Microphonic Starquad cable

[Tubetec]

I connected 40cm of Canford Starquad SQ-6.5 to the output transducer of a reverb tank , screen to tank housing .
It works fine on line input (20kohms)into my SSL2 interface ,nice and quiet ,but lacks sparkle
when I change over to HI-Z(1Mohm) , which is also on balanced TRS it sounds better ,brighter and cleaner and noise is also low ,
but the cable becomes highly microphonic , to the point if I tap it with a screwdriver it makes a sound nearly as loud as the spring output itself , if I move the cable the strands of wire inside are audible as a kind of clicking and scratching noise ,
It doesnt cause any serious problem when the wires are stable and free of vibration , but its still not something you want .

I think I'll connect a 1Meg pot as a rheostat across the transducer so I can vary the matching impedence and see how the sound of the spring changes ,
theres no issue with the cable feeding line impedence 20kohms , maybe an intermediate setting somewhere in between that and 1Meg wil give a better compromise with handling noise in the wires .

 

[Newmarket]

Do you need the starquad cable or is it simply what you have to hand ?

 

[Tubetec]

Well I wanted a balanced connection , I could have used low noise guitar cable otherwise , thats designed to reduce handling noise with a semi conductive layer .

Im opperating the reverb tank in close proximity to the PC just now and depending on orientation RF spikes are picked up well or nulled almost completely .
Line mode isnt nearly so suseptable to RF as HI-Z .

 

[Tubetec]

Just to make things more clear , Im using the SSL at maximum gain ,
When I use the hi-z balanced connection everything buzzes loudly , its only when I make contact with the tank housing does the induced noise go away .

I thought perhaps it was something to do with the transducer coil not having a ground reference ,
so I added 820kohms to ground from each leg across the input , still the same .

Everything is battery powered , SSL2 has two 5 volt rails , a Nicad pack for the audio supplies and a powerbank supplying the USB and Xmos side . There is no ground reference until I make contact with the reverb housing , right at the spot where the cable screen connects reduces noise the best , the further I move from that spot ,like making contact with the TRS jack housing and noise isnt reduced as effectively .
Im starting to think the arrangement with the metal sub tray suspended by springs forms some sort of induction loop , either way the floating sub assembly in reverb tanks relies on the springs for connection to ground , thats a very poor arrangement now Ive looked at it , it creates multiple paths ,all of which might have partial or intermittant contact , insulating the subassembly and giving it its own dedicated ground looks like a plan .

 

[JohnRoberts]

That seems unusual. The biggest difference from starquad is relatively high capacitance.

JR

 

[Newmarket]

Well I wanted a balanced connection , I could have used low noise guitar cable otherwise , thats designed to reduce handling noise with a semi conductive layer .

Im opperating the reverb tank in close proximity to the PC just now and depending on orientation RF spikes are picked up well or nulled almost completely .
Line mode isnt nearly so suseptable to RF as HI-Z .

Understood. I was wondering if you need the starquad cable rather than standard screened twist pair mic cable. Like for like starquad has greater capacitance so less easy to drive. Low noise cable understood. There is (maybe was) a twisted pair mic cable with the semiconducting screen. I just found this out a few weeks ago when I fell foul of not stripping it back properly on one connector. So I ended up with a cable that failed on my cable tester. Getting the meter back on it showed around 5K. I've just checked it and I think it's Europa cable. But I think that's a Studiospares brand. At least I've not seen it elsewhere. And when I looked recently Studiospares seemed to have pretty much given up on selling cable !

Now reading your later post - and I'm going to suggest that when set to 'Hi-Z' the SSL2 input is single ended. Despite the TRS connection.
It's usual for "Instrument" input option on a micpre etc.

It seems to me that what is needed is to Hi-Z buffer the signal as near to the output as practicable. Then a low impedance balanced signal to a balanced cable into the SSL2 line input.

 

[andYz00m]

What about a transformer based DI?

 

[ccaudle]

When I use the hi-z balanced connection everything buzzes loudly , its only when I make contact with the tank housing does the induced noise go away .

The screen is connected to the tank housing at the reverb end, and also to XLR pin 1 or TRS sleeve connector at the interface end? And the transducer is floating?

And is the transducer piezo or coil based?

