Info on Decca cartridge loading sought.

[Aleksunder]

Hi all,

does anyone have any knowledge/experience with these cartridges?

I have the spec sheet for my C4E, and it doesn't mention anything about loading, although it does give the DC resistance and inductance as 4.9K and 285mH (per channel). The coils do indeed measure dead-on that figure for resistance, but I don't have the means to test the inductance.

Generally, I gather people like to load them at less than the 'standard' 47K of most phono-stages, but I've heard anything from 33K down to 10K mentioned.

And would the lowest achievable capacitance be a good idea? I have a pair of DNM Reson interconnects I was thinking of using for the arm's flying leads, (with an earthed shield-braid around them) which are virtually capacitance-free (15pF per metre).

thanx.

[honinbou]

my SC4E/C4E spec sheet says recommended input impedence is 50K.

[rayfutrell]

Hi Aleksunder,

The load resistance should not be too low because the DC resistance of the cartridge and the load resistor form a voltage divider. My rule of thumb is that the load resistor should not be less than 10 times the cartridge resistance so a 50K load resistor looks correct at first. Another rule of thumb is that the Q of the cartridge should be between 0.5 and 0.7. The Q is calculated by the formula Q=R*SQRT(C/L) where SQRT means take the square root. This is the formula for the Q of a parallel resonant RLC circuit with R the load resistor, L the cartridge inductance and C is the total capacity in the cartridge, tone arm wiring, interconnects and preamp. For your cartridge inductance and a 50K load resistor, a Q of 0.7 calculates to a maximum of about 60Pf total capacity. Practically speaking, this low a value of capacity will be impossible to realize, so you will have to reduce the load resistor below 50K to reduce the Q.

Regards,
Ray

[Aleksunder]

Originally posted by rayfutrell
Hi Aleksunder,

The load resistance should not be too low because the DC resistance of the cartridge and the load resistor form a voltage divider. My rule of thumb is that the load resistor should not be less than 10 times the cartridge resistance so a 50K load resistor looks correct at first. Another rule of thumb is that the Q of the cartridge should be between 0.5 and 0.7. The Q is calculated by the formula Q=R*SQRT(C/L) where SQRT means take the square root. This is the formula for the Q of a parallel resonant RLC circuit with R the load resistor, L the cartridge inductance and C is the total capacity in the cartridge, tone arm wiring, interconnects and preamp. For your cartridge inductance and a 50K load resistor, a Q of 0.7 calculates to a maximum of about 60Pf total capacity. Practically speaking, this low a value of capacity will be impossible to realize, so you will have to reduce the load resistor below 50K to reduce the Q.

Regards,
Ray

That's very useful info, thank you. It would seem the DNM might well be a good cable to use - the phono-stage (Gram Amp 2 SE) is 150pF on it's own.

As an aside, I got the impression that, given the Decca's unique abundance of 'speed' and 'transient attack' (due the weird "postive scanning" cantilever/motor design), a lot of people trade some of this off against a less 'shouty' presentation by making them generate more current with low (or thst high?) and theoretically non-optimal DC-resistance loads.

Perhaps this also induces more 'eddy-current' damping of the cantilever itself, and consequently improves tracking/decreases distortion? (please correct me if I'm wrong).

Thanks again.

[rayfutrell]

Mr. Hagerman has a good paper on cartridge loading at http://www.hagtech.com/loading.html .

Assuming 50 Pf of capacity in the cartridge and tone arm wireing(a guess), 150 Pf for the preamp and 15 Pf for the interconnects calculates to a load resistor of about 25K for a Q of .7. If the load resistor in your preamp is the standard 47K, you can approximate the correct load resistor by shunting it with an external 47K resistor. You can experiment with the external resistor and you don't even have to open the preamp and modify it.

Regards,
Ray

[Groovey Records]

To all

I Found this valuable resource at the vinyl engine It is a interactive db of thousands of cartridges

http://www.cartridgedb.com/

Regards

Groovey

[cerrem]

WHile it is totaly good to figure the loading... I prefer to measure it..
Since this is a electro-mechanical device...just the electrical proporties alone do not show everything..The Q is also affected by the contribution from the mechanical side of things compliance/ mass of the cartridge, ect... So the "Static" measurements don't mean much when your cartridge is in a "Dynamic" state when it is being used...
Use test records with as good a square wave as possible and adjust the loading as need be....
I think I have some old CBS or DIN test records with descent square waves....it been a while since I used them...
But I found that to be fairly good indicator....
In many cases there was no science or engineering behind the reasons for data sheets and recomended loading....In most cases it was a quick review by ear and the data sheet was born...So don't fool yourselves into thinking the data sheet spec is the bible....

Chris