Tone caps THD test, with polyphonic signal, through guitar PU & wiring harness

[freefrog]

Hi there,

I’ve been silent these last weeks because I was way too busy: I had to prepare an event on which my career was depending… Now that it belongs to the past, I’m back and I can do what I had promised.


LET ME (RE)INTRODUCE MYSELF !-)

I’ve already talked about my experiments of self-taught hobbyist about guitar gear: I’m 48, I play guitar (with a variable talent) and modify my gear (with a variable success) for more than 30 years. When time permits, I work as a non official (and free) guitar tech for various friends… It’s a good occasion to touch many guitar devices and to collect experience. That’s how I pay myself. During the last summer, for instance, I’ve repaired / refreshed an old Rickenbacker and a Vintage L Series Strat for a pro musician who is not a rich kid.

Last but not least: for 8 years or so, I collect data on guitar pickups and gear, each time I can do it. It would represent hundreds of files that I would already have published if I hadn’t to deal like any human being with life and “irony of destiny” (see the next section about it if necessary).

One of my young brothers, who is guitarist and engineer in electronics, helps me here and there but most of the time, I try to educate myself like a big boy. The last years, I’ve experimented here and there with tone caps...

Hence my recent posts about this subject here on MLP.

Hence this new thread, too.

I’ll take the freedom to organize my post with subtitles, in order to let my readers skip what they don’t want to know.


MY INITIATIC PATH ON SUCH MATTERS (and with "Irony of fate")

I’ve understood for years the interest of frequency analyzers with guitar gear BUT it has not been a peaceful and flat road: during a long time, I’ve used an old PC “tuned” for me and devoted exclusively to guitar gear experiments. One day, while I was experimenting on some guitar device (a floorboard), static electricity has killed the mother board of the computer… The Hard disk has itself become impossible to read. Fortunately, we’ve succeeded to save some data but I’ve lost many files in this accident and even among the saved data, some docs are now impossible to open or to use. That's something that I've rediscovered recently, while I was trying to classify my archives in order to share them with some members here or elsewhere...

So, no, sorry, I won't always be able to share my data with those who want it.

Anyway, there's some things that I wanna keep for me, for instance when it involves my work of moderator for a luthier in my country...



Right now, I keep doing frequency analysis with two recent PC’s & calibrated soundcards, but I don’t use the same softwares than in the past (several of them were in the previous PC now dead).

Nor do I use the same hardware, in fact: I’ve already killed three low impedance “exciting coils” during my tests on guitar pickups. I should be more cautious!


THE TESTS BELOW:

-guitar pickup used: a basic "Eko" single coil with a ceramic mag glued below the poles. R=6k, L= 3H, roughly. I’ve chosen this pickup because…

a)it’s an old cheap stuff that I wouldn’t use for anything else, anyway;

b)it’s a good compromise between a regular SC with six magnets and a HB with a baseplate and a single mag.


-method involved: like Helmuth Lemme, I excite the guitar pickup with a low impedance coil (600Ohms) fed by a loudspeaker output. The coil has been glued on the pickup. The pickup is plugged through a guitar harness including…

a) a 500k volume pot;

b) a 500k tone pot;

c) small banana plugs allowing to plug and unplug different caps on the tone pot.

The whole stuff is plugged through a regular guitar cable to the calibrated soundcard which feeds itself a frequency analyzer.


-program used in this case: Right Mark Audio Analyzer (a tweaked version related to a tweaked soundcard; I’ll keep the details for me if you don’t mind; each passionate hobbyist has his “secrets&#8221.


-conditions of this test:
a) a “multitone” (polyphonic) signal of -10db is sent to the pickup through the exciting coil;

b) the pickup sends its signal to the calibrated soundcard which feeds the program mentioned above, in order to measure the THD along the audio spectrum…

c) keep in mind that volume and tone pots were FULL UP in each case;

d)in each screenshot, the white line shows the THD with the tone pot grounded DIRECTLY (without cap: the outer lug is simply connected to ground thx to a jumper).


