OTL Headphone Amp project.

[DrNomis_44]

Update:

I finished doing all the prep-work on the aluminium chassis for my 6N1P OTL Headphone amplifier build today, it is all spraypainted, labelled, and clear-coated ready for the final build process, here's a pic so you can see what it looks like now, I think it looks pretty good, will look even better once the final build-process is completed:

[DrNomis_44]

Update:

I have now started work on the final build-process, all the hardware and etc have been installed in the diecast aluminium chassis, all that's left to do is all the wiring up and soldering-in all the resistors, caps, diodes, and Leds, before doing all the testing to ensure it is working correctly, it's a bit late at night, so I will continue the work tomorrow, but for now, here are a couple of pics to give you an idea of what the OTL Headphone amp will look like once it is built and working:

[DrNomis_44]

Update:

I have finally finished doing all the internal electrical wiring-up inside the chassis, I did some basic tests with my multimeter and as far as I can see, everything looks good, there doesn't appear to be any short-circuits where there shouldn't be any, but, I'm going to play it safe for the time being and not plug the headphone amplifier into the mains to try it out and see if it works properly, reason being is that I want to do some important tests first to make sure the power supply is working correctly, to do that I need to wait until next week so I can buy a new Variac (Variable AC Transformer) from my local Jaycar Electronics store, a Variac will enable me to slowly bring the amp up to mains voltage and not risk blowing-up the power supply filter caps, I can use my multimeter to monitor the output voltage of the +HT supply as I slowly bring the amp up to full mains voltage, it will be easier for me to spot any abnormal behaviour, at full mains voltage the +HT power supply should put out about +348V DC at about 20mA or so according to the original article detailing the design and build of the headphone amp, I found on the internet, the Hammond 369AX power transformer I'm using in my build can supply up to 115mA of current so the voltages in my build may be a bit higher that those given in the article, I'll have to wait and see, besides, I need to work out the correct fuse size to put in the fuse holder.

Once my headphone amp passes all the important tests I want to do on it, and I am confident that it is all working correctly, I'll be happy with plugging it directly into the mains for a good try out to see how it performs and sounds with my new pair of Audio Technica ATH-50x Headphones, until then, I'm not taking any chances, I'm playing it safe.

[DrNomis_44]

Update:

Boy, have I got some news for you all, firstly, I managed to buy a new Variac, some new audio leads, and some other stuff from my local Jaycar Electronics store today.

I went through my OTL Headphone amp build and did all the important tests I wanted to do on it to ensure it is safe to plug into a 240V AC mains outlet, it passed all the tests and as far as I can tell, the amplifier is working correctly like it is supposed to, the +HT supply, and the Heater supply appeared to both be working correctly, so, I plugged in the three 6N1P Tubes in their sockets, and gradually brought the output voltage of the Variac up to the full 240V AC mains voltage, and then I used my multimeter to measure the voltages in the circuits, they all appeared to be normal, the +HT supply was putting-out just over 300V DC, the voltages on pins 3 and 6 of V2 and V3 were sitting at about +160V DC, roughly about half the full +HT supply voltage, I felt confident that the amplifier was indeed working correctly, so I tried feeding a 1kHz signal into the inputs and seeing if there were healthy looking output signals on the terminals of the headphone out socket, both the left and right output signals looked very healthy and clean, so, I plugged in my Audio Technica headphones and had a listen to the output signals, they sounded loud and clear/clean.

The amplifier is working perfectly, I can't hear any background hum at all, the amp is really quiet with no signal going through it.

Next, I tried connecting the 3.5mm stereo line out socket of my studio pc to the inputs of the headphone amp and playing some music mp3s, my goodness....this headphone amp sounds AWESOME!!!!!

Bass is really nice, mids are really nice, trebles are really nice and not ear-fatiguing, the amp has a really nice warm sound to it, crystal clear too, it does get pretty loud driving my Audio Technica ATH-M50x headphones which are rated at 38 Ohms impedance, plenty loud enough, so I'm Super Happy with it, I just know it'll sound good with my 43 inch Blaupunkt Smart TV, you can use it with pretty much any audio source, even a CD player.

I've been listening to some of my favourite Dub Reggae, Goa Trance, and PsyTrance tracks, I'll see what rock sounds like played through it, but definitely Dub Reggae and Reggae sounds really good.

I'll post a pic of the fully-working 6N1P OTL Tube Headphone Amplifier later on for you all.

It is working perfect, and sounds AWESOME!!!!

As promised, here's a pic of the amp fully working and sounding AWESOME!!!!



Note: The blue glow in the three 6N1P tubes is coming from a 3mm blue led located underneath each tube.

