OTL Headphone Amp project.

[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:



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.

[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.c.../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/...tage+Capacitor

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.