VG-88V2 Subsonic Input Filtering? (for RMC)

Started by Virtual Madness, July 06, 2008, 11:45:53 AM

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Virtual Madness

I was thinking of Richard McClish (RMC) when this question occured to me, but feel free to jump in if anyone has any info about this..Somewhere online i saw some technical info about switching out some electrolytic capacitors in the VG-8 front end (to filter out the subsonic frequencies that mess with the pitch shifting if you use Piezo Pickups), and I recently got the VS9F subsonic filter for the VG-99, (This thing is great Rich, it solved ALL my VG-99/Piezo problems!) but I can't seem to find subsonic filtering info ANYWHERE about the VG-88V2!..Does anybody have this info (what part numbers do i take out, and what part numbers do i put in) AND is there anybody here that would consider letting me pay them to do this work for me?..I've never soldered on a circut board before, and I don't want to start with the VG-88!..I would rather have it done by someone who is familier with the unit..Thanks everybody, Royce

P.S. Rich, are you still thinking about that Modified Fanout box you mentioned that would have the subsonic filtering for all the VG units built in?
The greatest power in the universe is imagination!

feloniouspunk

#1
VT-  What type of guitar are you using with the VG-99?  I have a baggs equipped Godin and I am about to head over to the currency exchange a send Rich the cash for the subsonic filter.  I dont think my problems are as severe as those guitars that are equipped with RMC systems, but then again I have nothing to compare my guitar to.  There are some things that do drive me crazy, like weird frequencies on the high e string (and sometimes the b string) when in alt tune mode.  And some of the synth models can be a bit goofy sounding especially bowed. 

I know what I should do, and I have the cash, I just need to be sure I am doing the right thing.  Times are rough...

EDIT:  Well, I just did.  I walked over and got the money order, now to put it in a envelope and send it.  How long does it usually take?
Lots of Gear. :)

Virtual Madness

#2
Quote from: feloniouspunk on July 17, 2008, 11:01:47 AM
VT-  What type of guitar are you using with the VG-99?  I have a baggs equipped Godin and I am about to head over to the currency exchange a send Rich the cash for the subsonic filter.  I dont think my problems are as severe as those guitars that are equipped with RMC systems, but then again I have nothing to compare my guitar to.  There are some things that do drive me crazy, like weird frequencies on the high e string (and sometimes the b string) when in alt tune mode.  And some of the synth models can be a bit goofy sounding especially bowed. 

I know what I should do, and I have the cash, I just need to be sure I am doing the right thing.  Times are rough...

EDIT:  Well, I just did.  I walked over and got the money order, now to put it in a envelope and send it.  How long does it usually take?

Hey Felonious,
I'm using a Graph Tech Ghost System on a vintage style Fender Telecaster and the subsonic filter really "fine tuned" the VG-99 for me..You did the right thing sending the money out for the filter!..It took about two weeks to arrive from the day i mailed the money out (i'm in the midwest)..So it shouldn't be long before your in VG-99 Heaven!
Royce  :)
The greatest power in the universe is imagination!

feloniouspunk

Thanks Royce, I am in the mid-west too.  Chicago.  I put it in the mailbox about two hours ago, so it should go out tomorrow.  I do feel like I made the right decision, but I could have bought the Behringer FCB1010 for the same price.  Oh well, that will just have to wait.  Thanks for your input.   ;D

Chris
Lots of Gear. :)

Virtual Madness

#4
Your Welcome!


The greatest power in the universe is imagination!

feloniouspunk

According to the UPS tracking website, I should have the Filter anytime now.  Stand by for report.   ;D
Lots of Gear. :)

dhaiphi

#6
Quote from: feloniouspunk on July 25, 2008, 06:36:49 AM
Stand by for report.   ;D
All owners of Godin with RMC pickup are very eager to read your future post !  ;)

feloniouspunk

Hello France!   ;D  I will report back as soon as I install the upgrade.  However, my Godin does not contain the RMC pickup, it has the older L.R. Baggs pickup which I don't think had as many problems with the 99 as the RMC.  I should have it within the next few hours, but this is Chicago so knows when it will really get here.   >:(  Stay Tuned.
Lots of Gear. :)

feloniouspunk

Well my initial reaction is WOW and why did I wait so long.  ;D  It seems like overall my patches are much clearer, like the cotton was just pulled out of my ears.  And much of the irritating blips and skronks are gone from the problem synth models.  My biggest issue was with the Bowed model and now it sounds much better. Had I only known.  The weird overtones produced on the high e and b strings from the alt tuning are still there, but not as pronounced.  I replaced the part about an hour ago, so I have really only been going through my patches one at a time.  I do get the feeling that I will have to re-eq a lot of things, but thats minor.

I will write a full review over the weekend. 

