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/stripboard-layouts/chime-amp-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/toplevel/top_level_otl_tube_headphone_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.com/2018/03/20/the-6n1p-otl-headphone-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

[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...


Cheers, I happen to enjoy a challenge, and this build project should be a good one.

[DrNomis_44]

Update:


Seeing as I'm really in the mood for doing some diy scratch-build Tube amp projects this year, the diy scratch-build OTL Headphone amp is officially scheduled for 2023, well, it'll give me something "constructive" to do with my time anyway, besides, it'll fun.

You know what? getting back into doing some diy builds this year again, has made me re-discover why I got into electronics in the first place, it's because it's a whole lot of fun to go through the process of getting together all the parts for your build projects, and put them all together into a finished working electronic device, you get a huge sense of achievement out of it knowing you built it yourself, you also learn far more by doing practical things, rather than sitting in a classroom, well that's how I seem to learn best anyway.

[DrNomis_44]

Here's a link to one interesting OTL Headphone amp design article, it looks really simple and easy to build, just notice how simple the circuit is compared to a solid-state design of similar performance capability:

https://headwizememorial.wordpress.com/2018/03/20/a-single-ended-otl-amplifier-for-dynamic-headphones/

[DrNomis_44]

Update:


I think I have finally found the OTL Headphone amp circuit I want to build, this particular design uses a set of three 6N1P twin triodes and it looks like it can drive load impedances down to about 64 Ohms, some of the other designs I was considering building aren't happy with driving low impedance loads as low as that, here's a link to the webpage:

https://headwizememorial.wordpress.com/2018/03/20/the-6n1p-otl-headphone-amplifier/


This one looks like a relatively easy one to build and I think Evatco stock both the 6N1P tube and the Hammond 269AX power transformer.

Turns out I was right, Evatco do stock the 6N1P tube:

https://www.evatco.com.au/audio-tubes/preamp-tubes/6n1p-tubes/

Evatco don't seem to have the Hammond 269AX power transformer in stock, but, Mouser Electronics do seem to stock it, so I'll have to order it online from there.

https://au.mouser.com/ProductDetail/Hammond-Manufacturing/269AX?qs=%252BHhoWzUJg4LsIANOOQQymA%3D%3D


I just found the perfect application/use for my diy scratch-build 6N1P OTL Headphone amp build, currently I'm using a pair of wireless headphones with my new Blaupunkt HDTV, while the wireless headphones works with it, it has a nasty habit of the rechargeable battery going flat at annoying times, I can't use a pair of wired headphones with the Blaupunkt because there's no headphone out socket, but, it does have stereo RCA Audio Line-Outs on the back, all I would need to do is connect up my 6N1P OTL Headphone amp to the stereo RCA line outs and plug in a pair of headphones.

[DrNomis_44]

One of my makerspace students is building one of these

http://www.parasitstudio.se/stripboard-layouts/chime-amp-punch-amp-v2


That looks like a really cool first diy project for all those wanting to get into amp-building, cheers for posting that.

[DrNomis_44]

Update:

Been making some steady progress on my OTL Headphone amp build, I've elected to go with this design:

https://headwizememorial.wordpress.com/2018/03/20/the-6n1p-otl-headphone-amplifier/?fbclid=IwAR2H8kGuMjUFIxAgwGhk8UmNvTHNm7IvaZsHiIXs KK_1kaIUo0sN21SxpNE

I looked on the Evatco website and as it turns out, they do stock 6N1P Twin Triode Valves, they may also stock the Hammond 269AX Power Transformer as well.

I'm going to be mounting all the components, minus the main power supply filter caps, on tagboards, so, I'm now in the process of designing suitable layouts for the tagboards, I'll need to make three of them, one for the power supply, and two others for the left and right channels, this is going to be a fun build.

I've made a hand-drawn copy of the full schematic and will post a scan of it later on......stay tuned!!

