Question about input impedance

[MusicGeek]

When I bought the Helix, the input impedance at guitar input was set at 1MOhm, but as there was also an "auto" setting, I chose that option because I thought that Helix automatically set the best impedance for the guitar.

But now I have a doubt: if "auto" seems a good option, why the default option is 1 MOhm?

 

[mikisb]

Hi,

 

while most modern guitar amplifires have a 1 MOhm input impedance, this would be a good value as general default. Some effects originally have other (mostly lower) input impedances, what affects the sound if these effects are put in the first place directly after a guitar. So if you set the value to auto, this behaviour is reproduced. But as some may wonder why a effect (delay for exemple) can change the base sound of the instrument as much when its in the first place and not, when its in another place i suppose, line6 had set the default to 1 MOhm.

 

I can't imagine that in "auto", the Helix is adapting the impedance to your guitar as he don't now what guitar it is. It is just reproducing the original device witch is the first after the guitar.

 

What's the influence of the input impedance? Each magnetic pickup is  - in the electrical point of view - characterized by a resonance frequency (usually between 1500 and 6000 Hz) and the Q value wich describes how much more output the pickup delivers at the resonance frequency in relation to the average.

Resistive loads lower the q factor, so the overwight of the resonance frequency gets smaller. So if the amp hast a smaller value as input impedance, the described peak is more flat, as the load is bigger. The resistive load like input impedance and the combination of volume and tone pots does not or just a little bit affect the frequency itself but just the strength of the peak.

 

The capacitive load, specially from the guitar cable, lowers the frequency itself. With a longer cable, the capacity gets bigger and the resonance frequency lower as with a shorter cable. This can change the character of your guitars sound dramatically - often more then a new/other pickup.

My main guitar delivers a resonance requencs of about 2500 Hz with a 2 meter cable. with a 6 m cable it falls down to about 2000 hz wich make a pretty noticable difference in sound if the Q factor is not to small and there is a clear peak.

Some pickups in certain environments have a small Q factor and no really peak, so the resistive Load/input impedance is a bit less important. If you have no peak at 3 or no peak at 2 KHz is about the same ;) But cause after the resonance frequency the curve falls down with about 18 dB/oktave, there still is a difference to hear. So between dull and a bit more dull :)

 

Just to makevisible  the relation between resistance and capacity and pickup: This is a pickup i wound yesterday and measured. Nearly all pickups measuring in the same way, only the output power, resonance frequency and Q factor are different. The load values, the pickup was mesaured with are writen in the picture

[spikey]

An easier way to use this setting is using your ears. Do what sounds best to you.  :)

[PeterHamm]

...I can't imagine that in "auto", the Helix is adapting the impedance to your guitar as he don't now what guitar it is. It is just reproducing the original device witch is the first after the guitar...

 

Yeah, I'm sure it's not doing that, too.

 

What is really nice about this is that if you decide to put a fuzz first in the chain (where it should ALWAYS be, even before a buffer on a pedalboard) then the impedance will switch, not based on the guitar plugged in, but on the effect's needs.

[MusicGeek]

Thanks for the explanation :-)

So if I understand correctly, 1MOhm is like overriding the true behavior of the first pedal in the chain

[mikisb]

...

 

Thanks for the explanation :-)
So if I understand correctly, 1MOhm is like overriding the true behavior of the first pedal in the chain

....if this pedal has'nt itself 1 MOhm, what's mostly the case, but not always.  But yes - that's the point.

[willjrock]

Hi,

 

while most modern guitar amplifires have a 1 MOhm input impedance, this would be a good value as general default. Some effects originally have other (mostly lower) input impedances, what affects the sound if these effects are put in the first place directly after a guitar. So if you set the value to auto, this behaviour is reproduced. But as some may wonder why a effect (delay for exemple) can change the base sound of the instrument as much when its in the first place and not, when its in another place i suppose, line6 had set the default to 1 MOhm.