Chris

Lots of Gear. :)

Virtual Madness

Congratulations Chris!..You are now the proud owner of a WORKING VG-99!..Isn't that filter awesome?..Hats off to Rich (RMC) for putting the time and effort in to get it to us Piezo guys!  :)
The greatest power in the universe is imagination!

Doc West

Royce ,
Did you ever get an answer about filtering for the VG88v2 ?
I want a subsonic filter for the VG88
This thread seems to deal only with the VG99.
Thanks, Doc West



Quote from: Virtual Madness on July 06, 2008, 11:45:53 AM
I was thinking of Richard McClish (RMC) when this question occured to me, but feel free to jump in if anyone has any info about this..Somewhere online i saw some technical info about switching out some electrolytic capacitors in the VG-8 front end (to filter out the subsonic frequencies that mess with the pitch shifting if you use Piezo Pickups), and I recently got the VS9F subsonic filter for the VG-99, (This thing is great Rich, it solved ALL my VG-99/Piezo problems!) but I can't seem to find subsonic filtering info ANYWHERE about the VG-88V2!..Does anybody have this info (what part numbers do i take out, and what part numbers do i put in) AND is there anybody here that would consider letting me pay them to do this work for me?..I've never soldered on a circut board before, and I don't want to start with the VG-88!..I would rather have it done by someone who is familier with the unit..Thanks everybody, Royce


Elantric

#11
QuoteI want a subsonic filter for the VG88

This is accomplished by swapping internal capacitors and resistors in the VG-88


Join the Yahoo VG-8 group and search for the thread describing the modification.

http://launch.groups.yahoo.com/group/vg-8/message/28119

RMC wrote>
VG-88 SUBSONIC SENSITIVITY REDUCTION MODIFICATION (RMC - 2001)

-=- FYI, in the VG-88 there are only 6 capacitors to be changed.
The capacitor numbers are C138, C166, C167, C174, C175 & C176. These
are 10uF capacitors which can be changed for 0.22uF caps in order to have a
-3dB subsonic rolloff (single-pole) at about 35Hz (instead of 0.7 Hz !...).
Note that 0.18uF caps will provide -3dB @ 40Hz but they may be a little
harder to find. In this low-frequency filtering application, the film,
ceramic or electrolytic (polar or non-polar) types are all good choices.



The modification above is recommended for Piezo Hex pickup users of the OLD VG-88

GK-2A/GK-3A users may not see much benefit of this modification.



And the old VG-8 / VG8EX Subsonic filter is described as follows:

VG-8 SUBSONIC SENSITIVITY REDUCTION MODIFICATION (RMC - 1996)

There are 12 capacitors to change in order to reduce the subsonic
sensitivity of the VG-8 and VG-88. This helps to significantly reduce the
objectionable thumping and/or momentary distortion which may occur in the
amplified sound when the string tension and/or the pickup to string
distance is changed abruptly.

The same problem typically arises with a GK-2 in proximity of a tremolo
bridge. Because the bridge moves up & down with changes in string tension
as you move the tremolo arm, this produces objectionable subsonic signal
components (which can be much larger than the useful music signals) and
adversely influence the A/D conversion in the VG-8.

The mod is a simple capacitor substitution which is of no consequence in
the audio spectrum. It simply helps to maintain the A/D converter in its
linear mode of operation.

1) Replace C25, C29, C33, C37, C41 & C45 (all 10uF) with .22uF (metal
film, electrolytic or ceramic) capacitors.

2) Replace C201, C202, C203, C204, C205 & C206 (all 10uF) with .33uF
(metal film, electrolytic or ceramic) capacitors.

The Roland p.c.board has legend information which makes locating the
components easy. The work takes usually less than an hour to perform. The
parts are worth less than US$10.

You want capacitors somewhere between 16V and 50V (it's mostly a size
issue) and you want radial caps (with both leads at the same end like the
originals, since these capacitors are standing vertically on the board).

You have a choice of either a standard aluminum electrolytic or a tantalum
electrolytic cap. They both work well. If you just look for something
small, it should work fine. This is not a critical application and the
portion of sound you're filtering is not in the audio spectrum. Let a
competent electronic technician handle the capacitor selection if you have
a problem deciding.


info@rmcmusic.com

Tonato

Quote from: Elantric on January 19, 2010, 10:22:45 AM
This is accomplished by swapping internal capacitors and resistors in the VG-88


Join the Yahoo VG-8 group and search for the thread describing the modification.

http://launch.groups.yahoo.com/group/vg-8/message/28119

RMC wrote>
VG-88 SUBSONIC SENSITIVITY REDUCTION MODIFICATION (RMC - 2001)

-=- FYI, in the VG-88 there are only 6 capacitors to be changed.
The capacitor numbers are C138, C166, C167, C174, C175 & C176. These
are 10uF capacitors which can be changed for 0.22uF caps in order to have a
-3dB subsonic rolloff (single-pole) at about 35Hz (instead of 0.7 Hz !...).
Note that 0.18uF caps will provide -3dB @ 40Hz but they may be a little
harder to find. In this low-frequency filtering application, the film,
ceramic or electrolytic (polar or non-polar) types are all good choices.



