Cartridge Impedance Matching

[d to the g]

Cartridge Impedence Matching

Ok, I'm kindof a noob when it comes to diy, so take it a little easy.

I'm currently building a pass pearl and am trying to find out how to correctly load the cartridge with the correct resistor and capacitors.

I'm using an AT 440MLa, and I've found values for the recommended load.

Coil impedance at 1kHz: 3.2 kOhm
Coil inductance at 1kHz: 490 mH
Recommended load resistance: 47 kOhm

Does this mean I would just put a 47kOhm resistor on R1? What about capacitance?

 

[Ghianni]

I think, this is going to help you: http://www.hagtech.com/loading.html

 

[d to the g]

I think, this is going to help you: http://www.hagtech.com/loading.html

and you were right, that did help. thanks!

 

[EC8010]

I'm afraid Jim Hagerman is wrong on this one. He's quite correct in showing that different loading capacitances and resistances change the frequency response, but his assumption that a flat electrical response is desirable is incorrect. You see, cartridge manufacturers use the electrical resonant circuit to equalise for the falling mechanical response of their generator. That's why early Ortofon VMS20E required 400pF-500pF of loading capacitance and too little capacitance actually reduced the treble heard. 1970s and 80s Shures were the same.

 

[Nordic]

I'm currently bussy with a layout for Graham Maynard's The "ACE" from 1975.

It has variable damping, variale input C variable input impedance, extended RIAA switch, Diffirential bass cut switch, balance control.

Most of the circuits I see take no notice of damping etc...

 

[neutron7]

nordic LOL @ your avatar even if it is "slightly" photoshopped

 

[d to the g]

I'm afraid Jim Hagerman is wrong on this one. He's quite correct in showing that different loading capacitances and resistances change the frequency response, but his assumption that a flat electrical response is desirable is incorrect. You see, cartridge manufacturers use the electrical resonant circuit to equalise for the falling mechanical response of their generator. That's why early Ortofon VMS20E required 400pF-500pF of loading capacitance and too little capacitance actually reduced the treble heard. 1970s and 80s Shures were the same.

what do you suggest then?

 

[EC8010]

I suggest that you follow the cartridge manufacturer's recommendations. It's a shame they don't specify capacitance. 1970s and 80s cartridges tended to want 400pF-500pF (that's arm wiring and pre-amplifier together). Using that amount of capacitance tended to resonate the equaliser at somewhere between 15kHz and 30kHz, but the response dropped off like a stone after that. In fact, you can often get a good idea of stylus tip mass by calculating the resonant frequency of the cartridge's inductance paired with its load capacitance. As I recall, the Shure SC35 resonated at 15kHz (it was a big robust broadcast beast), whereas cartridges with lighter cantilevers and rondels needed less equalisation and had a correspondingly higher equaliser resonant frequency.

Then, CD4 appeared with its quadraphonic subcarrier that needed a cartridge response up to 50kHz. That led directly to improved stylus profiles (Shibata etc) and the necessity of reduced loading capacitance to enable the equaliser's resonant frequency to be set to 50kHz or so. Pick-up arm manufacturers responded and started making arms with greatly reduced arm wiring capacitance (it wasn't difficult and enabled them to claim that they were CD4-ready). All of a sudden, load capacitance had fallen from 400pF-500pF to about 250pF, but the majority of the cartridges were the same and needed the old capacitance. Ortofon sold a 200pF loading capacitor that slid onto the back of their VMS20E to correct the situation.

So, if your cartridge is old, try 400pF-500pF, if modern, try 250pF. Try 250pF first, then, if the extreme treble seems weak, try 450pF. If it needs 450pF, the extreme treble will improve, if not, it will get even worse.

 

[sreten]

Hi,

Also note that capacitive loading is additional to cable capacitance, usually ~ 200pF.

/sreten.

 

[d to the g]

I think I'll go w/o any additional capacitance and see how it sounds. My klipsch are awful bright so if the high end is off I'll definitely notice.

 

[Nordic]

To quote Graham Mynard - I have become quite the fan of his work.

1) A moving magnet pick-up cartridge is an inductive transducer that must be resistively damped and reactively tuned to optimise reproduction. Hence I fitted a sub-miniature twin gang 500pF variable and screen earthed twin gang potentiometers directly to the input circuitry
It is only *after* you have actually used these input damping and tuning controls whilst music listening to optimise your own equipment line-up, that you can understand just how much mind distracting spin has been repeated about fractional 'dB' variation with respect to an ideal RIAA characteristic.
Cartridge to pre-amplifier matching has a much more significant effect upon reproduction than does the achievement of perfect RIAA equalisation !!!

http://www.zen22142.zen.co.uk/Circuits/Audio/vinyl.htm

 

[EC8010]

Seems a little extreme, but if you consider that the tolerance of cartridge inductance and mechanical losses are not likely to be good, it makes sense. Perhaps the reason why I never really liked moving magnet cartridges was that their equalisers were mismatched...

 

[Nordic]

I suppose getting a dual adjustable capacitor like those are hard now.... Otherwise there is no harm in trying...

 

[EC8010]

No problem at all, just raid an old valve radio or go to a ham radio shop. Typically, they were 365pF + 365pF, older ones were a little larger and might be 500pF + 500pF. Air-spaced, of course...

 

[h_a]

Don't you have the instruction manual at hand? Usually the recommended load capacity is mentioned there.

Or you could email Audio-Technica about the recommended load capacity - in my experience they reply quickly!

I would really recommend finding out the correct value! Otherwise you'll get accustomed to a non-linear frequency response which I cannot imagine is in your interest.

With kind regards, Hannes

 

[d to the g]

Don't you have the instruction manual at hand? Usually the recommended load capacity is mentioned there.

I do, and it isn't

Or you could email Audio-Technica about the recommended load capacity - in my experience they reply quickly!

I did, and then they did. 100pF I'm not sure why they don't include it in the manual


thanks!

 

[rayfutrell]

That is interesting. That is about what I would have calculated as the correct value. My Shure V15V has almost the same inductance and the calculated value of capacitance for a Q of 0.7 was 120pF.

Never could figure out why Shure recommended a 250 pF terminating capacitor when it sounded much better with a 120 pF termination.
Regards,
Ray

 

[Panicos K]

''Recommended''capacitance is not necessarily correct or best.With modern cartridges with their more linear responses,in most cases(excluding extreme cases)it is not even necessary to use any capacitance at all.The capacitance of internal arm wires and output cable is enough.

 

[jackinnj]

Thought I would refresh this thread -- I have an M78S cartridge set up for mono (coils in parallel) and took a look at the actual impedance -- sure looks like 500mH and 200p =~ 16kHz:

An externally hosted image should be here but it was not working when we last tested it.

 

[lcsaszar]

Typical phono cable capacitance is around 100 to 150 pF.