OTL Headphone Amp project.

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Originally Posted by JohnH

Good to see you're still at it doc!

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Cheers, usually when I start a project, I tend to see it all the way through till completion, besides, my main motivation for starting this build-project was because I had gotten tired of always having to leave my wireless headphones on charge for a long time before I could use it again, secondly, sometimes it takes a while for sound to come through the wireless headphones, I think that's a quirk of the wireless transmitter for my wireless headphones there's a delay between when it gets digital optical audio signals from my TV and when sound comes out of the headphones.

I'm currently doing a stock-take of all the components and stuff I already have collected for this OTL Headphone amp build-project so I can compile a list of parts and etc I need to order this year so I can finally get cracking, and start the build process.

Update:

I managed to make some more progress on my 6N1P OTL Headphone amp project over the weekend, did all my marking-out of the diecast aluminium chassis, and drilled all the pilot holes, I used a stepped drill bit to enlarge some of the holes to the correct size, but I'll need to wait till next fortnight to buy some hole saws from my local Bunnings store, to cut the holes for four can caps that will mount on the chassis top near the power transformer, here's a couple of pics showing the progress I made, won't be too long till I can do all the final assembly, wiring-up, and testing work:

Attachment 45141

Attachment 45142

Top pic shows the chassis all marked-out, I may need to drill a few more holes in it though, second pic shows the insides of the chassis with some components, and tagstrips mounted, mostly so I can get an idea of how it is all going to work, I'm happy with it, had to go through a few revisions of the chassis layout till I came up with this layout which seems to work really well.

All I need to do now is get the new power transformer, two dual 50uF/500V can caps, can cap mounting brackets, and some extra resistors ordered, and then on to the final assembly and etc stages.

The outside of the chassis will be spray painted matt white with a covering of clear satin to seal-in the labelling, should look great once it's all done.

Update:

Made some more progress on this build project, managed to get the mounting holes for the fuse holder and tube sockets drilled out to size, the can caps sitting on the chassis are just to give an idea of how it's going to look once the final assembly has been completed, I'm not going to be using those two taller light-blue can caps, I'll be using two black MLytic dual 50uF/500V can caps instead:

Attachment 45143


The two shorter can caps are the two 470uF/450V DC output coupling caps that couple the output signals on pins 3 and 6 of V2 and V3 to the headphone out sockets, pins 3 and 6 of V2 and 3 will be sitting at about half the full B+ HT supply voltage (about +348V DC), or about +176V DC, so, the voltage rating of the 470uF can caps needs to be high enough to withstand it, that's why I chose 450V DC can caps, 450V DC should be more than enough for the job, the last thing you want is your headphones copping 176V DC across the voice coils, which would burn them out in an instant, it is always a good idea to over-rate the caps for the job in a Tube amplifier for reliability's sake, sure, it may cost a little bit extra but well worth it in the long run, by over-rate I mean use caps with a voltage rating a bit more than what you're expecting to see in the circuit, in some cases you can get-away with using a cap that's the next value up from that specified in the circuit if the specified value is unavailable, most electronic circuits aren't all that critical when it comes to component values.

In the original 6N1P OTL headphone amplifier schematic, the output coupling caps are two 220uF caps in parallel, which gives a total of 440uF, since space in the chassis I'm using is a bit limited, I wanted to reduce the number of output coupling caps to one for the left channel and one for the right channel, now 440uF isn't exactly a common value of capacitance so I opted to up the value to 470uF, which is more common, it may result in a more extended bass range, but I reckon it should work fine.

How about we do a quick analysis of how the main amplifier circuit works?.....I won't go too heavily into the mathematics though...

Okay, looking at the schematic, the signal from our audio-source comes into the amp via the input sockets, and appears on the clockwise tags of the dual 100k Log volume control, the dual 100k Log volume control acts like a variable voltage-divider with the signal level on the wiper tags proportional to where we set the volume control, the signals are applied to the junctions of the 100 Ohm and 1M resistors connected to pins 2 and 7 of V1, the two triode halves of V1 are configured as common-cathode amplifier gain-stages, both of them have a 42k/2W anode resistor and a 750 Ohm/2W cathode resistor, these two resistors set the quiescent, or no-signal, operating conditions of the gain stages, the input signals applied to pins 2 and 7 of V1 appear on pins 1 and 6 of V1 amplified but inverted, pin 1 of V1 is direct-coupled to pin 2 of V2, and pin 6 of V1 is directly-coupled to pin 2 of V3, V2 and V3 are configured in a circuit topology known as a White Cathode-Follower circuit, a cathode-follower has a gain of 1, or slightly less than 1, but it provides current-gain and a lower output impedance (my guess is that the output impedance is most likely about 10 Ohms or so), this is important because the output impedance of the two gain stages of V1 is high, and if we try to drive a low impedance load via a high impedance output, we will get a much lower and distorted output because a high impedance output cannot supply enough current to drive a low impedance load.