The modification above is recommended for Piezo Hex pickup users of the OLD VG-88

GK-2A/GK-3A users may not see much benefit of this modification.


Hi, Thanks for the data Elantric. Is that configuration the same for the VG88 V.2?

Elantric

Quote
Hi, Thanks for the data Elantric. Is that configuration the same for the VG88 V.2?

Correct.

VG-88 "V2"  = simply a firmware update - no hardware changes compared to 1st release VG-88

pinkjimiphoton

hi guys, sorry to bump a zombie but i thought this may be relevent or useful for someone else.
i read this post after joining a couple days ago, and decided to have a go at modifying my vg88.
i use a gk2a with it (or a gk2 depending on which guitar) and on some of the patches the subharmonic crap was terrible.
really drove me nuts.
so i thought about the suggestions and solutions previously posted and figured as the guitar doesn't have much to it below around 50hz,
that i could go up in decades to "tune" each string accordingly. i wanted to leave some wiggle room for the guitars with wiggle sticks on them, so they could track divebombs and such. so with the help of another jim on aron's diystompboxes forum we figured out some "offsets" if you will for different caps for each sensor in the hex.
caps are kinda funny, you can make 'em twice as big, and they drop about an octave. same with when ya make them half the size.
so starting with the idea of me being too lazy to type much more, it's c/p time... <take it away, pink>

from http://www.diystompboxes.com/smfforum/index.php?topic=118915.new#new

so i'm thinking you want to have a range that will cover about an octave drop on the low e, and maybe a 3rd down on the high e.

not being particularly bright or versed in magic myth or science lore,

it seems to me that it would work better if EACH string had its own size of cap.

if i went with say 150n  for the low e, that should cover (ballpark'd, of course) me down to a low enough pitch to improve the tracking, reduce the noise, and still be able to whammy ridiculously.

so by my unschooled logic, the d string, being about an octave higher than the low e should then get a cap around .082n, and then the high  e about 47n, right? being only two octaves apart, this orter work i would think.

so i'm thinking around 100n for the a string, and around 68n for the b string.

does this make sense? the suggestion is to replace 'em all with the same sized cap, but it seems to me that that's not gonna fix the subharmonic problem across the fingerboard.

well, the deed is done.

i bricked the fuggin' thing. or so i thought. a quick factory reset and all was well again, i hadn't programmed anything into it yet anyway.

when all was said and done, with the skinniest string being #1 (i hope)
i used the following... figuring they would be close enough for rock n roll.

string 1,(e) c138 47n/.047uF
string 2,(b) c166 56n/.056uF
string 3,(g) c167 68n/.068uF
string 4,(d) c174 100n/.1uF
string 5,(a) c175 150n/.15uF
string 6 (e) c176 220n/.22uF

my immediate thoughts are after messing with it for the last couple hours that its a worthy improvement. i have a feeling i should have gone smaller than i did for the higher pitched strings.
they still have some of that annoying subfrequency noise, so i'd imagine high to low .022, .033, .047, .1 , .15. .22 would be even better.
can't say for sure without testing further, but i am pretty lazy and for my uses i think it's close enough for rock and roll. i bet going right up the scale in decades with the nearest closest value for whatever caps is fine.
it definitely tightened up the tracking of the low strings noticeably.

fwiw, i used all regular ceramic discs for the caps. they work great, are small, and @#$% them 10uF electros ;)

thanks elantrik and all the others who posted this info. i hope somebody finds this useful.
i think you can tailor each string to whatever frequency range you want it to reproduce, and avoid some of that subharmonic hassle.
a solder sucker, 10cents worth of caps and a half hour later, its very cool.
i'm lucky that i didn't totally brick it.

anyways, hope this helps. peace out.

