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Old 20th November 2009, 08:47 AM   #1
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Default DC Coupling for MC Cartridges - how much error voltage before coil damaging

There are two methods of coupling the moving coil cartridge to the prepre input:
1) AC coupling about a capacitor
2) DC coupling (without a capacitor)
By AC coupling I have lack in quality cause additional capacitor device in series to the moving coils, but by DC coupling I risk a damage of both internal cartridge moving coils in case of an error in the head amp circuit (frontend).
Therefore the question:
How much current and voltage is acceptable at the moving coils without the risc of damage from internal coil winding?
Second question: What kind of dc protection are there without disadvantages by the sonic transmission?

Example of dc coupling head amp are the follow:
Hiraga MC Preamp and the circuits about
Mpp
and ac coupling go to post #99 about
HPS 4.0 phono stage
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Old 20th November 2009, 02:01 PM   #2
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I have been wondering about the same issues for some time. Sorry no help for you though...

Regarding the Current I think it must be a few mA (maybe more for a very short moment).

Regarding the DC protection, I think the best solution is to fit a FET than shorts the inputs (either the windings of the coil or to Gnd) in case of failure.........
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Old 23rd November 2009, 08:21 AM   #3
oshifis is offline oshifis  Hungary
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I did this experiment and the result was fatal
I connected a Dynavector DV-23RS into the cathode of a tube (E88CC/6DJ8) run from 50V. The cathode current was 4-5mA. It sounded fabulous - for a while. Result: one channel broken...
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Old 23rd November 2009, 10:02 AM   #4
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Quote:
Originally Posted by oshifis View Post
I did this experiment and the result was fatal
I connected a Dynavector DV-23RS into the cathode of a tube (E88CC/6DJ8) run from 50V. The cathode current was 4-5mA. It sounded fabulous - for a while. Result: one channel broken...
Very interesting observation. That bring me to the next question:
What value of bias voltage about the moving coils of a certainly cartridge is necessary?

From too large spread of spiders for loudspeaker-suspensions, it is sometimes necessary to fit the voice coil of diaphragm with a bias voltage for the correct neutral position. I. e., the ideal offset voltage of amplifier isn't 0 volts.

Unfortunately I cannot judge, whether this is relevant for moving coils of record player pickups. But this means at the same time, that I cannot rule out that bias voltage is necessary for such cartridges.

Last edited by tiefbassuebertr; 23rd November 2009 at 10:04 AM.
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Old 26th November 2009, 09:49 AM   #5
oshifis is offline oshifis  Hungary
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Bias voltage is not necessary, in fact it is harmful. The stylus will be set in the center of the magnetic field when proper vertical tracking force and proper antiskating force is applied.

Off-topic question: who can repair a damaged MC cartridge (broken coil)?
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Old 26th November 2009, 09:59 AM   #6
brianco is offline brianco  Ireland
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Safety limits will surely vary from cartridge model to model? e.g. The most extreme low output /resistance MCs will have a lower safety margin than those high output/resistance cartridges. I imagine that the exceptionally low output MCs - such as the Be Yamamura MC1 version of the Audio Teckne or the earlier Kondo Audionote Io will be more at risk than a high output MC.

Regarding repairs, the acknowledged best repairer at a sensible price would appear to be the Expert Stylus Company in London. Is it not possible to have the cartridge accepted back as part of a makers exchange programme?

Last edited by brianco; 26th November 2009 at 10:02 AM.
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Old 26th November 2009, 11:33 AM   #7
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Quote:
Originally Posted by oshifis View Post
Bias voltage is not necessary, in fact it is harmful. The stylus will be set in the center of the magnetic field when proper vertical tracking force and proper antiskating force is applied.

Off-topic question: who can repair a damaged MC cartridge (broken coil)?
I would look to find the fusing current of the typically VERY fine wire. It could easily be mA's. If you are lucky the open is at a post and someone in the cart mod biz can help you.
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Last edited by scott wurcer; 26th November 2009 at 11:52 AM.
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Old 26th November 2009, 01:45 PM   #8
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Quote:
fusing current of the typically VERY fine wire.
The size of that fine wire has always been my question. Has anybody here ever taken apart defunct cartridges and measured the wire. Or does anybody have data from a manufacturer. I can imagine low melting point fuse wire going open at a couple mA, but for copper wire to go open would require mind-boggling small wire. Just glad I don't have to work with the stuff!

CH
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Old 26th November 2009, 07:17 PM   #9
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A little on line research was needed for this. I did not find fusing current but I did find current for a 25C rise. For 40 AWG it is 0.09 A. The smallest wire I could find was 50 AWG which witch will carry 0.009A or 9 ma. I have no Idea what size wire is used in MC cartridges. Maybe more on line research will revile some answers.
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Old 26th November 2009, 08:00 PM   #10
jcarr is offline jcarr  United States
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In MC cartridges, the range of coil wire used will be between 12um and 60um. 12um is very seldom used, as it tends to break too readily in production. 15um was used by some medium-to-high impedance Denon cartridges. For low-impedance MCs, 30~40um wire will be more typical (35~40um in my case). The most extreme of the Matsudaira-designed MCs (MySonic Labs) use 60um wire for an internal impedance of 0.6ohms.

Quote:
Originally Posted by oshifis
The stylus will be set in the center of the magnetic field when proper vertical tracking force and proper antiskating force is applied.
When proper VTF and antiskating are applied to MC cartridges, one would hope that the signal coils will be centered and aligned to the magnetic field (rather than the stylus). But as far as I can tell from the cartridge design documents that I have researched, apart from designs such as JVC's MC-L1, MC-L10 or MC-L1000, so far this has not been the case (or if it was, the manufacturers were careful to conceal what they had accomplished (^o^)).

hth, jonathan carr
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