Low Impedance pickup - prototype

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Low Impedance pickup - prototype

Postby Beate Ritzert » Thu Sep 20, 2018 9:02 pm

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.

Image

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

Postby Matthew Orifice » Wed Jul 31, 2019 11:25 am

almost done.jpg
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
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Re: Low Impedance pickup - prototype

Postby David King » Wed Jul 31, 2019 3:55 pm

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.
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Re: Low Impedance pickup - prototype

Postby Matthew Orifice » Fri Aug 02, 2019 10:43 am

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.
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Re: Low Impedance pickup - prototype

Postby Matthew Orifice » Fri Aug 02, 2019 11:00 am

maybe i'll grab some rubber mags this weekend and play with it , i'll let you know what i find
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Re: Low Impedance pickup - prototype

Postby David King » Sat Aug 03, 2019 5:24 pm

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
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Re: Low Impedance pickup - prototype

Postby Matthew Orifice » Fri Aug 09, 2019 10:33 am

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).
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Re: Low Impedance pickup - prototype

Postby Beate Ritzert » Tue Aug 20, 2019 7:43 pm

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.
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Re: Low Impedance pickup - prototype

Postby Alan Peterson » Wed Aug 21, 2019 9:15 am

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.
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Re: Low Impedance pickup - prototype

Postby Beate Ritzert » Wed Aug 21, 2019 11:46 am

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.
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Re: Low Impedance pickup - prototype

Postby Beate Ritzert » Thu Aug 29, 2019 3:56 pm

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.
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