Passive basses and the volume control, an explanation (science warning)

[leswatts]

Nice analysis!

I have done a good bit of spice analysis fo guitar passive circuits as well.

I guess I just don't like batteries/phantom power. I usually use varitone (switched capacitor) tone circuits.

I did not know Fender jazz had pickup hot to the volume wiper!

The next logical step would be to compare both variants, and maybe provide a third solution which actually acts as a passive mixer and neither mutes both pickups nor acts primarily as a tone control (but is more sensitive to cable capacitance and amplifier input impedance... with passive electronics there is always a tradeoff!) So stay tuned

Series summing resistor after volume pots?

Les

 

[l0calh05t]

Nice analysis!

I have done a good bit of spice analysis fo guitar passive circuits as well.

I guess I just don't like batteries/phantom power. I usually use varitone (switched capacitor) tone circuits.

I did not know Fender jazz had pickup hot to the volume wiper!

I didn't either... until a few hours ago. But the reasons are obvious enough...

Series summing resistor after volume pots?

Les

Exactly for the 250k pots I'm thinking 125k summing resistors for the most consistent load. OTOH a smaller series resistance would mean less loading...

 

[Mr. Ray]

Thank you very informative..job well done. (Now can you explain to me how I can keep the drummer from speeding up?)

 

[l0calh05t]

Thank you very informative..job well done. (Now can you explain to me how I can keep the drummer from speeding up?)

I believe this is the wrong forum for that question, but I'd suggest playing very "laid back" and simply not going along with him speeding up... (or get a replacement part )

 

[leswatts]

Exactly for the 250k pots I'm thinking 125k summing resistors for the most consistent load. OTOH a smaller series resistance would mean less loading...

Cool. Lots of interesting tradeoffs. Looking forward to your analysis.

Les
L M Watts Technology

 

[l0calh05t]

Ok, as noted in my previous posts, in jazz basses the hot side of the pickup is not connected to top of the potentiometer but to the wiper. Compare the lower circuit to the upper circuit in this schematic:
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As you can see, the volume potentiometers no longer act as voltage dividers put rather as a variable load followed by a series resistance. The result? The volume potentiometers affect tone more than volume, as seen here (volume set to -6dB):
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The highs are rolled off, but volume is almost unchanged. Only when volume is rolled off all the way the pickup is muted.

Why use this kind of circuit then? The reason can be seen in this schematic:
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In both circuits, one pickup is muted, and the other is at full volume. Look at resistor R6. It is effectively a short circuit to ground! So if standard voltage dividers are use an one pickup is rolled off all the way, the output is shorted to ground and both are muted!

Is it possible to have effective volume control (and not only on/off with rolled off highs in between?)? Yes, by using summing resistors after the voltage dividers a short to ground is prevented, as seen here:
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125k resistors were used so that the loading of each pickup remains approximately constant. Why isn't this kind of wiring always used? The series resistances R8 and R9 make the circuit far more sensitive to external load (cable capacitance and amplifier input impedance as modeled by C8 and R10), which results in rolled off highs:
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If you do use this kind of circuit a 1Meg potentiometer should be used for the tone pot.

Here's also a plot of the losses over the series resistors:
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[aborgman]

If anyone has any questions or anything to add, go ahead and post!

A few things to consider -

1) Cable resistance. Don't just model cable cap, but actually model cable resistance also.

2) If you're analyzing this using a spice variant, it's quite possible that it's overstating the resonant peaks amplitude.

--
Aaron (B.S. Computer Engineering, current job title Electrical Engineer)

 

[l0calh05t]

A few things to consider -

1) Cable resistance. Don't just model cable cap, but actually model cable resistance also.

2) If you're analyzing this using a spice variant, it's quite possible that it's overstating the resonant peaks amplitude.

1) Any cable resistance would be quite small and negligible in comparison with the very high impedances in the remainder of the circuit. The same is true for cable inductance and the the frequency range is far too low for modeling using transmission lines (wavelengths in the range of many kilometers).

2) Any exaggeration would be due to an incomplete model. As already mentioned, the pickup model used is fairly simple, but should be close enough. A more detailed model could for example include eddy currents (see: http://buildyourguitar.com/resources/lemme/ )

 

[aborgman]

2) Any exaggeration would be due to an incomplete model. As already mentioned, the pickup model used is fairly simple, but should be close enough. A more detailed model could for example include eddy currents (see: http://buildyourguitar.com/resources/lemme/ )

It isn't your model that is flawed - it's spice and its modeling of RC filter circuits. IME it tends to exaggerate (although not massively) the peaks relative to the actual circuit.

 

[l0calh05t]

It isn't your model that is flawed - it's spice and its modeling of RC filter circuits. IME it tends to exaggerate (although not massively) the peaks relative to the actual circuit.

Actually, that would be due to an incomplete model. In a real RLC-Circuit, all inductances will have a series resistance and a parallel capacitance (and other effects if they use a ferrous core) and all capacitances will have an ESR (and ESL), if you include all these effects and any parasitic capacitances, the SPICE simulation will be much closer to the actual circuit.

EDIT: also don't forget the input impedances of any measurement devices!

