Parafeed dc blocking/coupling cap help - diyAudio
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Old 28th March 2009, 05:48 PM   #1
nhuwar is offline nhuwar  United States
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Default Parafeed dc blocking/coupling cap help

Hi Guy's

I'm finally about to get my tube preamp finished but I need a bit of advise. It's a parafeed stage but I don't know how you pick the cap size that acts as a blocking / coupling capacitor.

Any help would be greatly appreciated


Thanks
Nick
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Old 28th March 2009, 08:05 PM   #2
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Not a prob.

What you have here is basically a high pass filter with a finite zero at w= 0. The pole frequency is:

w0= 1 / (Cc(Rg + Rl)) (w0= 2pif0)

What you need to do is figure out what Rg is, and that'll be basically r(p) || Rp (where Rp is the parafeed plate load at the lowest frequency you want to pass) and, of course, you will need to know what load this preamp will be working into.
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Old 28th March 2009, 08:35 PM   #3
nhuwar is offline nhuwar  United States
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Ok Thanks Miles I think I can handle the rest.
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Old 28th March 2009, 09:19 PM   #4
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It is. of course, not that easy. The inductance of the parafeed transformer enters into the formula. You can get an estimate of the importance of the inductance by converting it to impedance at a particular frequency. The formula for this is Z = 2piFL where F = frequency and L = inductance. Usually, for parafeed you would do this at 20Hz.

If the inductance is sufficiently high that it is not an issue (the reflected load is much lower), then the above resistive load formula will give you a decent estimate. Otherwise, adjust them for inductance.

This will still only get you close, particularly for a choke load where the choke's own inductance will come into play. Also, there are some weird bass humps that can show up unpredictably. There are a number of simulators you can download to make estimates easier, or you can do what the rest of us do which is buy 20 cheap 1u caps and keep adding more in parallel until it sounds right. Then, buy a good cap in that size.
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Old 29th March 2009, 12:33 AM   #5
nhuwar is offline nhuwar  United States
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Haha that what I was originally going to but I thought the scientific approach was a better bet.


Serves me right for think'en LOL


Nick
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Old 29th March 2009, 03:23 AM   #6
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Here is how I do it.

http://www.raleighaudio.com/parafeed_capacitor.htm

Dave
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Old 29th March 2009, 03:44 PM   #7
nhuwar is offline nhuwar  United States
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That what I was thinking about doing. Pushing a sweep to it say 10hz to 50hz and measure the output with my spectrum analyzer. Them get close and tweak it by ear the rest of the way.


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