Salas DCG3 preamp (line & headphone)

diyAudio Chief Moderator
Joined 2002
Paid Member
Hi all, this thread is about a JFET & MOSFET preamp I am cooking for sometime now. Its a 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 full circuit description & Click for the development story

Some THD plots when driving 1.5kOhm line and various actual 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 board has correct J3 designation, split ground between channels (can be bridged) and the relay is reconfigured to rest at ground: post #3207


  • Line100mA400mV.png
    19.3 KB · Views: 26,994
  • Grado150mA400mV.png
    17.5 KB · Views: 2,611
  • Grado100mA400mV.png
    17 KB · Views: 2,567
  • Fostex100mA400mV.png
    17.9 KB · Views: 23,409
  • AKG100mA400mV.png
    20.4 KB · Views: 24,749
  • Sen100mA400mV.png
    19.3 KB · Views: 25,469
  • LineFFT.png
    14.1 KB · Views: 26,804
  • DCG-3_Sch.png
    26.7 KB · Views: 14,262
    785.3 KB · Views: 3,788
  • DCG3_Sch_V1.03.png
    30.3 KB · Views: 4,587
  • Like
Reactions: 1 users
diyAudio Chief Moderator
Joined 2002
Paid Member
Some pictures

There is also a simple series DC stabilizer I designed for it


  • DSCF4623.JPG
    575.1 KB · Views: 10,045
  • 1DCG3detail.jpg
    387.7 KB · Views: 9,236
  • Brbrd.jpg
    382 KB · Views: 12,268
  • DCSTB_Raw_Sch.png
    5.7 KB · Views: 7,905
  • DCSTB_Sch.png
    12.4 KB · Views: 9,456
  • Like
Reactions: 1 users
diyAudio Chief Moderator
Joined 2002
Paid Member


  • Tham_C.jpg
    310.4 KB · Views: 7,990
  • Michael_V.JPG
    462.5 KB · Views: 8,783
  • Like
Reactions: 1 users
diyAudio Chief Moderator
Joined 2002
Paid Member
Is suitable for high impedance headphones?
What are the rail voltages?
Power consumption?

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)


    493.3 KB · Views: 6,740
diyAudio Chief Moderator
Joined 2002
Paid Member
I had those uPA NEC monolithic JFET flatpacks that had very nice curve matching consistency and I wanted to test ideas around them. So I beavered away for one and a half months and there it was. But I tested everything I had, even LND150 mini MOSFETS. The board has LSK389 positions too because not NOS. Closest to uPA THD profile. But uPA are still cheap and available because nobody uses them.

Has a sound smooth as tubes no worries. But combining resolution better than DCB1 even. See how it goes second harmonic as the load gets heavier in the THD attachments. Its the single ended output stage MOSFET nature. 100-150mA bias and virtually zero Ohm output impedance gives bass slam and definition too.

The headphones sound easily more alive bigger and smoother than they did on my Benchmark DAC1. Swiss knife preamp.
diyAudio Chief Moderator
Joined 2002
Paid Member
Here is IMD SMPTE when driving 120 Ohm for instance. That's strong 250Hz trying to modulate quieter 8kHZ. See the little bass tone modulations around the high tone's base. 0.0027% modulation at 101dB SPL with those AKG. Means clarity in busy music. Not easy to do that with SE tube headphone amp.


  • DCG_HP_SMPTE.jpg
    924.5 KB · Views: 5,673
diyAudio Chief Moderator
Joined 2002
Paid Member
Some oscilloscope shots. Bandwidth is 1MHZ with 20kOhm pot at middle value (worst case situation)


  • DS2_QuickPrint2.png
    24.8 KB · Views: 5,314
  • DS2_QuickPrint1.png
    32.7 KB · Views: 5,482
  • DS2_QuickPrint3.png
    32.9 KB · Views: 5,264