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  #1  
Old 12-02-2018, 12:07 PM
Monsum Monsum is offline
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Default 500k vs 10M impedance

I've bought some new gear which has a Di input with 500k impedance and I'm trying to see if this can be used with a K&K pure mini pickup.
So I recorded my guitar (with the K&K installed) directly through two devices one with 500k and the other with 10M impedance. I tried to match the volume as best I could.
Can you tell which sample has lower impedance? I'll reveal later the order of samples.



Another question regarding impedance - companies like Fishman and LR Baggs use very high input impedance in their preamps (10Mohm).
The reason everybody states is that piezo pickups need that. But is this necessary for active piezo pickups or just passive ones?
If it isn't, what is right(or good enough) impedance for active piezo pickups?
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  #2  
Old 12-02-2018, 12:36 PM
jonfields45 jonfields45 is offline
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They sound the same to me. High impedance (>> 10K ohms) is required for passive piezos and passive magnetic pickups. Whether too high can be a problem for some passive piezos (large multi-head designs), or if even higher is better for other passive piezos (passive USTs), is much discussed and if you can't hear it then for your equipment it does not matter. Impedance is a vector and you are being quoted only its magnitude in a data sheet. The capacitive reactance of that vector is also important for predicting its impact on a piezo which is why the tests reported on the AGF for this controversy can't tell the whole story. Also, variation across different manufacturing runs and limits in cost effective production testing makes me inherently suspicious of some of these specs.
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Last edited by jonfields45; 12-02-2018 at 12:42 PM.
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  #3  
Old 12-02-2018, 12:41 PM
SpruceTop SpruceTop is offline
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Not much difference but I'll vote for the first section of your comparison recording as being the 10 MOhm example as it sounds just a bit smoother than the second one.

Only passive pickups may need 10 MOhm impedance.

I think from following AGF discussions that the K&K Pure Mini likes a 1 MOhm impedance for best overall tonal performance.
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Old 12-03-2018, 07:51 AM
varmonter varmonter is offline
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1mohm is what i've heard as well.
I can't tell much dif in the two. perhaps
a tad more low end response in the second
recording. like Jon said if it sounds ok with
your equipment then its good. my kk equipped
martin has sounded good with their preamps and my felix.
at 1mohm. And it's sounded awful at open mics
where i'm at the mercy of whatever's there.
So i think impedance can make a difference.
Best test is how it sounds at volume at a gig.
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Old 12-03-2018, 11:31 AM
Monsum Monsum is offline
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Thanks guys for your input.
What you said confirms what I hear as well - not much difference between the two, in a blind test I cannot distinguish which one has lower impedance. By the way, the first part in the recording is 10Mohm, followed by 500kohm.

I know this subject has been discussed many times on AGF and I think I read it all. A statement often repeated is that 10Mohm will make K&K sound boomy and anything less than 1Mohm will sound thin and harsh.
This is not my experience or is a result of this test. I haven't included a 250kohm recording in this test but with a 250k, I can clearly hear some bass lost although it is still a useable tone.
Interestingly, here on AGF people associate too low impedance with lack of bass but in other places on the internet it's the opposite - losing highs. Which one is true?
It seems though that not many people actually do any tests themselves but rely on someone else's opinion (which again was also based one someone's opinion found online).
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Old 12-03-2018, 01:01 PM
jonfields45 jonfields45 is offline
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Quote:
Originally Posted by Monsum View Post
Interestingly, here on AGF people associate too low impedance with lack of bass but in other places on the internet it's the opposite - losing highs. Which one is true?
Turns out both are true.

For a piezo which is actually just a capacitor with a funny dielectric, low impedance will let that capacitor attenuate the lows.

For a magnetic pickup which is actually just an inductor wrapped around a magnet, low impedance will let that inductor attenuate the highs.

Now this gets even more complicated if the input impedance of your amp/mixer/etc. is not resistive (which for higher impedances it usually becomes at least somewhat capacitive).

