Simple headphone amp from spare parts

Luda

Member
2006-07-28 1:00 pm
Hi,

I would like to build my first (but not terrible) headphone amp from spare parts.
So instead of just randomly picking one of the transistors and building a class-A circuit with a resistor as the constant-current-source, I would like to ask you for advice:

- The pre-amp will be a DIY Pass B1 (unity gain), so I'm not sure if the amp needs voltage gain or if a simple follower would be enough.

- I have no idea which of the transistors (see below) are suitable for building a decent quality headphone amp.


My headphones are:
- Sennheiser HD570 64 Ohm, [email protected], max 100mW
- Sennheiser HD280 64 Ohm, [email protected], max 500mW
- Etymotic HF2 16 Ohm, 105 [email protected], max 120dB


Parts:
- a spare case
- a transformer 2x 0-12 Volts, 0.66 Amps each
- possibly different wall warts
- rectifiers, voltage regulators, ...
HTML:
FET			        U Drain-Source	U Gate-Source	U Gate-Drain	I Drain    	I Gate    	P total
5	BS250KL   	P-FET	-45 V        	+-20 V                	        0.18 A	                	0.83 W
6	ZVP3310A	P-FET	-100 V       	+-20 V                	        -0,14 A	                	0.33 W


20	BS170       	N-FET	+60 V	        +-20 V	                	0.5 A     	            	0.35 W
11	IRFP044N	N-FET	+55 V	        +-20 V	                	53 A          	                	
3	IRFP044N (salvaged)						                	
							                	
8	2SJ74        	P-(J)FET			        +25 V        	0.012 A     	-0.01 A     	
20	2SK170     	N-(J)FET	+40 V		        -40V        	Ic = 0.02 A?  	0.01 A      	0.4 W
8	2SK170 GR(F)							
								

Bipolar			U CBO	U CEO	U EB	I C		
10	BD135	NPN	+45 V	+45 V	+5 V	1.5 A		
9	ZTX 450	NPN	+60 V	+45 V	+5 V	1 A


and of course some resistors and capacitors.

I would appreciate any suggestions of simple circuits using these components.

Luda
 
Last edited:
So instead of just randomly picking one of the transistors and building a class-A circuit with a resistor as the constant-current-source, I would like to ask you for advice:


How about this?

Death of Zen Class-A - Use it as a headphone amplifier

It employs a CCS for the SE output stage.

The caveat is that you will need a large power supply for a headphone amp (2 amps @30 volts).

I breadboarded this circuit and it's very nice. I am considering building one.
 

Luda

Member
2006-07-28 1:00 pm
Thank you very much Fast Eddie D!

Wow, the power requirements are higher than I expected. The author recommends a transformer with 50 VA, 100 VA preferred.

I guess I could use BD135 (or maybe even the ZTX 450?) instead of the BD139, but I'm not sure if I should use BD135s as Q3 and Q5.

I'd prefer a circuit that works with the (2 x 12 V, 0.66 Amps each) transformer that I have left over, but if I can't build a circuit with these spare parts, I might give that DoZ a try ;-)
 
Here's a transformer that would fit the bill.

25.2V CT 2.0A Heavy-Duty Chassis-Mount Transformer with Lead : Transformers | RadioShack.com

Alternatively, you could experiment with lower bias current, but you might be leaving performance on the table.

The transistors are not that critical. I used what I had on hand and it worked fine. Look up the data sheets for the recommended transistors, then look up the data sheet for what you have on hand, and see if you have a match. This is addressed in the article accompanying the circuit.
 
I'd prefer a circuit that works with the (2 x 12 V, 0.66 Amps each) transformer that I have left over, but if I can't build a circuit with these spare parts, I might give that DoZ a try ;-)


You could configure your transformer for 25.2 volts center tapped and do a parallel 5532 circuit in Douglas Self fashion.

5532 parallel headphone amp

I prototyped a circuit similar to this. I found that 3-5532s in parallel (total of 6 buffers in parallel) with an 5532 front end makes a terrific headphone amp. I used nested feedback, with the front end gain set to 5, buffers set to unity gain, and the global feedback set to 3.
 