Tony Raven

Well, bless you for your adventurous spirit!! :) I'm pushing 60, & cannot (yet!) shake free of the notion I'm dealing with horrendously expensive hardware. :-[ I like the idea of the mod & may actually pursue it before I cash out. ;D

And in this era of capacitor fetshism -- Orange Drop! Bumblebee! Paper/Oil! -- it's pleasant to see someone willing to work with the basic tan disks. ;)

pinkjimiphoton

capacitor "mojo" is a bunch of hoodoo. electrons don't particularly care what type of cap is being used.
mylar, film, ceramic, not a whole lot of difference really.
orange drops are a waste of time in a guitar circuit really, too big. unneccessarily so.

i did a little shoot out of my own one day with some wires, some roach clips <hey don't judge lol > and an ASS load of caps including MOJO crap, sprague black cats, tropical fish, you name it.

the "mojo caps" ALL sounded warmer. BUT thats cuz they leak signal to ground and actually kill some of the output from your guitar.

best sounding caps to my ear after wasting a day listening? chicklets and ceramics sound the same, have lower loss and the guitar tone ended up stronger and clearer.

mojo caps added a nice fuzzy dirty edge... just like they do when leaking dc in a guitar amp.

bunch of hooey i tell ya. frequency, cutoff ranges, voltage and value matter more than type 99% of the time.

and in some cases, cheaper sounds fine. like in this case, where they're really only used for two purposes. one, to block any possible dc entering, and two to set the cutoff frequency of the pickup.

10uf (stock) is bloody huge. 1uf is bloody huge. you can look up frequency charts and find the proper values to use... its all ballpark stuff really. i just found it tracked cleaner with smaller caps on the higher strings, but the values i used still are an octave or more below the target pitch of each string, to give whammy bar wiggle room.

peace!

Elantric

Quote from: pinkjimiphoton on November 21, 2017, 01:13:00 PM
capacitor "mojo" is a bunch of hoodoo. electrons don't particularly care what type of cap is being used.
mylar, film, ceramic, not a whole lot of difference really.

http://conradhoffman.com/cap_measurements_100606.html

Measured Differences Between Capacitors for Audio Applications

There is general agreement that different capacitors can be demonstrated to have different sound qualities in audio applications under various conditions. Which capacitors in what circuits and under what conditions remains hotly debated. Not helping matters any, there is a tremendous amount of myth, lore and legend, plus marketing hype and defective test methodology out there that confuses the issue and causes a great divide between the objective and subjective camps of audio design philosophy.

Make no mistake, I sit solidly around the campfire with the objectivists. I have no tolerance for the viewpoint that sound quality is beyond our apparently primitive and useless measurement capabilities, that there is some "parameter X" we have yet to discover and that can't be measured with current technology. The cases that support such an argument invariably fall apart under a careful examination- measurements were never actually made, the wrong measurements were made or they were made poorly. Many times the claimed difference disappears under more strict listening conditions. It seems incomprehensible to the subjective camp that they're subject to all the same biases as the rest of us, but they are. I would never say that ferreting out sound quality differences is easy, in fact it can be notoriously difficult, but the tools and techniques do exist. The only item frequently in short supply is dogged perseverance.

I'm going to present test data for a small group of 0.01 uF capacitors to show first, that when people talk about "measurements" they're usually ignoring things that could affect sound quality and second, there are obvious measurable differences between all of the parts. Though I can't say at what level these differences become audible, I would strongly suggest that if two parts are very similar when subjected to a complete suite of tests, it can be said with almost complete certainty that, given a properly conducted listening test, their sound will be indistinguishable- regardless of cost, construction and flowery marketing prose. Unfortunately, it will also become apparent that every part type has its own distinguishing characteristics and the happy situation of equivalent parts arises less frequently than one would hope.

The capacitor group consists mostly of 0.01 uF film types because of the availability of a 0.01 uF Teflon capacitor I wanted to include in the tests (thanks Stefano!). A silver-mica and a ceramic are also included so that the line-up contains an example of most of the possible choices one might consider for a crossover or RIAA network. A few types were slightly different in value but close enough to compare for our purposes. Thus we have:

Audience Aura-T, 0.01 uF, 600V, 1%, Teflon- a moderately expensive cap aimed at the high end audio market
MIAL, 0.012 uF, 160V, 2.5%, polystyrene- an inexpensive cap often found in consumer electronics
Philips, 0.0095 uF, 63V, 1%, polystyrene (two 4750 pF KS radial units in parallel)- a premium commercial styrene
Panasonic, 0.0082 uF, 50V, 2%, polypropylene- a common and inexpensive polypropylene from Digikey (discontinued)
Acushnet, 0.01 uF, 600V, 10%, polyester (Mylar)- a very typical axial polyester
Component Research, 0.01 uF, 50V, 1%, polyester (Mylar)- an escaped aerospace component of excellent construction
CDC, 0.01 uF, 500V, 1%, silver-mica- the upper size limit of silver-mica caps, probably expensive
CGW, 0.01 uF, 50V, 10%, K5L- a Corning Glass Works "Spinseal" C0G/NP0 ceramic




The test suite consisted of the usual low level capacitance and dissipation factor measurements at 1 kHz, series model. A simple test for dielectric absorption was made, plus a few high frequency measurements. These would be the typical measurements one might find on a data sheet. In addition, the stray capacitance to a metal plate was measured. Some "boutique" capacitors tend to be very large and stray capacitance to surrounding objects like ground planes and chassis can become a concern, though the Audience cap tested here is only slightly larger than a polystyrene. Finally, a new test for non-linearity and distortion was added to the mix- nothing more than examining the bridge residual at null. More on that later. Note that I can easily think of half a dozen or more measurements that I haven't made, so this should not be considered as any final word on capacitor measurement, only an illustration that there's a lot more one can do besides the usual value and DF measurements.

