I recently had a lot of fun having a go at building an OTL headphone amplifier using three Svetlana 6N1P tubes I bought online from Evatco, there are a few reasons why I decided to have a go at building one, firstly, I was curious to find out what it would sound like, secondly, my 43 inch Blaupunkt HDTV does not have a headphone out socket, only an optical digital audio out, so I needed some kind of an amplifier, thirdly, I thought that it would make a good and fun diy audio project, so I did some searching for OTL headphone amp schematics and found a really interesting build article on the Headwize Memorial website by a guy named Bruce Bender, the article was based on another very similar OTL headphone amp, the schematic in the article looked easy enough to build, so I started ordering all the needed parts online, once I had all the parts I needed, I got to work on marking out all the hole centers I needed to drill and cut in the diecast aluminium box I used for the chassis, I used some matt white prime and paint to spray paint the chassis after drilling and cutting all the holes, then I used my labelmaker to make adhesive paper labels to stick on the spray painted chassis before using some satin clear coat spray to seal it all.
After doing all the internal electrical wiring, I plugged the amp into my newly bought 500VA Variac, turned the amp on, and slowly turned up the output voltage on the Variac while monitoring the +HT supply with a multimeter, after I had increased the Variac's output voltage to 250V AC, the +HT was measuring +306V DC, and as far as I could tell by doing voltage checks with my multimeter, the amp appeared to be operating normally as it was supposed to, so, I hooked up my new FNIRSI 100MHz Digital Dual Trace Oscilloscope/Function Generator and fed in a 1kHz sine wave signal into each of the amp's stereo line inputs and monitored the outputs with the oscilloscope, I saw a very clean looking 1kHz sine wave signal on each of the amp's outputs, next, I plugged my new pair of Audio Technica ATH-M50X headphones into the amp's headphone out socket and had a listen, I was pleasantly surprised to find that the amp could drive the Audio Technicas plenty loud enough even though the headphone is rated at 38 Ohms impedance, I tried playing some music from my studio pc through the amp, and if my ears aren't lying to me, the sound quality was awesome, really crystal clear with nice bass, mids, and highs, I could clearly hear the subtle nuances in the music, there's none of the "ear fatigue" with this amp that I get when listening to a solid state amp, apparently from what I read in the articles my amp build is based on, the frequency response is ruler-flat from 10Hz to 100kHz with gentle roll-offs, the amp sounds relatively free from any obvious distortion, I read somewhere that the 6N1P tube is regarded as being very linear, there is practically zero hum-noise even when the volume control is turned up fully, it is a very quiet amp due to the tube heaters being powered by a 6.3V DC supply that is deliberately floated above ground via a 220nF/630V film capacitor.
I'll see if I can post some pics of my OTL headphone amp build later on once the admins have approved my thread post.....stay tuned!!!!
After doing all the internal electrical wiring, I plugged the amp into my newly bought 500VA Variac, turned the amp on, and slowly turned up the output voltage on the Variac while monitoring the +HT supply with a multimeter, after I had increased the Variac's output voltage to 250V AC, the +HT was measuring +306V DC, and as far as I could tell by doing voltage checks with my multimeter, the amp appeared to be operating normally as it was supposed to, so, I hooked up my new FNIRSI 100MHz Digital Dual Trace Oscilloscope/Function Generator and fed in a 1kHz sine wave signal into each of the amp's stereo line inputs and monitored the outputs with the oscilloscope, I saw a very clean looking 1kHz sine wave signal on each of the amp's outputs, next, I plugged my new pair of Audio Technica ATH-M50X headphones into the amp's headphone out socket and had a listen, I was pleasantly surprised to find that the amp could drive the Audio Technicas plenty loud enough even though the headphone is rated at 38 Ohms impedance, I tried playing some music from my studio pc through the amp, and if my ears aren't lying to me, the sound quality was awesome, really crystal clear with nice bass, mids, and highs, I could clearly hear the subtle nuances in the music, there's none of the "ear fatigue" with this amp that I get when listening to a solid state amp, apparently from what I read in the articles my amp build is based on, the frequency response is ruler-flat from 10Hz to 100kHz with gentle roll-offs, the amp sounds relatively free from any obvious distortion, I read somewhere that the 6N1P tube is regarded as being very linear, there is practically zero hum-noise even when the volume control is turned up fully, it is a very quiet amp due to the tube heaters being powered by a 6.3V DC supply that is deliberately floated above ground via a 220nF/630V film capacitor.
