Quad cable - again - Capacitance

L

Laudanum

New member
I've been thinking about the high capacitance of quad type cables.  Looking at the Canare and Mogami specs on their sites  Canare site only states capacitance figures.  Mogami is more detailed, giving capacitance for several different scenarios.  I cant completely understand Mogamis diagram (link attached) but it got me thinking.  Canare gives the capacitance "between twin Blue and twin White conductors" and also between "conductors to shield".  I have an inexpensive LCR meter.  Pairing the blues and whites, I measure pretty close to the Canare listed capacitance.  Also pretty close if I measure conductor to shield.
But measuring single conductor to single conductor rather than pairing the conductors, the capacitance is much lower.  The Mogami site seems to list the capacitance for single conductor to single conductor, and it's pretty low.  Again, im not positive what their diagram is showing.  

I got to thinking about the capaitance because I re-wired a tonearm using the Canare star quad (smaller version).  I used just about a meter of the Canare quad for external wiring and I went by their spec (162pf/meter) figuring my tonearm cabling would fall between that and 180pf or so.  Seduction is 60pf and that would put me in the 220 - 240 pf range which would be prefectly fine for the MM carts (mostly vintage) that I like.  There is a fair amount of chatter on the net about NOT using quad for tonearm external wiring because of the high C but it was neat, convienent and appropriate from a capacitance standpoint on my situation with Seduction and vintage MM carts.  

Heres where I need clarification or correction ... it seems that capaitance of quad isnt really an issue when the conductors arent paired and the shield isnt being tied at both ends.  In the case of my tonearm cable, I measure around 90pf from the cartridge tags with the RCA plug not connected at the other end (or from RCA plug back to open cart tags).  If Im making any sense at all of the Mogami capacitance "schematic", this would make sense.  Im splitting the quad conductors into 2 pairs, one for left and one for right (unbalanced of course).  So one conductor to RCA pin and another for RCA shield, per channel.  AND the braided shield is NOT conected at the RCA plugs.  If wiring like this, apparently capaitance is fairly low (or atleast reasonable).  My understanding of the Mogami numbers and my measurements would seem to confirm this.  Assuming of course that my LCR meter is fairly accurate and seeing how I got close to Canares stated numbers, I would assume that it is relatively accurate.

So, can anyone who has beared with me for all this, tell me if Im thinking on the right track here? And if not, where i am way off course.   Heres the link to the Mogami quad page:

http://www.mogamicable.com/category/bulk/microphone/quad/

Wasnt sure to post this in general or technical so feel free to move it if you think it's better in general.
 
Desmond,

I follow your logic.  It looks like you are measuring properly. 

Remember all wire and cable has a capacitance.  Tone arm wire is small and all tone arms have that type, varying, of wire.  It has to be flexible, stranded, and light because of the tonearm movement and low mass. 

Then each cartridge wants to "see" a load of a given capacitance.  Knowing what your arm/interconnect to the phono stage capacitance is allows you to apply what additional capacitance is needed.

Additional capacitance can roll off high frequencies.  I'm assuming that is your concern, right?  When talking about pF capacitance you are not going to get much of a roll off.

Do you know what load your cartridge wants to see?  Are you using the small gauge Quad cable all the way to the phono stage or do you use an additional interconnect to the phono stage?  Do you know the capacitance there?

I'm also wondering if the Quad shield is grounded somewhere.
 
It's not really so much of it being a concern because I assumed around 160 - 180 pf of tonearm cabling, both internal and external when I rewired the arm.  That was based purely on the Canare spec of 162pf per meter, between two PAIRS.  I didnt thing about that at the time.  But after doing some reasearch, and as the more detailed Mogami specs seem to indicate,  it appears that capacitance is much lower between individual conductors.  Measuring with the capacitance meter appears to confirm it.  So, from what I am reading and measuring, it seems that the capacitance of quad cable is only high between pairs, or conductors to shield.  Not high between individual conductors.  I would like confirmation on this from the cable experts, but that's what it seems to me from what I can gather.

I wired the arm with a small awg teflon insulated stranded wire internally, then soldered to the Canare quad inside the tonearm base.  The arm tube ground, arm base ground and the quad's shield are all tied together inside the arm base along with an external ground wire.  The canare quad is splt at the RCA end into two pairs for left and right.  One conductor to RCA plug pin and the other to RCA plug shield at each, L and R, RCA plug.  An individual braided shield covers each pair from the split to the plugs but is NOT connected at the RCA shields. If the shields were connected at the RCA plugs then obviously there could be issues with noise AND the capacitance would probably also be quite a bit higher (again, from what Im reading).  Then of course, the seperate ground wire connects at the Seduction ground lug.  So what I really have is about 100pf of cable capacitance (measured) and about 60pf for Seduction.  So, 160pf total, not the 220 - 240pf  that I had originally assumed..  Most of the vintage MM carts that I have want about 200pf and up, depending on the cart.  The Shure M97 Im currently using wants, I think, 200 - 300pf.  It sounds fine, there likely isnt going to be much difference between 160pf and 200pf in terms of response.

