Parafeed dc blocking/coupling cap help

nhuwar · 28th March 2009, 05:48 PM

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

Miles Prower · 28th March 2009, 08:05 PM

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.

nhuwar · 28th March 2009, 08:35 PM

Ok Thanks Miles I think I can handle the rest.

dsavitsk · 28th March 2009, 09:19 PM

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.

nhuwar · 29th March 2009, 12:33 AM

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

David Davenport · 29th March 2009, 03:23 AM

Here is how I do it.

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

Dave

nhuwar · 29th March 2009, 03:44 PM

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.

Nick

28th March 2009, 05:48 PM	#1
nhuwar diyAudio Member Join Date: Jan 2007 Location: Cypress Texas	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

28th March 2009, 09:19 PM	#4
dsavitsk diyAudio Member Join Date: Jan 2005 Location: Chicago	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. __________________ http://www.ecpaudio.com :: http://diy.ecpaudio.com

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28th March 2009, 08:35 PM	#3
nhuwar diyAudio Member Join Date: Jan 2007 Location: Cypress Texas	Ok Thanks Miles I think I can handle the rest.

29th March 2009, 12:33 AM	#5
nhuwar diyAudio Member Join Date: Jan 2007 Location: Cypress Texas	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

29th March 2009, 03:23 AM	#6
David Davenport diyAudio Member Join Date: Jun 2004 Location: Raleigh North Carolina	Here is how I do it. http://www.raleighaudio.com/parafeed_capacitor.htm Dave

29th March 2009, 03:44 PM	#7
nhuwar diyAudio Member Join Date: Jan 2007 Location: Cypress Texas	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. Nick