And one last comment as a retired electrical engineer with a boat load of electronics manufacturing experience. Most specs are "guaranteed by design" which means the production line does not check if they were actually achieved. At the end of the day you must try your gear and decide on what you hear. Reading manuals and data sheets is interesting and might save you some trouble but won't guarantee you will like what you hear.
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Last edited by jonfields45; 12-03-2018 at 01:09 PM.
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  #7  
Old 12-04-2018, 12:55 PM
agedhorse agedhorse is offline
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In general, 1M is just about the sweet spot between flatness of response, noise and electrical damping of the pickup. There are some pickups that do benefit from >1M input impedance (the small piezo crystal based Underwoods for example) but in general my 40 year experience in both the pickup industry designing pickups (primarily ~30 years ago) and in the amp industry designing acoustic amps has led me to conclude that much of the hype behind the very high input impedance perspective comes from the marketing side.

A (piezo) pickup is an electro-mechanical device which can be crudely represented (or modeled) by a spring-mass system mechanical model. It will have inertia, momentum and resonance. The input impedance acts like damping on the spring, which can be underdamped (oscillates wildly), critically damped (oscillates in a defined, controlled manner) or overdamped (no longer oscillates, dies out as energy is extracted faster than it is able to be replenished). Since the piezo element is modeled as a capacitor, the the equivalent electrical equations would apply and these effects apply primarily at lower frequencies where the pickup has it's highest effective impedance.

Of course, this is difficult for marketing folks to effectively (and profitably) convey, causing them to more often focus on a simple, more easily discussed number (that conveniently fits into the "higher number is better" advertising model)
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Old 12-05-2018, 02:31 PM
guitaniac guitaniac is offline
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Quote:
Originally Posted by Monsum View Post
Thanks guys for your input.
What you said confirms what I hear as well - not much difference between the two, in a blind test I cannot distinguish which one has lower impedance. By the way, the first part in the recording is 10Mohm, followed by 500kohm.

I know this subject has been discussed many times on AGF and I think I read it all. A statement often repeated is that 10Mohm will make K&K sound boomy and anything less than 1Mohm will sound thin and harsh.
This is not my experience or is a result of this test. I haven't included a 250kohm recording in this test but with a 250k, I can clearly hear some bass lost although it is still a useable tone.
Interestingly, here on AGF people associate too low impedance with lack of bass but in other places on the internet it's the opposite - losing highs. Which one is true?
It seems though that not many people actually do any tests themselves but rely on someone else's opinion (which again was also based one someone's opinion found online).
I noticed that one of my Mini Pure-equipped guitars was much bassier through a Baggs PADI (EQ set flat) that through my old Platinum Pro EQ preamp (EQ set flat) even though both had a stated input impedance of 10Mohms. One has to suppose that there are other factors, aside from input impedance, involved in the differing results.

I recall when Doug Young did his experiment with his Grace Design Felix preamp/blender. The Felix enabled him to test input impedances of 10Kohms, 332Kohms, 1Mohm, 10Mohms and 20Mohms. I was interested in the Barbera Soloist pickup at the time, and recall that it sounded very thin and trebly with the 10Kohms input impedance. All the other input impedances seemed to work pretty well. (The recommended input impedance for the Soloist is 1Mohm.)

BTW, I once took apart a DTAR Wavelength system and tried preamping the passive Wavelength transducer with two different preamps which had stated input impedances of 10Mohms. My results were extremely thin and trebly. When I asked DTAR co-founder Rick Turner about it, he commented that the Wavelength transducer requires an input impedance "in the neighborhood of 100Mohms".
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Old 12-05-2018, 04:57 PM
jonfields45 jonfields45 is offline
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I guess I've posted this too many times, but Leo Fender when he studied tube application notes and decided to make guitar amps used a 1M ohm grid bias resistor for the first tube stage at the input of his amps. Ever since 1M ohm has been a kind of default standard for guitar pickup loads. A piezo pickup with sufficient oomph (like a K&K) loves 1M, where as the tiny polymer piezo in an Element or Matrix would sound terrible (that series capacitance would kill the bass).

