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Old 27th November 2013, 10:48 AM   #641
Crom is offline Crom  United Kingdom
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This looks like the perfect project to re-juvinate my Buffalo IIIse. I'm running dual mono into the tube-i-zator tube output stage but I've never been 100% satisfied as I think the impedance matching between the current output dac and the valves is well off and this looks like it may pay worthwhile dividends over/different to the Legato 3.1 which I also have. So...I'm going to give this a go! bigpandhahk has been kind enough to sell me his spare kit but I have a few questions...for starters:

1) I have a couple of toroids kicking about from an old project. They're rated 2 x 40-0-40vac output at 80VA per transformer. They are centre-tapped, is that OK? I noticed that on another of Owen's projects this configuration wasn't suited so I thought I'd check. I also understand that the recommended power rating is 50VA but it's not a huge problem is it?
2) I would like to try dual mono by bridging the outputs of each of the BIIIse's and using a single NTD1 board. I think the excess heat/space/FET matching requirements for dual NTD1's is probably overkill ;-) With this in mind, this has been discussed before and I understand that I need to halve the gain and IV resistors, which I am assuming are R13,14, 16, 17, 25, 26, 28, 29?
3) Also regarding dual mono, in this post there was mention of lowering the rails (BUILD THREAD - A New Take on the Classic Pass Labs D1 with an ESS Dac). Is there any guidance as to what to lower them to?

Thanks very much for a fascinating read so far and I am looking forward to posting some pics and progress reports when everything arrives!

Cheers,
Crom
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Old 3rd December 2013, 08:11 PM   #642
Crom is offline Crom  United Kingdom
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Urp, no responses in almost a week. I know that some of this has been discussed before but I can't find answers to the specifics in the various threads. Have I posted this in the wrong place?
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Old 3rd December 2013, 09:09 PM   #643
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Hi Crom,

Sorry for the delay... I've been busy packing up board orders and haven't been replying on threads as often.

To address your questions:

Quote:
1) I have a couple of toroids kicking about from an old project. They're rated 2 x 40-0-40vac output at 80VA per transformer. They are centre-tapped, is that OK? I noticed that on another of Owen's projects this configuration wasn't suited so I thought I'd check. I also understand that the recommended power rating is 50VA but it's not a huge problem is it?
This is fine for the NTD1 as it uses complimentary (+ and -) regulators, so the GND is common on the whole channel. Only the floating supplies on the new PSU require complete isolation of the AC inputs.

Quote:
2) I would like to try dual mono by bridging the outputs of each of the BIIIse's and using a single NTD1 board. I think the excess heat/space/FET matching requirements for dual NTD1's is probably overkill ;-) With this in mind, this has been discussed before and I understand that I need to halve the gain and IV resistors, which I am assuming are R13,14, 16, 17, 25, 26, 28, 29?
That is correct. You need to roughly halve each resistance value to get half the gain. This will double the current draw given the same rail voltage, which in turn doubles the power dissipation to nearly 100W. If I were to build such a thing, I would reduce the rail voltage down to +/- 32V and use two PSU V2 supplies instead of the on-board supply. This will make power dissipation a little more manageable, dramatically improve the supply performance and noise, and will also improve overall performance thanks to the higher idle current. I would consider the above arrangement as the "ultimate" NTD1. I can provide the NTD1 boards with the supply cut off for exactly this application.

Quote:
3) Also regarding dual mono, in this post there was mention of lowering the rails (http://www.diyaudio.com/forums/digit...ml#post2407111). Is there any guidance as to what to lower them to?
See above

I will post a modified schematic later this evening to show exactly what resistance value to use, and what the overall power dissipation would be.

Cheers,
Owen
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Old 3rd December 2013, 09:52 PM   #644
Crom is offline Crom  United Kingdom
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Thanks for coming back to me Owen. This sounds very interesting...

Quote:
This is fine for the NTD1 as it uses complimentary (+ and -) regulators, so the GND is common on the whole channel. Only the floating supplies on the new PSU require complete isolation of the AC inputs.
OK, this sounds good. Is the 'new PSU' that you mention above the PSU V2 mentioned below? If so, then I assume from your comment that I'd need 2 independent secondaries which are not centre-tapped?
Quote:
That is correct. You need to roughly halve each resistance value to get half the gain. This will double the current draw given the same rail voltage, which in turn doubles the power dissipation to nearly 100W. If I were to build such a thing, I would reduce the rail voltage down to +/- 32V and use two PSU V2 supplies instead of the on-board supply. This will make power dissipation a little more manageable, dramatically improve the supply performance and noise, and will also improve overall performance thanks to the higher idle current. I would consider the above arrangement as the "ultimate" NTD1. I can provide the NTD1 boards with the supply cut off for exactly this application.Click the image to open in full size.
I'm liking the sound of this ;-) Where can I find information about the new PSU?

