#### lt cdr data

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....

#### rayfutrell

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.
Regards,
Ray

#### EC8010

audiobomber said:
See this site for a good explanation of cartridge loading:

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.

#### audiobomber

EC8010 said:
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.

#### AndrewT

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.

#### audiobomber

AndrewT said:
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.

#### EC8010

audiobomber said:
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.

#### rayfutrell

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.
Regards,
Ray

#### EC8010

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.

#### jondavis

how about DC current? how much current can an MC cartridge tolerate? what's the typical DC resistance of MC cartridges?

#### EC8010

I wouldn't expect a cartridge to tolerate any DC whatsoever. DC in a loudspeaker increases distortion, so, by reciprocity, I would expect the same to be true of a cartridge.

Cartridge resistance for a low output MC can be anywhere from 3 Ohms to 20 Ohms.

#### rayfutrell

(it has to coincide with the vinyl/tip mass resonance, and is therefore an indicator of tip mass)

Regards,
Ray

#### EC8010

What you're trying to do when you resonate the electrical circuit is to make an electrical response that mirrors the mechanical response. Higher tip mass causes the mechanical response to roll off earlier, but with much the same shape. Ergo, you can deduce tip mass from the electrical resonant frequency. It's a long time since I went through all this, but if I recall correctly, the Shure SC35 had an electrical resonance at 15kHz. I looked at a number of cartridges at the time, and there was a strong correlation between tip mass and electrical resonant frequency.

If one had a collection of different cartridges, one could play clicks and scratches from the same record and capture them on a digital oscilloscope or soundcard to find the frequency vinyl/tip mass resonance and check this against the electrical resonant frequency. The reason I suggest this is that I'm deeply suspicious of some of the tip mass claims that I have seen.

#### jondavis

EC8010 said:
I wouldn't expect a cartridge to tolerate any DC whatsoever. DC in a loudspeaker increases distortion, so, by reciprocity, I would expect the same to be true of a cartridge.

Cartridge resistance for a low output MC can be anywhere from 3 Ohms to 20 Ohms.

how about those opamp-based mc preamp? with dual supply, the ground connection biases the inputs and bias current passes through the loading resistor and because the mc cartridge has lower resistance about 20 ohms, most current passes through the cartridge. Am I right?

#### analog_sa

Am I right?

You're right. Using bipolar opamps with an MC is not such a great design idea. Apart from the possible increase in distortion you may have to use a demagnetiser at some stage. Not to mention that if something kills the opamp output your cart is toast.

#### jondavis

analog_sa said:

You're right. Using bipolar opamps with an MC is not such a great design idea. Apart from the possible increase in distortion you may have to use a demagnetiser at some stage. Not to mention that if something kills the opamp output your cart is toast.
so, what shall i do to prevent this problem? shall i use coupling cap? i have an opamp-based mc stage right now.

#### peranders

Paid Member
analog_sa said:
You're right. Using bipolar opamps with an MC is not such a great design idea. Apart from the possible increase in distortion you may have to use a demagnetiser at some stage. Not to mention that if something kills the opamp output your cart is toast.
Are you really sure? We are talking about nA and to kill an opamp, not likely if a proper design is made.

A LT1115, LT1028, AD797 will work fine with direct coupled cartridge.

My LT1115 based amp works pretty good with no input coupling cap.

#### EC8010

If the cat leaps up and pats the stylus, your cartridge is ruined. But it's unlikely to happen. Likewise, why should an op-amp suddenly fail in such a way that it passes a destructive current through a cartridge?

Sonically, there's always a problem with DC. Any op-amp will always have a very small bias or leakage current, and if you AC couple, the required size of capacitor (to maintain low noise) ensures that its leakage current is greater than an op-amp's bias or leakage current. The only way to ensure that you don't pass DC into the cartridge is to use a transformer. Otherwise, there's no reason whatever not to use a bipolar op-amp.