Thor, an all discrete Lateral FET Headphone Amplifier. - Page 2 - diyAudio
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Headphone Systems Everything to do with Headphones

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Old 27th February 2012, 06:14 PM   #11
jcx is offline jcx  United States
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higher V also gives less distortion - can avoid instability from device parasitics that increase near saturation - at least several V extra, >2x Vth is a good idea

but as a intellectual puzzle can you tell me why if 2-3x over bias/current reserve is so good why isn't more V margin as important

since by publishing the circuit here you are implying others might want to build it - it is good to spell out the assumptions, limitations, particularly if the circuit is "optimized" for a particualr headphone

I wouldn't call ~7 Vrms to be the end point for headphone amp Vout


another point is that other builders would want to know how closely did you match input Q - how much performance degrades with mismatch, Vth can still vary despite Idss match

Last edited by jcx; 27th February 2012 at 06:20 PM.
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Old 27th February 2012, 06:30 PM   #12
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Originally Posted by jcx View Post

but as a intellectual puzzle can you tell me why if 2-3x over bias/current reserve is so good why isn't more V margin as important

since by publishing the circuit here you are implying others might want to build it - it is good to spell out the assumptions, limitations, particularly if the circuit is "optimized" for a particualr headphone

I wouldn't call ~7 Vrms to be the end point for headphone amp Vout


another point is that other builders would want to know how closely did you match input Q - how much performance degrades with mismatch, Vth can still vary despite Idss match
It is mainly built for 32-150 Ohm headphones but someone who wants to drive 600 Ohm headphones or need more output voltage could just increase the supply to +/- 18VDC instead of +/- 12VDC. +/- 18VDC is the limit of the DC Servo Opamp.

Input JFETs are matched to around 6mA with each pair being within 0.1mA of each other and the N and P channel pairs being with 0.2mA of each other.

This is just a fun little project, done in my spare time. I shared it so other people could build it if they want or get some inspiration from it.

It is what it is.

Last edited by Neutrality; 27th February 2012 at 06:37 PM.
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Old 27th February 2012, 07:03 PM   #13
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Also, the JFET Source resistor of 240 Ohm will give around 3-3.5 mA through each JFET if they are matched to around 6 mA IDSS. JFETs with higer IDSS will need a larger value JFET Source resistor. As an example JFET's with 10 mA IDSS will need a 470 Ohm source resistor.
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Old 28th February 2012, 03:40 PM   #14
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I took a few pictures today.

One will notice is that I did not make the correct Eagle model for the Silver Mica compensation caps so I had to bend the legs a little and they are also a little too close to the transistors than I would normally want them to be.

However the rest of the PCB layout works very well.
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Last edited by Neutrality; 28th February 2012 at 03:46 PM.
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Old 28th February 2012, 06:52 PM   #15
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If anyone is interested I have enough boards left for 4 or 5 headamps.

Selling these for $20 USD a pair + postage due to the suboptimal compensation cap placement.
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Old 28th February 2012, 06:58 PM   #16
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Count me in for a pair.
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Old 28th February 2012, 07:02 PM   #17
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Originally Posted by Neutrality View Post
It is mainly built for 32-150 Ohm headphones but someone who wants to drive 600 Ohm headphones or need more output voltage could just increase the supply to +/- 18VDC instead of +/- 12VDC. +/- 18VDC is the limit of the DC Servo Opamp.

Input JFETs are matched to around 6mA with each pair being within 0.1mA of each other and the N and P channel pairs being with 0.2mA of each other.

This is just a fun little project, done in my spare time. I shared it so other people could build it if they want or get some inspiration from it.

It is what it is.
You could use the LSK389 for the N channel pair. No equivalent P channel type is currently in production as far as I know.
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Old 28th February 2012, 07:10 PM   #18
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You could use the LSK389 for the N channel pair. No equivalent P channel type is currently in production as far as I know.
Thats the problem, no dual P-channel in production. I would rather just match both the N and P channel JFET's than use a dual N-channel JFET+2 matched P channel JFET's.

I'm a little OCD when it comes to doing schematics and layouts, I like it to be as symmetric as possbile. Using 1 dual + 2 single JFET's just isn't symmetric enough.

Last edited by Neutrality; 28th February 2012 at 07:14 PM.
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Old 28th February 2012, 07:17 PM   #19
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Thats the problem, no dual P-channel in production. I would rather just match both the N and P channel JFET's than use a dual N-channel JFET+2 matched P channel JFET's.

I'm a little OCD when it comes to doing schematics and layouts, I like it to be as symmetric as possbile. Using 1 dual + 2 single JFET's just isn't symmetric enough.
Well, if you were open to NPN and PNP matched pairs in the same package, then Analog Devices makes these things.
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Old 28th February 2012, 07:25 PM   #20
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Well, if you were open to NPN and PNP matched pairs in the same package, then Analog Devices makes these things.
Well, after I found the complementary floating JFET input stage topology I'm never going back to BJT's for input stages and output stages. For VAS, drivers and other stuff I would still use them.
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