The Results!

Brand   Audience   MIAL   Philips   Panasonic   Acushnet   Comp. Research   CDC   CGW   ASC x463
Dielectric   Teflon   Polystyrene   Polystyrene   Polyprop   Polyester   Polycarb?   Silver-mica   Ceramic   Polycarb
Temperature   72   72   72   72   72   70   72   73   72
Voltage rating   600   160   63   50   600   50   500   50   50
Value, mrkd   0.01   0.012   0.0095   0.0082   0.01   0.01   0.01   0.01   0.01
Value, meas   0.01006822   0.0122617   0.00952693   0.0082597   0.00929425   0.00988958   0.01000063   0.00999143   0.00982809
Tolerance %   1   2.5   1   2   10   1   1   10   10
Error %   0.68%   2.18%   0.28%   0.73%   -7.06%   -1.10%   0.01%   -0.09%   -1.72%
Dissipation   0.000023   0.000053   0.000028   0.000122   0.004739   0.000791   0.000168   0.000108   0.000705
Absorption   0.02%   0.02%   0.02%   0.04%   0.23%   0.02%   0.82%   0.34%   n/a
DCR, 100V   6.50E+13   3.00E+13   2.00E+15   3.50E+14   9.50E+10   1.50E+14   2.00E+12   3.00E+12   n/a
Phase, 2MHz   -84   -84   -84   -86   -84   -83   -86   -84   n/a
R, 2MHz   6.35   6   7.8   9.2   8.5   7.8   7.6   7.6   n/a
Self-res, MHz   4.5   7   7.7   9.7   7.5   9.7   8.4   9.2   n/a
Stray C   5.0   2.6   4.4   3.8   4.6   2.8   6.0   n/a   n/a
Bridge residual   low   low   low   very low   high   high   low   low   high
What does it all mean?

In a perfect world, you'd expect a capacitor to match its marked value exactly, with most other parameters being zero or infinity. The basic capacitance measurements here are unremarkable, as the parts mostly meet tolerance. All film capacitors have a significant temperature coefficient, so it's a bit silly to measure them on a standards bridge at part per million levels except for the fact that this same bridge allows very accurate dissipation measurements on nearly perfect dielectrics like Teflon. Much time was spent waiting for the capacitance value to stabilize after so much as touching the leads, which raises the temperature. One should consider the temperature coefficient when constructing an RIAA network and calculate the deviation when the capacitors go from turn-on on a cold day, to fully warmed up on a hot summer day. Most preamps don't have ventilation, and this would be a reason to consider it.

Dissipation factor is useful when evaluating the health of power supply electrolytics. It's effect on the sonic performance of signal capacitors isn't agreed on, but may be quite minor. The number represents internal losses, and can be converted to effective series resistance (ESR) if desired. ESR is not constant with frequency, but tends to be so low in high quality capacitors that it doesn't have much effect on circuit performance. If one were building high Q resonant circuits it would be a different story. Still, low dissipation factor seems to be a hallmark of good dielectrics, so a high number can be a tip off that more investigation is needed.

Dielectric absorption is possibly more disturbing. There is remarkably little published on the matter. It was a significant issue with early analog computers and some related books covered it. There was also a good paper by Bob Pease, published by National Semiconductor, that may be the best write up to date. I'll speculate that high dielectric absorption is not always serious, since silver-mica capacitors suffer from an excessive amount of it, yet can produce very good sounding RIAA networks. In later measurements of residual distortion, high dielectric absorption doesn't produce high residuals, but I'll still remain suspicious of it until it's proven harmless.

DC leakage measurements should have no effect on anything, as resistance should be very high in any signal capacitor. They do indicate one interesting thing about capacitor construction. With higher dielectric constant materials, less surface area is required and leakage can be almost unmeasurably low. Lower dielectric constant materials like Teflon, in spite of its basic high resistivity, may require so much surface area that leakage, possibly from the slightest contamination or impurity, can increase. DC leakage is probably a good quality control measurement, but not relevant to sound. Note that lower voltage rated caps were measured at 50 VDC instead of 100 VDC.

