Parallel Humbucker circuit

[cooltone]

I am submitting some ideas about the circuit design for a humbucker coils configured in parallel. I am planning to use these ideas on my Vintage V100 and would appreciate any comments/criticism/improvements from the forum.

Summary of circuit features
- Simplifies setting the characteristic guitar sound. L_hb, C_res and R_Q act like a an equaliser giving good control of the characteristic sound - experiment with C_res values first, then R_Q values.
- The characteristic sound is preserved at nearly all vol pot levels - treble bleed is not needed.
- Some provision for centre position volume mixing is available
- A second characteristic sound created when tone pot set to zero, which is user definable.

1) Humbucker resonance
First some background ideas triggered by the fine work of JohnH, thread ref: A better treble bleed circuit.

My approach is to think of the humbucker inductance as making a resonant circuit with the all the surrounding components, up to and including the cable and amp impedance.
With multiple capacitors there are multiple modes of resonance. Each mode of resonance is defined by a particular LCR combination which defines the resonant frequency and the Q factor. When R is high there is little resonance (low Q). LCR combinations with a low resonant frequency attenuate higher frequency resonance of other LCR combinations.

2) HB Parallel Coils (compared to series coils)
- the inductance is roughly divided by four
- the series resistance is roughly divided by four
- Coli capacitance is multiplied by four - in practice the multiplier is 2~3 times
(For reference see: www.buildyourguitar.com/resources/lemme/table.htm )

3) The MacSpice Net List for the design

***********************
ParallelHumbuckerCircuit
*
* By Cooltone

* Input voltage
vin 1 0 ac 1.0 dc 0.0

* Humbucker Parameters (estimated)
L_hb 1 2 1.65
R_hb 2 3 2K
C_hb 3 0 200p

*Load Capacitor and Resistor - to define resonant freq. and Q
R_Q 3 4 0.01k
C_res 4 0 1.5n

* Volume Pot
R_vol1 4 5 0.01k
R_vol2 5 0 550k

* Tone Pot and Capacitor
R_tone 4 6 550K
C_tone 6 0 10n

* Mixer Resistor
R_mix 5 7 50k

* Amp and Cable Load
R_amp 7 0 1Meg
C_cable 7 0 200pf

.control
* dispose of any 'save' statements from previous runs
delete all

* perform an ac analysis
ac dec 200 200Hz 4kHz

* plot the magnitude of the voltage at node 7 vs frequency
plot db(v(7)) xlog

.endc
.end
*******************

4) Circuit description
- At Vol pot = 10, L_hb and (C_hb +C_res) define the prime resonant frequency (see R_mix info). As the volume pot is reduced the resonance of L_hb and C_cable is attenuated by the resonance characteristic of L_hb and C_hb+C_res. This means the resonant frequency is quite stable over all vol pot levels.

- R_Q is used to manage the Q factor. Since series wiring reduces the pickup resistance, the Q factor is increased, R_Q is used to reduce the Q factor.

- R_mix, provides some mixing function when selecting both neck and bridge pickups. Since R_mix feeds into the R_amp (~ 1M), the signal loss is relatively small (< 1db). R_mix also provides further isolation of C_cable from L_hb which reduces its influence on the prime resonant frequency.

- C_tone is increased by a factor of four, since L_hb is reduced by a factor of four. As the tone control pot is reduced, the resonance of L_hb and C_tone attenuates the resonance of L_hb and (C_res + C_hb). When the tone control is close to zero, a new resonance is formed from L_hb and (C_res + C_hb + C_tone).

- Connecting the coils in series results reduced output compared to the parallel configuration, but I do not believe it is halved, I believe it is dependent on power transfer characteristics (a bit too complex for me to work out). However, I believe this can be compensated by turning up the pre-amp without introducing significant amp noise.

5) Caveats
- Many of the circuit values have been estimated, since manufacturers rarely provide characterisation data for pickups.
- I have only performed model analysis on MacSpice, so there may be some variations in practice
- Choosing a value for R_Q is just a guess. Setting R_Q to zero yields a Q of 13db, which is aggressive. 6.8k reduces this to 8db.
- I used 550k pots from CTS/Bareknuckle

I would really appreciate any comments and a sanity check!

