Where to get a choke for a class-A single-ended MOSFET power stage?
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From: Morris Slutsky on 4 Feb 2010 23:48 Thinking of a circuit like this. Simple and elegant-looking: http://img524.imageshack.us/img524/4038/simplefet.jpg Really like a very cheap version of a single-ended triode stage. Basic 'parafeed' arrangement except that no transformer is necessary. Resistors provide bias, negative feedback, and give the FET a bit of 'plate resistance' like a triode to lower output impedance. What's great about this cheap FET circuit is that it's cheaper than the tube equivalent, much, and might sound something like. No need for expensive output transformer. A cheap FET like a IRF510, compare to a 300B, yeah. No need for heater supply! And the choke . . . the choke doesn't need to be NEARLY as large. A parafeed triode amp, to get decent bass response, might need a choke in the tens or hundreds of Henrys range. Here, the current is much greater, the voltage is much lower, impedance is much much lower, so you can get by with a choke in the millihenrys. Should be easy to find one I think. But I'm not sure where. Any ideas where to get a 50-100 mH choke for this? Needs to take an amp or three of current without saturating. That's the hard part there I think. Lots of dual "common mode" 5-30 mH chokes for sale but I imagine those don't take much DC current at all before saturating, they're meant for AC circuits aren't they? If I could find a choke this would be a nice cheap thing to build though. Easy enough.
From: Stephen Cowell on 5 Feb 2010 00:21 "Morris Slutsky" <morris.slutsky(a)gmail.com> wrote .... > If I could find a choke this would be a nice cheap thing to build > though. Easy enough. Roll your own, Morris... it is not space technology. Obtain any copy of the ARRL Amateur Radio Handbuch and start windin'. Big stick of red-coded ferrite would probably get you there in a hurry... the choke can be *way* over and not hurt anything, right? Hell, use a PT primary. __ Steve ..
From: Phil Allison on 5 Feb 2010 00:40 "Morris Slutsky" > Thinking of a circuit like this. Simple and elegant-looking: > > http://img524.imageshack.us/img524/4038/simplefet.jpg > Any ideas where to get a 50-100 mH choke for > this? Needs to take an amp or three of current without saturating. ** The nearest is a Hammond choke: http://datasheet.octopart.com/195P5-Hammond-datasheet-7863.pdf A 195p5 is closest to your spec. 30mH is a minimum and say 1 ohm resistance max. Absolutely FORGET using ferrite for this application. ...... Phil
From: legg on 5 Feb 2010 06:36 On Thu, 4 Feb 2010 20:48:37 -0800 (PST), Morris Slutsky <morris.slutsky(a)gmail.com> wrote: >Thinking of a circuit like this. Simple and elegant-looking: > >http://img524.imageshack.us/img524/4038/simplefet.jpg > >Really like a very cheap version of a single-ended triode stage. >Basic 'parafeed' arrangement except that no transformer is necessary. >Resistors provide bias, negative feedback, and give the FET a bit of >'plate resistance' like a triode to lower output impedance. > >What's great about this cheap FET circuit is that it's cheaper than >the tube equivalent, much, and might sound something like. No need >for expensive output transformer. A cheap FET like a IRF510, compare >to a 300B, yeah. No need for heater supply! And the choke . . . the >choke doesn't need to be NEARLY as large. A parafeed triode amp, to >get decent bass response, might need a choke in the tens or hundreds >of Henrys range. Here, the current is much greater, the voltage is >much lower, impedance is much much lower, so you can get by with a >choke in the millihenrys. Should be easy to find one I think. > >But I'm not sure where. Any ideas where to get a 50-100 mH choke for >this? Needs to take an amp or three of current without saturating. >That's the hard part there I think. Lots of dual "common mode" 5-30 >mH chokes for sale but I imagine those don't take much DC current at >all before saturating, they're meant for AC circuits aren't they? > >If I could find a choke this would be a nice cheap thing to build >though. Easy enough. Up until today, I would have said Hammond, but their catalog seems to have been chopped. You're looking at 2.5lbs of part, measuring 4 x 2.5 x 2.25 inches, for the low end of the inductance range quoted, at 2A. Double the weight for the upper end - 4 x 4 x 3 inches. RL
From: JJTj up on 5 Feb 2010 07:58
>>Thinking of a circuit like this. Simple and elegant-looking: >> >>http://img524.imageshack.us/img524/4038/simplefet.jpg >> If it hasn't been mentioned, or you already know, I hope you visit and d/l all the free data available at: http://www.passdiy.com/ Mr. Pass in the KING of class-a design, offers enough free info to fill a book, and answers ?s in the forum. When reading, you will find that a choke there is big, heavy, and wasteful. Just use a constant current design (cheaper to build too). I've built his ZEN designs, which look alot like your pix using CCS(s). There are even sources for PCBs! I promise you you'll LOVE the site. JJTj |