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Old 27th December 2006, 08:35 PM   #1
bulgin is offline bulgin  South Africa
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Default Moving Coil cartridge experiment

Hi All

I suppose you're never too old to admit you're stoopid when it comes to some things to do with mc carts. I'm busy with some armature winding experiments and recently found some nice copper wire which I'm guessing is approximately half the thickness of wire I've used before. To start this experiment, I used 20cm of the previous wire I used and laid it out on a piece of card and put a blob of solder at each end. Next, I used the new thinner wire and did the same with that (20cm length and a blob of solder at each end).

I then measured resistance/impedance of both and found the thicker wire measures say, 7.0ohm and the thin wire 15.0ohm.

Continuing the experiment, I then wound the cart's armature (which before had a reasonable 0,35mV output), and used the same number of turns/channel as before.

I haven't yet done any measurements with the new thinner wire and the new 15.0ohm impedance but I could hear a slight loss of output, using the same dc-powered stepup (nominal gain 40dB and voltage gain 100X).

Following this experiment, I stripped the cart and rewound the armature, this time adding 30 extra turns of wire and increasing the impedance to 15.7ohm on each channel.

On trying it out, I found I had all the 'sparkle' and all the previous good properties back, including (I guessing here) its original 0,35mV output.

Something else I'm also pondering here is that the thicker wire used before, actually caused the armature and its coils to sit further forward (or away) from the magnet's flux. The thinner wire is clearly taking up almost half the mass loaded on to the armature core, is flatter when wound and clearly brings the coils in closer proximity to the magnet's flux. The other benefit would be that the armature core now carries probably half the coil mass than before, with obvious benefits of better frequency response.

Can anyone perhaps think of any NEGATIVES to this change? There is also the small matter of armcable capacitance...

I am reasonably happy that it has been a worthwhile change, unless...

Regards and a very fine, peaceful, prosperous & healthy New Year to all

bulgin

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Old 7th January 2007, 11:01 PM   #2
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Changing the coil impedance(7 or 15 ohm)and using the same input impedance on your phono stage,does not give you a clear view of how the two options compare.I would suggest that your phono stage input impedance should be around 20 times higher than coil imp. of your cartridge.Then,you will be able to have a more clear and valid opinion on the sound of different options to your cartridge.
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Old 8th January 2007, 02:46 PM   #3
bulgin is offline bulgin  South Africa
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Default Moving Coil Cartridge Experiment

Hi Panicos

Thanks for your kind opinion. I've since completed these experiments and have decided to stay with the thinner wire. You are probably right with the 20X rule of thumb but unfortunately I have only a couple of unadjustable phono pre-amps, plus a pre-amp with a switchable mc input between mm and mc.

There seems to be no adverse effect stepping the pin measurements up from 7.0 to 15.0ohm. I also think I have an extended frequency response due to the coil mass which has in effect been halved.

Regards

bulgin
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Old 8th January 2007, 05:51 PM   #4
tubenut is offline tubenut  South Africa
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Most MC phono pre amps just have resistor across the input to load the cart, you can not go up but you can certainly go down by paralel connecting a resistor across the input with a Y RCA splitter and RCA plug with resistor from signal to ground.

Typically a cart playing in a higher load then optimum can boost treble output but you know this I am sure. I would thus first adress the impedance and then listen again. I would expect microdynamics and detail to improve due to the decreased mass of the coil.

jm2c
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Old 8th January 2007, 06:57 PM   #5
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Tubenut,you are right about the impedance set resistors.If it is easy for bulgin to identify these resistors,I think he can go up also,by replacing them altogether.Also,I think it helps for better sound to have a single resistor than a parallel pair.If with the thiner wire the cartridge impedance is around 15 ohm,using resistors around 300 ohm will give an ''optimum'' load for the cartridge.And,bulgin if there are parallel caps to the input resistors,experiment by removing them completely and see how it goes.Usually,to my experience,by removing capacitors either in mm or mc phono preamps,makes the sound less harsh,more open and sweeter,unless the cartridge in use suffers from really high lift at high frequencies and therefore needs certain capacitance to tame it.
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Old 8th January 2007, 07:05 PM   #6
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By the way bulgin,which was the original cartridge you now experiment with?
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Old 8th January 2007, 08:08 PM   #7
Cobra2 is offline Cobra2  Norway
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I thought Grado did the opposite...unwind the coils for their high-end carts...(with low output as result).

Arne K
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Old 9th January 2007, 12:09 AM   #8
jlsem is offline jlsem  United States
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Quote:
You are probably right with the 20X rule of thumb but unfortunately I have only a couple of unadjustable phono pre-amps, plus a pre-amp with a switchable mc input between mm and mc.
It occurs to me that anyone capable of winding an MC cartridge armature should be able to change the load resistor in a pre-amp, no problem .
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Old 9th January 2007, 09:58 AM   #9
EC8010 is offline EC8010  United Kingdom
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When I make a (valve) power amplifier, I test with a variable resistance load to make sure that the optimum load resistance is the same as the one I designed for. I can't help thinking that bulgin needs adjustable cartridge loading to determine optimum loading quickly and easily. A pair of cheap rotary 2 pole (to allow for ganged stereo switching) 6 way switches wired like a decade box with 20 Ohm resistors between the contacts of the first and 100 Ohm between the contacts of the second would allow 0 to 600 Ohms in 20 Ohm steps at minimal cost.
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Old 9th January 2007, 10:26 AM   #10
bulgin is offline bulgin  South Africa
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Default Moving Coil Cartridge Experiment

Hello Everyone

A happy, Prosperous, Healthy 2007 to you all

I 've been watching this post since the day I did so, never giving up on receiving some useful advice

The perceived 'problem' started when a particular batch of copper wire for winding armatures ran out and could not be replaced with an equivalent. I had a particular sound and performance with a cart which I did not really want to lose and the only alternative was this new wire which is half the dia of the previous wire. So thin, that I had to teach myself an entirely new winding technique and special care in soldering the leadout wires to the cart pins.

Overall, as mentioned in the previous post, the change has been beneficial. The dreaded highfrequency emphasis is a load of baloney in this case

My own opinion (and that of the smart posters to this thread) is that halving the coil mass and being able to bring the coils closer to the magnet as the windings are flatter, retained most of the previous output of 0,35mV, plus the added benefit of better frequency response as is evident on watching its performance on a dualtrace scope, playing say, a Decca Frequency Test Record

The cart I messed-around with was reviewed in Analog Corner (Stereophile 12/06).

The next step is to heed the advice I received here and build a loading device

Regards

bulgin
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