Headphone amp help

[gareth]

Firstly I would like to say hello to everyone for making this a great site that has so far been a really helpfull place. Thankyou.

I currently have quite a few BJT's that I originally intended to build a pair of mono-block amplifiers with. This didn't materialise as I don't feel confident enough to build this yet, I do not have an oscilloscope or frequency generator, yet!

There are :- 21 of 2N5401
20 of 2N5551
2 of TIP31C
2 of TIP47
3 of TIP142
3 of TIP147
5 of BD238
5 of BD237
5 of 2SA1667 (it says A1668 on the device though!)
2 of MJ2955
2 of 2N3055H
2 of MJ15003
2 of MJ15004
...and I think that is it ! phew.

What I was hoping to do was maybe to use the 2N5401/5551 for the headphone amp. In the past I think I read somewhere that BJT's could be parallelled to provide higher output powers.

I will be feeding the headphone amp from the pre-outputs on my amplifier, and I am using two different headphones : the Beyedynamic DT770pro (80ohms) and the AKG K416P (60ohms).

Maybe someone could give me some advice as to where to go from here. I am a qualified electrician and am aware of electrical principles but as I have said previously I am not too confident yet.

I would appeciate any help I can get.

Thanks
Gareth

 

[Gordy]

A headphone amp is a good place to start.

It is unlikely that you will need to parallel devices, as your headphones will only require a few milliWatts to go very loud. You will be looking for a few volts swing and typically less than 100 milliAmps for most headphones.

Try poking around here for a couple of hours...
http://headwize.com/projects/index.htm

and have a look over this...
http://www.rane.com/hc6hp.html

 

[aparatusonitus]

...or you may start with this one, even have tryed and tested pcb layout witch is, I guess, important to you

http://www.diyaudio.com/forums/showthread.php?s=&threadid=114632

 

[gareth]

...or you may start with this one, even have tryed and tested pcb layout witch is, I guess, important to you

http://www.diyaudio.com/forums/showthread.php?s=&threadid=114632

I have looked the Diamante Diy and even though it looks good I wonder if it may be a bit too complicated for my first attempt. Also I would like to use the 2N5401's and 2N5551's and maybe the BD's that I mentioned earlier.

There is a similar design to what I was hoping for on another thread on this forum, it's called Discrete Single End True Class A Headphone Amp (mouthfull) and I am hoping that I will be able to swap the design BJT's for the ones that I have .

Based on what Gordy says and checking the specs of my headphones then I guess that no more than 1watt will be needed.

Do you think this is ok?

Thanks
Gareth

 

[aparatusonitus]

Hi Gareth,

It may look a bit elaborated then you like, but the basic topology is really simple, one voltage gain stage (differential amplifier on input + folded cascode) and one current gain stage (buffer)...One may say that the current mirror on top of input differential is another stage, but that is another story...Anyway, project have files for psb and that is very important, it is very hard to make a mistake if you simply follow schematic and pcb layout. You can do that, I'm sure...

About BJT in that preamp...they are so chip, just don't one drink in your pub, and you can buy a handfull of them, all right?

 

[gareth]

Hi Gareth,

It may look a bit elaborated then you like, but the basic topology is really simple, one voltage gain stage (differential amplifier on input + folded cascode) and one current gain stage (buffer)...One may say that the current mirror on top of input differential is another stage, but that is another story...Anyway, project have files for psb and that is very important, it is very hard to make a mistake if you simply follow schematic and pcb layout. You can do that, I'm sure...

About BJT in that preamp...they are so chip, just don't one drink in your pub, and you can buy a handfull of them, all right?

Very true, these are cheap transistors and as you say following the circuitry is easy enough. I think perhaps I will keep my transistors for something else.

Thanks
Gareth

 

[Gordy]

For 80 Ohm 'phones you can get 1 mW from approximately 4 mA at 280 mV. Allowing some reserve, say 20 mW, then you need about 16 mA at 1.27 V. Low numbers for sure.

As a general rule higher impedance 'phones will require higher voltage and lower current, while low impedance 'phones will require lower voltage and higher current.


(By the way, the Rane site that I mentioned earlier rates the DT770 Pro at 600 Ohms. However I guess that they may have made a lower impedance version).

 

[gareth]

For 80 Ohm 'phones you can get 1 mW from approximately 4 mA at 280 mV. Allowing some reserve, say 20 mW, then you need about 16 mA at 1.27 V. Low numbers for sure.

As a general rule higher impedance 'phones will require higher voltage and lower current, while low impedance 'phones will require lower voltage and higher current.


(By the way, the Rane site that I mentioned earlier rates the DT770 Pro at 600 Ohms. However I guess that they may have made a lower impedance version).

Thanks Gordy. 1.27 V and 16mA sure is low. Do you know if the devices I have can be used at this low an operation? I wonder if I the transisitrors will work properly.

Thanks
Gareth

 

[jaycee]

If you wanted to try something fairly simple, then you could try the "op amp follower" type circuit. I have built a circuit similar to this one with good results.

If you'd like to try discrete, then you can build a scaled down "Lin" topology amplifier. You often see these as pre-amps as well as headphone amps. For a good example, try Bora's "CLEPSIDRA" preamp mentioned here

Bear in mind that the PCB layout won't match the layout of the 2N5401/2N5551's - you could build such an amp on stripboard though if you were careful and cut tracks as short as possible.

 

[gareth]

Cheers jaycee.

Thanks
Gareth

 

[jcx]

re required power for music listening:
I always have to point out the difference between LOUDNESS and HEADROOM - read http://headwize.com/articles/hearing_art.htm twice, once for avoiding hearing damage from average LOUDNESS and again for the required dynamic peak HEADROOM of real music - that you should want to cleanly reproduce, but only for seconds per hour of listening

taking some dynamic range values from the headwize article you can see that while 1 mW average power into most headphones could lead to hearing damage with extended exposure, it wouldn't be totally unreasonable to want 100-500 mW peak power to reproduce natural music peak levels without clipping


for the given headphones I would want a amp capable of > 4 Vrms ~= +/-6 V, +/-80mA in round numbers
the "Glimore Lite" Class A design on Headwize could be fine, the idea of Class A paralleled to-92 output stage could also be adapted to a bipolar or even op amp front end