Low Impedance Pickup Research

[nickceulemans]

Hi bbsailor,

Thank you for your reply. Your initial ideas are what inspired me to try to put my spin on the way my future instrument will convert sound into electricity...

As an architect I am very much used to working on the boundary of physics and esthetics. This is what fascinates me. Sometimes inspiration comes from some sort of esthetic search, sometimes from a simple engineering solution.

As for this pickup solution I try to study as much as I can about basic theory, but as Helmholtz also explained, I seem to have massively underestimated the reason why the laminated core is there to couple the coils to the aluminum. I will definitely do some more research on this!

Would a split core CT sensor (or two?) work to directly attach to the aluminum frame? They seem to be doing the same thing?

Thank you,
Nick

[Helmholtz]

I always thought the laminated core e.g. Lace uses, was there mainly to attach/assemble the coils to the aluminum frame. My reasoning was that by leaving this step out I could "extract" more of the electrical 'pulses' directly from the alu loop to the copper coil...

The alumitone is based on the transformer principle. The aluminum loop is the primary "coil", the other coil the secondary. The signal current in the aluminum loop produces a tiny magnetic AC field. Signal transfer from primary to secondary is accomplished by magnetic coupling. The ferrous core of a transformer concentrates the coupling field. Transformers without core are inefficient because of insufficient coupling (meaning poor signal energy transfer).

[nickceulemans]

Helmholtz, Bbsailor,

Thanks, Got it!

Attached is a revised drawing for a v2.0. My idea now is to attach an off-the-shelf coil with an iron binding screw (maybe acting as a sort of DIY laminated core?). Somehow electrically insulated from the aluminum of course. The simplest form would be 1 coil attached to 1 central leg. In the attached drawing I still have 4 coils attached to 2 legs. That way I could potentially experiment with different coils, turns, materials ect. But I can also imagine possibilities such as coils in parallel or series, blending options etc...

What do you think? Would this work in theory?
Nick

[bbsailor]

Helmholtz, Bbsailor,

Thanks, Got it!

Attached is a revised drawing for a v2.0. My idea now is to attach an off-the-shelf coil with an iron binding screw (maybe acting as a sort of DIY laminated core?). Somehow electrically insulated from the aluminum of course. The simplest form would be 1 coil attached to 1 central leg. In the attached drawing I still have 4 coils attached to 2 legs. That way I could potentially experiment with different coils, turns, materials ect. But I can also imagine possibilities such as coils in parallel or series, blending options etc...

What do you think? Would this work in theory?
Nick

Nick,

If you are targeting feeding this pickup into a high impedance amp, you will need coils with 5,000 to 10,000 turns of very fine wire, assuming that there is sufficient magnetic coupling to transform the current into a useable voltage. Based on my experiments, I do not think this will work as there needs to be good magnetic coupling to get a good signal and to overcome noise that poor coupling would create.

The current transformers that I recommend using target a typical low impedance mic mixer that is rated at 150 to about 250 ohms actual output impedance but has an actual 2,400 ohm input impedance to act as a bridging impedance to minimize loading losses. When the output impedance equals the input impedance the voltage is reduced to one half or minus 6db. Bridging impedances are typically 10 or more times higher than the actual mic or pickup output impedance to limit the loading loss to less than 10 percent.

My selection of current transformers (CTs) with 500 turns was my engineering attempt to target mic mixer input impedances rather than guitar amp high impedance inputs. There are no commercially available current transformers that have any higher than 5000 turns. However 500 turn CTs when properly loaded with a thick enough copper string loop with the magnet in the center would produce about 10 millivolts peak output and effectively feed an XLR mic input. After doing many prototypes and taking a lot of measurements, I found that the resistance of the copper string loop will effectively control the output impedance and tonal quality. Here is how this works.

The CT output impedance is very close to this calculation. Multiply the DC resistance of the string loop by the CT turns ratio squared. My use of the Triad CSE-186L requires that the three turns of AWG 16 primary winding be removed to expose a square primary window to accommodate AWG 8 wire that is about .125 inch diameter and 52 micro ohms (.000052) per inch. by bending the AWG 8 into a long hairpin loop you can slip it into the transformer primary window and only need to make one very, very, very good joint to complete the string loop. I use a copper 18-10 crimp sleeve. Manufactured: GB Gardner Bender 10-310C. I compress the round sleeve into an oval and use it to put the ends of the string loop side by side and make a tight mechanical connection that I silver solder. I also use anti oxidant to clean all the touching copper areas to ensure a good low resistance joint.

