Power amp adapter

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Can I use the speaker output terminals of a power amp to drive headphones ?

Yes and No….

Yes, because high impedance (> 300Ω)  headphones can be connected directly to power amplifiers without damage
Yes, because some ortho-dynamic headphones need a lot of ‘power’ which can be delivered easily with power amplifiers.
Yes, when similar measures are taken as those found in most power amplifiers with a headphone socket.
No, because most low impedance headphones run a a high risk of being burned to a crisp when left unattended and the volume is accidentally turned up.
No, because of noise which could be higher than dedicated headphone amplifiers and could become audible.
No, When the amplifier used is of a so called ‘bridged’ design, such as a lot of class-D and car-audio amplifiers, unless 4 wire headphones are used.

So.. yes and no depending on the circumstances.

Below there is a lot of technical bla bla about powers and resistor values etc.
As most readers just want some info on a practical attenuation box for general purpose headphones you can skip most of what’s written below and go to the bottom of this page.There you will find how to connect a few resistors and what values you would need.

Amplifiers that drive headphones need to be able to deliver enough voltage to drive the headphones but not too much to fry them. Power amplifiers can deliver enough voltage. Since the impedances of headphones are higher than those of speakers headphones need less current so all power amplifiers have absolutely no problems to provide the needed current. Speakers in general have impedances between 4 and 8Ω, headphones between 32 and 300Ω.

Most speaker amplifiers can deliver between 15 and 300W depending on type and model. The numbers are generally given for impedances of 4Ω or 8Ω (sometimes even 2Ω or 16Ω) but these power levels will not be reached in headphones as the drawn power is determined by the output VOLTAGE of the amplifier and the load (IMPEDANCE), the higher the impedance, the lower the current at a given voltage.

NOTE: The section below is to show how headphones can be connected SAFELY to certain power amps (on the speaker connectors) so they won’t blow up.
If you want to know how much power your headphones can handle (NEVER connect earbuds and IEM’s to an amplifiers output !) you may find this INFO in this ARTICLE
ALSO you should realise you NEVER have to reach SPL levels of over 120dB peak ! so when a headphone is rated 200mW it is most likely you will never drive it above 20mW in practice.
This also means that the values in the tables below MAY leave you little travel in your volpot. Sounding loud already when the volpot is barely turned away from its minimum setting. If that is the case (depends on the used headphone) simply calculate resistor values for max headphone power that is at least 10x lower than the max power rating.
SO… the power tables below are thus made to ensure you don’t blow up your headphone and NOT to reach loud levels at the end of the volpot range. You may have reached those levels already when the volpot has barely left the minimal setting.

Not everyone likes calculus so to make it more graphic by creating a few tables.

On the left column the specified power of an amplifier into 4Ω, one column to the right of it specified into 8Ω, next to it the output VOLTAGE that belongs to those powers and is the important thing here as the maximum voltage a headphone can handle depends on its impedance and power rating. It should be noted that most power amplifiers can deliver a HIGHER voltage in reality than the one given here because of the substantial load (4 – 8Ω speakers) the speakers provide is absent and thus the often unregulated internal power supply voltages do not drop allowing for a somewhat higher voltage swing.

On the next columns you can see which power can be achieved when the amp is turned up fully without reaching clipping levels. This is given for the most common headphone impedances. Green cells show the headphone with that specific impedance can be connected directly to the speaker terminals and will give no risk of drivers being damaged by too much power, the orange cells show the upper range of the power ratings of the most common headphones. the red cells show the headphones with these impedances should not be connected to the output terminals directly as there is a serious risk of drivers being burned/damaged/blown. This table is for headphones with power ratings ranging from 200mW (0.2W) to 500mW (0.5W). Most of the common headphones have these kind of power ratings, the smaller IEM’s may have power ratings of just a few mW so NONE of those should EVER be connected directly. When using these tables make sure you use the power ratings as specified by the manufacturer at the given impedance. A 50W amp specified at 8Ω delivers the same output voltage as a 100W/4Ω amplifier.

HP pow 1

The table above shows it is no problem to connect high impedance (> 300Ω) directly to the output terminals of amplifiers capable of delivering  50W into 4Ω, Headphones with 600Ω can even be driven safely by 65W/4Ω power amps. Low impedance drivers, which are very common these days run a serious risk of being fried. A 32Ω headphone on a 40W/4Ω amplifier can receive 5W, way too much for drivers rated for just 0.2W.

The table below is similar BUT for headphones with power ratings between 1W and 3W.  A lot of D.J.- and professional monitoring headphones can have (and may need) such power ratings as they are sometimes used as little speakers hanging around the neck or lying on a console while playing loud acting as small desktop speakers.

