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Old 17th February 2013, 03:13 PM   #11
jcx is offline jcx  United States
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Quote:
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

Last edited by jcx; 17th February 2013 at 03:21 PM.
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Old 17th February 2013, 03:32 PM   #12
tinitus is offline tinitus  Europe
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how about lytics with the latest dry cap technology ?
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Old 17th February 2013, 05:52 PM   #13
gootee is offline gootee  United States
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Quote:
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.

Quote:
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.
Quote:
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.
Quote:
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

Last edited by gootee; 17th February 2013 at 06:16 PM.
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Old 19th February 2013, 11:00 AM   #14
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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?
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Old 19th February 2013, 11:09 AM   #15
DF96 is offline DF96  England
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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.
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Old 19th February 2013, 11:36 AM   #16
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what would the charging time be for 150uF? The warm up time of the is generally a few minutes, so I wouldn't be plugging headphones in until after, say, 90 seconds in any case - that would probably be enough 'lag' time to prevent any damage to the headphones?
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Old 19th February 2013, 12:25 PM   #17
DF96 is offline DF96  England
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Charging time depends on C and R. For 150uF and 300R the time constant is 45ms. It is the quick charge which gives the kick. Put a 10k resistor to ground, then without the headphones plugged in the time constant is 1.5s. Allow say 4 time constants, so you can plug in 6s after the amp itself has stabilised.
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Old 20th February 2013, 03:21 AM   #18
gootee is offline gootee  United States
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Quote:
Originally Posted by lordearl View Post
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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Old 20th February 2013, 04:28 AM   #19
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Broskie is quite good with the capacitor spacing – there’s about 10cm for an axial capacitor with a few different pcb hole options….
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Old 20th February 2013, 04:32 AM   #20
jcx is offline jcx  United States
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Bateman suggests using bipolar electrolytic when possible - full thickness oxide grown on both foils has lower measured distortion

motor start electrolytic may be your best bet for high V rating bipolar
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