[DrNomis_44]

Update:


As promised, here are scans of the full schematics for my 6N1P OTL Headphone Amplifier build I'm currently working on:

44587

Full schematic of the power supply:

44588

[DrNomis_44]

Update:


I just ordered three new Svetlana 6N1P tubes online from Evatco, for my 6N1P OTL Headphone amp build, Evatco don't stock the Hammond 269AX Power Transformer I need, but fortunately Mouser Electronics do have it in stock, so I'll order it online from Mouser Electronics, they may also stock all the resistors and caps that I'll need too, so I'm going to do some browsing on their website and see what I can find.

Jaycar stock this big diecast aluminium box which I will be using as the chassis for my OTL Headphone build, it's a little bigger in size than the one used in the original build article:

44621


Here's a link to the original build article:

https://headwizememorial.wordpress.com/2018/03/20/the-6n1p-otl-headphone-amplifier/


Oops....I just found out that the Hammond 269AX power transformer only has a 115/125V AC primary winding, so I'm going to have to go looking for a similar specced transformer with a 240V AC primary.

Evatco do have a Hammond 369AX power transformer in stock that looks like it'll do the job, it has a 250V CT @ 115mA secondary winding, a 6.3V CT @ 2A Heater winding and a 240V AC primary, so I'll order one of them instead.

[Simon Barden]

I’m wondering why you don’t just get a 240v to 6.3V transformer (if such a thing exists) and just use straight 240V instead of 250V for the secondary side. More efficient and you only drop the secondary voltage by 10V/4%.

[Simon Barden]

You could even use a simple 6.3V DC power supply for the valve heaters for reduced hum rather than the transformer, rectifier and capacitor combo. Plenty of adjustable output voltage DC power supplies available.

[Simon Barden]

I think I just realised that you probably need the transformer to allow the ‘neutral’ to be connected to ground without upsetting the mains it’s connected to. But you could probably save money by getting a 1:1 40VA isolation transformer and a DC supply than an all in one transformer.

[DrNomis_44]

I think I just realised that you probably need the transformer to allow the ‘neutral’ to be connected to ground without upsetting the mains it’s connected to. But you could probably save money by getting a 1:1 40VA isolation transformer and a DC supply than an all in one transformer.

I guess I could have gone with something like that, but I decided to stick to the power supply design as per the schematic, just to keep things simple, besides the real estate on the chassis is a bit limited, so there's room for only one small power transformer, I'm also using a 3 meter three-wire mains lead with the amp, Brown (Active), Blue (Neutral), and Green/Yellow (Earth), it's a bit too dangerous to derive the +348V HT directly from our Australian 240V AC mains so a proper power transformer is mandatory, since I'm going to be attaching the green/yellow Earth wire directly to the chassis for safety's sake, I'm also going to be including a Fuse holder in the Active wiring in accordance with current Australian mains power wiring regulations, the three Svetlana 6N1P Tubes I ordered from Evatco yesterday each require 6.3V at 600mA for the heater filaments which works out to about 1.8A in total so the 6.3V heater winding on the transformer needs to be able to supply at least 2A, I think going with a standard power transformer is going to be the best way, I'm not really too worried about spending a bit of money on parts for this build since it's a one-off build, so I don't mind paying a bit extra.

My new Blaupunkt 43-inch HDTV does not have a built-in headphone socket, but, it does have an Optical (Fiber Optic) Digital Audio out socket that I can plug a Fiber Optic cable into, this means I need some way to convert the Digital Audio into Stereo Analog Audio outputs for the headphone amp, so I bought this nifty little device from my local Jaycar Electronics store:

44623

The device itself is very small in size and is powered off a 5V DC plugpack, it has an Optical in socket, an RCA Coaxial in socket, and two stereo RCA Analog Audio out sockets, which is perfect for what I need.


The other reason for using a standard mains power transformer is that a lot of the modern HDTVs and UHDTVs use what's called a Switchmode Power Supply where the mains power is connected directly to the main Switchmode Power Supply circuitry, a standard mains power transformer will help to isolate the headphone amp electrically, of course the Optical cable will also provide electrical isolation too.

[Jetblack]

I got one these for my for my audio technica turn table, so to use it with a digital surround amplifier. The turntable did have the option for digital to analogue switching but in digital mode was muffling the sound quality. Now using a preamp I wired into the turn table and run it out to a phono box, works very nicely. ;)

[DrNomis_44]

Update:

Just got these in the post literally a few minutes ago, they are a set of three brand-new, never used Svetlana 6N1-P Dual Triode Tubes, I ordered these online from Evatco on Tuesday of last week.

