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Capacitor dielectric - high impedance headphones

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I recently built the 'Broskie Unbalancer' for use as a preamp, however the vrms of my source is strong enough to power my amplifier so I have removed it from the chain and plan to use it as a headphone amplifier instead.

I am using some Beyerdynamic DT880, which are high impedance @ 600R.
I notice on page 7 of Broskie's documentation he states:

"which with a 1µF coupling capacitor will give a -3dB frequency of 11.5Hz. If the BCF is used to drive 300-ohm headphones, a job that the BCF is amazingly good at, at least a 30µF coupling capacitor is needed"

Presumably I can get by with around 20µF, but there certainly isn't enough space in the chassis/casing or on the board for a film/PIO cap - can anyone suggest another suitable type of capacitor?
 
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.... polypropylene are better :confused:

ahh... well if nothing else, at least it have been a very reliable goldmine...and as it seems, will go on forever and never dry out :D

but if they really put their hart into making better polyester caps, I bet they would be the top ;) and that btw was not ironic or a joke :rolleyes:

might be more a matter of production quality ...things like clean materials etc
 
ahh... well if nothing else, at least it have been a very reliable goldmine...and as it seems, will go on forever and never dry out :D

but if they really put their hart into making better polyester caps, I bet they would be the top ;) and that btw was not ironic or a joke :rolleyes:

might be more a matter of production quality ...things like clean materials etc
well, it could be only some opinions from internets, but i looked in families of products from WIMA
some PP caps are audio rated, but none of their PE :)
 
tinitus: (hpeter: ) ".... polypropylene are better"

ahh... well if nothing else, at least it have been a very reliable goldmine...and as it seems, will go on forever and never dry out

And Teflon are even better. But the OP has a size constraint.

Pano: You need a high voltage electrolytic here. 30uF minimum, 250V. Not much way around that. Look for a good one and bypass it with a film cap.

DF96: If the capacitor has a sufficiently high value that it has low signal voltage across it, even at low frequencies, then it can't generate much distortion no matter how bad a cap it is. 100uF electrolytic would do. You can bypass it if you think this will help.

jcx: (DF96: ) "You can bypass it if you think this will help."
but it won't - unless you believe, and listen sighted.
I suggested polyester because the op suggested there was a size constraint

I believe that what's quoted above summarizes the correct thinking that has been posted so far.

Here is a good "capacitor reality check" article, by someone who is knowledgable and well-respected:

Capacitor Characteristics

To have a negligible effect on bass, use a -3 dB "cutoff" frequency that is at most one-tenth of the lowest frequency that you wish to have no effect on. ("Purists" might say one-hundredth.)

f (-3 dB) = f = 1 / ( 2 π R C )

C = 1 / ( 2 π R f )


Example:

If you want to not degrade the amplitude of frequencies down to 20 Hz, then choose f (-3 dB) ≤ 2 Hz, which gives

C ≥ 1 / ( 2 π 600 2 )

C ≥ 0.000133

C ≥ 133 μF


There is probably no reason not to use the largest capacitance that is available in an acceptable case size. The more capacitance you use, the better it should perform.

Also, using multiple smaller capacitances in parallel could enable better use of the available space and better ability to meet maximum dimension constraints, and should also lower the resultant ESR (equivalent series resistance) and ESL (equivalent series inductance), compared to a single larger cap, which should only be able to improve performance.

Regards,

Tom
 
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Thanks for the input. After some comprehensive reading (the Elliott article was excellent), I'm not too scared of electrolytic capacitors - I think it will be better to get the uF rating correct than be too concerned about having PIO, etc.

I think 150uF 450v should be perfect, just a matter of finding a reasonably priced, solid performing option.

Can anyone point me in the direction of the 'dry cap' electrolytic caps which Tinitus mentions above?
 
With a big cap you may need to think about how you are going to charge it up when switching on, otherwise it could deliver quite a kick to your headphones. A relay with normally-closed contacts, which opens a few secs after power up would do it. Cheapest option is a resistor, but don't plug in the phones until after switching on.

Switching off should be less of a problem.
 
Thanks for the input. After some comprehensive reading (the Elliott article was excellent), I'm not too scared of electrolytic capacitors - I think it will be better to get the uF rating correct than be too concerned about having PIO, etc.

I think 150uF 450v should be perfect, just a matter of finding a reasonably priced, solid performing option.

Can anyone point me in the direction of the 'dry cap' electrolytic caps which Tinitus mentions above?

How much space do you have fo the capacitor, and what is the spacing of the connection points for the leads?
 
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