I'll see if I can post some pics of my OTL headphone amp build later on once the admins have approved my thread post.....stay tuned!!!!
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Looks like the forum admins have finally approved my posts, so, as promised, here are two pics of my 6N1P OTL headphone amplifier build.
And here's a pic of my 6N1P OTL headphone amp, powered-up and fully working!
For those interested, here's a pic of the amplifier schematic for the left and right channels, minus the power supply circuit which is relatively simple and conventional.
The two halves of V1 (6N1P) are just standard common-cathode voltage amplifiers, the two halves of V2 (6N1P) form a circuit topology known as the White Cathode-Follower, the two halves of V2 act like a push-pull output stage, resistors R2, R4, and R5 set the operating conditions of the White Cathode-Follower output stage circuit, the value of R4 needs to be chosen so that the gain of the top half of V2 has a gain of 1 in order to enable the amplifier circuit to drive loads down to 32 Ohms without excessive distortion due to an imbalance of gain of the two halves of V2, the circuit uses two parallel-connected 220uF/350V capacitors to AC-couple the output at pins 3 and 6 of V2, to the headphones being driven, because space was limited in the diecast aluminium case I used as the chassis, I elected to replace the two 220uF/350V caps with a single 470uF/450V can cap, which seems to work fine with no detrimental effect on the sound of the amp, as far as my ears can tell.
I used 1W and 2W 5% Metal Oxide resistors for all the resistors in the amp circuits, and three 1k/5W ceramic resistors in the power supply circuit, I measured +306V DC at the output of the +HT supply, and about 160V DC on pins 3 and 6 of V2.
The blue glow you see in the three 6N1P valves while the amplifier is fully powered-up and operating, is due to a 3mm blue Led located in the center of each of the three 9-pin valve sockets underneath each of the three valves.
As you can see from looking at the amplifier schematic, practically zero negative-feedback is used, and the output at pins 3 and 6 of V2 is 180 degrees out of phase with respect to the input on pin 2 of V1, as far as I can hear with my ears, the 180 degree phase-shift appears to have absolutely no effect on the sound of the amp whatsoever, it sounds wonderful to my ears, crystal clear and very pleasant to listen to for extended listening sessions, I deliberately over-rated the components for the job they do in the amplifier circuit, for reliability's sake.
I used 1W and 2W 5% Metal Oxide resistors for all the resistors in the amp circuits, and three 1k/5W ceramic resistors in the power supply circuit, I measured +306V DC at the output of the +HT supply, and about 160V DC on pins 3 and 6 of V2.
The blue glow you see in the three 6N1P valves while the amplifier is fully powered-up and operating, is due to a 3mm blue Led located in the center of each of the three 9-pin valve sockets underneath each of the three valves.
As you can see from looking at the amplifier schematic, practically zero negative-feedback is used, and the output at pins 3 and 6 of V2 is 180 degrees out of phase with respect to the input on pin 2 of V1, as far as I can hear with my ears, the 180 degree phase-shift appears to have absolutely no effect on the sound of the amp whatsoever, it sounds wonderful to my ears, crystal clear and very pleasant to listen to for extended listening sessions, I deliberately over-rated the components for the job they do in the amplifier circuit, for reliability's sake.
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Hope it's okay to post links to webpages in the diyAudio forum threads, if not, I apologize in advance, anyway, if any of you are interested in having a look at the original articles, I based my 6N1P OTL headphone amp build on, here's a link to the main one:
https://headwizememorial.wordpress.com/2018/03/20/the-6n1p-otl-headphone-amplifier/
https://headwizememorial.wordpress.com/2018/03/20/the-6n1p-otl-headphone-amplifier/
Thank you for sharing. This project brought the memory of Headwize back to ~20 years ago.
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I have been using my OTL headphone amp ever since I first built it, so far it has given me virtually zero issues, it is a very well-behaved amplifier.
@DrNomis44, just for my curiosity... 🙂😉 :
You wrote that your OTL 6N1P headphone amp is able to drive 32R loads, but did you attempted to measure the effective output impedance of your little cool 😎 circuit ?
T
You wrote that your OTL 6N1P headphone amp is able to drive 32R loads, but did you attempted to measure the effective output impedance of your little cool 😎 circuit ?
