This is the second version of my tube preamp but this time has different goal, not to drive headphones but I wanted balanced outputs to drive power amp directly so everything is optimised to drive line-level loads.
Compared to the first version I have significantly upgraded power supply. High voltage part contains CRCRC filter before it goes to the mosfet + LD1086 regulator with lots of blocking and capacities around. Thanks to this I have basically battery-like psu for the tubes with excellent mains filtering.
For tube filaments I have LM337/317 balanced psu with again lots of filtering around and slow turn-on.
There is also regulated 5V1 psu for relays and controll of the preamp + to power raspberry pi board with DAC ES9038Q hat for wifi audio streaming.
Audio path / voodoo improvements over 1st version:
There is my own "invention/evolution" design of combined voltage and current feedback directly into input differential pair. Thanks to this, preamp has excellent stability, transient response and low distortion. Thats why its somehow VFA/CFA preamp, I have tried this in previous samples also with WCF output (white cathode follower) for unbalanced version with even better results of 0,0002% THD into 300R load
Audio extras
- audio part of PCB is standalone so its possible to wire it via IDC ribbon cable to whatever control board, I have designed two versions:
Analog "pure" one with 555+4060+4040 timers
Digital with attiny with IR remote control
Preamp schematic:
PSU
I/O
Control ("analog" without remote) // edit // actually I noticed one change in schematic, that I have not used T2, as that would send (audiodetect) reset loop all over the place from analog to digital gnd, so I have replaced this one with cny17f opto to have this competely clean and separated. Digi gnd is complately isolated from analog gnd, and only one way where they meet is from DAC RPI hat into analog input. By doing so there is absolutely pure connection without any mess in harmonics...
Some build pics
Mechanical design
I wanted to design future-proof sturdy enclosure with complete access to PCB from both sides... The main structure consists of separated rear panel (for easier label printing), 8mm thick machined anodised aluminium front panel, and to interconnect panels there is C-shaped profile on side which creates base for PCB and on other side U-shaped tunnel with the very same function but to also hold transformer and to cover all the wiring inside of it:
For absolute best mains filtering I have used custom audio-grade toroidal transformer with earthed primary/secondary copper shielding. Transformer windings are shielded on the outside by several layers of laminations. I potted whole assembly into iron cover with polyuretane potting compound. So as a result transformer is tripple shielded. Toroidal iron cover is actually hanging in the U-shaped tunnel by rubber vibration dempeners, so its also mechanically isolated from the rest of the chassis and "levitating".
Aand as upload sucks and cannot insert more images and when i do so on external server i get hated i will continue in next post
Compared to the first version I have significantly upgraded power supply. High voltage part contains CRCRC filter before it goes to the mosfet + LD1086 regulator with lots of blocking and capacities around. Thanks to this I have basically battery-like psu for the tubes with excellent mains filtering.
For tube filaments I have LM337/317 balanced psu with again lots of filtering around and slow turn-on.
There is also regulated 5V1 psu for relays and controll of the preamp + to power raspberry pi board with DAC ES9038Q hat for wifi audio streaming.
Audio path / voodoo improvements over 1st version:
- in signal path there is minimum coupling capacitors and if so all are audio-grade polypropylenes
- I wanted to minimise series relay connection as much as possible, so there is only one reed-relay at the input selection, for the output mute I choosed relay to short to the ground so there is no signal going thru the contact.
- All impedances are lower also in feedback loop
- I have improved current sinks in input differential pair = TL431 + low noise BC550, soldered from the back of the pcb to even smaller thermal drift and improved stability
There is my own "invention/evolution" design of combined voltage and current feedback directly into input differential pair. Thanks to this, preamp has excellent stability, transient response and low distortion. Thats why its somehow VFA/CFA preamp, I have tried this in previous samples also with WCF output (white cathode follower) for unbalanced version with even better results of 0,0002% THD into 300R load
Audio extras
- audio part of PCB is standalone so its possible to wire it via IDC ribbon cable to whatever control board, I have designed two versions:
Analog "pure" one with 555+4060+4040 timers
Digital with attiny with IR remote control
- there is automatic power-off function after 30minutes without audio signal to save electricity and tube life.
- stereo/mono switch
- preamp gain switch
- (tape)monitor switch
- hardware mute switch
- as mentioned in psu section above, one of the inputs is RPI with DAC hat for dlna audio streaming thru wifi
- first line input contains also remote-out +5V trigger so its possible to turn on phono preamp with selecting the one
Preamp schematic:
PSU
I/O
Control ("analog" without remote) // edit // actually I noticed one change in schematic, that I have not used T2, as that would send (audiodetect) reset loop all over the place from analog to digital gnd, so I have replaced this one with cny17f opto to have this competely clean and separated. Digi gnd is complately isolated from analog gnd, and only one way where they meet is from DAC RPI hat into analog input. By doing so there is absolutely pure connection without any mess in harmonics...
Some build pics
Mechanical design
I wanted to design future-proof sturdy enclosure with complete access to PCB from both sides... The main structure consists of separated rear panel (for easier label printing), 8mm thick machined anodised aluminium front panel, and to interconnect panels there is C-shaped profile on side which creates base for PCB and on other side U-shaped tunnel with the very same function but to also hold transformer and to cover all the wiring inside of it:
For absolute best mains filtering I have used custom audio-grade toroidal transformer with earthed primary/secondary copper shielding. Transformer windings are shielded on the outside by several layers of laminations. I potted whole assembly into iron cover with polyuretane potting compound. So as a result transformer is tripple shielded. Toroidal iron cover is actually hanging in the U-shaped tunnel by rubber vibration dempeners, so its also mechanically isolated from the rest of the chassis and "levitating".
Aand as upload sucks and cannot insert more images and when i do so on external server i get hated i will continue in next post
Last edited:
Below are more pics of pot wiring, ir sensor installation and backlight of knobs. I also did jumper on control board to be able to switch between cold white (modern) and yellow(vintage) knob backlight as I have many different power amps
Finished preamp
Some video what it does...
notice the "pop" when I mute, not sure why it does this, as previous version did not - but one thing is different and thats the muting relay is shorting output to the ground (via 100R resistor). Maybe some leftover DC of big caps or changing feedback loop impedance by shorting the output or what, I suppose that's the tax for not going with signal thru relay contacts voodoo hiend **** or what
Some measurement below, 2k load, spectrum calibrated to dBV... please see mains harmonics
Finished preamp
Some video what it does...
notice the "pop" when I mute, not sure why it does this, as previous version did not - but one thing is different and thats the muting relay is shorting output to the ground (via 100R resistor). Maybe some leftover DC of big caps or changing feedback loop impedance by shorting the output or what, I suppose that's the tax for not going with signal thru relay contacts
voodoo hiend **** or what Some measurement below, 2k load, spectrum calibrated to dBV... please see mains harmonics
Last edited:
Wiliks,
Looks good, performs good, nice job!
Because my latest tube amplifiers are balanced low power amplifiers, I can only use them with my XLR output CD player.
I should build a single ended to balanced preamp, so I can also use my balanced amplifiers with my LPs, FM tuner, DVD, and TV.
If I do design and build a single ended to balanced preamp, I am sure it will be simple and not all that good looking.
Looks good, performs good, nice job!
Because my latest tube amplifiers are balanced low power amplifiers, I can only use them with my XLR output CD player.
I should build a single ended to balanced preamp, so I can also use my balanced amplifiers with my LPs, FM tuner, DVD, and TV.
If I do design and build a single ended to balanced preamp, I am sure it will be simple and not all that good looking.