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Old 19th June 2005, 09:56 AM   #1
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Default moving coil loading

ok, moving coil cartridges are said to be less susceptible to capacitive loading than mm's.

But the coil inductance will still resonate with the lead and input cap., which the loading resistor damps.

so we should vary the load resistor to get a flat response.

Not much is said about the lead or input cap tho', surely to get the correct value resistor, we need the correct value cap. as well, so how do we work this out. We should know our cartridge inductance, so we can calculate damping resistor by square root of (L divided by C)

And why do mc stages still have input caps, surely they could just use lead capacitance?

And what's all this about using resistors to do mechanical damping, too? bit confused....
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Old 19th June 2005, 02:08 PM   #2
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See this site for a good explanation of cartridge loading:

http://www.hagtech.com/loading.html
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Old 20th June 2005, 07:34 PM   #3
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The formula for the resonant Q is Q=R*SQRT(C/L). Mr. Hagerman's formula for the load resistor R=SQRT(L/C) implicitly uses a Q of 1.0. You want the Q to be between 0.5-0.7. If the Q is higher than 0.707 the response starts to develope a peak as shown on Mr. Hagerman's website.

Of course with a low inductance MC cartridge the peak would be at an ultrasonic frequency F=1/(2*PI*SQRT(L*C)) unless you had a really big terminating capacitor that brought the resonant peak down into the audiable range. With high inductance MM cartridges the terminating capacity is critical.
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Old 22nd June 2005, 01:03 AM   #4
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by audiobomber
See this site for a good explanation of cartridge loading:

http://www.hagtech.com/loading.html
Although the maths and electrical theory is correct, a fundamental (and incorrect) assumption is unspoken. The mechanical response of the cartridge is not necessarily flat. The resonant electrical response is designed to equalise the mechanical response. That's why it was possible to have HF loss using an Ortofon VMS20E (induced magnet) with insufficient capacitive loading.

Moving coil cartridges tend to have lower moving mass, so I would expect their HF resonance to be far less critical. Just as well, really, as the frequency of HF mechanical resonance is determined by tip mass and individual vinyl compliance.
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Old 22nd June 2005, 02:47 AM   #5
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Quote:
Originally posted by EC8010
The mechanical response of the cartridge is not necessarily flat. The resonant electrical response is designed to equalise the mechanical response.
Yes, most MM's use this resonance to boost the highs so they can get to 20kHz. I believe the frequencies above the electrical resonance will be opposite in phase to the main output of the cartridge. No wonder most audiophiles prefer the highs from an MC.
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Old 22nd June 2005, 08:32 AM   #6
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Hi,
MM require quite small capacitance to balance out the treble peak. Usually in the range 50pF to 470pF. The input capacitance of the phono amp and the cable capacitance are comparable to the lower end of this range and so should be taken into account when adjusting the additional capacitance to correct the MM response.

MC require substantial capacitance (due to lower inductance and lower resistance) and the cable and input capacitance can be virtually ignored. I believe typical values are 500pF to 2000pF.
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Old 22nd June 2005, 02:44 PM   #7
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Quote:
Originally posted by AndrewT
MC require substantial capacitance (due to lower inductance and lower resistance) and the cable and input capacitance can be virtually ignored. I believe typical values are 500pF to 2000pF.
MC's are very insensitive to capacitance, as you state. But they do not require capacitance to shape their frequency response, as do MM's. The HF resonance of an MC is above the range of audibility, so extra capacitance (above that provided by the wires and phono stage) should not be added, because it will degrade the sound and cannot improve it. Extra capacitance will cause increased phase shift and no benefits.
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Old 22nd June 2005, 02:56 PM   #8
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Quote:
Originally posted by audiobomber
I believe the frequencies above the electrical resonance will be opposite in phase to the main output of the cartridge.
Because of the phase change through resonance. But it doesn't really matter because above resonance the response is the combination of the (falling) response of the cartridge generator system and the (falling) response of the electrical resonance. The word "plummet" comes to mind. All resonant equalisers have this brick wall response. I hate to think what the phase response looks like below resonance.
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Old 22nd June 2005, 08:52 PM   #9
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If you model the cartridge as a low pass filter in a circuit analysis program, as Mr. Hagerman does, you can look at the group delay (time delay) and see the effect of the capacitive loading on the phase linearity. For example, the Shure V15V has a very nice equiripple group delay with 80 pF load capacity. I can not figure out why Shure recommended a 250 pF load.
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Old 23rd June 2005, 12:10 AM   #10
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Quote:
I can not figure out why Shure recommended a 250 pF load.
Because it wouldn't satisfactorily equalise the mechanical system? I don't have the figures to hand, so I can't repeat your analysis, but presumably the change in capacitance causes a significant change in the amplitude response.

Incidentally, when I first looked at this problem (with the aid of a BBC Model B "computer"), I found that you could make a very quick assessment of a cartridge's relative quality by calculating the resonant frequency from the inductance and recommended loading (it has to coincide with the vinyl/tip mass resonance, and is therefore an indicator of tip mass). That was instrumental in my decision to replace our Shure SC35 catrridges with the recently introduced Ortofon OM Pro.
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