When some simple power IC project is finished people often need to connect headphones to it. There is more than enough power and in that way one does not need to build separate headphones amp, saving hardware and work. I compared several schematics used by Japanese companies and noticed that usually headphones output is connected in parallel with speakers output using just one series resistor in the range from 220R to 680R rated 0.5W or 1W. This is simple and indeed works quite well in practice. There is one downside that phones are driven from high impedance and that creates some frequency irregularities. Is there better way to connect headphones in parallel with speaker outputs. Would it be better to use two resistor voltage divider which will have impedance matched to the impedance of headphones? Any ideas?
Frequency and impedance are linked. So simply shoving a resistor works but isn't perhaps the best long term option. A small 8ohm to 32/300ohm transformer may be a better option?
Indeed works only for protecting headphones, but in most cases works terrible. Most of speakers, and headphones aren't exclusion, have mass-stiffness resonanse, which usually must be damped by lowish output resistance of the connected amp.
This is naturally broken in case of resistance inseries with amp's output.
There are one possible way to properly connect any kind of speaker (including headphones) to the amp's output - straightly.
There are only one exception - speakers specifically designed to be undamped.
The best way to proceed is indeed to insert a resistor divider rather than just a resistor in serie. The divider will provide a low impedance output, we're not aiming for any impedance matching.
See the attached, as suggested by Grado.
As most headphones have a common ground, you must check if your amp allows this (iow, if it hasn't bridged outputs).
See the attached, as suggested by Grado.
As most headphones have a common ground, you must check if your amp allows this (iow, if it hasn't bridged outputs).
Attachments
That Grado adapter (20 Ohm and 2 Ohm) is also how Rod Elliot suggests, but he gives a table of options for different amplifier power, and also an option to let the headphone see 120 Ohms rather than 2 Ohms.
I made one with 25 Ohm + 2.5 Ohm, and that 25 Ohm load gets really hot, so be conservative with the power rating there. Check for very low DC before connecting.
Project 100
I made one with 25 Ohm + 2.5 Ohm, and that 25 Ohm load gets really hot, so be conservative with the power rating there. Check for very low DC before connecting.
Project 100
The voltage divider solution mentioned above is explored in some detail here:
Power amp adapter | DIY-Audio-Heaven
You can skip most of the technical information and go directly to the bottom of the page. There you will find the circuit and the values of resistors you'll need.
Power amp adapter | DIY-Audio-Heaven
You can skip most of the technical information and go directly to the bottom of the page. There you will find the circuit and the values of resistors you'll need.
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I'm considering using transformers on the output of a class d amp, they would solve the problem of a common earth and presumably negate the use of the chokes/capacitors on the output.
The Rod Elliot's article has to be nuanced.
Many high impedance headphones have big variations in impedance vs frequency and really benefit from a low impedance output (senn hd600, hd650 being good examples). But they will need a bit more than 2vrms and the second table should be recalculated for that.
Otoh, Grado for example have a fairly flat impedance and a 120r output impedance (or even more) will not be a big problem. So you could use the first table (less wasteful), but again recalculated, to give 2vrms rather than 5Vrms, which is too much.
Many high impedance headphones have big variations in impedance vs frequency and really benefit from a low impedance output (senn hd600, hd650 being good examples). But they will need a bit more than 2vrms and the second table should be recalculated for that.
Otoh, Grado for example have a fairly flat impedance and a 120r output impedance (or even more) will not be a big problem. So you could use the first table (less wasteful), but again recalculated, to give 2vrms rather than 5Vrms, which is too much.
I did now some experiments, my result so far, it is a no-go for me.
I have an excellent single ended class A, transformer output power amplifier, which is a cheap one comparing to my 2A3 PSE properly buit, but comes quite close. On speaker (high sensitivity) I hear no hum at all.
First problem with headphone, there is hum. Maybe I could reduce that if I would want to, but, second problem is (and now with different sources), it does not sound good. Not bad, but the "monster" OTL headphone amp is just much better. I can not say anything wrong, but, just does not keep me listening.
Maybe I could try to divide, but, I think I just stop here. For me, the termination has to be 8 Ohms since it is a transformer output class A designed for 8 Ohms at the secondaries.
Now, I search for something smaller, lighter than these 10+ kg monsters.
Maybe I try this with a tube rectifier : The 6N1P OTL Headphone Amplifier. – HeadWize Memorial
Regards,
JG
I have an excellent single ended class A, transformer output power amplifier, which is a cheap one comparing to my 2A3 PSE properly buit, but comes quite close. On speaker (high sensitivity) I hear no hum at all.
First problem with headphone, there is hum. Maybe I could reduce that if I would want to, but, second problem is (and now with different sources), it does not sound good. Not bad, but the "monster" OTL headphone amp is just much better. I can not say anything wrong, but, just does not keep me listening.
Maybe I could try to divide, but, I think I just stop here. For me, the termination has to be 8 Ohms since it is a transformer output class A designed for 8 Ohms at the secondaries.
Now, I search for something smaller, lighter than these 10+ kg monsters.
Maybe I try this with a tube rectifier : The 6N1P OTL Headphone Amplifier. – HeadWize Memorial
Regards,
JG