T
The original build article I based my amp build on, mentions that the +HT supply tends to drift up in voltage if no 6N1P valves are installed in their sockets, so, in my build, I made a very minor modification to the +HT supply by adding a 220k/2W 5% Metal Oxide resistor from the output of the +HT supply to ground to stop the +HT supply from drifting upwards in voltage, the 220k/2w 5% Metal Oxide resistor acts like a bleeder resistor, which I think is a good idea, I did some basic Ohm's Law calculations and from the results I got, the 220k resistor is not even dissipating anywhere near its maximum power dissipation, about 400mW (milliwatts) if my calculations are correct, so, it shouldn't overheat.
Actually, no I didn't to be honest, but, the article/s I based my build on both mention that the amp is able to drive loads down to 32 Ohms, the article I based my build on, has a link to another article on the Headwize Memorial website, titled The Morgan Jones Mini Tube Headphone Amplifier, the article mentions that the amp has an output impedance of 53 Ohms and a power output of about 10mW (milliwatts) which I think is plenty to drive a pair of good headphones, the article my build is based on is actually an optimised version of the Morgan Jones Mini Tube Amplifier circuit, from comparing the two circuits, two resistors R2 and R4 appear to have been changed in value in order to optimize the amp so it can drive 32 Ohm loads, R2 was changed from 27k to 42k, and R4 was changed from 120 Ohms to 200 Ohms, this means that the top half of V2 has a gain of 1, or a bit less than 1, the top half of V2 also acts as a cathode-follower, cathode followers are only capable of a gain of 1, but, they also provide a low-impedance output.
My pair of Audio Technica ATH-M50X headphones is rated at 38 Ohms impedance, and my headphone amp build can easily drive it plenty loud enough with no trouble, I very rarely have to turn the volume control pas half way, and even then it's plenty loud enough, sound is crystal clear right across the entire audio spectrum, I even checked it on a scope, both channels look clean on a scope, my FNIRSI digital scope can do screen captures so I'll post some screenshots later on.
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I actually couldn't source any 200 Ohm/2W/5% Metal Oxide resistors, so, I used two 100 Ohm/2W/5% Metal Oxide resistors in series for R4, likewise, I couldn't source any 42k/2W/5% Metal Oxide resistors, so I used two 82k/1W/5% Metal Oxide resistors in parallel for R2, giving me a 40.7k/2W/5% Metal Oxide resistor, close enough to 42k, it seems to work fine.
In my build, I used some 8-way phenolic tagstrips I bought from my local Jaycar Electronics store for mounting the majority of the resistors and caps, and just ran some 7.5A Silicone Rubber insulated stranded wire from the tagstrips to the pins on the 9-pin sockets, the power rectifier diodes are all mounted on a tagboard that's mounted underneath a larger tagboard that you see in the pic I posted of the insides.
To be honest, I really have no idea what the real-world output impedance of both the outputs of my amp build actually is, but apparently it is low enough to drive my pair of Audio Technicas plenty loud enough for headphone listening sessions with no problem at all, it does get very loud at full volume, below half volume is plenty loud enough for my ears anyway.
C4 is specified as a 0.47uF/400V cap, I used a 0.47uf/630V metal film cap instead in my build since they were easy enough to buy online, seems to work perfectly well.
For the main +HT supply filter caps, I used two 50uF/500V dual can caps, and wired two 50uF/500V sections in parallel to effectively make a single 100uF/500V cap.
The power transformer I used in my build is a Hammond power transformer with a 250V CT/115mA HT secondary, and a 6.3V CT/2A heater secondary, I wired the primary windings for our Australian 240V AC/50Hz mains supply, seems to work great, the 6.3V DC heater supply is deliberately floated above ground via a 0.22uF/630V cap from the 6.3V DC heater supply negative to chassis ground, the reason why the heater supply is deliberately floated above ground is that the Svetlana 6N1P's heater-cathode voltage rating is only about 100V, and in operation, pins 3 and 6 of V2 are at about 160V above ground, floating the heater supply above ground prevents the heater-cathode voltage from being exceeded.
For the main +HT supply filter caps, I used two 50uF/500V dual can caps, and wired two 50uF/500V sections in parallel to effectively make a single 100uF/500V cap.