 

I can't imagine that in "auto", the Helix is adapting the impedance to your guitar as he don't now what guitar it is. It is just reproducing the original device witch is the first after the guitar.

 

What's the influence of the input impedance? Each magnetic pickup is  - in the electrical point of view - characterized by a resonance frequency (usually between 1500 and 6000 Hz) and the Q value wich describes how much more output the pickup delivers at the resonance frequency in relation to the average.

Resistive loads lower the q factor, so the overwight of the resonance frequency gets smaller. So if the amp hast a smaller value as input impedance, the described peak is more flat, as the load is bigger. The resistive load like input impedance and the combination of volume and tone pots does not or just a little bit affect the frequency itself but just the strength of the peak.

 

The capacitive load, specially from the guitar cable, lowers the frequency itself. With a longer cable, the capacity gets bigger and the resonance frequency lower as with a shorter cable. This can change the character of your guitars sound dramatically - often more then a new/other pickup.

My main guitar delivers a resonance requencs of about 2500 Hz with a 2 meter cable. with a 6 m cable it falls down to about 2000 hz wich make a pretty noticable difference in sound if the Q factor is not to small and there is a clear peak.

Some pickups in certain environments have a small Q factor and no really peak, so the resistive Load/input impedance is a bit less important. If you have no peak at 3 or no peak at 2 KHz is about the same ;) But cause after the resonance frequency the curve falls down with about 18 dB/oktave, there still is a difference to hear. So between dull and a bit more dull :)

 

Just to makevisible  the relation between resistance and capacity and pickup: This is a pickup i wound yesterday and measured. Nearly all pickups measuring in the same way, only the output power, resonance frequency and Q factor are different. The load values, the pickup was mesaured with are writen in the picture

AC PT Alni3 8000+9000 ser..jpg

Epic!!! ......put it in the manual.  ;)

[zooey]

Hey mikisb, great answer! Not sure I would have responded with that level of detail and background, but you did, and made it super clear and understandable. Nice work!

 

Just to say it, an active buffer in your guitar, or (some flavors of) active pickups make cable, amp, and stomp loading much less important, because they lower the output impedance of the guitar, so it's more able to drive whatever's out there. Active output also keeps the impedance low when you turn your guitar volume down, preserving the highs you'd otherwise lose to cable capacitance, more and more the further down you go.

 

That's why I typically use a buffer, to keep the tone consistent, at any volume, and in different environments.

[mikisb]

Thanks folks - as i'm just in my pickup winding phase, it was'nt too difficult to me ;)

But i'm happy that you understood what i mean cause my english is'nt really good.

If you ask me next week, i've probably forgotten the half of what i learned in the last weeks.

 

A last word when guitars come to active: As zooey said, the input impedance and cable capacity becomes way less important in this case.

But the sound changes for the same reasons, as the cable capacity is "lost" and therefore the pickups resonance frequency will rise. This can cause a harsh and shrill sound not everybody likes and wich the first active guitars where known for.

To sound active as with the former cable, just add an  impedance in the input of the buffer/booster in the guitar with about the same value as the cable had. Just put it from the hot wire to the ground. With 90 pF per Meter cable it's  quite ok and the guitar will sound like with cable before but with less cable and impedance influence ;)

I think in the wireless systems of line6, they include a input capacitor just to simulate the cable - this works without modelling just with a part for a few cents.

[mykejb]

From reading this I'm guessing that the impedance won't really matter if the Helix is being driven by the output of a G70? In this case is the best setting Auto or 1M ?

 

Mike

[mikisb]

Hi Mike,

 

do you know the output impedance of the G70?

Nearly every active device has a low output impedance compared to passive guitar/bass pickups, so it should be no difference it you choose 1 MOhm or auto.

[mykejb]

Not sure, I'll have to go check the specs and see if it's published.  Either that or I'll just wait till the Helix arrives and experiment :)

 

Mike