 

[Tubetec]

Its only 40 cm's of cable , Im measuring almost identical capacitance from each leg to ground ,
approx 320pF .
Its a little unusual to see a Hi-z balanced input , but I have verified the TRS is still active in hi-z mode ,
of course under normal circumstances a TS jack shorts the slieve and it works in the conventional unbalanced fashion . Half patching a TRS jack gave exactly 6 db reduction in signal level
It was also confirmed to me in correspondence with SSL that the Hi-z is fully balanced .

I get the idea of a putting a buffer in the tank itself , seems to solve the noise issue in the cable ,

Yeah I dont know how many times I had people with bad leads because they didnt strip the black carbon loaded plastic layer before soldering ,

Screen is connected at the TRS jack and to the corner of the tank closest to the pickup transducer .
I had the winding floated , then I tacked on 2x 820k resistors to ground , they hook to the same point as the cable screen , I thought that would help matters , it didnt .

I might try tacking on a ground connection to a near by radiator pipe , just to see if the buzz noise goes away without me having to make ground contact with my body .

I have seen twin screened with the low noise wrap , I have some somewhere ....
I only used the Starquad as I had a few short lenghts to hand and wanted to give it a try in this application . Theres isnt a great deal of output above 10khz from the spring .

 

[Tubetec]

Output transducer measures 370 ohms with the DMM ,
with the LCR
380 ohms @100hz
470 ohms @1khz
14Kohms @7.8khz
Ls-385mH

That coil normally drives a tube through a 500pf cap with a 100k grid resistance following .
A lower impedence coil would work with a step up transformer , Im not sure about using a passive DI to step down an already tiny signal to mic level but I can give it a try .
Active DI with 5meg input resistance I have used as spring recovery , a 15cm patch cable keeps the capacitance/distance down , from memory still sounds a little bit scratchy if you move the cable for any reason though .

Maybe theres wisdom in vintage cloth wrapped braided sheild cable to the tank , might have less handling noise than mic cables with twists and plastics making clicking noises inside it

The Fet based mic booster would be another option ,at the expense of more circuitry in the loop.

'A high impedance mic has many of the traits of an electric guitar, so the cable used for it is generally a coaxial instrument cable. The “hot” center conductor is insulated with a high-quality dielectric; shieldedelectrostatically to reduce handling noise and triboelectric effects; shielded with a braid, serve, or foil which is also used as the current return path for the signal; and jacketed for protection. This type of cable is discussed indepth in “Understanding the Instrument Cable.”'

'The triboelectric effect that causes impact-related “slapping” noise as the cable hits the stage or is stepped upon during use is related to capacitance, specifically the change in capacitance that takes place as the insulation or dielectric is deformed. This causes it to behave as a crude piezoelectric transducer, a relative of an electretcondenser microphone. Because such transducers are extremely high-impedance sources, the drastic impedance mismatch presented by a low-impedance microphone and its preamp or input transformer makes the extraneous noise generated by triboelectric effects negligible except in cases involving very low-level signals. In low-impedance applications, handling noise is best addressed by using soft, impact-absorbing insulation and jacket materials in a very solid construction with ample fillers to insure that the cable retains its shape. Note that it is totally invalid to evaluate the handling noise of a low-impedance mic cable without using a resistive termination
to simulate the microphone element. A cable with no termination essentially presents an infinitely high sourceimpedance, a situation that is beyond worst-case!'
http://www.rfcec.com/RFCEC/Section-...OPHONE/Understanding The Microphone Cable.pdf

 

[kags]

I have seen twin screened with the low noise wrap , I have some somewhere ....

Gepco makes the X-Band cable that is double-braided shield and low capacitance. Very good cable. I like it for cable that get handles a lot. It’s a tiny bit stiffer than other mic cables, but in a reassuring way.

 

[Tubetec]

https://c3.zzounds.com/media/inst_guide-83537d22b5560c48c0f48a17831b2c6c.pdf

'What does the electrostatic shield do?

As the cable is flexed and bent, the copper shield rubs against the insulation, generating static electricity. The electrostatic shield acts as a semi-conducting barrier between the copper shield and the center insulation
which discharges these static electrical charges. Without it any movement of the cable would result in obnoxious“crackling” noises being generated.

What are electrostatic shields made of?