HP pow 2

It is obvious the higher impedance headphones can be connected directly to amplifiers that can provide lots of power. The higher impedance headphones (>300Ω) can be connected to 100W amplifiers. The lower impedance headphones still run a risk when connected directly to amplifiers >20W. It should be noted these headphones would play incredibly loud at these powers and you couldn’t wear them on your ears but can use them as little speakers hanging around the neck.

Most of the newer Planar magnetic (Ortho dynamic) headphones can handle MUCH more power than normal headphones because of the way they are built. Most of them can be connected to power amps directly and will play beautifully on them. Headphones like the HiFiman HE5 and HE6 are best driven directly from power amps even. Below the table for 3W to 6W rated headphones.

HP pow 3

It’s easy to see these headphones are not easily blown up but it should be noted most planar magnetic drivers are somewhere between 32Ω and 60Ω in general. This means these headphones can be connected directly to amplifiers up to 40W but I would not recommend to connect them directly to higher power amplifiers.

There is, however, a cheap and easy ‘fix’ so most headphones can be connected to power amplifiers but NOT directly. They can be connected via a resistor.

single R

The bigger the output power of the amplifier the higher the output voltage and also the higher the added resistance needs to be. Below similar tables as above but this time with 3 different series resistances which are in effect the output resistance of the headphone output now (the amplifiers themselves are below 0.1Ω in general). The table below shows the output powers when connected DIRECTLY (so no added output resistor) to ‘normal’ headphones. Green is ‘safe’ to use, Orange is ‘safe to use when playing music’, Red shows a potential risk of frying these headphones. As you can see even low power amplifiers can potentially damage these headphones as the biggest part of the table is red.

HP pow 1

The table below is for the same types of headphones (200mW) but with a series resistor of 120Ω (3W rating). It is obvious low power amplifiers cannot damage the headphones any more and even a 15W/4Ω amplifier can be used with music signals. The blue cell shows these headphones may NOT play at substantial volume any more (depends on the used headphone).

120

Those resistors not only lower the output power rating but also the SOUND LEVEL.
The headphone will thus play a lot softer and some headphones may sound different as well.
Below a table that shows how much attenuation you can expect with different impedance headphones.
The table is made for 35Ω and 120Ω serie resistors.

reduction

Depending on the impedance variances of the headphone in question the frequency response may alter as well.
This is explained HERE.

When higher power amplifiers are to be used the resistor value can be increased. This is shown in the table below for a 330Ω (5W rated) series resistor.

330

More green areas so the resistor creates a bigger ‘safe to operate’ area and at the same time lowers the noise level of the amplifier (NOT the music signal). Amplifiers of 40W/4Ω can safely drive all ‘normal’ headphones and if we include the ortho dynamic headphones the whole table would be green.

For the sake of illustration below a table for a 680Ω (5W rated) series resistor. slightly more green but also more blue and not that much less red so increasing the resistance about 2 times (coming from 330Ω) didn’t do very much it seems.

680

The observant reader is seeing rather high output resistances. This is actually not something that is liked by a lot of headphones although some headphones could sound substantially more ‘Euphonic’ this way. Other headphones may become boomy and ‘fat’ sounding. To find out why read resistance, impedance and other issues.

Preferably the output resistance should remain below 120Ω although some headphones like the K701 for instance may sound a lot better from an amplifier with 330Ω or even 680Ω output resistance.

Voltage divider

divider

The solution is to use a voltage divider that lowers the voltage. With this simple trick the output voltage of the amplifier is lowered by voltage division, similar to what is described above, BUT because 2 resistors are used we can lower the output resistance ‘seen’ by the headphone. This is because the 2 resistors not only divide the voltage so it is lower but also create a lower output resistance. The circuit is described above. R1 and R2 for both the left and right channel are mentioned in the tables further down on this page.

The reason for that is we can say the output resistance of the power amplifier is (or negligibly close to it compared to the impedances and resistances that are connected). This effectively ‘parallels’ the two resistors that are used. The formula is simple. (1/R1) = (1/R2) = 1/Rout. The load the amplifier ‘sees’ is a bit lower as the R2 is in parallel to the impedance of the headphone. The load the amplifier sees = R1 + 1/ ((1/R2) + (1/Zheadphone)) and the output voltage  of the circuit can thus be calculated as.

Uheadpone = U amplifier x 1/(((1/R2) + (1/Zheadphone)) / R1 + ((1/R2) + (1/Zheadphone))). It is obvious there are many variables including the differences in efficiency the possible outcome of calculations would be too much to fit in a few tables.

A factor 3 in voltage reduction = -10dB will be sufficient to protect and drive most headphones out there.
Then there is the question of noise.
Most power amplifiers are designed to drive rather insensitive speakers and noise is not an issue on speakers, unless you have your ear against the tweeters.

Headphones, however, are way more sensitive than speakers so the noise levels of a quiet speaker amp may become quite audible when not attenuated.
-10dB may not be enough to get rid of the noise or to get a decent adjustment range on the volume control. In this case it is likely the headphones will play quite loud already when the volpot is barely turned up.

