of the pot, compared to using a 250k pot?  

3.   I'm still unclear on this: When the spin-a-split series blender pot is at zero, does that pot still impart a loading effect on the single coil sound? 
3b. If 'yes', would a bypass switch therefore be a useful addition in order to eliminate the series blend pot from the circuit when using the single coil alone?  

4.  The loading effect of the spin-a-split series blender pot is undesirable given that a series combination of two coils sounds less clear than a single coil alone regardless of a series blend pot being present aspect.  So what good measures can be done in a guitar's circuit to counter this in the spin-a-split design?   
Perhaps a bass cut control (as in PTB scheme), or a fixed 'bright' cap like the Jaguar high pass filter 'strangle switch'?


thanks

[reTrEaD]

Nov 29, 2018 7:31:04 GMT -5 Ro_S said:

Nov 25, 2018 17:48:34 GMT -5 JohnH said:

I really like the Type A design best. Its very smooth and clear. I use a 250k log no load pot and it has no tonal or loading effect at either end if its travel.

On my Strat, I have this, and I probably dont need it as no-load, since theres no audible change as it goes 'off' but YMMV.

JohnH - 

A few questions, please:

1.  Would a linear taper for the spin-a-split series blender pot provide better control over the variance of the series blend amount, compared with an audio taper? 

2. Would a 500k pot for the spin-a-split series blender - if you not using a no-load pot - help mitigate against the loading effect of the pot, compared to using a 250k pot?  

3.   I'm still unclear on this: When the spin-a-split series blender pot is at zero, does that pot still impart a loading effect on the single coil sound? 
3b. If 'yes', would a bypass switch therefore be a useful addition in order to eliminate the series blend pot from the circuit when using the single coil alone?  

4.  The loading effect of the spin-a-split series blender pot is undesirable given that a series combination of two coils sounds less clear than a single coil alone regardless of a series blend pot being present aspect.  So what good measures can be done in a guitar's circuit to counter this in the spin-a-split design?   
Perhaps a bass cut control (as in PTB scheme), or a fixed 'bright' cap like the Jaguar high pass filter 'strangle switch'?


thanks

You should probably value JohnH 's opinion over mine as he's had more first-hand experience in this area. But I'll try my hand at answering these questions from a technical analysis point of view.

1. Linear taper will only give 'better control' over the series blend in the portion of the range from both pickups contributing equally to just a slight reduction of the contribution of the lower pickup in the stack. With a linear taper that would occur between 'both' to the midpoint of the blend pot's rotation.

Over the entire range from 'both' to 'upper pickup in the stack only', audio taper will provide far better control.




2. Unless you're using strat pickups which are highly overwound, the 250k load on the lower pickup isn't really significant.



3. When the blend pot is at zero, the upper pickup is directly connected and the lower pickup is fully shunted. The contribution of the upper pickup in the stack should be the same, whether a bypass switch is employed or not. The blend pot won't affect the loading of the upper pickup.

3b. If you subscribe to the theory that unused but shunted pickups have some effect, you might expect a slight difference between bypassing the lower pickup (switching the upper pickup's (-) connection directly to ground) and having the blend pot at zero. Also, fully counterclockwise doesn't always get zero resistance from wiper to the CCW lug on some pots. In that case, you might notice a difference between fully CCW on the blend pot and a switched bypass.



4. I'm not sure what you're asking.

[JohnH]

Yes to what reTrEaD says.
J

[Ro_S]

reTrEaD and JohnH - 

Thanks for your replies.


reTrEaD ,  To respond to your points:
 
Re: Q1 -
Noted that an audio/log taper is better than a linear taper for this purpose.   Thanks, Very useful to know!

Re: Q2 -
Well, for my project, I'm intending to use two Strat/Fender style single coil pickups with Alnico magnetic pole pieces.  The principal pickup is a vintage type approx 6k ohms DC resistance.  But the other pickup, the one with the series blend pot controlling it, is very overwound; it is has approx. 11.5k ohms DC resistance!   
So do you think I should use a 500k pot rather 250k in this case?

Re: Q3 / Q3b -
Noted that the series blender pot when at zero shouldn't have a loading affect on the principal pickup.  Good to know!

Re: Q4 -
You didn't understand my question, so let me try to re-express and explain it more.  
Pots have a loading effect on a circuit which reduces the high frequencies present. The normal choice of pot values for the volume and tone controls on a guitar with single coil pickups is 250k.  That's because they sound brighter and the reduced loading effect of 500k pots compared to 250k would make them sound too ice picky bright.  As combining two single coil pickups in series is making a humbucker effectively or a dual coil pickup, they will sound darker due to the increased coil wind resistance in play.  In addition, the presence of the the spin-a-split series blend pot control (when not at zero) further loads the circuit, thereby having the undesirable affect of further reducing the high end frequencies.   Yet, on the other hand, the dilemma is that one wants to stick with 250k volume and tone controls for the normal single coil settings.
So the point of my question was ascertain what could be incorporated into the design of a guitar's circuit to counter the loading effect of the spin-a-split series blend pot when it's not at zero.  (And that's in addition to the other pots being only 250k rather than 500k, the latter of which is the norm for dual coil pickups.)

Thanks

[reTrEaD]

Hi Ro_S

I think we're to the point of splitting hairs. I'm not sure that's really necessary.

Re: Q2 -
Well, for my project, I'm intending to use two Strat/Fender style single coil pickups with Alnico magnetic pole pieces.  The principal pickup is a vintage type approx 6k ohms DC resistance.  But the other pickup, the one with the series blend pot controlling it, is very overwound; it is has approx. 11.5k ohms DC resistance!   
So do you think I should use a 500k pot rather 250k in this case?

