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Old 27th April 2006, 06:52 PM   #1
sbelyo is offline sbelyo  United States
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Default Help choosing capacitor value?

I'm gathering the parts for this OTL Headphone Amp:
Click the image to open in full size.

My question is on C1 and C2. The minimum value for C1 is 10uf. How whould I determine the right value to get the best frequency response?

My headphones are 48, 55 and 250 Ohms
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Old 27th April 2006, 07:06 PM   #2
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You are probably going to want a 470uF cap which is going to be an expensive part as you'll need a pretty high voltage. For how to figure this out, see my article here: http://www.ecp.cc/cap-notes.html
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Old 27th April 2006, 07:41 PM   #3
sbelyo is offline sbelyo  United States
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That article explains a bunch for me.

Just so that I can see I'm using the formula right

F = 1/(2 * pi * C * R)

what is the value obtained if C = 10uf and R = 250 Ohms
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Old 27th April 2006, 07:58 PM   #4
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Default f=63.66 cps

about 64 hz
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Old 27th April 2006, 07:58 PM   #5
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It's about 64Hz (10uF is .00001F).

Really, your caps are probably going to cost more and sound worse than a couple of cheap output transformers. I don't get the OTL thing in general, but I don't think it is particuarly appropriate for headphones, at least not low impedence ones.

One option might be to use the small cap, and add an impedence matching transformer -- something with a 10K primary and a 32 ohm secondary -- after the cap. Or, you could put a solid state buffer after the output cap. These tend to have a very high input impedence which would allow you to get away with a little cap -- something like 0.1uF perhaps?
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Old 27th April 2006, 08:03 PM   #6
sbelyo is offline sbelyo  United States
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Thanks a bunch... I'm begining to understand
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Old 27th April 2006, 08:07 PM   #7
sbelyo is offline sbelyo  United States
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pi=3.14 right?

I got it now
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Old 27th April 2006, 08:22 PM   #8
sbelyo is offline sbelyo  United States
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Now the real question is how low can I actually hear? I should get my ears cleaned and hearing checked anyway!
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Old 27th April 2006, 10:18 PM   #9
cerrem is offline cerrem  United States
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To calculate the cap size for a given low frequency response..
You need to know two R values...
The (2*pi*R*C) is an ideal equation that assumes you have an ideal source, which is never the case...
You need to know the equivelent impedance looking back into the amplifer from before the cap...and then ahead of the cap which you already know as you headphones.. I would use the smallest headphone impedance for the calculation, since anything higher is gravy...
Keep in mind you typically use a -3dB frequency much lower than 20 Hz, the reason is that in audio you are concerned with phase shifting.... Since the Phase shift at -3dB for a -20db/decade slope is 45 degrees, not really good for audio... Drop the freq then examine the phase shift until it is satisfactory.. I personally design for 6Hz or lower to obtain least phase shifting at 20Hz..

Chris
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Old 27th April 2006, 11:07 PM   #10
cerrem is offline cerrem  United States
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Did some number crunching...
The output Z of the circuit is roughly 220 ohms.....
Driving a 48 ohm load:
You would need a 100uF to get a -3dB at 6Hz.....roughly...

Driving 250 ohm load:
You would need 57uF to get the same -3dB at 6Hz....
Since they don't sell 57uF, you can parallel caps or use a 68uF that will bring you to -3dB at 5Hz...

Using 6Hz as your -3dB point will roughly bring you in to 12 degrees of phase shift....

Chris
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