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Old 4th November 2013, 11:01 AM   #1
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Default Huginn, High performance, Class A, Lateral FET headphone amplifier

Started with this : Midgard, a low distortion Lateral FET output Headphone Amplifier

But I only did some simulations and played around with a layout before I stopped working on it.

However, I went back and had a look at it again and implemented some big changes and I ended up with this : Midgård, compact, low distortion, high quality, Class A Lateral FET headamp

This amplifier module I actually built and spent a few weeks listening to it but I did not get around to make a finished amp and there was still some things I wanted to change.

Well, changes have been done and this is where I am today. See attached pictures for final schematic and layout.

The schematic does not show it, but as one can see on the layout there are 3 connectors on the board, for the inclusion of a DC servo on a separate PCB.

This is due to the fact that it makes it easy to choose between servo or non servo operation and it also makes it easy to experiment with different types of DC servo circuits without having to redo the whole HAB.

Also not shown on the schematic, the input JFETs are high frequency, low noise BF862, input cascodes are J111, current mirror is BCM62B, all from NXP.

Simulation results are incoming.
Attached Images
File Type: png Huginn-Schematic.png (47.3 KB, 477 views)
File Type: png Huginn-Layout.png (128.0 KB, 473 views)

Last edited by Neutrality; 4th November 2013 at 11:51 AM.
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Old 4th November 2013, 11:19 AM   #2
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Some simulation results :

5Vrms into 32 Ohm, THD20 : 0.001552%

5Vrms into 150 Ohm, THD20 : 0.000481%

5Vrms into 250 Ohm, THD20 : 0.000475%

5Vrms into 600 Ohm, THD20 : 0.000471%

It is stable driving up to 20nF capacitive loads, more than enough for any headphone/headphone cable available.

Attachments :

1 : Frequency response, 250 Ohm load.

2 : SquareWave, 20kHz, 2VPP Input, 10nS Rise/Fall time.

3 : SquareWave, 20kHz, 10VPP Input, 10nS Rise/Fall time.

4 : SquareWave, 100kHz, 2VPP Input, 10nS Rise/Fall time.

5 : SquareWave, 100kHz, 10VPP Input, 10nS Rise/Fall time.

Last edited by Neutrality; 4th November 2013 at 11:47 AM.
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Old 5th November 2013, 10:18 AM   #3
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Hi Neutrality,

The project looks great, congratulations. Maybe it is just me but I have problems interpreting your pcb color scheme. Where do you find the NXP products? Here in France, I could not find them @ my usual suppliers and Mouser does not seem to carry those either.

Jacques
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Old 5th November 2013, 11:01 AM   #4
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Quote:
Originally Posted by jacques antoine View Post
Hi Neutrality,

The project looks great, congratulations. Maybe it is just me but I have problems interpreting your pcb color scheme. Where do you find the NXP products? Here in France, I could not find them @ my usual suppliers and Mouser does not seem to carry those either.

Jacques
You can get them from from rsonline.dk(rs-components.com), however they should be available from digikey.com as well.

Attached is bottom layer and top layer.
Attached Images
File Type: png Huginn-GBL.png (23.7 KB, 77 views)
File Type: png Huginn-GTL.png (17.7 KB, 62 views)

Last edited by Neutrality; 5th November 2013 at 11:04 AM.
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Old 5th November 2013, 11:31 AM   #5
gootee is offline gootee  United States
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Why are the planes left with holes in the middle?
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Old 5th November 2013, 11:43 AM   #6
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Originally Posted by gootee View Post
Why are the planes left with holes in the middle?
Cooling, I will be dissipating 3.2 Watt in the heatsinks.

With the holes in the planes I can now put holes in the bottom under the PCB and in the top over the PCB, this will result in improved cooling compared to no holes in the planes.

Simple really.
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Old 5th November 2013, 12:00 PM   #7
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Quote:
Originally Posted by Neutrality View Post
You can get them from from rsonline.dk(rs-components.com), however they should be available from digikey.com as well.

Attached is bottom layer and top layer.

Thanks for the pcb layouts. Indeed, the local RS carries the NXP components.