Film capacitors generally have excellent high frequency capabilities but this is often compromised by large size and long lead lengths. You'll notice that the small radial Panasonic capacitor has a much higher self resonance (9.7 MHz) than the Audience capacitor (4.5 MHz). This isn't due to any failing of the high end Teflon cap, merely the fact that it has several inches of lead length and couldn't be connected near the body. The Panasonic cap has a fraction of an inch of lead length, and the body is smaller. If you need high frequency performance, possibly not for sound, but to maintain stability with high bandwidth semiconductors, keep size and lead length to an absolute minimum. For bypass duty, surface mount will beat leaded every time.

An often neglected parameter is how much capacitance a device presents to nearby objects. A physically large capacitor will have a significant capacitance between it's outer electrode and surrounding parts. There may be undesired coupling to a ground plane. Less appreciated is that the capacitor may not be surrounded with the same wonderful dielectric it was built with. It may well be wrapped with polyester, regardless of what the dielectric is. Thus, the quality of the unwanted capacitance may be poor. It would take a better setup (strict shielded 3-terminal connections to an enclosed measurement chamber) to get a reliable DF number for these low values, so that was ignored on this go around.

Inexperienced designers often simulate circuits in Spice, but rarely do they include the extra capacitive and inductive couplings that exist in real physical circuits. There's no substitute for bench experience. If you're building a precision network be sure to account for strays. The capacitance between a metal plate and the capacitor under test was measured with the cap leads shorted and the largest face of the capacitor in contact with the metal plate. The Philips part was a special case as the pair couldn't be placed in good contact with the plate. A separate Philips cap was tried on all surfaces for the maximum stray value, then the value doubled to represent the pair. All measurements were 1 kHz, series model. All the caps tested showed only a few pF to the plate, so in any normal audio frequency network application there would be little to worry about. If a cap with similar stray body capacitance were used across an opamp feedback resistor, or on the very high impedance grid of a tube, this amount of capacitance would need to be carefully considered. (yes, I forgot to measure the ceramic but as the body is small, the capacitance should also be small.)

The last item to talk about is bridge residual. This was measured using a Schering Bridge, an old General Radio Corp. 716C. This was chosen because the major adjustable impedances in the bridge arms are both high quality air capacitors. There are also some range and trim parts that use polystyrene and silver-mica capacitors, but overall the bridge contributes almost no non-linearity or distortion to a measurement.

Normally a traditional bridge is balanced using a tuned null detector. A single frequency is fed in and all but that frequency is filtered out when looking at the output. In that way the signal to noise ratio is tremendous and measurements can be made to high precision without worrying about hum and noise.

The bridge still works just fine without filtering the output, in fact early bridges were balanced using nothing more than sensitive headphones as a detector. Without filtering, one notices a curious phenomenon. Some capacitors can be balanced to a much deeper null than others. It means that the capacitor doesn't perfectly adhere to the standard model for a capacitor, a perfect reactance in series or parallel with a perfect resistive loss element, but distorts the signal slightly.

This may be another manifestation of dielectric absorption specific to the measurement frequency (or it may not), but it turns out to be a very useful way of looking at capacitor distortion. The usual tests for dielectric absorption are cumbersome and hard to carry out at the frequencies most interesting to audio folk, but this is not. As you look at the following scope signals you'll notice that they don't correlate well with the very low frequency dielectric absorption measurements already made (or much of anything else), but I believe they may have some relation to perceived sound, at least in the case of polyester (Mylar). I believe Bates did several articles on capacitor distortion in the UK publication Electronics and Wireless World and it would be interesting to see if the results bear any similarity to these.

The measurement setup was identical for all caps. The bridge was excited with 120 Hz at about 40 VRMS which gives about 3.6 VRMS across the capacitor under test. The output of the bridge was viewed with a high gain scope plug-in set to 0.5 mV/division. Thus, all the scope images can be compared directly. The residual for all caps tested was 360 Hz, the 3rd harmonic. This is obviously "the nature of the beast", but is a bit counter intuitive and rather undesirable. Various other caps were tested and the results were consistent by type, i.e. by dielectric material.

Here is the bridge residual for the Audience Teflon cap:

scope photo- Teflon distortion
Next the Philips polystyrene:

scope photo- polystyrene distortion
Next the Panasonic polypropylene:

scope photo- polypropylene distortion
Next the CDC silver-mica:

scope photo- silver-mica distortion
And finally, the Acushnet polyester (Mylar):

scope photo- polyester distortion
If nothing else, this may explain the consistent dislike for polyester capacitors in the high end audio community. As for the rest, I have to wonder if there's a preference for a non-zero amount of 3rd harmonic distortion, and that possibly "cap-rolling" is an attempt to hit a specific preference. Is it possible that at one end (polyester) we get the phrase, "hard and gritty", and at the other end (polypropylene) we get, "sucks the life out of the music"? Perhaps somewhere in the middle (Teflon?) lies just the right amount of "edge"? The engineer in me wants to reject all of this because the residuals are so small, but when you have a huge amount of anecdotal evidence, it's wise to at least speculate. I believe this would also be consistent with the almost universal high end rejection of amplifiers having near zero harmonic output, though admittedly the ability of people to distinguish different amplifiers in double blind testing has been shown nearly impossible.

