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Why loading a guitar pickup loose high frequencies
Why loading a guitar pickup loose high frequencies
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Old 4th October 2021, 09:05 AM   #1
Frederico Acardi is offline Frederico Acardi  Italy
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Default Why loading a guitar pickup loose high frequencies

Hello, why loading a guitar pickup do i loose high frequencies, i should just loose signal, why do i have less brightness ?


On some DI its very dark, on other its bright, i know its about loading and input impedance, but why it affects the high frequencies ?


Thank you.
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Old 4th October 2021, 09:43 AM   #2
dotneck335 is offline dotneck335  United States
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A guitar pickup is an inductor. Its impedance rises with frequency. Higher frequency---higher source impedance----more affected by load impedance.
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Old 4th October 2021, 09:47 AM   #3
Frederico Acardi is offline Frederico Acardi  Italy
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Ah ok, so it form a low pass filter, thank you!
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Old 12th October 2021, 11:37 AM   #4
Bill Coltrane is offline Bill Coltrane  Netherlands
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Variable impedance is ime a very useful feature in any preamp.
Not many have it unfortunately.
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Old 14th October 2021, 08:05 PM   #5
shanx is offline shanx  Canada
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You could build an active preamp DI, and make the input impedance variable.The passive DI units are pretty limited to the transformer characteristics.
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Old 14th October 2021, 09:36 PM   #6
gab far is offline gab far
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A few years ago I did some experiments with guitar preamp input impedance. I built a high impedance preamp -- input impedance of 3 megohms or so -- and then tried different shunt resistors across the input to see how they affected the sound. The guitars I plugged into it were a Fender USA Stratocaster of 1980s vintage, and a Gibson Les Paul of about the same age. With the Strat I was barely able to detect a loss of brightness (high frequency production) with about 200 or 300 kilohms for the shunt resistance (i.e. a 200-300k loading of the guitar and guitar cord). The volume and tone controls on the guitars were all full on. This was a barely detectable loss of highs, listened to through good quality headphones. Through a guitar speaker, you wouldn't notice it.

Interestingly, the Gibson, which had humbuckers and therefore, I would expect, a higher inductance, could drive a lower load resistance without noticeable loss of highs.

At the other end of things, you don't want too high an input impedance because it results in more noise, especially EMF interference. Things get noticably worse past 1 megohm.

So my conclusion was that, if you're only plugging the guitar into a single input, that input should have an impedance of about 330 - 470 kilohms. That's high enough not to kill much treble, but no higher, so that noise is minimised. In practice, a higher input impedance may be desirable because some people plug a guitar into two amps at once using a simple Y splitter on the cord; then you'd want about double the input impedance on each amp. So perhaps the traditional Fender amp input impedance of 680 kilohms is about right.
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Old 14th October 2021, 10:59 PM   #7
MiguelMatador is offline MiguelMatador  United States
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The pickup, the volume control, the tone capacitor, and the cable, essentially reduce down to a RLC resonant circuit. Increasing 'C' (via the cable) lowers (e.g. shifts down in frequency) the resonant peak of the combination of components, which makes the perceived tone 'darker'.

An active buffer between the output of the guitar and the cable will shield the RLC circuit from the capacitance of the cable, because the output Z of the buffer is small in comparison to the RLC circuit.
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Old 14th October 2021, 11:25 PM   #8
Bigun is offline Bigun  Canada
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Why loading a guitar pickup loose high frequencies
Guitar pick up is like a giant phono cartridge, so h.f. response is down to the capacitance of the looooong hook up cable and capacitance of the input stage of the amp. All the same tricks they talk about with phono amps and cables apply.
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Old 15th October 2021, 12:10 AM   #9
bucks bunny is offline bucks bunny  Germany
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Consider the factory wiring of a fender strat and you will find it loaded by the vol pot (250k or so), and by the tone pot as well. Thus input impedance above 500k will make no difference.

But if you rewire the guitar and put your pre-amp close to the P.U., i.e. you create an active p.u, you will notice differences upto 1meg load resistance. This is what I did.
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Old 15th October 2021, 10:59 PM   #10
egellings is offline egellings  United States
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Buffer the pickup at the guitar, if possible.
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