The two White Cathode-Followers formed by the two halves of V2 and V3 have a 200 Ohm resistor connected to pin 1, and a 220 Ohm resistor connected to pin 8, V2 and V3 also have their pin 3 connected to pin 6 which are connected to their respective output coupling caps, one other thing about cathode-followers is that the output signal is in-phase with the input signal, the 200 Ohm resistor and 220 Ohm resistor, as well as the 42k resistor, all set the quiescent operating conditions of the White Cathode-Follower output-stages formed by V2 and V3, in order for the OTL Headphone amp to be able to drive impedances down to 32 Ohms, the White Cathode-Follower output-stage has to be properly balanced so that the top triode has a gain of unity, or 1, otherwise excessive second-harmonic distortion will occur when the amp tries to drive low impedance loads, the amplifier itself doesn't really need to be very powerful since you only need tens of milliwatts to drive headphones to a good volume level.

Pin 7 of V2 and V3 is tied to ground via a 1M resistor, and a 470nF/630V cap couples the signal on pin 1 of V2 and V3 to pin 7 of V2 and V3, essentially, V2 and V3 act like a kind-of push-pull output stage.

The signals on pins 3 and 6 of V2 and V3 are AC-coupled to the headphones via the coupling caps, which, in my case, are two 470uF/450V DC can caps, they block the +176V DC on pins 3 and 6 of V2 and V3 and pass AC signals.

The 1000uF/16V cap in parallel with the 750 Ohm resistors, and the 2200uF/25V caps in parallel with the 220 Ohm resistors are bypass caps that allow each stage in the amplifier circuits to work at their best.

Here's the amplifier schematic for reference:

Attachment 45144

Since I now have a new Digital Scope that allows me to take screenshots of what's on it's LCD screen, I will be using that feature once the 6N1P OTL Headphone amp has been built and is working, it will be interesting to have a look at the signal-waveforms present as the amplifier is operating.

Update:

Finally managed to get the rest of the parts I need, including the power transformer, ordered online from Evatco on Tuesday this week, I paid the extra money for express delivery while I was at it, bought a few more things I needed from Jaycar earlier this afternoon, postal tracking for the stuff I ordered from Evatco says expected delivery should be on next Monday, so, hopefully by the end of next week I'll finally have this build project completed and working.

Will post some more pics of my fully built, and working 6N1P OTL Headphone Amplifier, plus some scope screenshots for you next week.....stay tuned....

I decided to do a minor mod to the design of the 6N1P OTL Headphone amp I'm building really just to spruce it up a bit, I was originally going to use a 5mm Red Led in a chrome bezel as the front-mounted power indicator, I've changed it to a 5mm Blue Led, also, I'm going to be adding three 3mm Blue Leds to each of the 9-pin tube sockets, the 3mm Blue Leds will be mounted in the centre hole of each 9-pin tube sockets, all the Leds will get their power from the 6.3V DC heater supply, I worked out that there will be about 200mA spare from the 6.3V DC heater supply after powering each of the 6N1P tube heaters, the power transformer can supply 6.3V AC at up to 2A, so it should be plenty.

So, I'll need to work out the value for the current limiting resistors for each of the Blue Leds.

Update:

Well, the parcel containing all the extra needed parts I ordered online from Evatco arrived in the post earlier than I expected, I had to pick the parcel up from my local Post Office on Cavenaugh St, anyway, I have finally collected all the needed parts to start the final build-process of the 6N1P OTL Tube Headphone Amplifier build project, all I need to do is pop-into my local Bunnings store to buy a set of hole saws to cut the mounting holes for the four can caps in the chassis, prior to giving the chassis a quick rub down with some 98.9% pure Isopropyl Alcohol which will get rid of any residue from the marking out, as well as any oils and contaminants, I will also give it a light sandpaper all over to help the matt white paint stick to the chassis, although it says on the can that it is prime and paint, after applying some labels made with my label maker, I'll give it a good spray with some satin clear coat to seal the labels so they won't peel off.

I did a quick trial fit of the can caps, the new power transformer, and the three 6N1P Tubes, and everything seems to be fine, even the four mounting holes for the power transformer all line up, I do need to pop into Jaycar to buy some rubber grommets so that the wires for the power transformer windings don't chafe against the chassis, I don't fancy getting a 340VAC/115mA or 240V AC/10A belt, so I'd rather do it all right the first time.

Hopefully if all goes well, I'll have the 6N1P OTL Tube Headphone Amp build finished and working by the end of this weekend....fun times ahead....stay tuned!!!!

Here's a pic to give you an idea of what the 6N1P OTL Headphone Amp is going to look like, the big black part surrounded by the blue can caps is the new power transformer:

Attachment 45155

I reckon it looks great, will look even better once the chassis has been spray painted/labelled/satin clear coated, hopefully it will work and sound great with my new Audio Technica ATH-M50X Headphones, and produce enough power to drive them to a decent volume.

Update:

Finally, at long last, I have everything I need to start work on the 6N1P OTL Tube Headphone Amplifier build project, right down to the last nut and bolt, I popped into Jaycar this afternoon to buy some rubber grommets, and some M4 washers, Bunnings had the Craftright Hole Saws in stock too, so I bought a 5-piece set, one of the Hole Saws has a diameter of 32mm which is about the right size for the can caps, the Hole saws are made from hardened steel which should be fine for cutting holes in diecast aluminium which is softer, I just need to let my cordless drill charge overnight, so tomorrow it's Game On!!!