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Old 22nd April 2013, 05:08 PM   #1
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Default Parafeed 45 amps

Hi,

has anyone built either of these parafeed 45 amps?

Kurt Strain - parafeed 45

Gary Pimm - all active 45

Do you have any build notes or extra details you could share, if you have?
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Old 22nd April 2013, 10:33 PM   #2
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I have an usual amount of experience with amps like this. Kurt's is pretty straight forward. The only change I would make is to replace the zener diode and cap in the current source with a 33K resistor in parallel with a 10,000uF/6V cap. (not originally my idea, this came from a very experienced designer)

Without spending a lot of extra money, a pair of 0D3's in series would give you a regulated voltage supply for your driver, allowing you to control the potential voltage across the MJE-350.

I don't like Pimm's amp at all (no offense Gary). Current sources like compliance, and going monkey feed means that you are giving up driver headroom. On the output stage, I would use the MJE5731AG in place of the MJE-350. If you found a suitable plate choke, you could maintain the topology and use a choke loaded driver instead of a CCS'd driver, and you'd have a nice piece of kit!

IMO, a completely shunt regulated amp will outperform either of these.

-PB
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Old 23rd April 2013, 12:27 AM   #3
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Do you have a schematic you could point me to? I'm using about 95db efficient speakers and I don't listen to loud music, so I thought 2-3 watts might be enough. I keep reading about the DHTs and I thought if I'm going to build a replacement amp at some point, I might as well build one I don't need to replace any time soon, if ever. The 26, 10Y and 45 seem to have a consistently good reputation.
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Old 23rd April 2013, 01:10 AM   #4
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The design I'm mentioning is not my property, and not permitted to be posted in the public domain (yet). It's also of sufficient complication that it makes a pretty rough DIY project unless you're comfortable with PCB design.

At 95dB, a 2A3 amp would be a pretty easy undertaking!
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Old 23rd April 2013, 12:48 PM   #5
45 is offline 45  Italy
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Quote:
Originally Posted by drlazybones View Post
Do you have a schematic you could point me to? I'm using about 95db efficient speakers and I don't listen to loud music, so I thought 2-3 watts might be enough. I keep reading about the DHTs and I thought if I'm going to build a replacement amp at some point, I might as well build one I don't need to replace any time soon, if ever. The 26, 10Y and 45 seem to have a consistently good reputation.
You might also evaluate if buying an OT without gap + Choke + a really good capacitor is better than a properly good SE OT. Using a CCS you get better noise rejection given the same PSU but you can always design a better power supply. I don't remember MagneQuest products to be cheap....
Many years ago parafeed was more common not only to save money but also because of poor quality of magnetic materials which couldn't give satisfactory perfomance in some applications. The latter is surely not your case as you are going to use small power devices. I don't think the 26 will give enough power.
You might also consider the 4P1L in triode connection which is quite linear and very cheap! Then you could invest the money you save in good SE irons and PSU.
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Old 23rd April 2013, 01:28 PM   #6
mp9 is offline mp9  United States
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If it helps any, i had a parafeed 300b amp which i had found too "solid state" sounding for my taste.

Last edited by mp9; 23rd April 2013 at 01:34 PM.
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Old 23rd April 2013, 01:30 PM   #7
mp9 is offline mp9  United States
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I have no trouble driving Mark Audio Alpair 12P 92db wide bandwidth drivers with 45 tubes in place of the 2A3's in a Fi2A3 amp (it's designed to be used with either output tube via a rectifier tube swap).

If not determined to do parafeed, here's another design to consider, www.wavelengthaudio.com/bugle.pdf
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Old 23rd April 2013, 02:36 PM   #8
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The only thing I think I'm sure of is that I'd like to build a single ended tube amplifier using excellent tubes and iron, and generating "just enough" watts to give me good dynamic range at listenable levels. I have tinnitus and I can't listen to very loud music. I measured with an app on my iPhone once and somewhere around 94dB was my max limit, though I'm sure some of the peaks were beyond that. I'm pretty sure 2 watts will be good enough, and from all the raving I've read about it the 45 seems like a great output tube. I don't yet know enough about design to intelligently comment, and I haven't heard these things because I don't know the right people, apparently. I'd be happy to try the plain old transformer coupled output, it will probably be the easiest thing to do anyway. The driver I suppose could be anything, but I've read a few posts about the 26/45 combination being excellent. Isn't the Wavelength design an SRPP? As far as I understand, that uses a push-pull driver arrangement with the output tube as a single ended follower. Doesn't that defeat some of the purity of a single-ended design? It looks like Kevin Kennedy has a very similar design, maybe a bit simpler:

http://www.kta-hifi.net/projects/amp...45_dht_all.pdf

Looks like Gordon Rankin uses parafeed output for his production 45 amplifiers, probably with SRPP drivers? Bottlehead uses it too. There just seem to be a bunch of examples out there, so there must be something to it.. I dunno.
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Old 23rd April 2013, 02:44 PM   #9
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The 45 and 26 would indeed be an alluring combo, but very difficult to troubleshoot!
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Old 23rd April 2013, 02:53 PM   #10
45 is offline 45  Italy
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Quote:
Originally Posted by drlazybones View Post
The only thing I think I'm sure of is that I'd like to build a single ended tube amplifier using excellent tubes and iron, and generating "just enough" watts to give me good dynamic range at listenable levels.
You don't need spend a fortune to get excellent results. The Tamura F-475 is just so right for the 45 (and 4P1L I would say). It also has the additional bonus of holding its value as this is really a time proven performer. To get really marginal improvements in 1-2 areas you need to spend 3-5 times more.....
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