[JohnH]

Hi cooltone, welcome to GN2. Great to see some careful thinking going on. Do you have a diagram of what you are discussing? It would be easier to comment if you can post one.

cheers

John

[cooltone]

John, thanks for the welcome. I'll post a diagram, but it may take a couple of days (8 week old twins to deal with!).

A couple of corrections:
- R_Q should be around 6.8k
- I think my comment about the coil output voltage is rubbish, it is halved. I realise what changes is the current drive. So the loss compared to series is about -4db.

cheers,

Stuart

[cooltone]

Hi John, here's the info.

Apologies first, MacSpice doesn't seem to do multiple plots, if anyone knows how to, please let me know. In each of the simulations below I have given the component values that were changed.

Schematic (note: R_Q=6.8k)



As per original netlist posted earlier with R_Q=0.01k


I changed R_Q=6.8k on all the sims below because I thought the Q was to high with R_Q=0.01k

Full Volume: R_Vol1=0.01k, R_Vol2=550k, C_res=1.5nF, R_Q=6.8k


Low Vol: R_Vol1=500k, R_Vol2=50k, C_res=1.5nF, R_Q=6.8k


V. Low Vol: R_Vol1=545k, R_Vol2=5k, C_res=1.5nF, R_Q=6.8k


Tone Responses at Full Volume
Mid Tone: R_tone=120k, R_Vol1=0.01k, R_Vol2=550k, C_res=1.5nF, R_Q=6.8k


Low Tone: R_tone=50k, R_Vol1=0.01k, R_Vol2=550k, C_res=1.5nF, R_Q=6.8k


V. Low Tone - Showing 2nd Resonance
R_tone=1k, R_Vol1=0.01k, R_Vol2=550k, C_res=1.5nF, R_Q=6.8k


Full Volume/Full Tone - with higher main resonance
R_Vol1=0.01k, R_Vol2=550k, C_res=1.0nF, R_Q=6.8k


You can see what's going on, but it's time for bed. I'll add some further comments in a couple of days (curry night tomorrow!)

Cheers S.

[JohnH]

OK, I think I get what you are doing now. So you wire the coils in parallel (which gives a good low impedance output), and then use R’s and C’s to tune the new brighter response, back down to the same tone as for series wiring, or other tonalities in between and beyond? A side effect is apparently less sensitivity to cable capacitance.

I agree you can get the original volume and gain back on the amp if needed.

Comments on the analysis:

The pickup inductance, if it’s 1.65H in parallel, must have been a very high value in its normal form? Most 8k humbuckers that I’ve seen data for, have about 4H inductance, which would become 1H in this parallel arrangement.

The cable capacitance at 200pF seems like a rather low value, unless you have very expensive or very short cables. I usually use 0.4 or 0.5nF, for a 10’ cable with about 30 to 40pF/ft, plus a bit more for the amp input. A lower value results in less change at lower volume.

I suspect that it’s OK to combine the pickup resistance with Rq, and also the pickup C and Cres. So I tried that on my spreadsheet, which has the ability to overlay two traces, and it is indeed possible to get an almost exact tone match, of parallel coils with the extra tuning parts, to the original series-wired tone. With the extra Cres involved, the cable capacitance seems to be a less important factor, and it is better then usual without treble bleed, but I reckoned still some treble bleed could further improve consistency, given that I was using a higher cable C than you.

The mixer resistors would help combine the pickups, but Id suggest testing with and without since they may change the tone by reducing interactions at full volume.

So what’s your plan? Will you build something like this with the coils hard wired for parallel, and then use R’s and C’s to change the sound?

Cheers

John

[cooltone]

John, many thanks for the feedback.

Yes, that is exactly what I doing. And yes I believe the tonality can be set by C_res to give different characteristics. This opens up a number of possibilities, such as switching cap values, I doubt if I will explore this though.

I have no equipment to characterise the pickup so I used the L values from the link given in the first post, 6.6H and divided it by four. My plan is to roll a few C_res caps until my ears tell me it's ok, starting around 1.5nF. It's annoying that pickup manufacturers don't spec their pickups.

