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
Sept 2023 extra measurements like multi-tone resolution at same level as here. Also THD vs Freq sweeps for alternative J1,J2 types: post#6941-42
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
Sept 2023 extra measurements like multi-tone resolution at same level as here. Also THD vs Freq sweeps for alternative J1,J2 types: post#6941-42
Attachments
(edit)
Some development and prototyping pictures. Also a series type PSU/DC stabilizer with Leds voltage reference and Sziklai pair pass transistors that I designed to accompany it. The DCSTB.
Some development and prototyping pictures. Also a series type PSU/DC stabilizer with Leds voltage reference and Sziklai pair pass transistors that I designed to accompany it. The DCSTB.
Attachments
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=URX22TNevNw&feature=youtu.be
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=URX22TNevNw&feature=youtu.be
Attachments
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)
Attachments
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.
Attachments
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.
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.
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.
