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Old 16th February 2010, 05:59 PM   #21
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I hope you two geniuses will be kind enough to give him a good price on his unfinished amp when it comes up for sale on the classifieds. Honestly, common sense is the hardest kind to come by some days.
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Old 16th February 2010, 06:17 PM   #22
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Originally Posted by leadbelly View Post
I hope you two geniuses will be kind enough to give him a good price on his unfinished amp when it comes up for sale on the classifieds. Honestly, common sense is the hardest kind to come by some days.
OooooooooooH, eXcuse us!!!!!!

We all need to simply bow and kiss the.... ah, er... floor you tread for allowing us the undoubted privilege of your obviouly superior intellect and knowledge. Oh, Yes, it was "common sense" , my error in recognition.
To thee, I now bow!!
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Old 16th February 2010, 06:36 PM   #23
tomchr is offline tomchr  United States
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I'm not sure if that's sarcasm or not, but basically from what I remember of reading it MJ is no fan of OTL's and also does not claim to be an expert in their design.
MJ is incredibly opinionated and it shows in his book. Read through the design of the single-ended scrapbox challenge amp and you'll see what I mean. In part that makes his book somewhat entertaining to read, but unfortunately it also means that some topics are not covered in any kind of detail beyond "this is a useless topology, don't even bother!".

That said, it's still a very good book as far as covering the fundamental building blocks at a level that's reasonably science-based yet still accessible to the beginner.

~Tom
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Old 16th February 2010, 07:20 PM   #24
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MJ is incredibly opinionated and it shows in his book. Read through the design of the single-ended scrapbox challenge amp and you'll see what I mean. In part that makes his book somewhat entertaining to read, but unfortunately it also means that some topics are not covered in any kind of detail beyond "this is a useless topology, don't even bother!".
Just in case you are misconstruing some of my comments as being anti-OTL, let me say that I am absolutely not, and have already socked away most of the parts for one when suitable tube prices started to shoot up a few years ago.

I am just saying that (1) recommending MJ as a good book to get for someone who wants to build an OTL is riculous, and (2) recommending a 40W scratch built OTL (with 6C19P's of all things, I guess about 30-40 are needed for 40W stereo?) as a 1st tube project is ridiculous.
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Old 16th February 2010, 07:21 PM   #25
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Right. This schematic is copyright by Atma-Sphere Music Systems, Inc. but you can use it for your personal projects. As always, all rights reserved and all that stuff.

The B+ for the power tubes runs about 135-140V. B+ and B- for the driver tubes is +300 and -300.

The place to start is to work with only a pair of power tubes and get the amp running, then wire the rest of the power tubes. The other elements of the circuit, driver, power supplies etc are the sort of thing you run into with a lot of amps, so no issues there. The nice thing about this is there is no guesswork to get the OPT right- there is none. I often run the amplifier with less than the full complement of power tubes. You will want to provide enough room on your chassis to space the 6AS7s about 3 1/2" apart (center to center) at a minimum.

This circuit does not use much feedback- about 1 db. If you want more, you are going to have to have more gain in the voltage amplifier, perhaps even an additional gain stage in front. If you want my advice, don't do it! Instead find a speaker that is easily driven by this amp without feedback. It **will** sound better.
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Old 16th February 2010, 08:04 PM   #26
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For a low impedance futterman check here
OTL Asylum
i requested the schematic a few weeks ago and he promptly emailed them though im not sure what the output power or minimum impedance is.

circlotrons are great but no one has seemed to mention that if a key componant blows (and this might only apply to the transistor variants) that high dc or other transiants from the mains B+ or what have could be applied to the speakers. at this point though the amp is probably shot so a crowbar circuit might not be a bad idea (why lose your amp and speakers when you can just lose the amp). Elliot has a nice crowbar circuit on his site Project 120 .

For a totem pole approach this page has alot of issues, dealing with the uneven input requirements power ripples ect ect.
OTL Design
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Old 16th February 2010, 08:09 PM   #27
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Default This discussion is of value (to me at least) ...

I have looked hard at doing an OTL design with the mentioned tubes, but must admit that I usually end up agreeing with Leadbelly. Seems like a pretty (and $$) heater in the end. (Not taking a side here just struck by how close this thread is to an on-going internal dialog (its sick, I know ).

And yet these designs are everywhere. And people swear by them. Could some of the OTL supporters answer the question:

"Why bother?" There MUST be a reason. The tubes are impressive -- how much does that enter in? How much heat do these things produce? Are they recommended below the 40th parallel ? How much do they weigh??

Regardless of the answer to the why question - I hope a design gets discussed. Hopefully, lower output options might be discussed too -- 80 watts of tube amp seems like a awful lot to me in my small listening room.
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Last edited by moonbird; 16th February 2010 at 08:15 PM.
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Old 16th February 2010, 08:16 PM   #28
TheGimp is online now TheGimp  United States
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Wouldn't the 6N6P be a good choice for the output tubes? Works well at low voltage, Good current handeling capacity. Nice plate resistance. Moderate plate dissipation. Nice Linearity.
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Old 16th February 2010, 08:23 PM   #29
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another good question if these are so good why has no one built one with one of the holy grail tubes (300B,2a3 or 845) two 845's per channel should put you where you want to be (60 watt) but then you run into price again. 74.95 each at tube depot : / and the power supply for tubes like that arent cheap either.
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Old 16th February 2010, 08:25 PM   #30
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another good question if these are so good why has no one built one with one of the holy grail tubes (300B,2a3 or 845) two 845's per channel should put you where you want to be (60 watt) but then you run into price again. 74.95 each at tube depot : / and the power supply for tubes like that arent cheap either.
Those tubes are not suitable for OTL. You want a tube that can deliver current.
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