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Old 16th December 2012, 02:47 AM   #11
qusp is offline qusp  Australia
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yep cascoded, or with a separate front end supply (the whole thing could still be about the same size as 1 x SSM part). I have in the past wondered how we could use the E-trim function to good effect.
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Old 18th December 2012, 08:32 PM   #12
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Been looking at the datasheet of BCV62 and BCM62B more closely and I am not too happy about what I see.

The current matching is not nearly as accurate compared to what the SSM2220 delivers.

They have higher gain than the SSM2220 and they are cheaper than the SSM2220 but as always you do not get anything for free.

I will probably stay with the SSM2220 as current mirror. They cost a great deal more and have less gain but current matching is just a lot better.

Last edited by Neutrality; 18th December 2012 at 08:34 PM.
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Old 24th December 2012, 10:44 AM   #13
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I am very close to the final layout.

There has been a few changes compared to what presented in my first post.

I added another transistor to the VAS and it is now a beta enhanced VAS, this reduced THD1 to around 0.0001xx% at close to full power. THD20 is a respectable 0.001xxx% at close to full power. Going from 32 to 250 to 600 Ohm loads, the THD1 and THD20 does not change significantly.

Final board size will be 50mm x 80mm so it is quite compact.

The end result is a high quality Class A headphone amplifier than can drive almost anything on the market, from 32 to 600 Ohm and with very low distortion

I am more than satisfied.

This is the design that I have felt the most excited for out of those I have designed and built so far.

I am also working on a high quality supply board for it. Current plan is a cap multiplier to lower ripple as well as getting rid of as much mains nosie as I can and then followed by a Sulzer regulator for truly low ripple and noise. Sulzer regulator alone show 50Hz and 100Hz down around -120dB, adding the cap multiplier before the Sulzer regulator should help getting this even lower.

But this is still a work in progress, I have only done some simulations for now, there is still a few weeks to go before I can show any schematic or layout.

Last edited by Neutrality; 24th December 2012 at 10:58 AM.
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Old 24th December 2012, 02:32 PM   #14
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Looks very promising. I am looking forward to building this.
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Old 26th December 2012, 02:44 PM   #15
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Most likely final schematic and layout.

I will give it a few days and if I am still satisfied with it, to the manufacturer it goes.
Attached Images
File Type: png Midgård_Schematic.png (45.1 KB, 317 views)
File Type: png Midgård_BOTTOM.png (13.5 KB, 300 views)
File Type: png Midgård_TOP_Silkscreen.png (13.5 KB, 293 views)

Last edited by Neutrality; 26th December 2012 at 03:01 PM.
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Old 26th December 2012, 04:57 PM   #16
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I am considering changing the green current source LEDs to green 0805/1206 Chip LEDs.

They really do not have to be visible from the top side so making them SMD instead of TH is just as good.
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Old 7th February 2013, 06:40 PM   #17
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It took a while but I am in the process of assembling a pair of boards.

I went with 1206 Chip LEDs instead of TH LEDs and a few component values have been tweaked but the current boards have more or less the same layout as shown in post #15.

If all goes well, I should have them up and running tomorrow or on Monday.
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Old 8th February 2013, 03:35 PM   #18
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Finished the boards today, used a pot to trim the bias, once I get the bias where I want it I will replace the pot with a fixed resistor of the same value.

Once the bias is adjusted the output stage bias does not drift more than +/-5mA and the DC servo does its job, keeping the DC offset at around +/- 1mV.

I just need to clean of the flux and the boards are done.

On monday I will do some listening. Cant wait.
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Old 8th February 2013, 05:00 PM   #19
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Quote:
Originally Posted by Neutrality View Post
......and the DC servo does its job, keeping the DC offset at around +/- 1mV.
Just to clarify, the DC servo keeps the DC offset at 0V, +/- 1mV.
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Old 9th February 2013, 05:17 PM   #20
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This is the final design and the one I am almost done assembling.

I made a name change from Midgård to Freya, thought it sounded better.
Attached Images
File Type: png Freya_Schematic.png (44.9 KB, 161 views)
File Type: png Freya_Gerber_Bottom.png (14.4 KB, 145 views)
File Type: png Freya_Gerber_Top.png (13.6 KB, 52 views)
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