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Old 16th July 2013, 12:27 PM   #11
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Or if that's to complicated someone posted an even simpler otl using
a white cathode follower output stage about 2 years ago. I spent 10min
looking for it but couldn't find it.
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Old 16th July 2013, 01:30 PM   #12
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Quote:
Originally Posted by woody View Post
With a power requirement that low you probably will not need any voltage amplification so could just use a current buffer. Doing this with a mosfet is
real easy. If I just had to have a 1/2 watt tube amp I would think about
modifying this circuite. I would replace the 2 voltage dain stages with just
one triode stage and just use one or 2 output tubes.
Trouble is, for 200mW into 4 ohms you need over 200mA of current. OK you can do that with a MOSFET but then there's little point having any tubes in there.

Cheers

Ian
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Old 17th July 2013, 04:17 AM   #13
roline is offline roline  United States
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A hybrid tube input with Mosfet class A follower works great. Several designs available, check out the headphone forum......
I've built several of them and they are wonderful, with great clarity and sound stage.
I'm not a glass purist, I'l mix sand in my glass....
I've used 12au7's at low voltage and triple triodes at 220VB+ with mosfets and other sand components, Black hole quiet till you crank it up.
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SO many tubes, SO little time!!!

Last edited by roline; 17th July 2013 at 04:24 AM.
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Old 17th July 2013, 06:50 AM   #14
ChrisA is offline ChrisA  United States
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Originally Posted by Thetwinmeister View Post
Thanks for clicking in!

I am new to the tube amp world, but would like to learn enough to one day build my own. I have to start somewhere however, and I need to drive an experimental set of headphones I am building based on the Neo3 PDRW tweeters over in this thread. To my displeasure I have discovered all of the various amps I can find on the internet aren't "rated" anywhere near down to the 4 ohm impedance of the neo3s. I don't want to introduce distortion or melt the amp. The good news is however, as they are isodynamic planer drivers, the impedance not only never drops below 4 ohms, but is actually entirely flat across the board....
This will actually be a very easy amp to design and build. I assume you don't need more than 1 watt per channel of power. This means you can use a Class A amp and not have to deal with phase splitters.

The single most important and expensive and massive part of a tube amp is the iron core transformers. Select those first. Theaaudio output transformers will have more effect on the soud than any other component. Al you need is a pair of transformers with a 4 ohm output. You can buy them with multiple taps on the secondary. with one with 4, 8 and 16 ohm taps and you can use the amp to drive different loads.

A very conservative choise for a transformer is a 1629SEA by Hammond buy that at $160 and you need two. Edcor has some good ones for a LOT less Just use a 6V6 tube and yu will have 10X more power then you need.

All tube amps need a transformer to match the very high impedance and volts from the tube to to 4 or 8 ohm load.
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Old 17th July 2013, 08:19 AM   #15
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You might look at Simplest OTL thread that started back on July 8 2005 .
Post 5 has a 3 watt otl using a pair of 6c33c tubes . You could probably
modify it to use a pair of 6080 tubes in stead of the more expensive 6c33c.




But as I said earlier sense you don't need any voltage amplification a
mosfet power follower would be much cheaper and easier to build.
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Old 17th July 2013, 05:37 PM   #16
ChrisA is offline ChrisA  United States
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Originally Posted by woody View Post
You might look at Simplest OTL thread that started back on July 8 2005 ....
What is the output impedance of that amp? Is it 4 ohms?

Also I don't like the idea of OTL for headphone amps. The only isolation from the high voltage power supply is one capacitor.

But maybe more importantly an output cap pretty much has to be an electrolytic (470 uF in this case) and you don't want those in the signal path. The sound is not the best.

You pretty much need a transformer (or solid state parts) to get to 4 ohms
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Old 20th July 2013, 04:16 AM   #17
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I'm still taking in all these very educational replies and don't want to hinder their development, but I would like to insert a question.

Would it be worthwhile to consider balanced topology? I have a great balanced source. I understand it may potentially make the amp twice as expensive, but I don't want to rule it out based on that alone.
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Old 20th July 2013, 08:42 AM   #18
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Quote:
Originally Posted by Thetwinmeister View Post
I'm still taking in all these very educational replies and don't want to hinder their development, but I would like to insert a question.

Would it be worthwhile to consider balanced topology? I have a great balanced source. I understand it may potentially make the amp twice as expensive, but I don't want to rule it out based on that alone.
With a tube design, for a balanced amp topology you need a transformer at the input and one at the output. For a balanced source but an unbalanced amp topology you need a transformer at the input and one at the output. The galvanic isolation and pretty much all the CMMR takes place in the input transformer, so unless you are expecting huge amounts of interference to hit the amp itself I would say a balanced amp topology has little to offer.

Cheers

ian
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Old 20th July 2013, 04:27 PM   #19
ChrisA is offline ChrisA  United States
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Originally Posted by Thetwinmeister View Post
Would it be worthwhile to consider balanced topology? I have a great balanced source. I understand it may potentially make the amp twice as expensive, but I don't want to rule it out based on that alone.
That was how the old Hammon Organ "tone cabinets" worked. and YES the number of componets seems to be double, but you also double the gain and the power. There is a rather large payback in that you don't need a phase splitter in the power amplifier. That saves a tube but also the distortion of that stage.

In their system the organ console produced three channels of balanced output signals from the tone wheels and spring reverb and this was cabled out to any number (from 1 to 6 or 8) cabinets. Each cabnet had three balanced amplifiers, a power supply and speakers in a wood box the size of a home washing machine. The only drawback was the cost. A large installation could cost literally as much as 10 new cars. But that was "cheap" compared to a "real" pipe organ.

The Hammond system was truly balanced in that there was no input transformer. The two sides ran 180 degrees anti-phase all the way to the output transformer. There were two negative feedback loops.

So you'd not be the first. Other very high end professional audio systems were build this way but home HiFi never used balanced signals because of the double cost.

Today to cut costs, many systems that accept balanced inputs convert the balanced input to single ended by using an input transformers but those are NOT "balanced amplifiers" They are single ended amps. You would, no doubt, get lower noise with a true balanced amp and if you cary that balance all the way through the power section you avoid the phase splitter. One big design problem is making sure the gain is the same on both sides. NFB takes care if that but it must be the same on both sides.

It would be a fun experiment but not a good first project. I'd go with a simple class-a single ended amp as a first project. Build a 5W amp with a 6V6 tube first. Given the right speaks it can be "way loud." Yes it can. I have a speaker rated at 105 dB/W at 1M. So it can break 100 dB with the volume control way down at "2".

Last edited by ChrisA; 20th July 2013 at 04:34 PM.
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Old 20th July 2013, 04:39 PM   #20
ChrisA is offline ChrisA  United States
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Originally Posted by ruffrecords View Post
Trouble is, for 200mW into 4 ohms you need over 200mA of current. OK you can do that with a MOSFET but then there's little point having any tubes in there.

Cheers

Ian
At 4 ohms he is going to be using an output transformer. (or as you say solid state.) A tube based OTL is just a "circus stunt". Seriously I don't know what else I'd call a pair of EL34 (or like) tubes driving headphones. Use transformers and then a 12AU7 tube makes a good power tube

Also with headphone you kind of want to have isolation for the B+ voltage. Yes yu can do that with a huge electrolytic cap but they will one day fail, maybe even shorted and then you have 200 volts across your headphones.

Last edited by ChrisA; 20th July 2013 at 04:41 PM.
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