The there is the question do we really need deafening levels when the volpot is at maximum setting ? Do we rather not need a usable volpot range ?

To get a better volpot range a reduction of 20dB (10x attenuation) may possibly be better though. Amplifier background noise will also be reduced by 20dB as well which may be needed.

So, I will make life easier and just will give some values that will yield a relatively low output resistance (below 10 Ohm) that will work on most power amplifiers (rated between 30W and 100W into 8 Ohm) and the majority of all ‘normal’ headphones out there.

The load resistance on the amplifier will be ‘light’ (about 35Ω) so the amp doesn’t need to work hard and won’t get hot.

divider

R1 = 33Ω (5W)
For both the 10dB and the 20dB version R1 will be the same.

To have a about factor 3 reduction (-10dB) R2 needs to be 10Ω (3W to 5W).
The output R will be 7Ω which is low enough for most headphones.

To have about a factor 10 reduction (-20dB) R2 needs to be 3.9Ω (3W to 5W)
The output R will be 3.5Ω.

So there you have it.

R1 = 33Ω (5W) and R2 could have values between 10Ω and 3.9Ω (3W to 5W) depending on how much attenuation you want/need.

Thus buy 2x 33Ω, 2x 10Ω and 2x 3.9Ω  for instance or values between them to try.
resistors don’t cost that much.
The box and connectors will define the build costs.

If you want to know how much power your headphones can handle (NEVER connect earbuds and IEM’s to an amplifiers output !) you may find this INFO in this ARTICLE
ALSO you should realise you NEVER have to reach SPL levels of over 120dB peak ! so when In case of doubt or other questions feel free to contact me (Solderdude) in this thread in our FORUM

BALANCED amplifiers (such as car audio or some class-D or class-T amplifiers) can NOT be used with headphones UNLESS the headphone is intended for balanced operation (4 wire/4-pin plug). When unsure do NOT connect the ground (-) connections of these amplifiers together. This may destroy the amplifier.

Balanced

The adapter as shown above will work with most equipment.
Just not with balanced outputs.
Normal amplifiers have a single signal (one for each channel) which is connected to a common return wire (often called ground)
Balanced amplifiers basically have 2 amplifiers which have the same signal on it but in opposite phase. The picture below illustrates the principle. The drawing is from www.tubecad.com which is a great source of info about amplifier designs.

balanced.png

A balanced amplifier has double the output voltage of a normal amplifier.
There is a small ‘but’ here because the maximum output voltage will only double when the output stage is able to deliver double the current as well.
Some devices simply aren’t able to do this and limit the max. output power due to current limiting of the amplifier design.

Most balanced amps (by design) would have to be able to provide double the current as well though. Certainly NO problem when using a power amp to drive headphones.

With a single ended amplifier the output voltage, for instance, would be 1V (just a random number used here for illustrative purposes only) and this would be 31mW in 32Ω.
When the same signal is sent out balanced the ‘other’ amp that is now connected also will have a 1V output signal but in opposite phase.
This means: when one amp is at the ‘top’ of the signal there is +1Vrms and the other active amp at that same moment is -1Vrms.
The voltage difference between the 2 amp outputs (where the headphone is connected to) thus is 2Vrms. It is doubled in voltage. Double the voltage in the same resistance also means double the current.
Power = voltage x current so the power is 2 x 2 = 4 times higher = 128mW in 32Ω.
4x the power (= 2x the voltage) is an increase of 6dB in amplitude.
That is quite audible….

The output signal of the Left and Right channel thus are NOT connected like in normal headphone outputs.
Furthermore balanced outputs must NEVER have any of the Right an Left channels connected. You run a chance of blowing up the amplifier in question.

So to be able to use a headphone on a balanced amplifier you need to have 4 wires (2 pairs of 2 wires) which must be connected to a 4-pin connector or 2 pieces of 3-pin connectors.

There is a catch here which is that unlike ‘normal’ 3-pin headphone plugs (The so-called TRS Jacks) which regardless of size (2.5mm, 3.5mm or 6.3mm) have a pretty standard configuration. Tip = L, Ring = R, Sleeve = ground.

Below the universal schematic for a balanced headphone amp attenuator.

stereo-32 Ohm adapter tube balanced

As can be seen there are no connectors specified. The reason for that is that there is no standard and not everyone uses the same pinning.

Source side is the amplifier side, headphone side is of course the side that must be connected to the headphone.
The red (or +) connector of the amplifier must be connected to Right + (and for the left channel the Left + and the black (or -) connector of the amplifier must be connected to Right (and for the left channel the Left).