After using this amp to listen to some music, I've noticed a few things about it's "sound", I'm just describing what my ears are telling me me though, it doesn't get ear-fatiguing during long listening sessions, it also seems to have a pleasant habit of revealing all the subtle nuances in the music, the stereo image is really good too, for some reason it seems to make the audio from YouTube sound a lot better as well, unless my ears are outright lying to me.

I tried listening to some vintage Led Zeppelin, sounded really nice too.

[Trevor Davies]

Well done. It looks great, and wonderful that it also sounds so good and that you are really happy with it. One day I will have to give an amp go!

[DrNomis_44]

Well done. It looks great, and wonderful that it also sounds so good and that you are really happy with it. One day I will have to give an amp go!

I definitely recommend it; I got a huge amount of enjoyment out of the whole build-process from start to finish, a lot more than I would have gotten if I had elected to do a solid-state build instead.

I think I have posted a link to the original build-article I found on the HeadWize website in one of my earlier posts, in case anyone else wanted to have a go at building one for themselves, I reckon it's a great first Tube amp project, but I would still recommend familiarizing yourself with all the safety procedures for working on, and building high voltage electronic circuitry first, for safety's sake.


A bit later on this week, I'm going to measure all the voltages in my build with my multimeter, and post a full list of them in this thread, so that other builders can use them as diagnostics for their build in case they encounter any tech problems/issues, I will also post scans of updated versions of the schematics for my build with voltages at key points in the amplifier and power supply circuits as well, oh, that reminds me, I'll need to do a series of screenshots of the signal waveforms I see on the LCD of my Digital scope too.

[DrNomis_44]

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...

I just recently finished building the amplifier, and I have to say that it has exceeded my expectations, it is a real joy to listen to, at first, I wasn't sure if it was able to drive my new pair of Audio Technica ATH-M50x headphones without generating too much distortion, mainly even harmonics, but, I was very pleasantly surprised, not only was it able to drive my Audio Technicas plenty loud enough, the sound was crystal clear, my Audio Technicas are rated at 38 Ohms, my understanding is that some OTL headphone amp designs have trouble driving low impedance headphones lower than about 100 Ohms, apparently not this particular design I chose, it seems perfectly happy with 38 Ohms.

I tried connecting the line outs of my studio pc to the amp, sounded great, also tried connecting it to the headphone out socket of my new Asus gaming laptop, it also sounds great, with nothing connected to the amp itself, it is practically dead-quiet even at full volume settings, the heater supply is 6.3V DC which is floated above ground via a .22uF/630V cap, so as not to exceed the heater/cathode voltage, I put a 220k/2W metal oxide resistor on the output of the +HT supply to ground to stop the caps from over-voltaging with no load on the +HT supply, seems to work a treat, I'm getting about +306V DC or so from it under load, pins 3 and 6 of V2 and V3 are sitting at about +160V, roughly about half the full +HT voltage, I checked a few other voltages with my multimeter, and as far as I can tell, everything looks normal.

[DrNomis_44]

Update:

I managed to set up my test bench so that I could take a screenshot of the two outputs of my OTL Tube Headphone Amplifier build working into a 33 Ohm resistive load, I used two 33 Ohm/1%/1W metal film resistors to simulate a pair of 33 Ohm headphones, it may not be entirely accurate, but for this quick test, it is as they say "close enough for government work".

I adjusted the volume control for the highest output signal level with the least amount of distortion, but you will notice some soft-clipping on the bottom peak of the output waveform, in the screenshot, the yellow trace is the left-hand channel going into a 33 Ohm load, and the pink trace is the right-hand channel going into a 33 Ohm load.

So, it looks like the amp can easily put about 600mV of drive signal into a 33 Ohm load, I'm guessing that the signal level into a 38 Ohm load would be higher, since a 38 Ohm load would present a lighter load to the amp, it looks like the amp has a low-enough output impedance to easily drive loads down to 32 Ohms impedance.

I just did some calculations and, assuming my calculations are correct, my OTL Tube Headphone Amplifier is capable of putting 4.968 milliwatts RMS into a 33 Ohm resistive load, I'm guessing that it is most likely capable of putting a total of 5.0 milliwatts RMS into my pair of Audio Technica ATH-M50X headphones that are rated at 38 Ohms impedance, due to the lighter loading.

[fender3x]

So, how does it sound?

[DrNomis_44]

So, how does it sound?

To my ears, it sounds really good, the sound is crystal clear with very good bass, mids, and highs, the highs are very pleasant and not ear-fatiguing even after extended listening sessions, and I swear that I can hear all the subtle nuances in the music, the amp itself is able to drive my pair of Audio Technica's (an ATH-M50X) plenty loud enough, the amp exceeded my expectations.