Quote:
I will post a modified schematic later this evening to show exactly what resistance value to use, and what the overall power dissipation would be.
Great, thank you. I'll take a look when you post the schematic and get back to you.
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Old 4th December 2013, 01:13 AM   #645
opc is offline opc  Canada
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Hi Crom,

Attached is the schematic for the lower gain version using 32V supplies. This setup gives exactly 2VRMS out at 0dBFS with a mono ES9018 DAC. It also draws 180mA for each leg, giving a total dissipation of 46W which is pretty much what the original version does.

Also attached is the simulated distortion plot at -6dBFS. Since it's a perfect simulation of a differential circuit, the even harmonics are non-existent, but in the real world small imbalances between each leg will result in some even order distortion. Based on measurements, the even order products are about 10dB lower than the odd order harmonics, so you can use your imagination to fill in the blanks.

In the end you get a better power supply, lower noise, lower distortion and you don't give up anything in terms of dissipation.

As for the supply, I was referring to these:

"The Wire" Official Boards for All Projects Available Here! BAL-BAL, SE-SE, LPUHP

Cheers,
Owen
Attached Images
File Type: png NTD1 Mono.png (22.9 KB, 137 views)
File Type: png NTD1 MONO OUTPUT -6dBFS.png (76.1 KB, 141 views)
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Old 5th December 2013, 03:34 PM   #646
Crom is offline Crom  United Kingdom
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Hi Owen,

Thanks for the Schematic. The closest I can get to for R3& R4 is 100R (Caddock MP930). Also for R1&2 the closest would be 200R. Does this level of accuracy matter?

I'm also having a challenge matching the components from the main schematic and it also looks like some values of other components has changed between the original schematic and this one. Is the below is correct?

Using the schematic here as a reference to compare with the above schematic: http://www.diyaudio.com/forums/attac...pdac-final.pdf

New schematic opdac-final.pdf schematic
R13,14 10kR R12,30 30k9R
R11,12 2k3R VR7,9 5kR
C3,4 2200uF C35,C45 10uF

Do I follow the new schematic for these other component values or the original?
If I can only get 100/200R values is it possible to re-run the simulation with those values and will this give different component values for the other components?

Sorry, probably stupid questions but I'm still learning ;-)
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Old 5th December 2013, 06:31 PM   #647
Crom is offline Crom  United Kingdom
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Quote:
Originally Posted by Crom View Post
The closest I can get to for R3& R4 is 100R (Caddock MP930). Also for R1&2 the closest would be 200R. Does this level of accuracy matter?
This problem has been solved. texas components will create custom values, which is great. This means that I can get the exact values that you specify for the gain and IV but need to understand whether I need to source difference values for the components above as well, or, stick to the original bom.
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Old 6th December 2013, 12:55 AM   #648
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Hi Crom,

The circuit I posted was only for the voltages and the resistor values of the 4 gain/current set resistors.

The rest of the changes are only for simulation purposes, so you should follow the main schematic.

As for the resistor values themselves, it would be best to have them custom made if possible, but using a 100 and 200 resistor wouldn't be too far off. It just means less current and a little more gain, neither of which are good things...

As an alternative to the custom parts, you can order the 100 and 200 ohm resistors, and just parallel a 1206 thin film SMD part between their pins that would give you the exact value you're looking for.

Cheers,
Owen
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Old 6th February 2014, 01:45 PM   #649
Crom is offline Crom  United Kingdom
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Thanks Owen, I am now off and running with my build. I didn't want to clog up this thread so I'm posting to here:

I have managed to get hold of one of your original matched FET quads (kindly from bigpandhahk). It's group 10, see details here:

Click the image to open in full size.

Is there any particular position within the quad I should be using these in? For example, pairing the nearest numbers in a particular column or balancing the highest value with the lowest...or something like that.
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Old 6th February 2014, 01:58 PM   #650
opc is offline opc  Canada
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You got a good quad!

I would probably pair them according to transconductance, so 57/142 in one channel, and 45/146 in the other. Overall though, these are such a good match that it probably won't make any difference.

In addition to these parts, matching on the four resistors is also critical, so it's worth paying for 0.1% or 0.01% parts here, or hand matching using the parallel high value resistor technique described above.

The more identical the two legs of each channel are, the better the cancellation of second harmonics will be.

Cheers,
Owen
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