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-   -   Salas DCG3 preamp (line & headphone) (https://www.diyaudio.com/forums/analog-line-level/296406-salas-dcg3-preamp-line-headphone.html)

asanden 14th January 2017 09:19 PM

Quote:

Originally Posted by Salas (https://www.diyaudio.com/forums/analog-line-level/296406-salas-dcg3-preamp-line-headphone-post4951262.html#post4951262)
A fully modern preamp system configuration. Congratulations. What attenuator impedance you calibrated for via the microcontroller? You will be probably discovering even more about sonic traits after familiar. Keep us posted.

Thanks!
I calibrated the attenuator impedance as 12K and the load impedance as 330K. I will try some NSL-32SR3 LDR's which have higher resistance/lower THD in a couple of weeks.
The preamp is mainly driven by a Soekris DAC (raw R2R signal, Rout=600R)
BTW; DC offset is max. 1mV

Salas 14th January 2017 09:23 PM

Will it allow you set attenuator impedance as 33k also?

asanden 14th January 2017 09:32 PM

Yes, I will try...

Salas 14th January 2017 09:41 PM

Try see how it compares to your now setting. Because this more than doubling in attenuator's impedance affects the bandwidth there should be some signs of bit different sonic presentation. Let us know what setting you preferred if you will think you could hear some differences indeed. If its worthwhile audible in your system you could try an intermediate point like 24k also for yet another comparison.

nashbap 15th January 2017 01:19 AM

Quote:

Originally Posted by Morde (https://www.diyaudio.com/forums/analog-line-level/296406-salas-dcg3-preamp-line-headphone-post4951377.html#post4951377)
My offset measurements were with the op-amp in place.

After the device running for about 45 min with the lid on the temps were 50 C on the DCG3 sink and 57 C on the PS sinks (the middle ones). I think I'm going to change the R10s to 10 ohm, since it seems to be giving around 130 mA bias with other users (Asanden). That should be plenty I think.

What size are your heat sinks and does the case have any ventilation holes? Thanks.

ammel68 15th January 2017 02:48 AM

Quote:

Originally Posted by Salas (https://www.diyaudio.com/forums/analog-line-level/296406-salas-dcg3-preamp-line-headphone-post4951088.html#post4951088)
1) Either solution will work. First one will look nicer, second one you can hand pick individual Leds for Vf if symmetry must be spot on in your application. In the first you can still vary the currents with R1 R2 to effect Vf total or add voltage with RxJ RyJ so to trim.

2) 5V Vin-Vout with such circuits is good to have across. Not that they won't work with less but for better performance. When using Schottky diodes you will be near there with 24VAC though. The mains stay usually higher than nominal also.

Okay, so I need to have at least 5V more going in or at least 17V for a 12V output?
If so, I would just use a 36V CT transformer.

Because I don't really understand how this PS works, I'm not clear on what you mean by "varying the currents with R1 and R2" if I short out some of the Kingbrights. :confused:
Will raising their value from 33 ohms lower the output voltages or would I need to lower their values to get lower voltage?

Can a 100 ohm trimmer be substituted for R1 and R2 to vary the output voltages?

Thanks!

Morde 15th January 2017 06:28 AM

2 Attachment(s)
Quote:

Originally Posted by nashbap (https://www.diyaudio.com/forums/analog-line-level/296406-salas-dcg3-preamp-line-headphone-post4952005.html#post4952005)
What size are your heat sinks and does the case have any ventilation holes? Thanks.

The sink is like this and the case is from eBay. You can see the ventilation holes on the top cover in my earlier post here. I have also drilled some additional ventilation holes between the sink's fins, as you can see in this post. I guess you could always have better ventilation, but this is the best I could do with my limited metal machining tools.

Edit: I added a couple of photos to clarify my enclosure.

Morde 15th January 2017 09:00 AM

Quote:

Originally Posted by Morde (https://www.diyaudio.com/forums/analog-line-level/296406-salas-dcg3-preamp-line-headphone-post4951377.html#post4951377)
My offset measurements were with the op-amp in place.

After the device running for about 45 min with the lid on the temps were 50 C on the DCG3 sink and 57 C on the PS sinks (the middle ones). I think I'm going to change the R10s to 10 ohm, since it seems to be giving around 130 mA bias with other users (Asanden). That should be plenty I think.

Now that I changed the R10s to 10 ohm my bias is running at 136/137 mA in L/R channels. The temperatures were after about 45 min with top cover on 46 C for DCG3 and 50 C for DCSTB middle sinks. I'm satisfied. Let's see when I find the time to complete wirings and get to finally listen to this beast. :eguitar:

xrk971 15th January 2017 09:41 AM

2 Attachment(s)
I just finished my DCG3 (Teabag board) tonight. The build was very smooth and it fired up the first time with 100mA bias. I had to adjust DC offset from 500mV down to 0mV. Then installed servo opamp. Listening to a few test tracks right now - very powerful headamp. I am using the shunt regulated PSU board from Prasi's HA project. Teabag's board is a pleasure to assemble - went together in less than 3 hrs and works the first time. I am using Idss matched DN2540's and Vgs matched IRF69610's, and Hfe matched BC560's. The LSK389's are from Linear Systems direct, and have been sitting in my bin of parts for 3 years and finally have a home here.

Amps sounds very nice. Thank you Salas and Teabag. :)

https://www.diyaudio.com/forums/atta...mp-test-01.jpg

https://www.diyaudio.com/forums/atta...mp-test-02.jpg

Salas 15th January 2017 10:20 AM

Hi X. Nice to see your build. Metal can Jfets utilized too. Cool. What headphone you use?

*That PSU has high output Z (as much as its noise cleaner section series resistor). Its a series chip regulated type with just a shunt noise cleaner. Otherwise it would have big sinks. Good for low noise but not to be sharing highly current swinging channels. I advise for either two of them or a low Z type, still better if two. Thick & short power rails cables use anyway also. There will be DC voltage loss too both on that PSU's series input and output resistors as well as on the long thin cabling. All those advised measures will be improving on slam, instruments differentiation, and resolution.


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