Low Impedance pickup - prototype

[Beate Ritzert]

Just finished the prototype of a LoZ pickup. Works nicely into a microphone input, and sounds very natural / open. As "acoustic" as one might get from flatwound strings. This is a current transfomer, primary a single loop of 10 mm^2-wire combined with a secondary of 500 turns (Talema AX-500). Same principle as the Alumitone, but low impedance / large bandwidth. I could imagine that it would sound great on archtop guitars.



TODO: control the bass response - it is a bit too low for an electric bass. Find a place for the secondary loop.

[Matthew Orifice]

8 string bass with a 12 string treble harp

I've been working with this concept as well, ( i think we have the same transformer type). i'm planning on experiment with wiring pairs of them to get the top range cancellation that happens with Humbuckers. and having a mini switch to kill one coil for a more S-ish sound

[David King]

Cool execution Beate, I wonder if bass response could be affected at all by using different magnet types. People hear all kinds of changes to tone when there are 5000 or 10000 turns of primary coil and the magnetic material is exchanged. I wonder if going to rubber magnets ( as used in Lace sensors) might dampen the sound a bit.

[Matthew Orifice]

i went with available, but from what i've read the type of magnet doesn't have a huge effect ( counter intuitively) i've been considering doing a double coil to get some high cancellation, possibly with a push button switch to make it switchable from double to single coil. the bass has good response. but the neo's really make you keep the pickup down.

[Matthew Orifice]

maybe i'll grab some rubber mags this weekend and play with it , i'll let you know what i find

[David King]

Most paper clad printed refrigerator magnets are magnetized in very close lines of alternating polarity. They do this to increase the holding power but they won't work for pickups. Search for flexible magnets or rubber ferrite magnets. Something like this: https://www.adamsmagnetic.com/flexible- ... gnet-strip

[Matthew Orifice]

thanks , have you put any thought into Pots for a circuit, i've been trying to do research on it but not getting much of anywhere. I'm finding conflicting answers about whether standard pots and caps will be okay (the bass harp doesn't seem to be effected by the strat volume and blend control).

[Beate Ritzert]

The pickup demands for a microphone amplifier, both regarding sensitivity as well as input impedance - which should not be larger than 2 kOhms in oder to damp the treble resonance. The latter is important - if undamped it could harm any tweeter. In these respects these príckups are very similar to traditional low impedance pickups: my prototype is as loud as the traditional LoZ pickup with 500 turns on the guitar is showed above.

It is actually possible to build such pickups with the sensitivity of, say, a tele neck or a strat pickup: use a current transformer with more windings, use an additional step up transformer. Like in any other guitar with low impedance pickups. These prototypes would run very nicely with the circuitry of the Gibson Les Paul Recording.

But anyway - i would use an active cricuit, a preamp stage with ~20 dB amplification (which would yield normal guitar pickup output) and then an adjustable lowpass (state variable) filter, or maybe one per pickup. Such things are commercially availble but admittedly rare.

[Alan Peterson]

On the topic of state variable filters, any opinion as to how this may work with the Craig Anderton "Super Tone Control", found almost anywhere on the web (as well as his books)?

I'm thinking if you drop one of those into a guitar body along with two of your Lo-Z pickups, you basically have a homebrew Alembic.

[Beate Ritzert]

Short research - Craig Anderton "Super Tone Control" tells me nothing - this seems to be an implementation of a SVF. It'll probably need a preamp stage to raise the signal level. Keep in mind that the treble resonance has to be thoroughly damped out to linearity with any guitar pickup. Unless You want to achieve "special effects" - the idea of the SVF is to model a variable treble resonance.

Here a summary of the theory: https://www.electronics-tutorials.ws/fi ... ilter.html

And here - from talkbass - a sketch of the alembic implementation: https://www.talkbass.com/attachments/al ... g.2361102/

Instead of using active circuits it is also possible to manipulate the treble response as usual using RC circuits (C-switch) - but of course due to the low impedance the capacity values become unhandy. In addition to modifying the treble resonance peak i find it useful also to add a traditional active tone control, maybe a (big muff alike) PI control.

[Beate Ritzert]

Just another use of that approach: hum cancellation coils. A single closed loop of thick wire (or maybe a few) and a suitable Talema AX to transform the level of the noise induced into the loop. The products of loop area times turns of the transformer must match the loop area of all windings of that pickup. Estimate that value for the PU (roughly), estimate the area of that loop and compute the number of turns necessary in the seconadary of the transformer. Than take the closest matching Talema. Wire that in series and chose the polarity so that hum cancellation occurs.

In the example below (the Sharkfin bass i recently showed You) one single turn of 6mm² and a Talema AX-1500 were matching. A larger electronics-cavity would have made fewer turns on the Talema possible.

The resistors and the condensor do not belong to the hum cancellation unit; this is simply to compensate for the load of the pots and some fine tuning of the sound.

[Rob Kidd]

This is a really interesting thread. I was at the namm show in '06 ( or was it '07...?) when I saw/heard the aluma-tones for the first time. Have been interested in this stuff ever since. Is the output V of your prototype comparable to a regular high inductance pickup?

the idea of the SVF is to model a variable treble resonance.