44658

[DrNomis_44]

Thought I would post this link to the Headwize Memorial website where I found the build article for my 6N1-P OTL Headphone amp, there are some solid-state designs on the website as well as Tube and Hybrid designs that look interesting: https://headwizememorial.wordpress.com/library/diy-projects/

[DrNomis_44]

Going to be needing to order two of these online from Soundlabs Group in order to build the power supply for my OTL Headphone Build:

https://www.soundlabsgroup.com.au/p/MU-MLYTIC-50-50-500V-Dual/50%2B50uF+500VDC+MLytic+HV+Dual+High+Voltage+Capac itor

Need to order two 50uF/500V Dual Can Caps.

[DrNomis_44]

Update:


Hopefully this week I'll be able to order some more parts from Mouser Electronics for the OTL Headphone build project, need to order some resistors and caps, some of the resistors are non-standard values like 750 Ohms and 43K, the 43K resistors need to be 2W types, but I might just buy 2W types for all the resistors, since I like to over-rate the components for the job for reliability's sake.

[Simon Barden]

Then surely you should get 5W resistors for the ones currently listed as 2W?

It’s the higher rated resistors that are going to get the warmest, and from watching amp repair videos, it’s often theses ones that end up underrated and on the edge to cut costs and often overheat.

The logic really follows that all resistors should be at least one rating step higher than currently specified.

[DrNomis_44]

Then surely you should get 5W resistors for the ones currently listed as 2W?

It’s the higher rated resistors that are going to get the warmest, and from watching amp repair videos, it’s often theses ones that end up underrated and on the edge to cut costs and often overheat.

The logic really follows that all resistors should be at least one rating step higher than currently specified.


The original 6N1-P OTL Headphone build article I'm working with specified 2W resistors for the two 43K resistors and the rest of the resistors as 1/2W ones, there are three 1K/5W resistors (article specifies 1K/2W) but they are located in the power supply circuit for the +348V HT, I was originally planning on using some 1W metal film resistors for all the 1/2W in the amp circuit, but I didn't have the values I needed in my stash of 1W metal film resistors, in my book it's always better to over-rate a resistor for the job it has to do than to under-rate it, by that I mean use say a 1W resistor if a 1/4W resistor is specified, or a 5W resistor if a 2W or 3W resistor is specified, I think that a resistor should only get slightly warm to the touch when a circuit is operating normally, the resistor should not get scorching-hot which indicates too much current is flowing through it.


I will make a minor mod to the +348V HT supply circuit, which involves adding a 470k/1W bleeder resistor on the output of the HT supply to ground to stop the supply filter caps from over-voltaging when no valves are installed.

According to Ohm's Law the current flowing in the 470k/1W resistor can be calculated using this formula:

I (current)= E (Volts)/R (resistance)

Or

I= 348V/470,000

Which gives us about 740uA, assuming my calculation is correct, so a 470k resistor will be a light load for the power supply.

Or, I could use a 220k bleeder resistor, which means about 1.6mA will be flowing through it.

And, if we use Ohms Law again to calculate how much power the 220k resistor would be dissipating:

P (power)=I (current) X E (volts)

P= .0016 X 348 = .5568W

So a 220k/1W will do the job fine as a bleeder resistor for the +348V HT supply in my OTL Headphone amp, the two amplifier circuits for the left and right headphone outputs will only be drawing about 20mA or so from the HT supply, an extra 1.6mA won't really matter much since the power transformer I'm going to be using has an HT secondary that can supply up to 115mA.


Hopefully my calculations are all correct, but if anyone wants to check that, please feel free to do so.

One other mod I could do is something that was done in the Sloclone schematic, and that's to raise the center-tap of the 6.3V/2A heater supply above ground using a resistive divider from the +348V HT to ground, using maybe a 1M resistor and a 100k resistor, then I'd be able to eliminate the use of four 1N5404 400V/3A power diodes.