Put a thin piece of tape on the transformer lamination on each side of the primary window to prevent the copper string loop from shorting against the metal laminations.

If you need to bend the string loop down 90 degrees you need enough length after the flat layer under the strings. in this case, an 8 inch length of AWG 8 will make a string loop that is 52 times 8 or 416 micro ohms. Now multiply this number by 250,000 or the CT turns ratio squared. This gets you about 104 ohms and a very good impedance to target an XLR low impedance input. To minimize noise do this.
1. Obtain 2 conductor shielded mic cable
2. Attach the shield to the string loop and the CT metal frame by cleaning off all glue to get a good solder joint and pin 1 on the XLR connector
3. Attach the CT output to the two inner mic cable conductors and pins 2 and 3 on the XLR connector.

If you were to use a 1000 turn CT, you would have an output impedance close to 416 ohms using AWG 8 wire. You would need to use AWG 4 and a toroid transformer to handle the .187 inch wire diameter to get the output impedance near 200 ohms. Once the string loop gets very thick, you now face the skin effect that does not allow higher frequencies to penetrate to the center of the wire, making higher frequencies generate less current and thus affecting to tonal response. Science and physics tells you why something happens but your ear tells you what sounds good, better or worse.

Bottom line. You need a good transformer lamination to transform the current flowing through the aluminum frame (transformer primary) to an appropriate number of secondary turns to drive either a high impedance input or a low impedance input amplifier or other matching device. Get a low impedance pickup to work as described, then tinker with alternate designs.

Joseph J. Rogowski

[bbsailor]

Helmholtz, Bbsailor,

Thanks, Got it!

Attached is a revised drawing for a v2.0. My idea now is to attach an off-the-shelf coil with an iron binding screw (maybe acting as a sort of DIY laminated core?). Somehow electrically insulated from the aluminum of course. The simplest form would be 1 coil attached to 1 central leg. In the attached drawing I still have 4 coils attached to 2 legs. That way I could potentially experiment with different coils, turns, materials ect. But I can also imagine possibilities such as coils in parallel or series, blending options etc...

What do you think? Would this work in theory?
Nick

nick,

See this thread:https://music-electronics-forum.com/...ically+curious

Go to post number 1 that I started in Sept 2009.

Look for small audio transformers that are rated 8 ohms to 10K, or 8 ohms to 15K or 8 ohms to 20K.

Alligator clip the 8 ohm transformer side across a string on an acoustic guitar behind the nut and bridge. Attach the high impedance transformer output to a guitar amp. Hand hold a magnet near the string and pluck the string and listen.

Joseph J. Rogowski

[nickceulemans]

Look for small audio transformers that are rated 8 ohms to 10K, or 8 ohms to 15K or 8 ohms to 20K.

Hi bbsailor,
Thank you! a lot to unpack in your 2 comments. Will start to experiment next weekend!

Nick

[Helmholtz]

Helmholtz, Bbsailor,

Thanks, Got it!

Attached is a revised drawing for a v2.0. My idea now is to attach an off-the-shelf coil with an iron binding screw (maybe acting as a sort of DIY laminated core?). Somehow electrically insulated from the aluminum of course. The simplest form would be 1 coil attached to 1 central leg. In the attached drawing I still have 4 coils attached to 2 legs. That way I could potentially experiment with different coils, turns, materials ect. But I can also imagine possibilities such as coils in parallel or series, blending options etc...

What do you think? Would this work in theory?
Nick

Yes, this nice design should work in principle. But a laminated high permeability core would provide much better signal transfer. Also your "cores" don't provide a closed loop for the magnetic flux which reduces coupling and provides little interference rejcection.

[bea]

Hi, maybe the question / the idea might sound absurd - but it should be possible to use a loop/CT device for hum cancellation, shouldn't it?

Background: i have just built two basses, both with tele neck like pickups. A first estimate for a large dummy coil fitting into the electrics cavity yields about 1100 turns (of 0.2mm wire) not taking into account the effect of the alnico magnets in the pickup.

I find that a bit unhandy ... so what about using ONE closed loop to catch the noise signal and a CT with, say, 1000 or 1500 turns to transform that up?