Electrostatic shields first appeared in cable as a layer of rayon braid. Nowadays carbon-impregnated dacron“noise-reducing tape” is a common element in any good high-impedance cable. Increasing in popularity are conductive-plastic (carbon-loaded PVC) electrostatic shields. Conductive PVC is extrudable just like an insulation, which guarantees 100% coverage of the insulation with a very consistent thickness and a very low coefficient of friction. The superior conductivity of C-PVC makes it much more effective than the semiconductive tape in bleeding off the small electrical charges that cause “the crackles.” Extruded C-PVC is also thinner and more flexible than dacron tape, which is applied longitudinally and restricts the “bendability” of the cable.
Although conductive plastic (with a copper drain wire) has been used to completely replace copper braid or serve shields, its effectiveness falls off above 10 kHz.

Why are some cables microphonic?

As was noted previously, the center conductor, insulation and shield of a coaxial cable form a capacitor; and, as many a microphone manufacturer will tell you, when the plates of a capacitor are deflected, a voltage is generated. (This is the basis of the condenser microphone!) Similarly, when the plates (conductor and shield)
of our “cable-capacitor” are deflected (for instance, by stepping on it or allowing it to strike a hard floor), a voltage is also generated. Unfortunately, this voltage generally pops out of the amplifier as a distinct “whap,” and can be very hard on ears and loudspeakers alike. Effects of this type are called triboelectric noise.

How can cable noise be reduced?

The electrostatic shield’s charge-draining properties help greatly to diminish triboelectric effects. Triboelectric impact noise is also reduced by decreasing the capacitance of the cable with thicker and softer insulation because the deflection of the conductor is proportionally reduced. This is the main reason that the single-conductor
coaxial configuration remains superior to the “twisted pair” for high-impedance uses—it allows thicker insulation for a given overall diameter. Triboelectric effects are accentuated by high source impedances, and are at their worst when the source is an open circuit—for instance, a cable plugged into an amplifier with no instrument at the sending end. Testing for this type of noise requires termination of the cable with a shielded resistance to
simulate the source impedance of a real instrument. '

 

[ccaudle]

into my SSL2 interface

When I use the hi-z balanced connection everything buzzes loudly , its only when I make contact with the tank housing does the induced noise go away .

Is that your interface with the highly modified power architecture?
Do you have an easy way to find that set of postings again to review? Maybe one of the changes has either decreased input CMRR for some reason, or has disconnected some of the audio circuitry reference from chassis, so that when the shield is connected to the reverb tank and the audio interface pin 1 or sleeve connection, that isn't effectively tying it to the circuit reference node.
The second would be my guess since the interface is running on batteries now.

Even high quality input designs have finite CMRR, and if the entire tank assembly is floating there will be some amount of power line noise induced onto the chassis through inductive or capacitive coupling. If the audio circuit ground does not have a connection to chassis to keep the common mode differences small then you can end up with a common mode signal so large there is no way the input circuitry can fully reject it.

 

[Tubetec]

As I said when I physically make contact with circuit ground at the tank all is well , noise drops back to that of the interface , I can then orientate the tank for minimum RF noise ,a null of 60db or more is possible within a few feet of a laptop running off mains power, the spurs being easily visible in REW/FFT.

Maybe the grounding arrangement I made when I modified the unit causes the problem Im seeing, but things in hi-z mode were already problematic before , its still a possibillity though .

The audio side of the interface is run off Nicad's ,
in standalone mode a USB powerbank supplies the Xmos via the USB connector ,
when you want to connect to the computer it supplies USB bus power .

 

[Tubetec]

The Canford starquad comes in at 80pf per meter
so I expect 40cm of it to add around 32pF to make up the total reading of 320pF to screen from each conductor .
Into 1Megohm thats siginfigant top loss , but its due to the coils construction mainly , a marked fall off in amplitude above 10khz is visable on FFT ,
Reducing cable capacitance in this case doesnt help me very much,

 

[Tubetec]

So it turns out starquad or even regular mic cable in this application isnt a good choice at all , there is the extra capacitance , but because the cable construction itself is prone to handling noise at hi-z.

