Thor, an all discrete Lateral FET Headphone Amplifier. - diyAudio
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Old 26th February 2012, 10:39 AM   #1
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Default Thor, an all discrete Lateral FET Class A Headphone Amplifier.

Thor, an all discrete Lateral FET Class A Headphone Amplifier.

PCB design for the amplifier modules is finished and the modules have been mounted and are now up and running.

180 mA bias, supply from +/- 12 to 18V. Gain is set at x2 but will happily work at x10 gain if needed.

DC coupled with a DC Servo to handle offset issued. DC Offset stays at around +/- 1mV even after several hours off playing.

The same stability is seen in the bias, once it stabilizes at around 180 mA it doesn't drift more than 10 mA in each direction.

Modules are simple to get up and running. Match the input JFET's to around 6mA IDSS, use a pot to trim the Lateral FET output bias, read the pot value and then use the fixed value resistor that comes closest and use that as your bias resistor.

Upcoming additions are a No-thump board, the design is done, just need to get the boards manufactured, mounted and tested. 2 Sec turn on delay and AC detection that turns off the output relay when AC to the amplifier supply is turned off. No thumps/pops/clicks in this amp.

Lastly a supply board, nothing special, just your standard LM317/LM337 combo but with a decent amount of capacitance both before and after the regulator. This design is done as well, just need to get the boards manufactured, mounted and tested.
Attached Images
File Type: png Thor_Schematic.png (51.8 KB, 2302 views)
File Type: png Thor_No-thump.png (43.5 KB, 2140 views)
File Type: png Thor_Supply.png (50.4 KB, 1989 views)

Last edited by Neutrality; 26th February 2012 at 11:01 AM.
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Old 26th February 2012, 01:10 PM   #2
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Looks really nice!
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Old 26th February 2012, 03:29 PM   #3
qusp is offline qusp  Australia
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interesting, as someone else in the other thread said, pretty straight forward mod to make differential in/out and i might just try it with some left over alfets as i have everything else in my parts bin. what transistors have you used in the network T5-T10? BC550/560 ok?

Last edited by qusp; 26th February 2012 at 03:32 PM.
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Old 26th February 2012, 04:05 PM   #4
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Quote:
Originally Posted by qusp View Post
what transistors have you used in the network T5-T10? BC550/560 ok?
I used Fairchildsemi KSC3503 and KSA1381.
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Old 26th February 2012, 04:38 PM   #5
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But BC550/560 would most likely work just as well.

Last edited by Neutrality; 26th February 2012 at 04:47 PM.
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Old 27th February 2012, 04:18 PM   #6
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Well, sometimes **** hits the fan.

The basic design is good, however I might have been a little too focused on getting very low THD-20 distortion numbers at the cost of stability in some cases.

2 test headphones worked very well, but my own cheap Sennheiser HD415 headphones caused some instability. Switching the 2 compensation caps from 10pF to 39pF fixed it right up. THD-20 went from 0.0005X% to 0.0008X% in simulation with 5.6V p-p output.

So I would advise anything from 39pF to 150pF as compensation caps. 150pF still gives a low THD-20 0f 0.001X% in simulation with 5.6V p-p output.

If you want to be on the 100% completely safe side then use 150pF, otherwise lower values could work just as well, depending on your headphone and the headphone cabling.

Revised schematic attached.

PS : It sounds absolute fantastic with just a pair of cheap Sennheiser HD415 headphones.
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File Type: png Thor_Schematic.png (51.7 KB, 1899 views)

Last edited by Neutrality; 27th February 2012 at 04:46 PM.
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Old 27th February 2012, 04:51 PM   #7
qusp is offline qusp  Australia
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nice, still perfectly good numbers with 150pf, my headphones and cables are all pretty low capacitance, I dont have a scope but i guess i can just check for current draw or just play it safe with 150, you used np0, mica or styrene?. ok we'll see how much drive i have to make more amps after the current crop of projects, i'm trying to get some perspective.....
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Old 27th February 2012, 04:59 PM   #8
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Yes, the numbers are still perfectly fine with 150pF compensation caps. However, it is just a simulation, reality might show numbers that are higher.

I do have access to 2 different audio analyzers at work, someday I might hook up my amp boards and see what they can really do in a real world test.

Checking for current draw is most likely the only thing you can do without a scope. But if you play it safe and go with 150pF compensation caps I cant really imagine it going wrong.

I use Silver Mica.
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Old 27th February 2012, 05:44 PM   #9
jcx is online now jcx  United States
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why such huge output Q?

2sj76, 2sk213 are more appropriate for headphone drive than the 7 A devices

+/-12 V supply is also a little small for these discrete parts, most headphones I know of that need several 100s of mA – as implied by your bias current

stability with output loading - most often cable C - can be improved with a few uH series inductor (parallel with R for damping if you care about 100 kHz ringing) - can be air core if you obsess about such things
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Old 27th February 2012, 05:56 PM   #10
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Originally Posted by jcx View Post
why such huge output Q?

2sj76, 2sk213 are more appropriate for headphone drive than the 7 A devices

+/-12 V supply is also a little small for these discrete parts, most headphones I know of that need several 100s of mA as implied by your bias current

stability with output loading - most often cable C - can be improved with a few uH series inductor (parallel with R for damping if you care about 100 kHz ringing) - can be air core if you obsess about such things
In general 32 Ohm headphones do not need alot of voltage across them, but they do like some current reserves. On the other hand 600 Ohm headphones do need alot more voltage across them, but do not really need that much current.

I personally feel that +/- 12V and a few hundered mA of bias current is a good balance. Besides, higher bias gives lower distortion.

I also like to oversize things and using a pair of big 7 A devices does not cost much more than the significantly smaller and less powerful 2sj76, 2sk213 Lateral FET's. Also, the 2sj76, 2sk213 Lateral FET's are hard to come by, while I on the other hand have a good source of the 2SK1058/2SJ162 Lateral FET's from a supplier where I can get them for about 5$ a piece.

With regards to stability I have already considered both an output resistor or an output inductor//resistor but decided that increasing the compensation caps would be far simpler and work just as good.

Bottom line is that it works and sounds great as well.

My own personal goals for this headamp have been more than met.
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