 

[RyreInc]

If you go to a single volume control and put the tone ckt or ckts before that, the effects change.

Some basses and guitars are wired this way.

 

[aborgman]

Actually, that would be due to an incomplete model. In a real RLC-Circuit, all inductances will have a series resistance and a parallel capacitance (and other effects if they use a ferrous core) and all capacitances will have an ESR (and ESL), if you include all these effects and any parasitic capacitances, the SPICE simulation will be much closer to the actual circuit.

Closer - but it still seems (especially in PCB applications) to overemphasize resonant frequencies in standard RLC filter systems.

I mean, spice tends to do a better job with them than with complex active filter networks or with non-linear circuits - but it still (compared to the real article) tends to exaggerate resonance.

EDIT: also don't forget the input impedances of any measurement devices!

Yep... that is the problem with passive pickups - you're driving with a relatively high impedance source.

 

[l0calh05t]

If you go to a single volume control and put the tone ckt or ckts before that, the effects change.

Some basses and guitars are wired this way.

Yes, they do. If you are thinking about a specific instrument and know the pickups parameters (DC resistance, inductance and capacitance) I could make a simulation for that instrument. Otherwise, I could make a simulation using the same pickup model as before.

 

[l0calh05t]

Closer - but it still seems (especially in PCB applications) to overemphasize resonant frequencies in standard RLC filter systems.

I mean, spice tends to do a better job with them than with complex active filter networks or with non-linear circuits - but it still (compared to the real article) tends to exaggerate resonance.

"Especially in PCB applications" makes me think that the problem might be missing parasitics from the interconnects, but I think going into this too deply might alienate a few people . Suffice to say that no simulation - spice or otherwise - will ever exactly match a real circuit. (The lumped circuit model is just that... a model!)

Yep... that is the problem with passive pickups - you're driving with a relatively high impedance source.

...and thus the circuits are very sensitive to external loading.

 

[aborgman]

"Especially in PCB applications" makes me think that the problem might be missing parasitics from the interconnects, but I think going into this too deply might alienate a few people . Suffice to say that no simulation - spice or otherwise - will ever exactly match a real circuit. (The lumped circuit model is just that... a model!)

Definitely - and it's a model that you often have to tweak a bit to get giving real world answers, although it's great for off the cuff/back of the envelope type analyses.

...and thus the circuits are very sensitive to external loading.

Yep... as an Electrical Engineer passive pickup systems have a huge flaw in that regard, but I still play passives because I think the flaw is generally outweighed by the reliability.

 

[RyreInc]

Yes, they do. If you are thinking about a specific instrument and know the pickups parameters (DC resistance, inductance and capacitance) I could make a simulation for that instrument. Otherwise, I could make a simulation using the same pickup model as before.

That's your choice; you've already done so much! I've run simulations comparing these two configs before, so I know what the differences are. (I even thought I had some pics relating that, but I can't find them...)

For the sake of the general audience you may consider doing it if you'd like...

 

[ehque]

Hi localhost.

I would like to know if you could model an active bass. Most active basses have the first buffer/gain stage after the blend, and i would like to know how this loading would affect the pickups.

Also, modelling the Audere preamp (where each pickup is individually buffered) as a comparison, would be nice.

Is this doable? A 3 way comparison of a Jazz bass would be very informative to me.

 

[therex]

awesome info dude, i really wanted to know the theory behind the volume control on bass

 

[SGD Lutherie]

Bad news... I noticed that I didn't use "correct" jazz-bass wiring for my simulations. In a jazz bass (according to [Invalid or Expired Link Removed] ), the hot leads of the pickups are connected to the wipers of the volume pots. In my schematics I connected the wipers to the output.

So, what's the big difference? My wiring uses the pots as voltage dividers in the way you would normally use for attenuating a signal, and the fender version presents a variable load to the pickups instead! What does this mean?

Right, they do that so turning down one volume doesn't also effect the other pickup, as it does on a Gibson Les Paul.

As you pointed out, at anything but full volume, you have some loading of the pickup, and you lose some high end.

Another interesting thing to add to your post is the higher the impedance of the pickup, the greater the effect of the volume and tone pots have in loading the signal. So the resonant peak will be lower.

This is why EMG pickups, which have lower output impedances than passive pickups, require a large tone cap (.1uF), and even then the tone control doesn't have as much of an effect as on a passive hi-z pickup.

Very interesting post.

 

[l0calh05t]

Hi localhost.

I would like to know if you could model an active bass. Most active basses have the first buffer/gain stage after the blend, and i would like to know how this loading would affect the pickups.

Also, modelling the Audere preamp (where each pickup is individually buffered) as a comparison, would be nice.

Is this doable? A 3 way comparison of a Jazz bass would be very informative to me.

Of course it's doable, but that would be material for another thread, I think. So let's finish with the info on passive pickups first, ok?

Another interesting thing to add to your post is the higher the impedance of the pickup, the greater the effect of the volume and tone pots have in loading the signal. So the resonant peak will be lower.

Yes, that is true. The resonant peak will not only be lower, but it will also generally be at a lower frequency since a hotter (higher impedance) pickup will have more windings and thus higher inductance and higher capacitance.