Conrad R. Hoffman
Rev. 1, 6/6/2010

Addendum & Thoughts

6/7/2010 I'm not completely comfortable with ranking the caps based on the bridge residual. Unlike amplifier THD measurements, where it's almost impossible for distortions to cancel, a bridge is based on cancellation between two "identical" paths and might allow cancellation of internal and external errors. Thus, I have to wonder if there is some slight contribution by the bridge that can change the ranking between the nearly perfect caps. It just eats at me that the Teflon isn't the best of the bunch. There's no chance of that concerning the polyester cap, which is just plain bad, but I need to find a way to confirm "zero". Ideally that would be a giant 0.01 uF air capacitor, but such a thing doesn't exist. 0.001 uF is the largest air cap I have, and I want to confirm the readings on the same scale as the original measurements. It may be possible to perform the same measurement with the GR1615A standards bridge, which uses only air reference caps and a ratio transformer, though keeping hum and noise pickup down is a bit tougher with that unit.

6/14/2010 Hooking up 3 big GR air capacitors in parallel gave about 0.0035 uF, allowing a test to be made on the same range as the test group. Hum contamination was a huge issue since no shielded cables could be used- the cables would be an additional dielectric that would affect the results. The test was inconclusive as there was still a small residual, similar to that seen with the better caps in the test group. Another test was run using the 0.01 uF range extender capacitor from the standards bridge. This is well shielded and the residual was below anything visible at the scale the tests were run at. It is, however, not an air capacitor. It's probably polystyrene so the same question of canceled distortions remains. It appears that hum pickup can confuse things greatly so the frequency was changed to 200 and 400 Hz to separate the effects. More caps were checked with the following conclusions:

the test is basically valid though exact ranking of the best caps could be different
no cap with high DF was good, but low DF isn't the controlling factor
polyester (Mylar) ranges from bad to terrible and is to be avoided unless you like the sound
miniature film caps seemed worse than larger ones
there are few rules of thumb to rely on- construction quality may be very important
6/15/2010 Made a big effort to reduce hum pickup by disconnecting nearby equipment from the line and putting a complete shield over the test area of the bridge. I added an ASC x463 polycarbonate cap and a General Radio calibration cap (huge silver mica foil stack design- nothing to write home about for audio use). This test was at 60 Hz because the residual seems to be larger at low frequencies. It's very non-linear with test voltage- need to investigate that further. Here is a new ranking- the numbers are in mV of residual signal.

ceramic   0.12
polypropylene   0.2
polystyrene (Philips)   0.3
polystyrene (MIAL)   0.32
Teflon   0.4
silver-mica   0.4
silver-mica (GR cal.)   0.6
x463 polycarbonate   1.1
polyester (Mylar)   3.0
An Excel scatter plot showed no corelation with dielectric absorbtion, but some caps do fall on a line against dissipation factor. Note that DF is plotted on a log scale. The polystyrene and Teflon didn't fall on this line, as shown below.



graph of residual vs dissipation factor
7/11/15 Found CGW ceramic cap data and added to description. It's a unique cap of exceptionally low temperature coefficient and stability, sadly discontinued.

7/19/15 It was suggested by Henry P. Hall (ret. from GR, designed the Digibridge) that I check the caps for voltage coefficient and dielectric absorption. Setting up the 716-C with the GR recommended bias adapter, the change in capacitance was nearly unmeasurable for all the caps in question. The Mylar cap was about 0.0001%/volt using a 250 VDC bias. That doesn't seem like the source of any problem so at some point I'll do better DA tests.



pinkjimiphoton

yeah, all of the above.
but in the end its a bunch of hooey, as a cap that is "close enough" for what we tend to do with them is fine.
unless precise capacitances are required, a cap is a cap is a cap. some may have lower esrs or tighter tolerances, but
we're talking electric guitars here, not audio guru nelvana. ;)

i reiterate. take some roach clips, do your own shootout for preference. me? i don't care for the so called "mojo" caps at all and have no use for them. a leaky cap may sound nice in a les paul, but it doesn't make it better. just overpriced. ;)

when dealing with units of capacitance, what WE usually deal with are fairly small.  there ain't a hell of a lot of difference in most cases within 20% or so.  very few people could tell the tonal difference between a 10u cap and a 10n cap on a higher pitch... lower pitches are where it becomes noticeable.

caps are all about ballpark in the real world.

i personally don't adhere to any generally accepted viewpoints on the subject, i deal with it in real world applications (well, in electronic guitar applications) everyday.

