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Old 14th December 2012, 08:44 PM   #1
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Default Midgård, compact, low distortion, high quality, Class A Lateral FET headamp

Well, it started with this : Midgard, a low distortion Lateral FET output Headphone Amplifier

However, things turned out in such a way that I did have neither time nor money to pursue it further.

A few days ago I decided to take a look at it again and there has been some rather major changes.

List of changes :

Dual matched NPN monolithic SSM2212 transistors for the input pairs.

Dual matched PNP monolithic SSM2220 transistors for the current mirrors.

BC850C are used in the LED based current sources.

BC860C for the VAS.

Semelab ALF08NP16V5 dual N+P channel Lateral FET is used for the output transistor.

LTP bias current is 2.6mA through each of the input transistors, VAS bias current is 5.2mA, output stage bias current is 115mA.

Almost everything is SMD so the board is a lot smaller this time. Old board were 100mm X 100mm for 1 channel, the new board is 90mm X 50mm, which is 45% of the previous size and it might end up even smaller. Something like 70-80mm X 50mm should be a possibility.

Smallest components are the SOT23 BC850C/BC860C transistors, resistors and capacitors are 0805 and up.

The input RC filter will be using either a C0G/NP0 ceramic capacitor or a PPS film capacitor. The same goes for the Cdom capacitor, that is also either C0G/NP0 or PPS film. I have not decided yet.

100nF bypass capacitors will either be some kind of film capacitors or X7R ceramic capacitors.

I know a lot of people do not like ceramic capacitors for audio use but X7R are great for supply bypassing and NP0/C0G are just as good as some of the best film/silver mica capacitors when it comes to low distortion. On top of that they are extremely stabile over temperature and are free from the usual piezo electric effect seen in X5R/X7R and lower quality ceramic capacitors.
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Last edited by Neutrality; 14th December 2012 at 09:26 PM.
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Old 14th December 2012, 09:02 PM   #2
cotdt is offline cotdt  United States
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Put me down for 4 PCBs.
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Old 14th December 2012, 09:15 PM   #3
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Put me down for 4 PCBs.
Well, I might need some time to get done with the layout as well as actually building a pair of boards.

I should have a pair of boards up and running sometime at the end of January if all goes well and I can find the time for it.

It is a hobby after all.
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Old 15th December 2012, 12:57 PM   #4
qusp is offline qusp  Australia
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you might look at some of the interesting dual transistors mentioned in the SW-OPA thread, lower noise and distortion again, as well as physically smaller than the SSM duals.

what size CDOM are you looking at? the AVX Accu-P thin film caps, but only available up to 12pf for the 50v parts, and 22pf I think for te 25v parts. these are more stable and linear again than C0G at very high frequency. there are some new thin filmVishay parts up to 10pf in 25v also.

the transistors I speak of are ones like
NXP BCM61B (2xNPN) and BCM62B (2xPNP)
NXP BCV61 and BCV62 (dual NPN and PNP current mirrors)
MMDT4401/4403 (dual NPN+NPN, PNP+PNP) (all specifically made for use as current mirrors) or MMDT4413 (complementary)

even new dual N-Channel Jfets in SMD in the form of MCH5908. some R-Ohm duals etc etc. lots of great little parts if you dont mind SMD, apparently a boom of AM radio in China is driving some new parts useful for us.

Last edited by qusp; 15th December 2012 at 01:06 PM.
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Old 15th December 2012, 01:13 PM   #5
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Originally Posted by qusp View Post
you might look at some of the interesting dual transistors mentioned in the SW-OPA thread, lower noise and distortion again, as well as physically smaller than the SSM duals.

what size CDOM are you looking at? the AVX Accu-P thin film caps, but only available up to 12pf for the 50v parts, and 22pf I think for te 25v parts. these are more stable and linear again than C0G at very high frequency. there are some new thin filmVishay parts up to 10pf in 25v also.

the transistors I speak of are ones like
NXP BCM61B (2xNPN) and BCM62B (2xPNP)
NXP BCV61 and BCV62 (dual NPN and PNP current mirrors)
MMDT4401/4403 (dual NPN+NPN, PNP+PNP) (all specifically made for use as current mirrors) or MMDT4413 (complementary)

even new dual N-Channel Jfets in SMD in the form of MCH5908. some R-Ohm duals etc etc. lots of great little parts if you dont mind SMD, apparently a boom of AM radio in China is driving some new parts useful for us.
Thanks, have not seen those currents mirrors before.

I think I will keep using SSM2212 for the input pair, but some of those dual PNP current mirrors are quite interesting.

EDIT : I think I will use NXP BCM62B for my current mirror, high gain, low noise and as a bonus, small size.

Last edited by Neutrality; 15th December 2012 at 01:41 PM.
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Old 15th December 2012, 03:51 PM   #6
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cool, actually my bracketed comments didnt work out correctly, I was referring to everything before I said 'all specifically designed as current mirrors' when I said that, not just that line. the BCM and BCV are the best parts of the lot too IMO, great choice! some of the builders may not thank me haha, but hey if the results are good its all good right?

thats what Scott Wurcer specifies in his SW-OPA (the BCV62A) so thats a pretty strong recommendation if ever ive seen one!
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Old 15th December 2012, 04:01 PM   #7
qusp is offline qusp  Australia
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what would be cool, since youve got a bit of space up the top of the board, would be if you put a hole in the PCB to allow alternate mounting of the mosfet under the board, then you could make a cute, but good and masculine chassis design with 2 x conrad or similar heatsinks like MF15-75/MF15-100 as the walls with the amps screwed to them and the PSU in the center. or of course the floor of a case if it was chunky enough

another interesting set of parts you may not have seen

Last edited by qusp; 15th December 2012 at 04:10 PM.
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Old 15th December 2012, 04:16 PM   #8
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what would be cool, since youve got a bit of space up the top of the board, would be if you put a hole in the PCB to allow alternate mounting of the mosfet under the board, then you could make a cute, but good and masculine chassis design with 2 x conrad or similar heatsinks like MF15-75/MF15-100 as the walls with the amps screwed to them and the PSU in the center. or of course the floor of a case if it was chunky enough
Interesting idea, I thought about something similar, like using the board as a small pure Class A amp, putting out 8 Watt into 8 ohm.

However, for the moment I am too keen on adding that option to the board.
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Old 15th December 2012, 04:24 PM   #9
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Those are really interesting, single chip matched input pair+current mirror, that is something special.
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Old 15th December 2012, 05:34 PM   #10
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Those are really interesting, single chip matched input pair+current mirror, that is something special.
Hmm, maximum Vds for all of them is 10.6V so a cascode is needed across the input pair. But still interesting.

However, using a dual matched N-channel FET with a cascode as the input pair and then a dual matched P-channel FET as a current mirror should work well.
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