Here a summary of the theory: https://www.electronics-tutorials.ws/fi ... ilter.html

And here - from talkbass - a sketch of the alembic implementation: https://www.talkbass.com/attachments/al ... g.2361102/

A few years ago, I was part of a TB thread that looked at simplified versions of the alembic/wal SVF implementation. I posted a few schematics using a sallen-key LPF with a switched resonance control. One of the other guys over there has turned the same idea into an open-source bass preamp design. I have a variation of the circuit in a bass with a musicman style parallel HB. It is very effective with this sort of pickup.

Here's the thread:

https://www.talkbass.com/threads/a-diff ... r.1146653/

hum cancellation coils. A single closed loop of thick wire

I think this idea is patented and on the market as a pickguard with a loop underneath. I can't remember the product name though...

but of course due to the low impedance the capacity values become unhandy.

Have you thought about winding your own transformer secondaries?

[Beate Ritzert]

This is a really interesting thread. I was at the namm show in '06 ( or was it '07...?) when I saw/heard the aluma-tones for the first time. Have been interested in this stuff ever since. Is the output V of your prototype comparable to a regular high inductance pickup?

The one You quoted is a compensation coil which ich taylored to a large output by the large size of the primary loop.
My pickup prototype was low output, suitable für typical microphone inputs. So it corresponds to typical traditional LoZ pickups. It is possible to raise the output signal to the level of HiZ pickups. That requires more windings in the CT - like with the alumitones. A german guy has made a humbucker according to this princible with roughly the output of a strat pickup.

As i would like to respect the policy of this site, i can unfortunately not post a link to the corresponding forum thread - only as PM.


BTW: getting the output up will also affect the voicing. The bandwidth will reduce.

[Mark Wybierala]

I'm at a complete loss of what you folks are discussing Anybody got a link to describe this low impedence pickup thing and transformer stuff?

[Beate Ritzert]

If it was not against the policy of this forum, i would have posted to links into other forums where that stuff is being discussed at length.

But let me try to describe what's going on: imagine a traditional magnetic pickup. its 5000-10000 windings lead to the well known behaviour - relatively large output voltage, large impedance, narrow frequency range.

now reduce the number of windings to, say, 500. output lower by a factor of 10-20 in the range of dynamic microphones, frequency range a lot wider. You can use a traditional transformer or an autotransformer to rise the output voltage (had been done in the Gibson LP recording). But this would again decrease the bandwidth of the system. Instead You can of course feed that into the microphone inlut of a mixing console.

further reduce the number of windings, to the extreme of 1 winding. again massive reduction of the output, and the easiest method to achieve at least a microphone signal is to use a transformer 1:500. An easy and cheap setup for this is to use one single loop of really thick wire as the primary and a cheap current sensor unit as the secondary.
again, it is fully up to You how to chose the parameters - You can of course use a rigid metal frame around a magnet and a current sensor with 5000-10000 windings. Which will give You a signal close to that from a HighZ pickup, including the limited bandwidth. Such a concept is realised in the alumitone pickups.

[David King]

I, having never read through the MIMF policies, feel quite free to post a link to MEF. That said Beate sums it up quite nicely and understandably.
Link: https://music-electronics-forum.com/sho ... php?t=5447

[Beate Ritzert]

Well, if such links are ok, here two links to a german forum where the theory is explained as well (all with references to te MEF but independent) and with a few interesting experiments which are not shown in the MEF discussion. One is about high output variants of the principle, AFAIR the 2nd thread:

https://www.gitarrebassbau.de/viewtopic.php?f=35&t=7924
https://www.gitarrebassbau.de/viewtopic.php?f=35&t=7855

[Mark Wybierala]

Thank you for the clear explanation. I very much appreciate explanations that are as clear as yours. With modern electronics being much more efficient than in the '50s, '60s and '70s (when most of the traditional pickup designs were developed), there probably are advantages that haven't been exploited yet. I'm playing around with rare earth magnets and looking at the advantages and complications. Designing pickups is totally hit or miss. The current idea in my head is that a stronger magnet at a greater distance from the string is less susceptible to minor changes in string distance from the pickup. You could actually separate the location of the magnet from the structure of the pickup coil. A magnet that does not have a restricted location or field alignment in reference to the coil may yield interesting results. ...variable coil alignment... Its a can of worms and I'm sure its already been done but its fun just the same.

[Veronica Merryfield]

These current transformer based pickups always sound more trebly and less bassy. There is going to be a d/dt limitation at the bass end of the spectrum anyway but with an increased high end it simply sounds less bassy, but they do give a very clean output. The tonal aspects can be manipulated with post processing. I rather like them personally.

Anyone ever used a hall effect device on the single turn to make an active version?

SVF in an analog implementation require 3 op-amps, 4 if you need a notch output. They do give you control of F and Q though and these days with cheap high quality SMT op-amps, the circuitry can be made much smaller than a ganged pot for control.

[Beate Ritzert]

The main reason why a currently not use them is that they are pretty large preventing from e.g, making hexaphonic humbucking pickups or very narow or very flat pickups. This seems to be better done with traditional low impedance pickup designs. From my prototype i conclude that the narrowst shape will be a strat pickup. the transformer itself is actually too large to fit into a tele neck shape.

I can imagine them really well as tranducers of the acoustic sound of steel stringed instruments. Guitars, maybe double bass.