The power transformer I used in my build is a Hammond power transformer with a 250V CT/115mA HT secondary, and a 6.3V CT/2A heater secondary, I wired the primary windings for our Australian 240V AC/50Hz mains supply, seems to work great, the 6.3V DC heater supply is deliberately floated above ground via a 0.22uF/630V cap from the 6.3V DC heater supply negative to chassis ground, the reason why the heater supply is deliberately floated above ground is that the Svetlana 6N1P's heater-cathode voltage rating is only about 100V, and in operation, pins 3 and 6 of V2 are at about 160V above ground, floating the heater supply above ground prevents the heater-cathode voltage from being exceeded.
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Here's a link to the Morgan Jones Mini Tube Headphone Amplifier article on the Headwize Memorial website, I think it is quite an interesting article to read:
https://headwizememorial.wordpress.com/2018/03/20/the-morgan-jones-mini-tube-headphone-amplifier/
It is actually based on a device called the EarMax miniature headphone amplifier, the article is really a reverse-engineer of it.
https://headwizememorial.wordpress.com/2018/03/20/the-morgan-jones-mini-tube-headphone-amplifier/
It is actually based on a device called the EarMax miniature headphone amplifier, the article is really a reverse-engineer of it.
For each of the 3mm Blue Leds located underneath each of the three 6N1P tubes, in the middle of each 9-pin socket, I used a 2k2/1% 1/2W metal film resistor to limit the current to the Leds, from doing some basic Ohm's Law calculations, we can figure out how much current is being supplied to the Leds:
I=E/R
I= 6.3V/2200
= 0.0028636363636364
Or roughly about 3 milliamps per Led, the 6.3V secondary of the power transformer I used in my build can supply up to 2A maximum, the heaters of the three 6N1P tubes each draw 600mA from the 6.3V DC heater supply, giving a total current draw of 1.8A, that left 200mA spare, so I decided to add the three 3mm Blue Leds, plus a 5mm Blue power on indicator Led to my build.
I=E/R
I= 6.3V/2200
= 0.0028636363636364
Or roughly about 3 milliamps per Led, the 6.3V secondary of the power transformer I used in my build can supply up to 2A maximum, the heaters of the three 6N1P tubes each draw 600mA from the 6.3V DC heater supply, giving a total current draw of 1.8A, that left 200mA spare, so I decided to add the three 3mm Blue Leds, plus a 5mm Blue power on indicator Led to my build.
With 2K2 resistors, your blue 3mm LEDS must be very bright... But it is probably necessary to blue-light enough the 6N1P tubes ? I must confess that I never experimented that kind of tube lightening "by the centre of the socket".
Below, for the pilot light of my MICRO-AMP, I used a blue 5mm LED with a 33K resistor, from a 6.8VAC voltage.
That said, I should test your idea on this amp to see what it looks like, since the center of the sockets is easy to access :
Why not, after all ? I can also test other colors, notably Orange, to underline the heaters...
Wait and See ! 😉
T
Below, for the pilot light of my MICRO-AMP, I used a blue 5mm LED with a 33K resistor, from a 6.8VAC voltage.
That said, I should test your idea on this amp to see what it looks like, since the center of the sockets is easy to access :
Why not, after all ? I can also test other colors, notably Orange, to underline the heaters...
Wait and See ! 😉
T
Those Blue Leds I used in my build are high-brightness types, and the encapsulation is clear coloured, I have been thinking about increasing the 2k2 resistors to something like maybe 10k to reduce the brightness a bit, the main reason why I added the three 3mm Blue Leds was to make the amp look a bit more high-endy, personally, I can't see why you couldn't use other coloured Leds besides Blue ones, which reminds me, one time I was doing some servicing work on an old 80's 28-Ch Soundcraft mixing desk for a friend of mine who runs a recording studio in Darwin NT Australia, some of the Vu meter lamps had gone faulty, and my friend suggested replacing them with Leds, which I thought was a cool idea, along with some yellow Leds, he bought some multi-colour Leds that he wanted to put in the two Vu meters for the master mix tracks on the mixer, which actually looked really cool, I had to do some Ohms Law calculations to work out what resistor values I needed to use for the current limiting resistors.
From looking at the original EarMax mini tube headphone amp, it looks like the designer used either Red or Orange Leds.
From looking at the original EarMax mini tube headphone amp, it looks like the designer used either Red or Orange Leds.
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Here's a hand-drawn schematic of how I have the 6.3V DC heater supply wired-up in my amp build.