Jacques
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Old 5th November 2013, 12:51 PM   #8
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Quote:
Originally Posted by Neutrality View Post
Cooling, I will be dissipating 3.2 Watt in the heatsinks.

With the holes in the planes I can now put holes in the bottom under the PCB and in the top over the PCB, this will result in improved cooling compared to no holes in the planes.

Simple really.
To expand on this, PCB mounted heatsinks usually rely on natural convection, which basically means that if you get an increase in the airflow around the heatsink and within the heatsink fins you also increase the amount of power you can dissipate in the heatsink.

Cooler air is sucked into the cabinet through the holes in the bottom cover, through the PCB planes holes, through the heatsink fins and then leaves the cabinet through the top cover holes above the heatsink. This gives you a chimney effect.
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Old 7th November 2013, 03:13 AM   #9
gootee is offline gootee  United States
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Well at least you have a rationale behind it. And using convection like that is a wonderful thing: a system that tends to self-regulate. i.e. The hotter the heatsinks get, the greater the convection effect gets, causing more airflow, cooling the heatsinks more. It's a beautiful thing.

The reason I asked is that since each hole makes its plane into a loop, with a significant enclosed loop area (see Faraday's Law), every time-varying magnetic field will induce a corresponding time-varying current in the loop. It might not be a significant concern, since it's not an "obviously bad" loop like it would be if it was a loop that included, say, the amplifier input resistor. Nevertheless, the induced currents will induce voltages across the impedance of the copper itself, which will present themselves between whatever pairs of connections can be found connected to the loop.

The two possible problem scenarios, if there would even be any problem, would probably come from your transformer and its primary and secondary leads, since the fields associated with those could be relatively large in amplitude because of their close proximity, and, from RF (radio frequency) fields from external sources, since they are high frequencies, to which the copper will be a much more-significant impedance (although the RF's fields are usually mostly electric fields, rather than magnetic, after they get more than one or two wavelengths from the transmitting antenna).

I don't have any idea about whether or not any possible effects from those might be significant. Just wanted to mention the theoretical possibility.

If there were to be a problem, I guess you could always simply solder some copper mesh (screen) across the holes, to eliminate the enclosed loop area, eliminating the induced currents, while still allowing the airflow.
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Last edited by gootee; 7th November 2013 at 03:21 AM.
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Old 7th November 2013, 11:15 AM   #10
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Originally Posted by gootee View Post
Well at least you have a rationale behind it. And using convection like that is a wonderful thing: a system that tends to self-regulate. i.e. The hotter the heatsinks get, the greater the convection effect gets, causing more airflow, cooling the heatsinks more. It's a beautiful thing.

The reason I asked is that since each hole makes its plane into a loop, with a significant enclosed loop area (see Faraday's Law), every time-varying magnetic field will induce a corresponding time-varying current in the loop. It might not be a significant concern, since it's not an "obviously bad" loop like it would be if it was a loop that included, say, the amplifier input resistor. Nevertheless, the induced currents will induce voltages across the impedance of the copper itself, which will present themselves between whatever pairs of connections can be found connected to the loop.

The two possible problem scenarios, if there would even be any problem, would probably come from your transformer and its primary and secondary leads, since the fields associated with those could be relatively large in amplitude because of their close proximity, and, from RF (radio frequency) fields from external sources, since they are high frequencies, to which the copper will be a much more-significant impedance (although the RF's fields are usually mostly electric fields, rather than magnetic, after they get more than one or two wavelengths from the transmitting antenna).

I don't have any idea about whether or not any possible effects from those might be significant. Just wanted to mention the theoretical possibility.

If there were to be a problem, I guess you could always simply solder some copper mesh (screen) across the holes, to eliminate the enclosed loop area, eliminating the induced currents, while still allowing the airflow.
I wont say you are wrong, because you are absolutely right about it.

However, I am not really worried about it that much.

First of all, the transformer is outside the case, since I am using a external wallwart to supply it. I do not like working with mains voltage if I can avoid it.

Secondly, RF induces currents, that is a bigger issue and to be honest I have no idea if it would be a problem or not.

Might have to do some thinking.
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