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?
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?
I believe even polyester/Mylar measured better than Electrolytics in Bateman's tests
but you could get 10x value Al electros for a fraction of the price
but you could get 10x value Al electros for a fraction of the price
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 😀
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 🙄
might be more a matter of production quality ...things like clean materials etc
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.
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 🙂
ok thanks, I built this board as a 'paint by numbers' project so don't really know what 'bypass' here means - are you suggesting just soldering a 30uf capacitor on top of the current film cap (0.68uf pio), piggyback style?
Bypass with film cap with different structure is not good idea for quality of sound for HF and MFreq....Main output cap must be only one and with High quality.
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.
but it won't - unless you believe, and listen sighted
I suggested polyester because the op suggested there was a size constraint
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And Teflon are even better. But the OP has 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?
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.
Switching off should be less of a problem.
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?
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.
How much space do you have fo the capacitor, and what is the spacing of the connection points for the leads?
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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