Pretty much a toss-up but I think I'd still go with a 250k no-load. That allows full brightness of the contribution coming from the overwound pickup when the blend is at 10 but should fade smoothly.

Re: Q4 -
You didn't understand my question, so let me try to re-express and explain it more.  
Pots have a loading effect on a circuit which reduces the high frequencies present. The normal choice of pot values for the volume and tone controls on a guitar with single coil pickups is 250k.  That's because they sound brighter and the reduced loading effect of 500k pots compared to 250k would make them sound too ice picky bright.  As combining two single coil pickups in series is making a humbucker effectively or a dual coil pickup, they will sound darker due to the increased coil wind resistance in play.  In addition, the presence of the the spin-a-split series blend pot control (when not at zero) further loads the circuit, thereby having the undesirable affect of further reducing the high end frequencies.   Yet, on the other hand, the dilemma is that one wants to stick with 250k volume and tone controls for the normal single coil settings.
So the point of my question was ascertain what could be incorporated into the design of a guitar's circuit to counter the loading effect of the spin-a-split series blend pot when it's not at zero.  (And that's in addition to the other pots being only 250k rather than 500k, the latter of which is the norm for dual coil pickups.)

One thing to note here: The blend pot doesn't load the entire circuit. Only the lower pickup in the series combination.

For the blend pot, I'd use whatever you have on hand. Preferably a 250k no-load but if you're reworking a guitar that already has a 500k or 250k pot that isn't currently in use, I'd go with either of those and not lose a minute of sleep over it.

[Ro_S]

reTrEaD

"The blend pot doesn't load the entire circuit. Only the lower pickup in the series combination."  - noted, thanks.

Well I don't have a no-load pot and I don't want to buy any.    I can use either a normal 250k or 500k pot, and any taper type, though.  Any are easily available for me, so disregard whatever the guitar currently has. 
I'm sensing a 500k value would be better for my intended application?     (Noted from discussion previously that you advise that I use an audio taper.)   
Thanks. 

[JohnH]

Nov 30, 2018 16:12:59 GMT -5 Ro_S said:

reTrEaD

"The blend pot doesn't load the entire circuit. Only the lower pickup in the series combination."  - noted, thanks.

Well I don't have a no-load pot and I don't want to buy any.    I can use either a normal 250k or 500k pot, and any taper type, though.  Any are easily available for me, so disregard whatever the guitar currently has. 
I'm sensing a 500k value would be better for my intended application?     (Noted from discussion previously that you advise that I use an audio taper.)   
Thanks. 

Definitely an audio taper, and unless it is a very hot pickup, personally Id try a 250k. Maybe if you are not sure, you could try a test. Before committing to which pot, put a 250k pot (any taper, for a test) across one coil of the pickup , and see if you can hear any difference. If not, then 250k is the go, if there is a significant different shunting one coil with 250k, use 500k. 250k will give a nicer spread of tones though.

[Ro_S]

Dec 1, 2018 4:41:42 GMT -5 JohnH said:

Definitely an audio taper, and unless it is a very hot pickup, personally Id try a 250k. Maybe if you are not sure, you could try a test. Before committing to which pot, put a 250k pot (any taper, for a test) across one coil of the pickup , and see if you can hear any difference. If not, then 250k is the go, if there is a significant different shunting one coil with 250k, use 500k. 250k will give a nicer spread of tones though.

Hi,

I'm intending to use two Strat/Fender style single coil pickups with Alnico magnetic pole pieces. The principal single coil pickup is a vintage type approx 6k ohms DC resistance. But the other singel coil pickup, the one with the series blend pot controlling it, has approx. 11.5k ohms DC resistance. 
I consider the latter as very hot for a single coil; hence why I was concerned about 250k and was wondering that 500k would be more appropriate for my scenario.

thanks

[JohnH]

OK sure, that 11.5k is pretty hot! So I agree that a 500k log pot might be the best guess for the type A series blander, if you cant run a test.

[jhng]

Hello JohnH -- I know it's a while since you did this work on series blending. However, I think you still use similar set ups in some of your guitars. Do you have any views on how series blending compares with a 'broadbucker' set up in terms of tonal utility?

Looking at the NxB combo on a Strat, for example, have you found that blending a small amount Neck in with the Bridge (using Type A blending) is more or less tonally useful than partially bypassing the Neck with a cap? Are the two significantly different?

I had been trying to encompass both options in a design, but I'm wondering if that's overdoing it and I should just do one or the other.

In general, I am a huge fan of series combos on a Strat, much more so than the classic parallel combos. (Ironically, the only parallel option I use with any regularity is N+B.) That's why I was looking at expanding the tonality in the series direction with blending and/or broadbucker.

[JohnH]

hi jhng

I think what I like best can depend on the specificguitar and pickups, and what direction I reckon I'd like to push the tone. On coils of a single hb pickup, I like the fixed cap bypass on one coil and find it more interesting than a resistor bypass.

Across two pickups, the type A blender is great and it let's you dial back the series mix so it doesn't get muddy. Out of N and B series mixes, my favorite is full N with B mixed in. It adds a good mid-range dose of extra thickness and there are good tones all through the blending range. It works on my HSS where the B component is one humbucker coil. B mixed with a bit of N is also good.

I don't think that these series blends need be available with both cap or no cap. But it might be good to rig up the guitar to try them before deciding which.

[jhng]

Thanks, John. That's really helpful.