Last edited:

Luda

Member
2006-07-28 1:00 pm
Hi Eddie,

Thanks for the suggestion, but I'm looking for an amp that I can build from the parts that I already have, and I don't have any 5532s.
The only ICs I have are some (3 to be exact) old "741" Op-Amps* and some voltage regulators. (and some microcontrollers ans darlington-arrays and stuf, but I don't think they will be any use ;) )

I would like to use the SJ74 JFETs though. I bought them because I thought about modding my Zen V4 monos, but they aren't a good match for my speakers anyways. I'll probably keep them in their "original" state and build some F5s when I find the time and money.

I still have some SMD-Package ZVP3310s that could be used, though the table in the first post says otherwise... (edit: updated the parts-list/table)


*Vcc +/- 18 V
Vid +/- 30 V
Vi +/- 15 V
Ptot 500 mW
and 170 mA or so output current
 
I'm not sure if the amp needs voltage gain or if a simple follower would be enough.
Depends on your source. e.g. Max output from a CD player is 2V rms, so you wouldn't need voltage gain with that.

I'd prefer a circuit that works with the (2 x 12 V, 0.66 Amps each) transformer that I have left over,
That should be fine. You only need a couple of volts output, and current in the tens of mA.
 
Hi Eddie,
Thanks for the suggestion, but I'm looking for an amp that I can build from the parts that I already have

I would like to use the SJ74 JFETs

You can certainly do something with those parts. For something a bit different, look at Figure 3.17 in Douglas Self's Small Signal Audio Design book. If you don't have it available, it's essentially a differential pair with an emitter follower as the output stage which is driven by the inverting output of the diff pair.

Your J74 FETs might work as the output stage similar to this:
P-amp Ver. 1.0 Headphone Amplifier Article By Grey Rollins
 

00940

Member
2003-09-03 12:17 pm
Liège
Honnestly... I think this kind of amp would be a better use of those jfet : The α20 Line Stage Amplifier (no need to order a pcb, use it as inspiration). More current output, some gain if you need it :)

Getting the extra parts (a few transistors and a pair of trimmers) would cost you next to nothing from reichelt, you already have the most expensive parts. You can use it on a +/-12v supply made with 7812/7912.
 

00940

Member
2003-09-03 12:17 pm
Liège
Well, you could replace the VAS transistors by more sk170, sj74, depending on the idss you've got (do you have complementary matched pairs ?).

You really need to get PNP transistors though to get decent current without an output cap. If you go on ebay.de, you can easily get 10bd139-bd140 pairs for 5€. That will allow you to build a much better amp.

Here is the general idea (R1-R2 is a 200r trimmer to set the output offset due to mismatch in jfet, r13 a 2K trimmer to set the output current, R5-R6 set the current in the VAS and have to be adjusted according to the idss of your jfet, the more the better, R7-R8 set the gain):

attachment.php


If you insist on using your parts... just make a kind of darlington with 1 2SK170 and 1 bd135and stick it on a 100maq ccs made with another bd135. You'll have no gain and you'll need an output cap. No need for input cap if you use a +/-12v supply.

PS: there might be a virtue with output caps in discrete design. No risk to get murdering offset at the output if your parts aren't well matched enough or trimmers go bad.
 

Attachments

  • parts.JPG
    parts.JPG
    35.3 KB · Views: 262
Last edited:
robbo33

Hi i have just built a headphone amp & use the same headphones as you. I found a site call William Neo audio & headphone tutorial site DIY Simple Headphone Amp Tutorialcant give it enouge praise. On the site (top right) under most viewed, click on the 4 options & there is all the info you need. I built one of the easier ones, (based on a cmoy) but used 470uf 16v instead of 220uf 16v & used a ic AD712jn i origanally used a TL072 cos that was the only one the shop had, it sounded abit distorted. Perhaps the Blurr Brown that are sugggested would be as good as the AD712 as i used, it give better deth/bass NO distortion. There are also class a mosfet amps dia. & parts list which i may have a go at. I want to build an amp with bass/treble next, i've got diagram but need more detailed parts list & pcb lay out. Hope this site is of help to you or anyone else. Good luck. DIY Simple Headphone Amp Tutorial