Regarding cable capacitance, again it another guess, so thanks for the info on cable capacitance.

I have revised my thinking about how the resonance works and the effect of the cable capacitance. I believe that there is only one resonance not multiple ones. I expanded the sim up to 100kHz looking for any second resonance, but I couldn't see one. It seems that the cable capacitance pulls the main resonance. I believe this can be seen between the Full Vol graph and the Low Vol graph ~ roughly 200Hz - ish!
So if the cable capacitance is doubled, it should pull the resonant peak lower. I think the Q will be less affected so the treble response should not be affected too much. I'll have to check this out.
I take your point on the mixer resistors, I'll probably try it without first. I have reversed the bridge magnet so I'm not too concerned about the mixing facility.
My last concern is about the Q factor, what is a reasonable value to aim for. I can't set this by ear and I have no idea what I will get in reality with the values I have chosen.

Next step is to get the soldering iron out! I will go for a fixed R_Q and C_res.

One last thing, I was thinking that since the inductor impedance is lower, maybe the inductors could withstand a load of 250k pots, which may improve the tine control response. I would need to re-check the impact on the Q factor though.

Cheers,

Stuart

[JohnH]

A useful table of values for inductance, resistance and capacitance of pickups can be found here , by Helmuth Lemme and explained here:

The Secrets of Electric Guitar Pickups

Here are some more, measured by a chap on the SD forum:

Pickup data

(the cap values look too high to be just the pups alone though)

I use the above with this spreadsheet:

GuitarFreak


With your parallel wiring, you probably can use 250k pots, which will also help to lower output impedance.

I dont know what Q is 'right', but what I do when comparing tone is put in my best guess of a perfect set up that I want to match - eg a PAF humbucker with 500k pots, and use that as a reference. You could do that, then change to the parallel values and drop the pots down to 250. You might find that the lower value pots mean you need less Rq.

Having done all this though, it would be a shame not to have a simple switch to cut out the Cres and Rq, to get the full parallel tone, which is a really good clear and bright sound on its own,

John

[cooltone]

Once again, many thanks for the info John.

From the pickup data, the average Neck HB is 4.9H and Bridge 4.1H when in series, so I'll use these divided by four for my sims.

And, I agree with your comments about the 250k pots and R_Q.

Prior to this analysis, I rewired the thing too many times that I just want something finished. So phase 1 will be without a switch.

For Phase 2, I'm thinking about a switched cap circuit, rather than a switch as below:

L=1.225H
C_hb=200 pF
C_res=1.0 nF
C_stack=12 x 10 nF + 12 pole switch, common connected to node 4
C_cable=400 pF
R_tone and C_tone are removed

- The total capacitance (Total pF)= C_res+(C_stack/n)+C_hb+C_cable where n is the switch position
- F. Res below was calculated with the standard LC resonance equation.
- The stack is just a stack of 12 x 10nF caps in series with the last cap grounded and the top cap open circuit. The 12p sw picks off between each cap.

SW	Stack(pf)	Total pF	F. Res
12	833	2433	2,915
11	909	2509	2,871
10	1,000	2600	2,820
9	1,111	2711	2,762
8	1,250	2850	2,694
7	1,429	3029	2,613
6	1,667	3267	2,516
5	2,000	3600	2,397
4	2,500	4100	2,246
3	3,333	4933	2,047
2	5,000	6600	1,770
1	10,000	11600	1,335

I located a 12p sw on ebay, but my only concern is that it will be too stiff for a top hat.

The circuit is below. C1 to C 12 are 10nF each, so that the capacitance of the stack is C/n, where n is the switch position.




I'm not going to be posting for a while, so thanks for all your help John and wish you a great holiday and fab 2013!

Cheers,

Stuart

[cooltone]

Sorry it's been a while - 6 month old twins=no time!

I will be posting an update in a month or so.

In the meantime I found a really interesting site, which has some detailed analysis of pickups. I particularly like the analysis of humbuckers.

www.moore.org.au/pick001.htm

cooltone