Passive Bass Reveals Poor Quality Cable (A Tale Of Capacitance)

[HowieBass]

A few weeks ago I welcomed my latest addition to the family, a Squier Vintage Modified Precision and, since this is a passive bass and all others in my collection are active, I expected and did see a difference in volume. I could switch from one active bass to another and not need to change the gain setting on my amp but the passive Precision needed a bit of a boost. So far so good. One thing I did notice however, was the fact that the Precision's tone control didn't seem to have much depth to it, some rolling off of treble but not much. I'd previously settled on which cables to run from bass to my Zoom effects unit and from the effects unit to amp, based on those that were quietest and there the story might have ended until the other day when by chance I tried running the Precision straight to the amp using the cable normally between the Zoom effects console and the amp... and the difference was incredible. Not only was the Precision louder but the tone control's range expanded in both directions; better treble when fully open but also much better cut of highs and thicker lows with the tone control closed. I grabbed another bass (a Westone Spectrum LX) and this happens to have both passive and active tone circuits and is switchable between them; again the difference was marked when running in passive mode where output volume increased and tonal range expanded to include better treble and better rolling off of treble. There was obviously something 'bad' about the cable I'd been using between instrument and the Zoom unit.

So, I then started checking a few things because I realised that impedance and capacitance matter when it comes to passive tone circuits. I couldn't measure the capacitance of my cables but I did measure that the resistance was about 20 ohm for the offending 20ft cable and about 2 ohm for my other similar length 'good' cables and AFAIK that's not enough to make much/any difference. That left me with capacitance... and the offending cable did have some 'clever' solderless plugs which I'd always wondered about (in terms of how good is the connection with the plugs employing a central spike to make contact with the 'hot' central wire). I did have an old cable with poor screening that has solder free plugs at each end (of a different, old-fashioned design, with connections made via small screw terminals) so I decided to use these at each end of the 'bad' cable to see what difference they'd make and suddenly the cable went from bad to good... the cable resistance also dropped from 20 to 2 ohm but I suspect what really changed was cable capacitance, in that the new-fangled design solderless plugs offered a poor metal to metal connection and I'd introduced, what was, in effect, another capacitor to my bass's passive tone circuit.

So there we have it, I'd finally proved that old audiophile myth that some cables are vastly better than others... well that some solderless plugs are better than others...

Another outcome of this is that the Squier Vintage Modified Precision actually has a hotter output than my active basses! A win-win situation!

[pfretrock]

Had a similar experience. Got my first bass 9 months ago (Fender MIM P) Was using a cheap £5 cable from a local shop. There was very little change on the tone pot, I thought this was normal as I'd heard this problem mentioned a few times. Fender combo and Laney practice amp both the same result. After a few months the signal started to break up. I unscrewed the connector and noted the braid earth connection was fraying so decided to re-make the joint. After a few snips with the side cutters I decided the cable was useless, braid falling out as I stripped it. Quick dash to a local shop before closing time to buy a decent low noise cable and now the P tone control works! Goes from almost muddy to fairly bright. Wish I'd measured the old cable capacitance before I threw it out. Both were 3m length. A puzzle as to what was going on as I'd expect the cable capacitance to be rather high to make the tone pot inoperable, but the output was fairly bright, it just did not change much either way with the pot.

[HowieBass]

What threw me, as mentioned, was the fact the cable was giving me what I expected with my active basses so I'd no reason to think it was defective... of course now I know, my cables are the first thing to check for any sound/tone issues. As for the inherent capacitance, I'd say that would be associated more with poor contact between the jack plug and the conductors in the cable.

