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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Salas DCG3 preamp (line & headphone)
Salas DCG3 preamp (line & headphone)
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Old 7th September 2016, 01:48 PM   #1
Salas is offline Salas  Greece
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Salas DCG3 preamp (line & headphone)
Default Salas DCG3 preamp (line & headphone)

Hi all, this thread is about a JFET & MOSFET preamp I am cooking for sometime now. Its two stage single ended Class A circuit.
It was developed on the bench involving much experimentation with topology and a range of active parts. Simulations came second.

Some speakers are insensitive, some amps also, or a source is relatively low output like a phono stage. So a line preamp with gain is good to have.

Its named DCG3 because it has DC servo and three MOSFETS. Its standard gain setting is also times three.

The main schematic is attached* Click for circuit description

Some THD plots when driving 1.5kOhm line input and various headphones are also attached.

*(12 Oct 2016 updated and build guide/bom pdf added)

erratum: C6,C7 should read 4x in the BOM. J3 in preamp's schematic is named Q5 on the actual board.

About J3/Q5 possible IDSS tolerances and a matching R3 value: post#452

V1.03 boards have split ground between channels (can be bridged) and the relay is reconfigured to rest at ground: post #3207
Attached Images
File Type: png Line100mA400mV.png (19.3 KB, 12213 views)
File Type: png LineFFT.png (14.1 KB, 11856 views)
File Type: png Sen100mA400mV.png (19.3 KB, 11657 views)
File Type: png AKG100mA400mV.png (20.4 KB, 11328 views)
File Type: png Fostex100mA400mV.png (17.9 KB, 10304 views)
File Type: png Grado100mA400mV.png (17.0 KB, 1218 views)
File Type: png Grado150mA400mV.png (17.5 KB, 1194 views)
File Type: png DCG-3_Sch.png (26.7 KB, 6127 views)
Attached Files
File Type: pdf BOM-SCH-GUIDE_DCG3-DCSTB.pdf (785.3 KB, 1694 views)
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Old 7th September 2016, 03:26 PM   #2
Salas is offline Salas  Greece
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Salas DCG3 preamp (line & headphone)
Some pictures

There is also a simple series DC stabilizer I designed for it
Attached Images
File Type: jpg Brbrd.jpg (382.0 KB, 6029 views)
File Type: jpg 1DCG3detail.jpg (387.7 KB, 4487 views)
File Type: jpg DSCF4623.JPG (575.1 KB, 4765 views)
File Type: png DCSTB_Sch.png (12.4 KB, 4154 views)
File Type: png DCSTB_Raw_Sch.png (5.7 KB, 3450 views)
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Old 7th September 2016, 03:34 PM   #3
merlin el mago is offline merlin el mago
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Old 7th September 2016, 03:39 PM   #4
merlin el mago is offline merlin el mago
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Is suitable for high impedance headphones?
What are the rail voltages?
Power consumption?
Etc....
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Old 7th September 2016, 03:45 PM   #5
Salas is offline Salas  Greece
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Salas DCG3 preamp (line & headphone)
Beta build samples by friends. Thanks for the good early feedback to Michael V and Tham C especially.

And clips by Tea tampering away in his basement following no particular order of instructions
https://www.youtube.com/watch?v=QSQSrmGxBAY https://www.youtube.com/watch?v=URX2...ature=youtu.be
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File Type: jpg Michael_V.JPG (462.5 KB, 4417 views)
File Type: jpg Tham_C.jpg (310.4 KB, 4046 views)
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Old 7th September 2016, 03:49 PM   #6
merlin el mago is offline merlin el mago
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What are these lights at the psu?
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Old 7th September 2016, 04:00 PM   #7
Salas is offline Salas  Greece
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Salas DCG3 preamp (line & headphone)
Quote:
Originally Posted by merlin el mago View Post
Is suitable for high impedance headphones?
What are the rail voltages?
Power consumption?
Etc....
Of course. Sennheiser HD40 you see measurement for is 600 Ohm for instance. Its an old insensitive for SPL headphone I got. Those measurements were done like in the O2 tests to can have perspective. It was loud enough with gain 3X but you can go 4X. 1.5K R6 will do that. With AKG K612 PRO 120 Ohm its very loud at 3X and hiss is absent.

Rail is +/- 17.3V. DC power consumption about 7W for two channels (when with 100mA output stage bias)
Attached Images
File Type: jpg DCG_HP_FOOBAR.jpg (493.3 KB, 3538 views)
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Old 7th September 2016, 04:05 PM   #8
merlin el mago is offline merlin el mago
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What about the SQ?
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Old 7th September 2016, 04:09 PM   #9
Salas is offline Salas  Greece
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Salas DCG3 preamp (line & headphone)
Quote:
Originally Posted by merlin el mago View Post
What are these lights at the psu?
Bargraph Leds. No need to match Vref Leds for DCG3 because it does not care about exact same +/- rails. Bar Led packs are compact, straightforward to solder, looking great.
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File Type: jpeg DCSTB (2).jpeg (129.3 KB, 2873 views)
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Old 7th September 2016, 04:10 PM   #10
merlin el mago is offline merlin el mago
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Quote:
Originally Posted by Salas View Post
Bargraph Leds. No need to match Vref Leds for DCG3 because it does not care about exact same +/- rails. Bar Led packs are compact, straightforward to solder, looking great.
Thanks.
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