I hadn't forgotten the cable capacitance, just assuming it's a constant. I'm figuring that for the price of a few caps it's worth trying. The bigger question really is what type of cap and tolerance would it be best to experiment with?
It's an Empire LTD 750 - NOS. The phono stage is a Dynavector P-75.
It's an Empire LTD 750 - NOS. The phono stage is a Dynavector P-75.
You're right, I do want to reduce capacitance. Isn't the formula for series caps the same as for resistors in parallel? i.e. 1/C=1/C1+1/C2. Plugging 470 pF and 68 pF into that equation gives an effective capacitance of 59.405 pF.
As I said, I'm comfortable with the theory, prepared to experiment and would appreciate some guidance on what type of caps/tolerances, etc. Obviously at 4.5 mV output, voltage rating isn't relevant.
I don't really want to remove capacitance from the phono pre as I also use it with other carts.
As I said, I'm comfortable with the theory, prepared to experiment and would appreciate some guidance on what type of caps/tolerances, etc. Obviously at 4.5 mV output, voltage rating isn't relevant.
I don't really want to remove capacitance from the phono pre as I also use it with other carts.
I totally agree with Sreten..there is something seriously wrong in this idea.
But---since you insist on trying,-any cheap capacitor, 20% or better, just for the sake of experimenting. If your idea holds water at all, the type or brand of cap will only affect the subtleties of the matter.
I'm not at all saying that capacitor types don't matter - just that their influence is grossly overrated, in general terms......
But---since you insist on trying,-any cheap capacitor, 20% or better, just for the sake of experimenting. If your idea holds water at all, the type or brand of cap will only affect the subtleties of the matter.
I'm not at all saying that capacitor types don't matter - just that their influence is grossly overrated, in general terms......
If your preamp has a 470pF input capacitance , then it must be due to a capacitor soldered across it's input. No standard circuit will have an input capacitance of 470pF in a phono stage without the addition of an external capacitor.
Your solution would be to find the input capacitor and remove it.
Note that this must be a capacitor in parallel with the input and not any series capacitor which might be there to block dc .
The tone arm cables generally have a capacitance in the region of + 100pF per meter. Finding cables lower than 70pF per meter might not be easy. So even with zero capacitance in the preamp you might not be able to get a 60pF load.
Unless of course you install the phono preamp board right under the turntable ( inside).
Then the contributing capacitance will be only the wires in the tone arm and the preamp input. You might have to change the input capacitor in the preamp to achieve the 60pF loading.
In any case you probably have a very rolled off HF performance right now with a 470 pF plus 120pF cable = 590pF
You could
1. Remove the input capacitance on the preamp and
2. keep the phono preamp right next to the turntable with a cable less than 1 foot in length.
You might just be able to manage close to 60 pF with a low capacitance cable. Check Belden or Canare for a low capacitance coaxial cable.
The best method would of course be to install the phono board inside the turntable . Some turntables do come with inbuilt phono stages. I have no idea how good those are. But it certainly helps with capacitance AND eliminating the performance of phono cables (?).
Cheers.
Your solution would be to find the input capacitor and remove it.
Note that this must be a capacitor in parallel with the input and not any series capacitor which might be there to block dc .
The tone arm cables generally have a capacitance in the region of + 100pF per meter. Finding cables lower than 70pF per meter might not be easy. So even with zero capacitance in the preamp you might not be able to get a 60pF load.
Unless of course you install the phono preamp board right under the turntable ( inside).
Then the contributing capacitance will be only the wires in the tone arm and the preamp input. You might have to change the input capacitor in the preamp to achieve the 60pF loading.
In any case you probably have a very rolled off HF performance right now with a 470 pF plus 120pF cable = 590pF
You could
1. Remove the input capacitance on the preamp and
2. keep the phono preamp right next to the turntable with a cable less than 1 foot in length.
You might just be able to manage close to 60 pF with a low capacitance cable. Check Belden or Canare for a low capacitance coaxial cable.
The best method would of course be to install the phono board inside the turntable . Some turntables do come with inbuilt phono stages. I have no idea how good those are. But it certainly helps with capacitance AND eliminating the performance of phono cables (?).
Cheers.
I think the 60pF requirement must be a typo .
Many older generation MM cartridges needed 470 pF but nowdays they are closer to 250pF. Aren't there some requiring 125pF capacitance ?
In any case cable capacitance + tone arm wiring capacitance+preamp input capacitance need to be accounted for.
It's easy to add capacitance but impossible to remove what is already there.
It's clear the only two places where you can do something to reduce capacitance is
1. Reduce length or type of coaxial interconnect cable
2. Change the capacitor at the input of the preamp.
Cheers.
Many older generation MM cartridges needed 470 pF but nowdays they are closer to 250pF. Aren't there some requiring 125pF capacitance ?
In any case cable capacitance + tone arm wiring capacitance+preamp input capacitance need to be accounted for.
It's easy to add capacitance but impossible to remove what is already there.
It's clear the only two places where you can do something to reduce capacitance is
1. Reduce length or type of coaxial interconnect cable
2. Change the capacitor at the input of the preamp.
Cheers.
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