OTL Headphone Amp project.

[DrNomis_44]

Hey everyone,


I've got a new build project to add to my list of build projects to tackle, this time I want to have a go at building an electronic device called an OTL Headphone Amplifier, so, exactly what is an OTL Headphone Amplifier?

Firstly, OTL is short for Output Transformerless, now most amplifiers that use tubes or valves to amplify, or make bigger, an audio signal, will need to use an electronic component known as an Output Transformer, it does at least a couple of essential jobs in a tube or valve amplifier, firstly, it isolates the loudspeaker from the high +HT supply voltages that the amplifier needs in order to operate correctly, secondly, it "Transforms" the impedance of the loudspeaker up to a level that the power tubes are happy with, most power tubes used in guitar amps nowadays don't really like working into low-impedance loudspeakers.

One other thing is that an Output Transformer enables a type of amplifier operation called "Push-Pull", in this case at least two power tubes are used to increase the output-power capability of the amplifier, you've most likely seen some Marshall or Fender amps with two, four, and maybe more, my mid 70's Fender Super Twin amplifier uses six 6L6 power tubes, in Push-Pull operation, one power tube "Pushes" while the other one "Pulls".

So, why build an Output Transformerless Headphone amp when most tube amps need an Output Transformer to operate?, well, the OTL Headphone amp I want to build is usually part of a home HiFi system, it is even considered high-end audio equipment, usually when it comes to building a tube amp for guitar amplification purposes, we are not really concerned with achieving HiFi performance since a tube guitar amp is used with an electric guitar as part of the tonal-shaping system, where the guitar and the amp work in synergy to create the final resulting sound, ever noticed how plugging a guitar into say a Vox AC30 amp can produce a sound that's different to what you would get if you plugged the same guitar into say a Fender Twin, or a Marshall JTM45, the amp that the guitar is plugged into is playing a part in creating the resulting tone.

Now when it comes to HiFi equipment, we are more concerned with making a reproduced signal, louder or bigger, but still sounding like the signal we started off with, we don't want to make it sound distorted and fuzzy, we want to reproduce the signal as faithfully as possible, that's the whole idea behind HiFi, or "High Fidelity".

As it turns out, the Output Transformer has a tendency to produce it's own form of distortion, so if we can somehow eliminate the Output Transformer, we can eliminate one source of distortion, of course, I could build the Headphone Amp using modern Solid-State technology if I wanted to, and there are a few designs on the internet, but seeing as I'm a bit of an old-school electronics kind of a guy, I thought that an Output Transformerless Headphone Amp would be a cool project to tackle, it would be fun as well.


Stay tuned for more updates, by the way, comments are most welcome......


For those of you who are reading my posts in this thread, please feel free to post your comments if you wish to do so, all comments are welcome.

[Dedman]

*still waiting on my sharks with frikken "lazer" beams........

[DrNomis_44]

*still waiting on my sharks with frikken "lazer" beams........

Hahahahaha.....thanks for reminding me about that one mate....cheers.

[Fretworn]

We’ve got mutant sea bass............

[DrNomis_44]

We’ve got mutant sea bass............

Hahahaha....ah, but they aren't just any old ordinary mutant sea bass....they are ill-tempered mutant sea bass....heeheeheehee....

[Old Tooth Hopkins]

One of my makerspace students is building one of these

http://www.parasitstudio.se/stripboa...p-punch-amp-v2

[Marcel]

Bon Voyage my friend on your transformer-less tube journey.... It is a fairly obscure path but at least it is a path that a few others have ventured before...

In the back blocks of my memory I do remember seeing only the one time in a Melbourne meeting of interested individuals a stereo DC coupled transformer-less tube hifi amp. It was in the early '70's as I entered my teenage years, and it was an event that had all who witnessed it in an absolute tizzy... although at the time I didn't see/hear what the fuss was about and frankly I was just a little disappointed as I considered the audio source and loudspeakers were not truly up to the task ...

Since that day I have heard fully DC coupled systems and have been incredibly impressed, and by all reports that I've encountered on transformer-less tube amps they are equally incredibly impressive devices to hear...

Good luck... keep us informed... and postcard to us plenty of pictures...

[DrNomis_44]

Was thinking of basing my OTL Headphone amp on this design:

http://astersart.net/amp/headphones/..._amplifier.htm


There may be some issues with regards to the output current capability not being enough to drive my pair of Audio Technica ATH-M30X Headphones, cause it's quoted impedance is 42 Ohms.

[DrNomis_44]

Found this interesting OTL Headphone Amp design that has got me really piqued, it uses three 6N1P twin Triode tubes:

https://headwizememorial.wordpress.c...one-amplifier/


After some further googling for info on the internet, I've found that you can substitute an ECC88 tube for a 6N1P, I was doing some quick reading of some service manual PDFs for my Telequipment D33R Oscilloscope, and found that it uses a few ECC88 Tubes, since some new 6N1P tubes from Evatco will probably set me back something like Au$20 each or so, I'm going to try using three ECC88 tubes from my Telequipment D33R scope to build the OTL Headphone amp instead of three 6N1P tubes, I've read in a forum that the ECC88 sounds better than the 6N1P.


From looking at the circuit of the 6N1P OTL Headphone amp, one half of V1 is used as a common cathode voltage amplifier, it's anode is connected directly to the grid of one half of V2, which then has it's cathode connected to the anode of the second half of V2, this forms a low-impedance output stage, it's an interesting design, very simple for an OTL Headphone amp.



Update:

Turns out there was only one ECC88 tube in my Telequipment D33R tube oscilloscope, the rest of the tubes in it are either 6BL8, or ECF804 small-signal Triode-Pentode tubes, so I'll have to order some 6N1P and ECC88 tubes from Evatco after all.

[DrNomis_44]

After some ummmm-ing and ahhhhhhh-ing, I've elected to mount all the resistors, caps, and power diodes on some tag boards with solder-tags on two sides of the boards, rather than wire it all up point-to-point style, so I'll need to order some suitable tag boards from Evatco along with all the other needed components, I'm still debating in my head what I'm going to use as the chassis for my OTL Headphone Amp build, once I've got everything worked out and all the needed parts, I'll start the build-process and will be doing a write-up as I go along, will also include some photos as well.

I've just finished drawing some layouts for the tagboards which will be used to mount the components.


Evatco sell two different types of tag boards I could use to mount all the resistors, caps, and power diodes on, one of them is a dark brown phenolic 36-way tag board with two rows of 18 tags, it looks like this:

https://www.evatco.com.au/36-way-tag-board-2-rows-of-18

The other one is a lighter coloured fibreglass tag board that has two rows of 28 tags on it, and looks like this:

https://www.evatco.com.au/tag-strip-28-x-2