Basically, Im just trying to understand the Quad cable in regard to capacitance.  It has a reputation for being high C type of cable but again, I think it really depends on how it's wired.


Thanks Grainger.
 
Desmond I am following just about everything but the grounding scheme:

Laudanum said:
  .  .  .  The arm tube ground, arm base ground and the quad's shield are all tied together inside the arm base along with an external ground wire.   .  .  .  An individual braided shield covers each pair from the split to the plugs but is NOT connected at the RCA shields.    .  .  .  Then of course, the separate ground wire connects at the Seduction ground lug.   .  .  . 

Thanks Grainger.
Click to expand...

Is the turntable grounded to the power ground somewhere?

Is the shield for the right and left inputs to your Seduction grounded on the source end?  Without at least a chassis ground on the source end then it doesn't shield. 

The ground wire at the Seduction is coming from the TT chassis, right?  Is it the point where the arm tube ground, base ground and Quad Shield is tied?

I'm just trying to get it firmly in my mind.

Thanks,
 
The external cables shield is tied to the arm tube and arm base ground and and external ground wire is run.  So only the arm is is directly connected/grounded to the Seduction ground lug. The stock Rega arm wiring used a coaxial cable as external wiring.  The arm tube and arm base ground are tied to the left channel shield so there is no seperate wire for connection to a ground lug. The turntable itself has no provision for grounding the motor or bearing to the arm and seeing how it is noise free, I didnt see any reason to create one.  It's a Rega clone (actually I believe it's a Systemdek made table like the i/900).  My Music Hall table is the same way.  The higher end tables may be different, I dont know but I dont know of any of the more recent Rega like, wood plinth decks that ground the arm to the motor or electronics.  Im not sure why they would want to?  It would seem to me that would introduce noise more likely than not and I would only consider it if there was a noise problem in the stock configuration that couldnt be remedied in other ways.  This is the way my table (both of them) and many others, atleast the "modern" wood plinth, fully manual tables that I know of, are configured.

Thanks Grainger
 
Desmond,

I have it now.  I was looking for the earth/power system grounding point.  All are tied to the Seduction chassis and through its power cord to ground.  It sounds like a good grounding scheme to me.

The electronics and bearing don't really need to be grounded that I can see.
 
The meter was purchased years ago, mainly for measuring inductors in crossovers.  But it has come in very handy aside from that.  I've used it a lot for basic resistor matching (measured values).    It wasnt an expensive meter and does not have a zero feature.  The lowest C range is 2nf (2000pf) and the stock leads measure .008nf (8pf) so I deduct that from whatever I meaure. But I have already done what you suggest, years ago, checking against low value silver micas.  I have a bunch of 100pf silver mica caps (they are 5% I believe).  I also checked several of them while fooling with the cable measurements.  They measure from 97 - 101 pf (after deducting the 8pf for the leads).  I figure that makes the meter close enough for my non-critical measurements of the cables.
I have three VOM's.  I havent checked the older one but the two I did check dont seem to be very reliable with measuring those 100pf caps ... But the little LCR meter seems capable enough for this application.

Thanks
 
Desmond,

I bought some Wima 1nF 2.5% caps off ebay (England) for a project a few months ago.  I measured them with my Fluke and got 1.5nF for all of them.  So I ordered the exact same caps from Mouser and got the same measurements. 

I contacted a buddy who was a Fluke repair technician.  He said it was a very accurate meter for capacitors.  Somebody is wrong!  I stay confused about these things.
 
Grainger49 said:
Desmond,

I bought some Wima 1nF 2.5% caps off ebay (England) for a project a few months ago.  I measured them with my Fluke and got 1.5nF for all of them.  So I ordered the exact same caps from Mouser and got the same measurements. 

I contacted a buddy who was a Fluke repair technician.  He said it was a very accurate meter for capacitors.  Somebody is wrong!  I stay confused about these things.
Click to expand...

Well, that's probably better than my 3 VOM's.  One wont read a 100pf or a 510pf cap, all over the place.  The second reads them both well high.  The 3rd meter is 50% high with the 100pf cap and close with the 510pf cap using the leads.  When I plug the caps into the slots, it reads the 510pf cap close but is still 50% high with the 100pf. 

Only that little LCR meter will read them accurately (based on their value)  and consistently.  I guess Im lucky to have it.  My understanding is that the good ones are a couple hundred dollars or more and up.  I got it from MCM before they were bought out (I think Newark owns them now eventhough the MCM website still exists).  I remember them having the meter available with or without calibration certificate.  It was signifigantly more expensive with it so I bought it without.  This had to be 10 years ago now. 

The last Tenma VOM I bought is on the fritz after only about a year.  A real P.O.S.  But that little LCR meter and the Tenma solder station I've had for 10 years are both going strong ... knock on wood.
 
I have one of these:
http://www.sparkfun.com/products/9485
It's advertised to go as low as 1pf. Not sure if it really does though. But it was cheap!
 
I've got some old silver mica caps pulled from one of the Dynaco PAS preamps I modified.  They are 5%, or were.  I'll see what happens with them.
 
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