I've just come across this concept and haven't seen any discussion of it or the theory involved. Apparently you "drive" the inner shield (somehow) to keep it at the same impedance as the source thereby eliminating any leakage current or capacitance seen by the source.
Any one provide any "light" or references on the subject? And how would you do this - what kind of driver and how should you CORRECTLY connect the cable on both ends. Make any sense to use in a preamp?
Any one provide any "light" or references on the subject? And how would you do this - what kind of driver and how should you CORRECTLY connect the cable on both ends. Make any sense to use in a preamp?
It was done in the past, I think by EMT. It makes no sense for a domestic enviroment to be honest.
First of all, triax cables are horribly expensive. They are used, e.g., with electrometers, where pA currents or really small voltages of high-impedance sources are measured. The guard is held at the same potential as the signal. This avoids leakage currents and capacitive effects in high-impedance measurements. "High" means hundreds of MegOhms up into the Tera-Ohm range.
So all in all nothing for "normal users".
So all in all nothing for "normal users".
Your diagram has confused me Alighszem. The triaxial cable I just looked at - the first that came up in a search - has two concentric (but insulated) shields and one center conductor - and costs a bit under $4/foot.
Ampex apparently used it for the repro tape head wiring in their recorders - specifically the ATR-102.
So you are all saying you don't believe it "makes any sense" in the audio world. How about versus balanced connection to a MM cartridge or tape head (with inductance in the 200-400mh range). Mitigating interconnecting cable length/capacitance?
THANKS
Ampex apparently used it for the repro tape head wiring in their recorders - specifically the ATR-102.
So you are all saying you don't believe it "makes any sense" in the audio world. How about versus balanced connection to a MM cartridge or tape head (with inductance in the 200-400mh range). Mitigating interconnecting cable length/capacitance?
THANKS
It might reduce capacitance. If two core cable is used it would be reduced to inter-core capacitance which is few times lower. With two concentric shields it would be very low.
But it doesn't seam practical to me. Too complicated with probable side effects just to let you keep phono preamp apart from the turntable. I would rather keep the preamp next to the table and use 20cm or less cable.
But it doesn't seam practical to me. Too complicated with probable side effects just to let you keep phono preamp apart from the turntable. I would rather keep the preamp next to the table and use 20cm or less cable.
What he shows his a conventional XLR cable, but not a triax,so you describe the correct type. Regarding prices of triax cables (used for measurements), have a look here:
https://www.digikey.de/de/products/detail/pomona-electronics/5223-60/737191
but these cables are surely not of the kind you would use. Btw, all the triax cables I used were very (!) stiff.
For a phono cartridge I would use a well shielded XLR cable, where the screen is properly connected to the chassis (and not to signal ground via pin 1, as many XLR connectors do). And as @chip_mk mentions, keep it short.
Covered in Horowitz and Hill... The inner shield is driven from a unity voltage buffer so it is kept at the same potential as the inner signal wire, so no capacitive loading occurs (at low frequencies at least - doesn't work with RF). Just another application of voltage bootstrapping. Was commonly used for very high impedance AC signals like EEG and ECG - these days we often just use a head-amplifier at the sensor itself so the signal is already buffered before the cable - after all a SOT23-5 FET opamp is way cheaper than special cable.
Many people have had the idea of integrating a phono preamp into cartridge, headshell or tonearm.
The voltage at the middle of RG in an instrumentation amp is the common-mode voltage of the input. Drive the driven shield with that to reduce the leakage from the signal pair to the shield. As pointed out above "instrumentation amp with driven shield" would be a good search term. "Guard ring" would be another good search term. The driven shield is basically an extension of a guard ring.
Not only are Triax cables horribly expensive. So are the connectors. The cable will also always be heavier than a simple shielded cable, which could make it challenging to use in a tonearm.
Tom
Not only are Triax cables horribly expensive. So are the connectors. The cable will also always be heavier than a simple shielded cable, which could make it challenging to use in a tonearm.
Tom
There is no signal pair, as it is no balanced cable. Just one centre conductor with two concentric shields around it.
To me, it sounds like an interesting approach to minimize the capacitive load on the cartridge.
To me, it sounds like an interesting approach to minimize the capacitive load on the cartridge.
Oh, true. A Triax would be a single conductor with two shields (one of which is driven). I was responding to Post #4 which shows a signal pair.
I agree that a driven shield would be an interesting way to reduce capacitance, but I wonder about the weight of the cable.
Tom
I agree that a driven shield would be an interesting way to reduce capacitance, but I wonder about the weight of the cable.
Tom
You could at least use it for the part outside the tonearm, where the weight of the cable is less of an issue.
True that. How big of an issue is capacitance in phono inputs?
Personally I'd move the RIAA stage into the turntable and have it output 2 V RMS line level. But that's me.
Tom
Personally I'd move the RIAA stage into the turntable and have it output 2 V RMS line level. But that's me.
Tom
Moving the phono preamplifier into the turntable is also what Nick Sukhov does. It allows him to use very non-standard termination impedances that give less noise and better transient response with many types of cartridge, see https://www.diyaudio.com/community/threads/nick-sukhov-su-xxi-mm-phono-stage-85-dba-sn-ratio.387375/ You could do something similar with triax cable if you don't want to put the amplifier into the turntable.
In general, cable capacitance has quite an influence on the response above 10 kHz when you have a high-inductance cartridge (say around 500 mH or above). Nonetheless, the recommended load capacitance for some high-inductance Shure cartridges is pretty high. @Hans Polak started a thread about modelling the cartridge response with load including the mechanical response, based on lots of measurements, see https://www.diyaudio.com/community/...ts-with-individual-transfer-functions.397815/
In general, cable capacitance has quite an influence on the response above 10 kHz when you have a high-inductance cartridge (say around 500 mH or above). Nonetheless, the recommended load capacitance for some high-inductance Shure cartridges is pretty high. @Hans Polak started a thread about modelling the cartridge response with load including the mechanical response, based on lots of measurements, see https://www.diyaudio.com/community/...ts-with-individual-transfer-functions.397815/
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Signal guarding is discussed in Horowitz/Hill starting in Chapter 5.a5.3D (page 358) and Figure 5.78. Again, concept seems only important with "high impedance inputs - somehwere I've seen mention of devices 600mH and above effecting things "above 10Khz". But the op amps shown also use balanced inputs. Would concept still be effective with unbalanced (single ended) inputs?
Definitely effective with single ended signals - in fact it can't work for differential signals unless each signal wire is individually guarded. The circuit in #4 doesn't make sense to me as it bootstraps only the common-mode signal, which is then ignored by the instrumentation amp!
If he stray capacitance of the input cables differ than this degrades the CMRR of the instrumentation amplifier. The driven shield ameliorates the problem.
Ah, true that could be very important for high impedance situations with a lot of common-mode noise...
Interesting discussion and ideas. I used Triax every day for about 20 years - but the big stuff. It is the de facto standard cable for broadcast video cameras. It carries a lot of information and power over moderately long distances - and in two directions. It even comes in a green jacket so you don’t notice it on the golf course. 🙂
Much bigger than you would use for audio, usually 1/3 to 1/2 inch in diameter. Belden 8233A is a good example.
I’m intrigued to learn how triax might be used in audio.
Much bigger than you would use for audio, usually 1/3 to 1/2 inch in diameter. Belden 8233A is a good example.
I’m intrigued to learn how triax might be used in audio.