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

Dimdim 17th January 2017 08:07 AM

1 Attachment(s)
It's alive!

https://www.diyaudio.com/forums/atta...1&d=1484643176

For the moment it's powered by my LM317/337 "test" power supply.

I've set bias at 150mA since I'm testing it with power-hungry Fostex T50RP MK3 headphones.

All 4 of my Ebay-sourced uPA parts measured exactly the same, so no matching was necessary.

I measured ~20 x BC560C to find 4 of exactly the same Hfe.

The BF256Bs that I got from Mouser were pretty strong. I used 220R for the two R3s, getting 4.3mA and 4.5mA Id respectively.

I bought 4 x DN2540 from Mouser, and used the 2 of them that measured the closest. The end result was about 10mA of difference in bias between the 2 channels.

My total current consumption is ~330mA for the positive rail and ~320mA for the negative one.

DC offset drift after warmup and adjustments was +/- 3mV without the op-amp installed. Went to 1mV with the op-amp. It might be even less, my meter won't go lower.

It had gotten pretty late so I didn't do any serious listening, but first impressions are very very postive. I'll do more listening and with a better source and power supply in the days to come.

AndrewT 17th January 2017 08:40 AM

Quote:

Originally Posted by marcus1 (https://www.diyaudio.com/forums/analog-line-level/296406-salas-dcg3-preamp-line-headphone-post4953917.html#post4953917)
I have two of the same multi-tapped transformers (found another one!) with secondary winding's of: 0v-9v-12v-15v-18v-21v-24v-30v

Each DCSTB requires an AC input of AC-CT-AC.
I'm not sure: Using my transformers, should I use the following taps for AC-CT-AC: 0v-9v-18v?

Thanks

if you make the 15Vac tap equal to your zero volts you end up with:15, 6, 3, 0, 3 , 6, 9, 15Vac
or if you make the zero volts at the 18V tap you get:
18, 9, 6, 0, 3, 6, 12Vac. Notice this has only one symetrical pairs.
Whereas the first offers three symetrical pairs.
You also have:
9, 0, 3, 6, 9, 12, 15, 21Vac when the 9Vac is made the zero volts
and:
12, 3, 0, 3, 6, 9, 12, 18Vac when the 12Vac is made to be zero volts.

I can see symetrical options of 3Vac, 6Vac, 9Vac, 12Vac and 15Vac
but these are NOT all available at the same time. You have to choose the option that is most useful and then keep that Zero Volt tap fixed at that tapping.

Salas 17th January 2017 08:50 AM

Quote:

Originally Posted by Dimdim (https://www.diyaudio.com/forums/analog-line-level/296406-salas-dcg3-preamp-line-headphone-post4954494.html#post4954494)
It's alive!

Nice initial phase work. Good luck down the road :cool:

Morde 17th January 2017 02:27 PM

2 Attachment(s)
I got my project playing music for the first time today. Just an iPhone and Spotify as a source for quick test, but everything seemed to be working ok. Infected Mushroom seemed to pound the bass as it should with my LCD-2's. Still need to finish and tidy up the wirings and make a hole in the front plate for 6,3 mm headphone connector. I think I might have to lower the gain, since even with iPhone I had volume only on level 25/50. I have LDR vol. control and it's set to 20 kohms impedance. My current gain is 3x, but I still need to test the DCG3 with my Genelec G3 active speakers before tweaking.

Salas 17th January 2017 03:12 PM

Good first run. Especially positive now to know in practice about driving the Audeze :up:

Salas 17th January 2017 03:21 PM

...Does "Audeze" mean Odyssey in some funky spelling way? :scratch:

xrk971 17th January 2017 04:42 PM

I think it might be more along lines of "Audio" and "Ease" or contraction of "easy audio"flipped. At least that is what I think where it comes from...

ammel68 17th January 2017 04:45 PM

Salas, adding a 15k ohm resistor across R14 didn't do anything for the wandering offset.
Any other suggestions?

Salas 17th January 2017 05:06 PM

Quote:

Originally Posted by ammel68 (https://www.diyaudio.com/forums/analog-line-level/296406-salas-dcg3-preamp-line-headphone-post4954971.html#post4954971)
Salas, adding a 15k ohm resistor across R14 didn't do anything for the wandering offset.
Any other suggestions?

Tryouts for hand picked best result sample JFET input technology op-amp from your collection. If the 0-1mV wandering seems cyclic then its the normal integration time of servo loop, so no worries.

Salas 17th January 2017 05:47 PM

6 Attachment(s)
Quote:

Originally Posted by xrk971 (https://www.diyaudio.com/forums/analog-line-level/296406-salas-dcg3-preamp-line-headphone-post4954969.html#post4954969)
I think it might be more along lines of "Audio" and "Ease" or contraction of "easy audio"flipped. At least that is what I think where it comes from...

Cool explanation scenario, makes sense.

BTW I took some loopback shots of my "ancient" interface that runs properly on Win XP only. Two are with a meter long cheapo cable on HiZ and LowZ input config (1MegΩ with jack, 1.5kΩ with RCA to XLR adapter) last one is with short cable sans adapters. You should be able to achieve better than your now base line of interference and residual THD with the modern Focusrite I would suppose and its worth investigating as the DUT included loopback result looks suspiciously like the interface loopback alone times gain and the more exposed area to interference same type of pick up. Its not a measurements thread I know, and we better don't expand in general lab issues talk, but I include them since that thing came along and they might give you a lead.


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