-depiction of the screenshots shared:
a) the single one shows the THD of the PU with its tone pot disconnected (green line) then with the tone pot connected to ground without cap / through a jumper (white line), as explained above;

b)the second one shows the same thing in white (tone pot to ground without cap) and, in green, the THD obtained with 4 different Mylar caps of various values (22, 33 and 2x100 nano);

c)the third picture involves a ceramic cap of 22n (left, top), a 47n PIO (left, bottom), a 22n Orange Drop (right, top), a 47n Orange drop (right, bottom).


IMPORTANT FOOTNOTES:

-I’ve NOT found the time to test each cap twice;

-I WON’T be able to redo the very same tests: trying to unglue it, I’ve destroyed the exciting coil used in these screenshots. I have been forced to wind and wire entirely a whole new one, with different materials. It has different specs and works, but… differently. Sorry about that;

-I DON’T want to decipher these pictures that I share: I admit that it could translate some experimental artifacts and I’m not sure that the results would be consistent if I had a chance to redo the whole process. I’m even wondering now if the DIRECTION in which each cap is plugged is not more important than its material…

The real specialists will comment my work and thoughts.

In the future, I’ll try to do other tests but for the reasons explained above, it will be done…

a) If time permits;

b) With a different testing gear.

Thx for your understanding and HTH.

 

[David Collins]

Thanks so much for following up and posting this. It's a crazy busy weekend for me, I'm a bit tired and will have to re-read this tomorrow with a clearer mind to digest it all, but I definitely wanted to thank you for going through the trouble.

 

[David Collins]

Well I had the chance to look it over again, and though I can't find any faults in the methods and it seems you were very good in keeping controls, I still have to say that there are enough differences there to leave me concerned that something must have slipped through. Even if the caps did cause a difference, I do believe there are deviations in those charts that could not be reasonably credited to this one factor.

I'm not familiar with the Right Mark Audio Analyzer, but are these shots spectrum analysis of an instantaneous moment of signal, or of a spectrum analysis of a section of at least a few hundred milliseconds of signal?

I do appreciate your time invested in this, and I will do my best to find time to repeat a similar test on my end. Like you however, I am currently pretty swamped with other obligations (my racks of waiting repairs are more than double-parked, with a 6-8 week backlog even on basic work), so I'm not sure when I'll get to it. I will make sure I do this when I find the time though.

 

[freefrog]

I'm not familiar with the Right Mark Audio Analyzer, but are these shots spectrum analysis of an instantaneous moment of signal, or of a spectrum analysis of a section of at least a few hundred milliseconds of signal?

It's a spectrum analysis of several seconds of signal.

The audio stimulus used in this program is basically a polyphonic series translating a kind of musical scale, with notes reminding what you can hear through a reverse echo effect...

 

[Metroboyd]

Hello, before I ask my questions, I want to let you know I have been in test and measurement (metrology) for over 20 years and have experience in many measurements and many types of test equipment. So, I am not trying to be a time waster and I do have a genuine interest.

At the risk of sounding newbish... what is the interest in THD of pickups with relation to caps?

The polyphonic signal, is that a swept signal or a audio noise source generator ?

From my understanding of what you wrote the test is conducted by applying the polyphonic signal to the wires of an EKO p'up that is in proximatey to the p'up being tested? Am I right?

 

[5F6-A]

cheers!

 

[freefrog]

Hello, before I ask my questions, I want to let you know I have been in test and measurement (metrology) for over 20 years and have experience in many measurements and many types of test equipment. So, I am not trying to be a time waster and I do have a genuine interest.

At the risk of sounding newbish... what is the interest in THD of pickups with relation to caps?

I was trying to decide if a cap affects the response of a pickup even with the tone pot full up.

I had previously done tests with a mono signal, showing the heigth of harmonics. I've thought that a THD test with a polyphonic stimulus would give a kind of "double check".

The polyphonic signal, is that a swept signal or a audio noise source generator ?

It's a series of "chords" along the audio scale. EDIT: I'm not talking about guitar chords that I'd play. These are recorded polyphonic stimuli (always the same) injected in the PU...