The 6N1-P OTL Headphone amp build article specifies using four 1N4008 power diodes for the +HT supply, a 1N4008 power diode has a PIV (Peak Inverse Voltage) voltage-rating of 1,200V @ 1A according to the datasheet, I have some 1N4007 power diodes which I could use as a substitute because they have a PIV rating of 1,000V @ 1A, and the +HT supply is only 348V DC at the most.

[DrNomis_44]

Update:

I'm currently in the process of ordering some resistors and caps from Mouser Electronics for my OTL Headphone amp build, and so far Mouser Electronics seem to stock all the resistor values I need including the 750 Ohm and 43k resistors, I've elected to go with all 2W Metal Oxide types for the resistors and Axial types for the caps, I still need to order a Hammond 369AX power transformer from Evatco and two 50uF/500V Dual Can Caps from Soundlabs Group.

Mouser Electronics won't sell me two 43k/ 5%/ 2W Metal Oxide resistors, they will only sell them in lots of 2500, unit price per resistor is Au$0.055, which is just over 5 cents per resistor, so, after a bit of mmmmm-ing and ahhhhhhhh-ing I decided to go ahead and order a bulk lot of 2500 43k/ 5%/ 2W Metal Oxide resistors, which works out to Au$137.50 all up for them, I only need two so, I don't know what I'll do with the 2498 leftover 43k resistors, I'll most likely need them one rainy day.

Arghhhh.....turns-out the 43k/ 5%/ 2W Metal Oxide resistors are going to have to be despatched in another 20 days cause they're on back-order now.

[Simon Barden]

You could try selling the 43k resistors in packets of say 10 & 50 with a hefty mark-up on eBay. Though that might be a good place to look for some in case someone has done the same thing.

[DrNomis_44]

You could try selling the 43k resistors in packets of say 10 & 50 with a hefty mark-up on eBay. Though that might be a good place to look for some in case someone has done the same thing.

Yeah, I guess I could, it'd be a good way to recoup my money.

[Simon Barden]

Have you tried looking elsewhere for smaller quantities?

[DrNomis_44]

Have you tried looking elsewhere for smaller quantities?

Not really, because 43k isn't one of the common resistor values.

[Simon Barden]

Could you make one up from a couple of standard ones in series or in parallel?

[DrNomis_44]

Could you make one up from a couple of standard ones in series or in parallel?

Two 82k resistors in parallel would give about 41k, there are three resistors in the 6N1-P OTL Headphone circuit that are critical to it's operation, the 43k anode resistor for the triode gain stage, the 200 Ohm resistor, and the 220 Ohm resistor in the push-pull output stage, these three resistors set the balancing of the push-pull output stage for minimum distortion, and the ability to drive impedances down to 32 Ohms, if the balance is out the output gets badly distorted when driving headphones as low as 32 Ohms.


The resistor values used in the 6N1-P OTL Headphone amp circuit optimise it so that 6N1-P Valves operate properly, but the HT supply needs to be at least +348V DC for them to perform at their best.

And since the push-pull output stage is directly-coupled to the 43k resistor, the 43k resistor effectively sets the bias point of the push-pull output stage.

https://headwizememorial.wordpress.com/2018/03/20/the-6n1p-otl-headphone-amplifier/


Remember, this is a piece of Valve HiFi gear I'm making, although there's no reason why it can't be used for guitar purposes, I reckon it'd make a great line-amp for recording purposes if you want to record an acoustic/electric guitar with piezo pickups into a Daw, you could also use it with an FX processor pedal, like the Digitech RP 50, then you'd have a good practice setup so you could practice your guitar playing and not annoy the neighbours.

[Simon Barden]

If you look at the parts list, that 43k resistor is actually given as a 42k resistor.

So either the 43k on the diagram is a typo, or the builder chose 42k over 43k as being more readily available. The "261-42K" Mouser part number no longer exists, so i have no idea what tolerance resistor they'd opted for.

Note that the OrCad diagram and the hand-drawn sketch also shows a 42k 2W resistor.

So I feel that 42k is the correct value. But you can obviously build it using 42k, measure the bias current and look at the output waveforms and decide whether you need 43k. In which case you could simply wire a 1k resistor in series with the 42k resistor.