Reasonable or crazy?

(i might also use an opamp to add the signals and to do a fine adjustment...)

[Helmholtz]

Hi, maybe the question / the idea might sound absurd - but it should be possible to use a loop/CT device for hum cancellation, shouldn't it?

Background: i have just built two basses, both with tele neck like pickups. A first estimate for a large dummy coil fitting into the electrics cavity yields about 1100 turns (of 0.2mm wire) not taking into account the effect of the alnico magnets in the pickup.

I find that a bit unhandy ... so what about using ONE closed loop to catch the noise signal and a CT with, say, 1000 or 1500 turns to transform that up?

Reasonable or crazy?

(i might also use an opamp to add the signals and to do a fine adjustment...)

Sounds like an excellent idea!

A larger loop area will produce more signal and will reduce step-up requirement (number of CT turns) and/or will allow fine tuning with resistors. As always loop wire gauge should be heavy to ensure good low frequency transfer. I guess you intend series wiring with the PUs?

[bea]

Yes, i would go in series. If i want to use a 500 turns Talema i would need to fold a large loop into 2 or 3 turns but use the Talema only on one of the turns, wouldn't i?





At least in the end both basses shall get active electronics, so addition and fine tuning would be easy (just a virtual mass amp with two iputs should suffice). But if i could obtain noise reduction as long as they are passive that would be nice.

[Helmholtz]

Yes, i would go in series. If i want to use a 500 turns Talema i would need to fold a large loop into 2 or 3 turns but use the Talema only on one of the turns, wouldn't i?

I am tired now, but my preliminary evaluation would be this:

3 primary turns will triple induced primary voltage. Transformer step-up ratio is determined by the ratio of secondary to primary turns. Thus feeding only one of the turns to the core will produce 3 times the secondary (output) voltage compared to feeding all three turns.
But feeding only one of the turns means more uncoupled external (leakage) serial inductance and this means that the output signal will contain less high frequency interference signal.
Any primary leakage inductance will be stepped up by the transformer's turns ratio and appear in series with the PU signal, reducing treble.

If you are able to increase the loop area to around 100cm², a single loop/turn should be sufficient and favorable with a 500 turns CT.
This would be comparable to the Ilitch Backplate, which uses around 400 turns embedded in a strat tremolo cover plate.

[bea]

I'm aware of the Illitch concept and similar approaches. But there is not enough space for such a large loop - it must fit into the e-cavity of that bass, which is in its upper horn:



So maybe i should try just two loops - which should give less voltage but possibly be closer to the noise signal of the pickup (1100 vs de facto 1000 turns) and hence some compensation and do the fine tuning with the active circuit.

[Helmholtz]

Beautiful and extravagant instrument!

So maybe i should try just two loops - which should give less voltage but possibly be closer to the noise signal of the pickup (1100 vs de facto 1000 turns) and hence some compensation and do the fine tuning with the active circuit.

Does the PU only have 1100 turns? In this case the loop area doesn't need to be much larger than the PU coil area using 2 turns and a 1:500 CT.( My 100cm² example was based on the assumption of a 5000 to 8000 turns PU.)

The high frequency roll-off caused by the series inductance of the CT depends on its load current (caused by controls and cable capacitance in passive circuits). Using active electronics you should be able to minimize loading and preserve most of the high frequency content.

Remember that hum cancellation gets worse if your loops are further away from the PU.

[bea]

No, the pickup has 6k DCR and hence heve something in the range of 5000-8000 turns. I did my estimate with 6600 turns.

Unfortunately the cavity is the only place for a dummy coil.

BTW: i am aware that the best solution in this case was to use a split humbucker - two coils around two pole pieces each.

[Helmholtz]

No, the pickup has 6k DCR and hence heve something in the range of 5000-8000 turns. I did my estimate with 6600 turns.

Then your loop area needs to be at least 4 times larger than the PU coil area (with 2 turns).
I don't have a proof but assume that the alnico magnets somewhat increase the hum field in the PU, maybe by 20%. So I would go for larger loop area.
Unfortunately 2 wide loops mean increased wire length and resistance which might cause some low frequency roll-off in the compensating signal.

The loops need to be insulated from each other and should be well fixed to the body. If they can vibrate they will produce some signal using the leakage field of the PU magnets.