"mojo" caps leak and sound "warmer" and "fuzzier" because they aren't as good as simple ceramics.
ceramics, particularly modern monolythic ones are great for most uses. the only place its really critical is in timing circuits or radio circuits.
ceramics used to be much more expensive to produce than some others.
they get maligned often, but truth is... if the capacitance is right, its gonna work if it can handle the voltage and temperature thrown to it.

getting back to the original thing in the thread, roland chose those values so that the vgXX could "read" pickups mounted various ways, with various guages of strings tuned to various pitches. but to my ear, the subharmonics were harshing the sound far too much, so i played with the modification i found cited here and found what works for me, with what i had on hand for relatively standard values. they aren't strictly chosen for frequency, but are "in the ballpark" of an octave or so and the improvement is, to me, at least, noticeable.

you could of course analize it to the point where very specific frequencies are rejected, but its really overkill.

a .022 cap is a .022 cap even if it reads .022485083 or whatever. cuz in the end, its close enough.

one of them arguements that never end, but in the real world, if its close enough for rocknroll ....... ;)

peace

(ps: not to malign the oped posted, but it makes a lot of subjective claims that really end up being meaningless in the field)

Tony Raven

In the late '70s, I started doing repairs on the side. A couple of my first patients had the ceramic disks broken (one split down the middle, the other separated into halves) because, well heck, it's rock & roll ;D & the guitars got bashed around ~nightly onstage. The local Radio Shack had a better selection of green Mylars for about the same unit price (like 87¢), & I figured they'd tolerate physical abuse better. Next thing I know, I'm getting kudos for imparting some sort of magic. ::) So that's MY part in the myth.

Really, I enjoyed reading ALL the foregoing. I'm not doing radio or servos or even "home hi-fi" crossovers, & can readily see where greater awareness of actual real-world factors is important, even critical. Over the years, I've met homebrew builders who worry over WHAT their resistors are made of... yet never thought of measuring the darned things to establish their actual ohmage!! And I won't even open the 741 wormcan... :-X

In any case, my thanks again to pinkjimiphoton for the creative upgrade. It's affordable, & not at all difficult, & stands to improve VG-88 performance significantly. When I get around to it, I intend to do a proper A/B with a stock 88.

pinkjimiphoton

thanks bro. i mean no disrespect (other to the snake oil cap salesmen) but have found repeatedly in the world of electric/tronic guitar, EE stuff becomes pretty meaningless in many cases.
i remember a couple years back, tc electronic touting "improved" product after a small shop started customizing the input stage. "on paper, it shouldn't have worked, but in practice, it was a significant improvement".

i can't take any credit. i am no EE, i'm a monkey with a breadboard ffs, but i've learned after 5 or 600 fuzzboxes to use caps to achieve what i need 'em to do.

the monolythic ceramics i use are a much tighter tolerance than the bog standard ceramic discs, which often have a +20/-80% tolerance (especially in the old days)

i used to use them green poly caps too... and still do in some cases. but many times, when dialing in a customer's guitar, they end up choosing with their ears... and its VERY often ceramic. little grainier, touch of microphonic ring, but a tone they prefer.

its all subjective hoodoo mojo baloney for the most part. what matters are
the physical size of the cap
the voltage rating
the capacitance.
if its close to the specc'd value, will handle the voltage, and will fit in the circuit, odds are it will be fine.

further, replacing the stock 10uF electros in this app with almost ANYTHING will be a sonic improvement, as electrolytic caps in general are lousy for audio. they use them cuz they are CHEAPER than better quality, small caps.

in the end, it all comes down to price point other than us more adventurous end users.

all the stuff elantrik posted is dead nuts on the money, BUT again, its all subjective.

i built a bunch of honeydrippers (madbean's clone of the colorsound diphonizer) a few years back. it was popular, so i ended up building around 10 of them. the FIRST one, i used all "quality" caps. it sounded good.
second one i used ceramics. it sounded better than the dang poly caps.
since then i just use whatever i have handy, but the point was the customers chose the "sound" with the "cheap ceramics" over the one with the poly caps.

i'm not eric johnson, sooooo...
if it works, it works, and i'm good with it.

mega props to the guy who figured this out in the first place!
in the fuzzbox world, we control frequencies all the time with different sized input caps, so its not even my idea.

i just figured it would be easier to do it this way than to build an interface box, and that if it nuked most of the subsonic crap, that would be good. thats where the "ballpark" caps came in. to limit subharmonics more than an octave or so below the fundamental. it made a - to me, anyway- noticeable improvement.

jme... ymmv ;)