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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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
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.
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.
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.
Attachments
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
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
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 $$) heater in the end. (Not taking a side here just struck by how close this thread is to an on-going internal dialog (its sick, I know
).
And yet these designs are everywhere. And people swear by them. Could some of the OTL supporters answer the question:
"Why bother?" There MUST be a reason. The tubes are impressive -- how much does that enter in? How much heat do these things produce? Are they recommended below the 40th parallel? How much do they weigh??
Regardless of the answer to the why question - I hope a design gets discussed. Hopefully, lower output options might be discussed too -- 80 watts of tube amp seems like a awful lot to me in my small listening room.
I have looked hard at doing an OTL design with the mentioned tubes, but must admit that I usually end up agreeing with Leadbelly. Seems like a pretty (and $$) 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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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.
The 6N6P is a good tube but its a lot of extra work, because you will need about twice as many as you will of 6AS7s.
As I mentioned earlier, you do not need any special speaker protection circuitry, even a fuse, as long as the primary for the output tube B+ is properly fused. For 6 6AS7Gs running at 135V that will be about 2 to 2.5Amps slow. We've been building these things for over 33 years now and if they were eating speakers we'd be in a hurt locker. We *have* seen a few problems; almost all of them have to do with people putting a different power tube type in the sockets of the amps! So eliminating those incidents, overall we have seen one driver damaged in 33 years and 1000s of amps. In that case the cone shattered but the voice coil was fine. So a crowbar or the like seems like overkill to me.
As I mentioned earlier, you do not need any special speaker protection circuitry, even a fuse, as long as the primary for the output tube B+ is properly fused. For 6 6AS7Gs running at 135V that will be about 2 to 2.5Amps slow. We've been building these things for over 33 years now and if they were eating speakers we'd be in a hurt locker. We *have* seen a few problems; almost all of them have to do with people putting a different power tube type in the sockets of the amps! So eliminating those incidents, overall we have seen one driver damaged in 33 years and 1000s of amps. In that case the cone shattered but the voice coil was fine. So a crowbar or the like seems like overkill to me.
Yes. But for headphones you can also use the 6SN7, 6BL8 and the like. We make a preamp (the MP-1) that uses a very similar circuit, and not only can it drive headphones but it does quite well driving my 97db loudspeakers. That whole idea that a small amp can't control big woofers? Rubbish. My speakers have two 15" woofers in them. If my preamp can do it, an amp can too
atmasphere>Very cool of you to share your schematics. This I think is pretty much exactly what I was asking for.
If one were to half the number of output tubes compared to the commercial version, i.e. only use 4. How would that affect the numbers?
I really like the idea of starting with two and then adding tubes to reach the desired wattage.
If one were to half the number of output tubes compared to the commercial version, i.e. only use 4. How would that affect the numbers?
I really like the idea of starting with two and then adding tubes to reach the desired wattage.
I'm still curious about the source impedance of these Circlotron circuits. That will very much determine what sorts of speakers could be used without wild frequency response swings.
I've got a column speaker design underway which will have about a 200 ohm nominal impedance- this may be perfect for a 6528-based OTL.
I've got a column speaker design underway which will have about a 200 ohm nominal impedance- this may be perfect for a 6528-based OTL.
Thank you,
Atmasphere for sharing with us.
And also thank you SY, MarkusG maybe we can keep this thread alive and learn rather than the usual thing of OTL threads dwinding to nothing....
Now where is Wavebourn??? His Russian tube knowledge and willingness to wade in where others of us fear is exactly what we need!!!!!
Earth to Wavebourn..... are you there????
Atmasphere for sharing with us.
And also thank you SY, MarkusG maybe we can keep this thread alive and learn rather than the usual thing of OTL threads dwinding to nothing....
Now where is Wavebourn??? His Russian tube knowledge and willingness to wade in where others of us fear is exactly what we need!!!!!
Earth to Wavebourn..... are you there????
Markus, running 4 6AS7s per channel seems to be in the 20-25 watt range.
There are several ways to determine output impedance, apparently based on what you believe in. I'm not a fan of what I call the Voltage Paradigm as it tends to lead to the use of negative feedback in a lame attempt to get the amp to look good on paper.
Gentlemen, what looks good on paper has little to do with how we perceive sound. The bench specs measure things that are not important and for the most part ignores the things that are.
So I measure output impedance by seeing what its actual impedance really is. Most techniques used by the Voltage Paradigm are actually putting a number on the servo gain of the feedback loop. Feedback enhances the odd orders used by the human ear/brain system to measure the volume of a sound, if we tamper with that the system will have a false loudness and a brightness. If you want it to sound like real music feedback has to be avoided. Consequently the method of measuring the output impedance looses is value as soon as the feedback is reduced or eliminated.
We have used this formula from the Radiotron:
2Rp/u + 2
where Rp is the plate resistance of all the power tubes in the circuit, as they are in parallel.
In the case of 4 power tubes this comes to 33.75/4 or 8.44 ohms. In practice our measurements come well within 10% of this formula.
If you use the Voltage Paradigm rules the 33.75 ohm value is what you will get. FWIW as I write this I am listening to an amp that only has 3 power tubes per channel, and its driving our test speakers (High Emotion Audio Bellas) quite well. Obviously a high damping factor is not all its been cracked up to be. I think that myth was started to sell the Voltage Paradigm to the public, and they bought it hook line and sinker. We all paid a price for that too but now I'm way too far down the rabbit hole...
There are several ways to determine output impedance, apparently based on what you believe in. I'm not a fan of what I call the Voltage Paradigm as it tends to lead to the use of negative feedback in a lame attempt to get the amp to look good on paper.
Gentlemen, what looks good on paper has little to do with how we perceive sound. The bench specs measure things that are not important and for the most part ignores the things that are.
So I measure output impedance by seeing what its actual impedance really is. Most techniques used by the Voltage Paradigm are actually putting a number on the servo gain of the feedback loop. Feedback enhances the odd orders used by the human ear/brain system to measure the volume of a sound, if we tamper with that the system will have a false loudness and a brightness. If you want it to sound like real music feedback has to be avoided. Consequently the method of measuring the output impedance looses is value as soon as the feedback is reduced or eliminated.
We have used this formula from the Radiotron:
2Rp/u + 2
where Rp is the plate resistance of all the power tubes in the circuit, as they are in parallel.
In the case of 4 power tubes this comes to 33.75/4 or 8.44 ohms. In practice our measurements come well within 10% of this formula.
If you use the Voltage Paradigm rules the 33.75 ohm value is what you will get. FWIW as I write this I am listening to an amp that only has 3 power tubes per channel, and its driving our test speakers (High Emotion Audio Bellas) quite well. Obviously a high damping factor is not all its been cracked up to be. I think that myth was started to sell the Voltage Paradigm to the public, and they bought it hook line and sinker. We all paid a price for that too but now I'm way too far down the rabbit hole...
Personally It doesn't matter to me what the numbers are as long as it sounds good.
However I've always been told that low source and high load impedance is important?
My speakers have 5ohm at the lowest and 16ohm at the highest (I think).
Normally a 8ohm output impedance would put me off but if you say I'll be ok, I'll believe you.
Doesn't this however result in very low efficency?
However I've always been told that low source and high load impedance is important?
My speakers have 5ohm at the lowest and 16ohm at the highest (I think).
Normally a 8ohm output impedance would put me off but if you say I'll be ok, I'll believe you.
Doesn't this however result in very low efficency?