Or if you've got the OrCad programs mentioned (they will have moved on since the 20+ years since that web site was created) you could try loading the simulation files and try with a 42k vs 43k resistor.

[DrNomis_44]

If you look at the parts list, that 43k resistor is actually given as a 42k resistor.

So either the 43k on the diagram is a typo, or the builder chose 42k over 43k as being more readily available. The "261-42K" Mouser part number no longer exists, so i have no idea what tolerance resistor they'd opted for.

Note that the OrCad diagram and the hand-drawn sketch also shows a 42k 2W resistor.

So I feel that 42k is the correct value. But you can obviously build it using 42k, measure the bias current and look at the output waveforms and decide whether you need 43k. In which case you could simply wire a 1k resistor in series with the 42k resistor.

Or if you've got the OrCad programs mentioned (they will have moved on since the 20+ years since that web site was created) you could try loading the simulation files and try with a 42k vs 43k resistor.


I have been thinking about getting myself a copy of the Orcad software, I did play around with another bit of circuit simulator software called Circuitmaker 2000 if my memory serves me right, it did occasionally produce some funny results though, I wonder if you can still get Orcad.

[Simon Barden]

Yes, I found their web site.

https://www.orcad.com/

But it now looks very professional, so could be expensive.

But their are free trial versions, which could be enough to see if it will load the 21-year-old configuration file. The format may well no longer be supported,

Otherwise you’d have to recreate the design from scratch.

[DrNomis_44]

If you look at the parts list, that 43k resistor is actually given as a 42k resistor.

So either the 43k on the diagram is a typo, or the builder chose 42k over 43k as being more readily available. The "261-42K" Mouser part number no longer exists, so i have no idea what tolerance resistor they'd opted for.

Note that the OrCad diagram and the hand-drawn sketch also shows a 42k 2W resistor.

So I feel that 42k is the correct value. But you can obviously build it using 42k, measure the bias current and look at the output waveforms and decide whether you need 43k. In which case you could simply wire a 1k resistor in series with the 42k resistor.

Or if you've got the OrCad programs mentioned (they will have moved on since the 20+ years since that web site was created) you could try loading the simulation files and try with a 42k vs 43k resistor.


I just remembered that I've got a whole heap of 1W/1% Metal Film resistors, and I may be able to find four 82k resistors amongst them, so, two of them in parallel will give me a 41k/1% 2W Metal Film resistor.

As luck would have it, I can only find three 82k/1W/1% Metal Film resistors amongst them, and four are needed.

[DrNomis_44]

Update:

Just got the last lot of caps and resistors I ordered from Mouser Electronics, delivered to my front door a few minutes ago, just have to wait till they send the 43k/2W resistors I ordered since they were on back-order, all that's left to do now is to order the new Hammond 369AX power transformer that I need from Evatco and then I'll be able to start work on building the new 6N1-P OTL Headphone amp.

[Simon Barden]

As I said, the 43k is a typo and the design is for a 42k resistor. It probably won’t have a huge effect on the bias, just your wallet.

[DrNomis_44]

As I said, the 43k is a typo and the design is for a 42k resistor. It probably won’t have a huge effect on the bias, just your wallet.


I had actually ordered 43k/5%/2W resistors, so since they are 5% types they will measure anywhere within +/- 5% of 43k, on the low side they could actually measure:

43,000-5%= 40,850 or 40.85k

And on the high side:

43,000+5%= 45,150 or 45.15k

But note that in each case, that's for a worst-case scenario.

Hopefully my maths is correct, but I'm happy to be proven wrong.

[DrNomis_44]

I will also need to order two dual 50uF/500V can caps from Soundlabs Group, along with the new power transformer.

[DrNomis_44]

Just out of curiosity, I did some calculations to see what values of resistances I would get if I put two garden-variety 82k/5%/1W carbon film resistors in parallel, here's what I would get in the worst-case scenarios.....


82,000-5%= 77,900 or 77.9k

Two 77.9k resistors in parallel would give us:

77,900/2= 38,950 or 38.95k

On the low side.

82,000+5%= 86,100 or 86.1k

two 86.1k resistors in parallel would give us:

86,100/2= 43,050 or 43.05k

On the high side.