One could easily make an attenuator with just 2 resistors which will work just as well.
BUT the 2 resistors are there for 2 good reasons.
A: The balanced signal stays perfectly balanced
B: When accidentally the L and R load are connected (by accidentally using a single ended headphone) the amplifier will NOT blow up and is protected against over-currents.

Connectors

Now we know what the input and output signals are we must find out which connectors are used and what their pinning is. In other words which signal(s) should go on which pin(s).

connectors TRRS etc

Above on the left there is the well known TRRS jack (usually 3.5mm) which is found on many headphones these days. These are NOT used for balanced signals in general but there are manufacturers that (mis)use these connectors for this purpose.
Never connect headphones with a TRRS jacks to balanced amplifiers when these headphones have a microphone and/or small remote in their headphone cord.
The 4th connection in this case is needed for the microphone and/or remote control.
There are 2 ‘standards’ for these plugs CTIA (Apple devices) and OMTP (most other brands) so the remote/mic of headphone A may well work with device B but not with device C for instance.

So be careful with the TRRS jack.
Below a few possible pinnings of gear with a 4 pin TRRS jack connected.

TRRS

As most of these aren’t specifically for headphones below a listing of the 4 main configurations of this TRRS plug.

TRRS audio.png

You need to find out what pinning is the correct one for the headphone AND is present on the amplifier/source.

The most common connector is the TRS jack (also in 2.5mm, 3.5mm and 6.3mm)
TRS.png

These are rarely used for balanced signals and in that case (Pono Player) you need 2 of those plugs, one for R and one for L.

One can never use a single TRS jack for balanced stereo signals but is almost always used for (single ended) stereo signals.

The mono plug only has a sleeve and tip and is often found on the end of a microphone.
It can only handle one signal (left or right for instance). 2 of these can be used for stereo signals (even balanced is possible with 2 of these plugs) but ONLY if the socket is a ‘mono’ socket as well.
NEVER plug a mono plug in a stereo socket. This could lead to damage of the connected source.

Another plug that is sometimes used for headphones is the 3-pin XLR (or mini XLR) or the 4-pin (mini) XLR.

The 4-pin XLR is suitable for balanced stereo signals.
You need 2 of those 3-pin XLR’s for balanced stereo headphones.
Below the most common configuration for 3-pin XLR plugs. Note that the pin numbering drawing of the XLR plugs above is valid when seen from the actual plug side.
When soldering wires onto these plugs you must realize what the pin numbering is on the solder side (mirrored from the plug side !).

XLR-3

The most common usage is balanced. This is standardized and all XLR sockets in (pro) XLRaudio equipment is usually connected that way. Most (balanced) high-end equipment often uses this plug. Sometimes the XLR socket is even combined with a TRS socket (see picture on the right).

Pin 1 is the ground. The pins (male connector) are all equally long but in the socket (female) the 1 socket is slightly longer. This way the ground is connected before the signal wires preventing loud ‘hums’ when plugging and un-plugging.

The 4-pin XLR is not used that much in audio but is gaining in popularity for balanced headphones as you need just one (professional quality) connector.
Below the most common usages for this plug. The HIFIMAN connector is by no means a standard but most manufacturers still use this pinning.

XLR-4

Connecting the attenuator.

Once you have determined what plug(s) you need and how they must be connected you can create an attenuator that has the same properties (in impedances and damping) as the G1217 adapter except for balanced amplifiers headphones ONLY.

It can NOT be used with single ended (3 wire) headphones.
In case one makes a mistake with wiring the headphone or accidentally or connects a 3-wire (TRS jack) headphone you can rest assured the amplifier/source will not be destroyed in the process. The ifi one does not offer such ‘protection’ when used in balanced mode.

Because of the many possible configurations and connectors such an attenuator has to be custom made. Some manufacturers offer converters or conversion cables from one plug to another. Beware that here too you will have to pick the right one for the job.

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Comments
  1. Syd says:

    Hi Frans

    Is the voltage divider update still going to be written? I have tried other sources but they all assume a little more initial knowledge than I possess.

    Thanks, Syd

    • Solderdude says:

      Yes, I still have to complete the article and had forgotten about it.
      What’s missing is voltage divider networks and still have to figure out how to ‘present’ is due to the many variables involved.

      The trick will be to maintain a defined output resistance and figure out how they are affected by the load and how to incorporate different voltage levels (power levels in 4/8 Ohm)
      Had started on it a long time ago but other projects required more attention.

      I do plan to complete the article though…

  2. Syd says:

    That’s great to know Frans. I look in this bit from time to time but finally decided to ask. This whole area is excellent but is it readily ‘discoverable’ to newcomers? I always go to Ian’s ‘Welcome’ post then on from there, or am I missing something?

    Syd

  3. Solderdude says:

    Whenever I update or add something I make a note on the ‘home’ page.
    https://diyaudioheaven.wordpress.com/
    checking that page should tell you what has been added or updated.

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