Edited September 1, 2014 by HowieBass

[KiOgon]

Just out of interest - I have the ways & means of measuring these parameters so here you go! I had a Fender Gold Bass 12' Cable in front of me, (a bass dedicated, directional 'highest quality cable'), so I used that. If their claims are to be believed - there must be a 'lot worse' cables out there I suspect these reading would be hard to improve on
Tip to Tip = 0.2 Ohms / 2.24 nF
Screen to Screen = 0.1 Ohms / 0.18 nF
Tip to Screen Open ended = Open Circuit (Infinity) / 1.79 nF
Tip to Screen Shorted = 0.2 Ohms / 3.08nF

Cheerz, John

[KiOgon]

- - - - but I did measure that the resistance was about 20 ohm for the offending 20ft cable - - - - -


That would be completely useless as a instrument cable- possibly OK as a washing line

[pfretrock]

[quote name='HowieBass' timestamp='1409566226' post='2540847']
What threw me, as mentioned, was the fact the cable was giving me what I expected with my active basses so I'd no reason to think it was defective... of course now I know, my cables are the first thing to check for any sound/tone issues. As for the inherent capacitance, I'd say that would be associated more with poor contact between the jack plug and the conductors in the cable.
[/quote]
That would make sense as the active circuit could drive almost any cable effectivly but in passive mode the inductance and resistance of the pickup would come into play. I'd previously done some Spice modelling of my amp and have just added a pickup model (guestimated inductance of 7.5H, resistance 7.5K, capacitance 70p) If I change the cable capacitance, the tone cut 3dB frequency is (this is with the pot flat out at 250k):

Nothing added: 3.8kHz
100p: 6.1kHz
220p: 5.68kHz
470p: 4.6kHz

Cables seem to be 16 - 50pf per foot, depending on brand.
A lot of guesswork in my model, but it shows cable can have an effect with a passive circuit. However it's probably best to use what sounds good and not worry too much about numbers!

[quote]
Tip to Tip = 0.2 Ohms / 2.24 nF
[/quote]

John, Just found a chart of cable capacitance, this Fender cable seems to be one of the highest Don't think resistance will be much of an issue.

[pfretrock]

[quote name='KiOgon' timestamp='1409571680' post='2540938']
Just out of interest - I have the ways & means of measuring these parameters so here you go! I had a Fender Gold Bass 12' Cable in front of me, (a bass dedicated, directional 'highest quality cable'), so I used that. If their claims are to be believed - there must be a 'lot worse' cables out there I suspect these reading would be hard to improve on
Tip to Tip = 0.2 Ohms / 2.24 nF
Screen to Screen = 0.1 Ohms / 0.18 nF
Tip to Screen Open ended = Open Circuit (Infinity) / 1.79 nF
Tip to Screen Shorted = 0.2 Ohms / 3.08nF

Cheerz, John
[/quote]

here is the cable chart I found:

[url="http://www.ovnilab.com/articles/cablechoice.shtml"]http://www.ovnilab.c...blechoice.shtml[/url]

What's interesting is his comment that the lowest capacitance should give the brightest sound, whereas my (simple) model shows there is an optimum capacitance for best response. I suppose that's the danger of taking a component in isolation, you can only really gauge its performance when in circuit where in inter-reacts with all the other stuff. And ears are always the best final test.

[dincz]

[quote name='KiOgon' timestamp='1409572044' post='2540942']
- - - - but I did measure that the resistance was about 20 ohm for the offending 20ft cable - - - - -

That would be completely useless as a instrument cable- possibly OK as a washing line
[/quote]

I wouldn't expect even a few tens of ohms to make any audible difference considering that pickups are typically several kohms and amp inputs more like 1Mohm.

EDIT: but yes, capacitance is a killer - especially with long cables.

Edited September 1, 2014 by dincz

[rmorris]

Well the 20 Ohm itself wouldn't be significant wrt a typical input impedance of 100K - 1M0 on an amp or even a typical 10K of a line input. But it does indicate something wrong with the construction of the cable since it shouldn't be that high.
Since it seems possible to measure a dc resistance then that indicates it's not a complete break in the cable itself. Something in the plug(s) appears to be introducing excessive signal to screen capacitance. Possibly strands from the signal wire being displaced such that it comes very close to the screen ?