From my understanding of what you wrote the test is conducted by applying the polyphonic signal to the wires of an EKO p'up that is in proximatey to the p'up being tested? Am I right?

The polyphonic signal is injected in a low impedance coil (like a loudspeaker without magnet nor cardboard structure).

This low impedance coil "excites" the Eko guitar pickup.

The guitar pickup is plugged to a wiring harness itself plugged in a 1M input.

This input feeds a calibrated soundcard itself linked to a frequency analyzer "tuned" for the soundcard.

It's not supposed to be a pro test. I just do my best to do the clearest measurements possible and I share the results...

 

[jonesy]

The guitar pickup is plugged to a wiring harness itself plugged in a 1M input.

This input feeds a calibrated soundcard itself linked to a frequency analyzer "tuned" for the soundcard.

It's not supposed to be a pro test. I just do my best to do the clearest measurements possible and I share the results...

Just curious, what is the DC resistance of the guitar pickup? And what is the value of the pots and caps in the wiring harness? How is it wired modern or 50's style? Is the 1 Meg the load of the input like an amp would be or something different?

Thanks for your time and sorry for all the questions, just trying to understand your test methods and results a little better.

 

[freefrog]

Just curious, what is the DC resistance of the guitar pickup? And what is the value of the pots and caps in the wiring harness? How is it wired modern or 50's style? Is the 1 Meg the load of the input like an amp would be or something different?

Thanks for your time and sorry for all the questions, just trying to understand your test methods and results a little better.

Pickup DCR: 6k.

Pots used: 500k volume and tone pots, modern wiring.

The 1M input mimics the input of a guitar amp.

 

[jonesy]

Pickup DCR: 6k.

Pots used: 500k volume and tone pots, modern wiring.

The 1M input mimics the input of a guitar amp.

Thanks. Have you tried your tests with a different pickup like 8-9K with 50's wiring, or 250K pots with the 6K pickup? Are the volume and tone pots always on 10 or have you run tests with them rolled down to 8 or 6? And if so how much did the test results change?

 

[freefrog]

Thanks. Have you tried your tests with a different pickup like 8-9K with 50's wiring, or 250K pots with the 6K pickup? Are the volume and tone pots always on 10 or have you run tests with them rolled down to 8 or 6? And if so how much did the test results change?

General answer : I've done enough tests with guitar gear to forget many of them... but I've not an infinite free time to redo or to recall them in details... not to mention that I'm not extremely lucky in my attempts (see below why).

Precise answer: yes, I remember to have tested more powerful PU's with 250 and 500k pots, tone pots in various positions (from 10 to 0) and so on... but I struggle to give the desired consistency to my findings (see below why).

The reasons of my statements above: see my first post about the archives that I've lost in the past because of a dead PC... and please, take in account that I've recently crashed another of the two PC's that I use for my tests. This time, it was because of an Internet virus but I've been forced to format my hard disk and once again, I've lost data (including those posted in this topic): 6 months of archives, 1 year of emails gone because of a A55 H 0 LE who has found amusing to hack my computer!

It seems that Destiny doesn't want me to achieve my technical tests of hobbyist : each time I try to continue them, Murphy's law annoys me...

If I find something interesting to answer to your question in the data that I've disseminated here and there, I'll post it, though.

See ya!

 

[Metroboyd]

Freefrog, have you cruunched any of the numbers? Were you expecting to be able to determine if a cap would give a more distorted sound?

 

[freefrog]

Freefrog, have you cruunched any of the numbers? Were you expecting to be able to determine if a cap would give a more distorted sound?

My goal was to "see" if a tone cap changes the tone in a subtle way, even with the pot full up.

THD measurements with a polyphonic signal were supposed to be a way among others to show this possible difference.

I had previously tried to show it by measuring the variable height of harmonics according to the caps used, with a mono signal injected in the pickup.

But as stated in my first post, I don't want to comment my screenshots: although I've done my best to produce consistent tests, I honestly don't know if the results that I obtain are due to the caps or to another parameter that I don't understand...

If my humble opinion on my own tests has any interest, I'll express it, but not now.