Headphone socket circuit - a mod for small valve amps

For Telenut62
Here is my headphone socket circuit for a "10 watt" WEM Dominator 25 Mk.II 2xECL82 into a 15ohm speaker.

Note 1 - this will only work for a relatively low power amp and the component values must be recalculated according to your normal speaker load and your headphone impedance (parallel both cans, mine AKG K240 are 55ohm each).

Note 2 - the power soak resistor will get very hot even if of suitable rating, so use long leads otherwise the solder connections may melt (it did that).

Top diagram = original (theoretical circuit) with 20ohm which blew the solder off.

Middle diagram = (practical circuit) using a pair of 51ohm 5W in parallel (20.5ohm) also note that 15ohm 5W, this is a series current limiter for the cans.
NB - very complicated stereo jack switching socket. I think one makes and two break, it was cobbled together by butchering from a "break" jack and a "make" jack. Sorry for poor drawing, I'm going to have to let you figure this one, I'm having a senior moment remembering what it meant when I drew it in 2009 (it's very late here 00:06h). I think the arrow points are supposed to be touching the line below them and the plug inserts through the rectangle to the left of the mess.

Bottom diagram = final (theoretical) showing load ~13ohm, which is near-enough (do bear in mind that an 8ohm impedance speaker has a DC resistance of ~6ohm).

You may need to adjust values to get suitable headphone volume at the sweet spot of the amp. You do not want to have the amp cranked up with cans on!
Because (1) you will go stone deaf and (2) you may damage the cans or the amp (if it goes open circuit).

I have used it for many years without harm to the amp even though it fried the solder off the 20ohm 5W (the then 10ohm provided a safety net load)

Caveat
It is not intended that anyone use this circuit as-is but to adapt it for own amp use.

Sorry too cryptic for my little noggin :lol:

Telenut62, Sorry you couldn't use the diagram. Let me try again.

*** I need the following information ***

What amp is it?
What maximum power is your amp?
And how high do you crank it when you play it / where is its sweet spot?

What impedance are -
a) your speaker?
b) your headphones?
***


BTW ... it's the bottom diagram.

The speaker is disconnected by the jack. It is an attenuator. But my values are for 10W max running at about 6W into nominal "15ohm" load using "55ohm" cans (which don't see the all that power).

Telenut62, Sorry you couldn't use the diagram. Let me try again.

*** I need the following information ***

What amp is it?
What maximum power is your amp?
And how high do you crank it when you play it / where is its sweet spot?

What impedance are -
a) your speaker?
b) your headphones?
***


BTW ... it's the bottom diagram.

The speaker is disconnected by the jack. It is an attenuator. But my values are for 10W max running at about 6W into nominal "15ohm" load using "55ohm" cans (which don't see the all that power).


Amp is 5F1
Max power??? 5watts but at RMS could be double?
Speaker is a WGS G10C 75watts
Headphones...I don't have any, would be a cool option for the buyer of the amp to use in his/her headphones. So can't answer that.

I like it about 75% cranked :wink:

You want a headphone socket but don't have headphones?

How on earth can I work out what resistors to use if I don't know their impedance?

If memory serves a 5F1 is a single 6V6 so about 4 watts tops, watts is watts (never "watts RMS")

And I do need the speaker impedance for the same reason as the headphones?

From that I can work out the Vrms from the output (yes it is RMS this time) and so decide how to divide it between ballast resistor and headphones with a voltage divider attenuator.

One has to trick the amp into believing it has the same load as the speaker when on headphones whilst limiting the current and hence the power to the cans so they don't blow up.

Domestic/consumer headphones can take about 300mW but studio cans can run up to around 3W (they're not supposed to, but will take it). Common impedance 55ohm or 75ohm each ear, and it makes a lot of difference to the circuit which one it is.

How loud you run it tells me where to limit the current i.e. a 3W 75% feed needs to be dropped to around 250mW (somewhere between earwax and permanent hearing impairment on 11).

I build these amps to sell, and a 5F1 is prob the best bedroom amp to have. So installing a headphone jack would be a cool selling point, but it may lead to more trouble than it's worth. I usually use 8ohm speakers

sqrt(4x8) = 5.7Vrms

So if you substitute a pair of 10ohm resistors in the bottom diagram it will give an effective load of 7.89ohm using 55ohm stereo headphones in parallel.
The split is 10:37.5, so only ~1/4 of the output goes through the headphones.
A 10ohm ballast and a 7.5ohm series gives around 7.78ohm and 10:35
and 10ohm and 5ohm give 7.6ohm and 10:32.5
and 10ohm and no series R gives 7.3ohm and 10:27.5
Ideal would be around 7ohm

If you are building amps to sell, you should be able to see how that works.
And work out a switching headphone jack socket.

If you use 75ohm cans or need more or less volume you need to re-jig the calculation.
Some of it is trial and error to get the sound level right.

But you must present the amp with the right load.

Suggest 10W resistors on long legs too because they can get hot enough to melt their solder, allow for heat.