Coax cable can have less pF per meter because it allows greater spacings and the low noise semi conductive tape reduces handling noise,

Can I handle a balanced signal over a pair of coax cables? Twin-ax

I should have a lenght of low noise guitar cable to hand , so I can check out this approach later.....

So I went away and rewired the tank output with two identical pieces of typical low noise instrument cable stuffed into TRS jack the far end ,
All of a sudden the microphonics are no longer an issue , completely cured the problem ,
not only that but the whole thing is now happy and hum free without the need for me to touch the tank . The proper CMRR is restored ,happy as a pig in muck now.

The other benefit is instead of seeing 320pF off each terminal with the starquad , Im now seeing 130pF on each leg with only a fraction pF difference between the two . This also translates into a bit more vibrancy in the top end from the spring .

I was also able to test the susceptabillity to nearby lighting dimmer hash , its very much reduced from what it was with the starquad . Adding an extra metal screening panel to the open end of the tank cures the very last traces of RF buzz and Im back to the internal noise generated by the interface .

I was thinking of trying one of the FET mic boosters as an impedence matcher for a tank , that probably only has a few pF of input capacitance , and perhaps would allow very much better behaviour from the transducer at high frequencies ,

 

[kags]

So I went away and rewired the tank output with two identical pieces of typical low noise instrument cable stuffed into TRS jack the far end ,

Did you twist these cables or just run them parallel? Seems like an interesting approach for difficult situations like yours - maybe using mini coax like that used for pedal boards.

 

[Tubetec]

Christmas Cheers Kags ,
I didnt twist them , just ran them side by side with short sections of heatshrink to keep things nice and tidy ,I trimmed back just less than 50% of the outer jacket on both cables for the first 40mm where they enter the Neutrik TRS jack .

The hi-z cable I had to hand is nothing special , it comes out at around 140pF/per meter,
in this case Ive got two 50cm lenghts , it adds 70pF x 2
A good modern low capacitance guitar cable can get you 52pF/m

(Guitar Cable Capacitance Chart • Comparison of pF Ratings by SHOOTOUT! Guitar Cables UK)

I was looking at 320pF from each conductor to ground with the starquad ,
The two strands of bog standard low noise guitar cable gets me a value of 130pF per conductor , 30cm of low capacity cable should get me down to around 75pF per conductor ,
When your matching into 1Meg theres still signifigant top loss due to the coils self capacitance , I tried a 140kohm resistor across the ends of the transducer and it made a favourable difference to the sound .

 

[Newmarket]

Its only 40 cm's of cable , Im measuring almost identical capacitance from each leg to ground ,
approx 320pF .
Its a little unusual to see a Hi-z balanced input , but I have verified the TRS is still active in hi-z mode ,
of course under normal circumstances a TS jack shorts the slieve and it works in the conventional unbalanced fashion . Half patching a TRS jack gave exactly 6 db reduction in signal level
It was also confirmed to me in correspondence with SSL that the Hi-z is fully balanced .

I get the idea of a putting a buffer in the tank itself , seems to solve the noise issue in the cable ,

Yeah I dont know how many times I had people with bad leads because they didnt strip the black carbon loaded plastic layer before soldering ,

Screen is connected at the TRS jack and to the corner of the tank closest to the pickup transducer .
I had the winding floated , then I tacked on 2x 820k resistors to ground , they hook to the same point as the cable screen , I thought that would help matters , it didnt .

I might try tacking on a ground connection to a near by radiator pipe , just to see if the buzz noise goes away without me having to make ground contact with my body .

I have seen twin screened with the low noise wrap , I have some somewhere ....
I only used the Starquad as I had a few short lenghts to hand and wanted to give it a try in this application . Theres isnt a great deal of output above 10khz from the spring .

Thanks for the extra detail.

 

[Tubetec]

A few further ideas came to mind overnight ,

Ive found in the past adding the outputs of two tanks together in mono smoothens the reverb effect noticably ,
Uptil now I recovered each spring seperately , then combined them in the mixer .

I was thinking of wiring the recovery transducers of two tanks in a hum bucking fashion ,
When the two tank enclosures are brought together with both recovery transducers at the same end each is a mirror image of the other ,which should improve rejection of external magnetic fields .

Each transducer coil has it own associated capacitance to the sub frame which is grounded to the housing , that might be minimised further by insulating the tray , I'll give that a try next .