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Old 6th April 2013, 01:37 PM   #1
Luda is offline Luda  Germany
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Default Simple headphone amp from spare parts

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, 102dB@1V, max 100mW
- Sennheiser HD280 64 Ohm, 102dB@1V, max 500mW
- Etymotic HF2 16 Ohm, 105 dB@0.1V, max 120dB


Parts:
- a spare case
- a transformer 2x 0-12 Volts, 0.66 Amps each
- possibly different wall warts
- rectifiers, voltage regulators, ...
HTML Code:
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 by Luda; 7th April 2013 at 11:41 AM.
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Old 6th April 2013, 02:33 PM   #2
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Location: Skokie Il
Quote:
Originally Posted by Luda View Post
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.
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Old 6th April 2013, 03:23 PM   #3
Luda is offline Luda  Germany
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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 ;-)
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Old 6th April 2013, 05:10 PM   #4
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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.
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Old 6th April 2013, 05:17 PM   #5
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Quote:
Originally Posted by Luda View Post
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 by Fast Eddie D; 6th April 2013 at 05:20 PM.
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Old 7th April 2013, 11:41 AM   #6
Luda is offline Luda  Germany
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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
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Old 7th April 2013, 03:29 PM   #7
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by Luda View Post
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.

Quote:
Originally Posted by Luda View Post
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.
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Old 10th April 2013, 12:45 PM   #8
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Quote:
Originally Posted by Luda View Post
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
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Old 10th April 2013, 01:13 PM   #9
Luda is offline Luda  Germany
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Hi, I'm trying to build a "Beast with 8 JFETs": Beast with a Thousand JFETs
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Old 10th April 2013, 01:43 PM   #10
00940 is offline 00940  Belgium
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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.
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