[Simon Barden]

You could afford to get a bunch if the common 82k resistors and test them with a meter and pick the best ones to give you close to 42k. Or 41k or 43k. Whichever value gives the best results for biasing.

[Simon Barden]

Another option would be a 39k 2W resistor in series with a 2W wirewound 10k multi-turn resistor (about $7 each here). You can then easily trim the bias resistance for optimal results and you don’t need to worry about tolerances.

[DrNomis_44]

Another option would be a 39k 2W resistor in series with a 2W wirewound 10k multi-turn resistor (about $7 each here). You can then easily trim the bias resistance for optimal results and you don’t need to worry about tolerances.

Yeah, that would be a good alternative, I'd need to do some Ohms Law calculations to see exactly how much power the 10k multi-turn adjustable resistor would be dissipating, I wonder how much a standard blue-coloured vertical-mount ten-turn 10k trimpot can dissipate before it burns-up? real-estate in the chassis is a bit limited, I've had to play-around with the layout a bit to see if I could get everything to fit in it, I'm using some 8-way tagstrips to mount most of the components, and I'll be using some stranded wire to wire from the tagstrips to the valve sockets.

I might do some Ohms Law calculations to find out exactly how much power the anode resistor of the first triode gain stage is dissipating, I'd have to work out the voltage-drop across it, and the maximum current flow through it though, if I take the anode voltage of the first triode gain stage, 172V, away from the + HT supply voltage, 348V, we get the following:

348-172= 176V

So, that 41k/2W anode resistor should have a voltage drop of 176V DC across it by my calculations.

So, if we apply Ohms Law to work out the current flow in that 41k/2W resistor we get the following:

I=E/R

I= 176V/41,000

= 0.0042926829268293

0.0042926829268293 X 1000= 4.292682926829268

Or about 4.29 milliamps if my calculations are correct.

If we do the power dissipation calculation according to Ohm's Law, we get this:

P=I X V

P= 4.292682926829268 X 176V

= 755.5121951219512

755.5121951219512/1000

= 0.7555121951219512 milliwatts.

So therefore that 41k/2W anode resistor isn't even dissipating the full power it is capable of dissipating.

[DrNomis_44]

Another option would be a 39k 2W resistor in series with a 2W wirewound 10k multi-turn resistor (about $7 each here). You can then easily trim the bias resistance for optimal results and you don’t need to worry about tolerances.


Since the needed 42k/2W resistors are going to be hard to order online in small quantities, I'm going to buy four 82k/1W carbon-film resistors from my local Jaycar Electronics store next week, and give them a try in my 6N1P OTL headphone amp build and see what results I get, I can always desolder them and swap them out with two 42k/2W metal film resistors at a later date, I'm going to compile a list of the parts I do have so I can work out what parts I need to order online this year, once I've got the 6N1P OTL amp build up and running and working properly, I'm going to do some tests on it using my diy XR2206 Function Generator, my DSE Q1804 20MHz Dual Trace Scope, and two 33 Ohm/1W carbon film resistors which will be used to, roughly, simulate a pair of 32 Ohm headphones plugged into the amp, so I can view the output waveforms the amp produces when driving 32 Ohm headphones, I'll post some screenshots while I'm at it.

While I'm doing the tests on my OTL headphone amp build, I'll see if I can set the output level of the diy XR2206 Function Generator to the output level of the Digitech D/A converter I'll be using to convert the optical-out of my 43 inch Blaupunkt TV to stereo analog line-outs, mainly so I can evaluate how the amp is going to perform, if necessary, I'll be able to make adjustments to the resistor values to maximise the performance, that's going to be the most fun part of the build-project, seeing if I can get the best performance out of it, its well-known that valves of a particular type tend to vary quite a bit due to manufacturing tolerances, so because of that there's no way to tell how well my three Svetlana 6N1P valves will perform in practice compared to the 6N1P valves used in the original design, plus the designer used some circuit simulator software in their design process, and the valve simulations in the software a usually idealized for consistency, it will be very interesting to see how well my build performs compared to the original design.

[JohnH]

Good to see you're still at it doc!

[DrNomis_44]

Good to see you're still at it doc!

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.

[DrNomis_44]

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.