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Old 19th August 2013, 02:00 PM   #1721
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Hi All

Just got this idea about the operation of the D1

If the 30K9 and the 5K pot "just" is dividing the +45V, in order to bias the mosfet; - why not replace the 5K pot, with like 2 green LED (which is much lower noise, and more "stiff" voltage-reference), and a small pot?

PEter
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Old 24th August 2013, 02:26 PM   #1722
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Quote:
Originally Posted by MisterRogers View Post
There's a build thread. The BOM's there.
BUILD THREAD - A New Take on the Classic Pass Labs D1 with an ESS Dac is the build thread, with BOM
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Old 10th October 2013, 04:33 PM   #1723
opc is offline opc  Canada
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Default How did nobody catch this??????

Hi Guys,

It has been a long time since there were any updates on this page, but I've recently taken this design out and dusted it off to take a fresh look at how I can reduce the power consumption without reducing the performance.

I was running some simulations when I had a bit of a "Aha!" moment.

Can anyone here take any guesses? It's painfully obvious as soon as you see it.

Take a look at the negative supply, and ask yourself, "What is this doing for us?".

The answer is... nothing, except dissipating 50% of the current power dissipation.

The voltage at the source of each fet is roughly 1.75VDC, and since this is a common gate stage, the voltage at this node does not swing by more than few mV even at full output. That means the negative supply can simply be tied to ground, and the lower resistor values adjusted accordingly, and tada, half the power dissipation with absolutely no functional change to the circuit itself!

This means we need two less regulators, and we will now be dissipating roughly 23W at 45V rather than the previous 45W! I would imagine this will be a welcome change to anyone currently struggling with heat!

If you currently have the stock NTD1 as shown in the build thread schematic, all you need to do is replace the four 400 ohm resistors with 15 ohm resistors, and short the negative supply to ground by shorting across the pair of 2.2uF decoupling caps on that rail. You also have to make sure you remove the regulators for both negative rails.

This also means you probably only need one transformer and you can use each secondary to power each channel since we now only need one supply.

I have tried this, and measured it, and it works perfectly! I strongly suggest everyone else take the time to do the same. Consider it an act of kindness to the environment

Cheers,
Owen
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Old 10th October 2013, 04:45 PM   #1724
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Hi Owen,

If we short across the 2.2uF decoupling caps then we don't need them anymore?

Also, for everyone, could you show a picture of the changes?

Once this is done, do we have to re-adjust?

Thanks
Do
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Old 10th October 2013, 05:35 PM   #1725
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Hi Do,

You are correct, the 2.2uF caps are no longer required. They should be removed and replaced with either small copper shorts, or you can use the appropriate size of 0 ohm resistor if you want it to look pretty.

As for re-adjusting, I would certainly suggest it. It will be close to whatever it was before, but it's still prudent to double check and re-adjust everything.

Also, as qusp pointed out some time ago, the best way to set the voltage at the source is to measure the AVCC voltage on your actual BII/BIII/Acko unit, divide it by 2, and set the voltage at the source to match. The BII and BIII both use closer to 3.5V for AVCC which means the voltage at the source (input of the NTD1) should be set to 1.75V.

I will see if I can get a picture of my modified (and now much cooler running!) unit posted tonight.

Regards,
Owen
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Old 10th October 2013, 05:38 PM   #1726
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By removing the decoupling caps it must also sound a little better no? Caps have a tendency to color so I assume it would be a little better, but what about DC offset since they're gone?

Thanks
Do
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Old 10th October 2013, 05:53 PM   #1727
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Hi Do,

Ooops! I should be clear about this. I'm referring to the 2.2uF decoupling caps on the power supply rail (C36, C37, C42, C43) and not the 22uF output coupling caps!

The output must still be AC coupled using the 22uF film caps.

Essentially what this modification is doing is shorting the negative supply to ground (using the above mentioned caps) and reducing R16, R17, R25, and R26 from 400R to 15R in order to maintain the same bias current in the design (116mA).

As you can see if you look at the schematic, we need to drop roughly 46.75V across the 400 ohm resistor to get a bias current of 116mA while keeping the source of the fet at +1.75V. This results in a power dissipation of 5.4W per resistor. All we're doing here is grounding the negative supply so that we only have to drop 1.75V across the lower resistor, which reduces power dissipation down to 0.2W for the same 116mA bias current. This saves about 21W of dissipation and does not change the circuit operation at all.

Regards,
Owen
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Old 10th October 2013, 05:57 PM   #1728
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Sorry, my bad!! I don't know why I had 2.2uF as output caps in my head... Probably the dot makes all the difference...

Thanks
Do
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Old 13th October 2013, 03:20 PM   #1729
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So, I can use 1W resistors for the 15ohm instead of Caddock 30W and the NTD1 can be directly mounted onto the floor of case instead of the big heatsink.

Thanks Owen. But I may not complete the DAC within this year.
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Old 19th October 2013, 09:07 AM   #1730
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Hi Owen,
thank you for this information!

Normaly the distortion of this stage will be higher using a 15R instead of 400R.
I thought about changing the power supply and the source resistor more than one year before and was canceling this idea.
Can you please make a simulation with high signal input level (higher current) comparing THD+N
If I find the time for it this weekend, I also will make a new simulation with Multisim.

May DAC runs fine since a long time now. I am very happy with the sound quality. My output copplers are 100F Black Gate parallel to 56nF silver Mica and a 3,3nF KP capacitor. The red bipolar Black Gate condensors are very important for the SQ. I know 100F is much, but with big values there is a audible difference in bass SQ. 47F will also be ok.
The most important update was changing the four Trident shunt regulaters to the newer versions 3.3V to V3 and 1.2V to V3 and the AVCC to V2.1
The AVCC V2.1 needs a long burn in (at least 20 hours) to sound better than the old version.
But then you will get much more transparency. The greatest dfference of the NTD1 compared with other DACs is the incredible "live sound"
Other DACs who use OPVs in the output stages are sounding "compressed" with less dynamic compared to the NTD1.

Last edited by Salomon; 19th October 2013 at 09:12 AM.
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