OTL Headphone Amp project.

[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.