Can I be the first chimpanzee to measure input capacitance on my phono ?
- Thread starter Chris
- Start date
This IMHO is well worth doing. BUT: Don't just plug a pF range cap meter into the phono input on your DIY phono pre-amp -you should be able to find the input capacitor fairly easily -if you need to add to that-look at the DB systems capacitor kit-he's been selling these for 30? maybe 40 ?years -they just plain work. If you want to spend a lot of money on this -dB systems can't help you there -pretty darn close to dirt cheap.
Educated guess - you can't with any kit that you are likely to own. More likely would be a ball-park calculation, another educated guess.
Is the idea not a bit the wrong way around?
Why would you not change cap' externally and listen, rather than rely on some numbers?
Do you know the cap' of the ICs in use?
Chris
pfm Member
Hi
Hi Vinny,
Johnny 7 told me the cap of my tone arm cable, now I need the cap of the phono input, then I subtract the sum of these from the 400pF that A&R quote and I have the value of the cap I’m going to solder inside a Rothwell blank attenuator plug. Unless they have a capacitance too, must remember to ask them.
Hi Vinny,
Johnny 7 told me the cap of my tone arm cable, now I need the cap of the phono input, then I subtract the sum of these from the 400pF that A&R quote and I have the value of the cap I’m going to solder inside a Rothwell blank attenuator plug. Unless they have a capacitance too, must remember to ask them.
Everything has capacitance at this level - pF is a minute measuremet/unit so any cable will easily make a difference.
Is the figure that you have from cart' tags to phono plugs that connect to the stage?
Jez or Martin (Clark) are really who you need to see this thread - there MAY be a close approximation that can be made for valve phono stages, maybe not, and they will know.
You can do no damage, so in the absence of numbers, I would just add some cap' and see/hear what changed. Hopefully somebody with an accurate grasp of this will be along.
Is the figure that you have from cart' tags to phono plugs that connect to the stage?
Jez or Martin (Clark) are really who you need to see this thread - there MAY be a close approximation that can be made for valve phono stages, maybe not, and they will know.
You can do no damage, so in the absence of numbers, I would just add some cap' and see/hear what changed. Hopefully somebody with an accurate grasp of this will be along.
Arkless Electronics
Trade: Amp design and repairs.
Do not use a blank attenuator plug. Open up the phono stage and see what value is fitted. Add the arm capacitance to this. It doesn't need to all that precise so don't worry about +/- 20pF. If more is needed then fit larger caps to the phono stage or add caps in parallel.
Still aplicable even with the op's valve stage?
Arkless Electronics
Trade: Amp design and repairs.
Actually that's a good point. In all my valve and hybrid stages I make sure this isn't an issue but yes it could be relevant here. Without a circuit diagram I can't say. There could be up to about 200pF from the Miller capacitance of the valve but this varies massively with the valve type and exactly how it's used. The most commonly used ECC83 is about the worst here. I'd allow about 60pF if it's the usual ECC83 but a circuit diagram needed for more accuracy.
Craig B
Re:trophile
I think that what @Vinny and @Arkless Electronics are getting at here is that the valve itself presents its own capacitance(s), and calculating the net value of such requires knowing the specific valve specs and stage gain. Specifically, the various capacitances are plugged into the following formula:
Cin = Cgk + Cgp * (A + 1)
where:
Cin = input capacitance
Cgk = grid-to-cathode capacitance (composed of internal capacitance plus stray capacitance)
Cgp = grid-to-plate capacitance (composed of internal capacitance plus stray capacitance)
A = stage gain
Cgk = grid-to-cathode capacitance (composed of internal capacitance plus stray capacitance)
Cgp = grid-to-plate capacitance (composed of internal capacitance plus stray capacitance)
A = stage gain
Can you scan and post the circuit diagram here? Maybe include some internal pictures of the valves and components beneath the valve sockets.
At a guess, the 51pF is there to bring the total input capacitance up to a preferred value. As this is a modern design, perhaps bringing it up to 100pF or 150pF. I doubt anyone would aim for more with a MM stage these days.
Mine was an educated guess, Jez was speaking from a point of knowledge 
LOL. I would not put my shirt on it
That would just be too logical.
That said, Jez's approximation and 51pF would come close to a nice round 100pF, so 100 or 150 seem not out of the question.
LOL. I would not put my shirt on it
That would just be too logical.
That said, Jez's approximation and 51pF would come close to a nice round 100pF, so 100 or 150 seem not out of the question.