K&K and JJB could have designed their PUPs to need a higher impedance and it would be as simple as making the heads (much?) smaller, but that 1M ohm default was no doubt in their minds as they sought a fairly usable no preamp design.
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Old 12-06-2018, 08:05 AM
guitaniac guitaniac is offline
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Quote:
Originally Posted by jonfields45 View Post
I guess I've posted this too many times, but Leo Fender when he studied tube application notes and decided to make guitar amps used a 1M ohm grid bias resistor for the first tube stage at the input of his amps. Ever since 1M ohm has been a kind of default standard for guitar pickup loads. A piezo pickup with sufficient oomph (like a K&K) loves 1M, where as the tiny polymer piezo in an Element or Matrix would sound terrible (that series capacitance would kill the bass).

K&K and JJB could have designed their PUPs to need a higher impedance and it would be as simple as making the heads (much?) smaller, but that 1M ohm default was no doubt in their minds as they sought a fairly usable no preamp design.
Thanks for posting that again, Jon. The info about Leo Fender is new to me.

I do seem to recall some comments on the AGF that there's a downside to using a greater-than-necessary input impedance, but I don't recall exactly why that is.
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Old 12-06-2018, 08:21 AM
agedhorse agedhorse is offline
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Quote:
Originally Posted by guitaniac View Post
Thanks for posting that again, Jon. The info about Leo Fender is new to me.

I do seem to recall some comments on the AGF that there's a downside to using a greater-than-necessary input impedance, but I don't recall exactly why that is.
Drawbacks of higher than ideal input impedance are increased circuit noise, increased RFI potential, loss of damping of the piezo element, increased triboelectric cable noise, etc.
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Old 12-06-2018, 08:52 AM
guitaniac guitaniac is offline
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Quote:
Originally Posted by agedhorse View Post
Drawbacks of higher than ideal input impedance are increased circuit noise, increased RFI potential, loss of damping of the piezo element, increased triboelectric cable noise, etc.
Thanks for the info. I'm about to do an experiment where I'll be swapping out an Element UST for an Open To Source Sensor UST (designed by David Enke, formerly of PUTW). I'm concerned that excessive input impedance might be an issue with the Element's "barn door" style preamp. The swap-out itself will be very easy, as the pickup connects with a mini-plug, and can be threaded up through the same slanted access hole in the bridgeplate. At worst, all I'll need to do is widen the access hole a tad for the slightly wider (1/8") OTSS UST.

Last edited by guitaniac; 12-06-2018 at 08:58 AM.
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Old 12-06-2018, 04:50 PM
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Quote:
Originally Posted by varmonter View Post
1mohm is what i've heard as well.
I can't tell much dif in the two. perhaps
a tad more low end response in the second
recording. like Jon said if it sounds ok with
your equipment then its good. my kk equipped
martin has sounded good with their preamps and my felix.
at 1mohm. And it's sounded awful at open mics
where i'm at the mercy of whatever's there.
So i think impedance can make a difference.
Best test is how it sounds at volume at a gig.
You've touched on a peeve of mine. Yes, every open mic should have something to match impedance, but I'm constantly amazed at the uninformed out in the world, both from the player and the host.People put K&Ks in their guitars and show up at open mics, and hosts have impedance switches on their boards and haven't a clue. So showing up at an open mic without a DI is not a good thing to do if you care about your sound.
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Old 12-06-2018, 06:01 PM
jonfields45 jonfields45 is offline
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Quote:
Originally Posted by agedhorse View Post
Drawbacks of higher than ideal input impedance are increased circuit noise, increased RFI potential, loss of damping of the piezo element, increased triboelectric cable noise, etc.
Well assuming you need multiple heads in your piezo PUP because they produce a different signal (some have more high E and some more low E, for example), they are not putting out the same signal in the same phase. As such they will load each other and form a low pass filter (one that is quite strange sounding depending on the signal difference in amplitude and phase). In such a situation you want a "dominant time constant" which is that 1M input impedance load on your pickup. Otherwise with an unbounded large load impedance, the out of phase different signal heads can load each other with all sorts of weird low pass characteristics.
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Last edited by jonfields45; 12-06-2018 at 08:55 PM.
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