Hey, usually I post only images of already finished devices here but I thought it could be nice idea to begin a build thread and add news as it goes 
I am building tube OTL headamp, but this one is special in a way that I planned it the best I could in terms of parameters with lots of additions like inputs switching, analog meters, some modes etc. It will be just analogue, very simple, no MCU. Only "digital" switching thingy is raspberry pi on one of the inputs for DLNA, but its powered on demand. Its just a concept and not sure how it will evolve and what the parameters will be, but simulation looks very promising
I am in very begining and gathering all resources atm but for now I will share the schematics what it's all about...
PSU
There is +190V LM317 regulated PSU for tube preamp supported by high voltage mosfet. Very simple and very quiet
Balanced +-12,6V slow turn on regulated PSU by LM317/337 is for tube filaments and VU log amp part OPs
+5V "digital" PSU is just super simple LM1085. Powering all the relays and some cmos. In case of DLNA input selected it is powering rasp pi zero w with boss mini dac.
AMP
I have built some really very good performing OTLs in past, but this time I wanted to do it even better. For this purpose I needed bigger open loop gain to get more feedback and get THD as low as possible. Begins with ecc83 tube diff stage with common cathode CCS with 2N5551 controlled by TL431 reference. Negative voltage for CCS is taken from filament regulated -12,6V psu regulated even more by 7909. Next stage ecc83 again looks like srpp but its not, its normal amp stage with anode load, well tube ccs basically, to get exactly 1/2 voltage on anode for next direct coupled wcf stage by 6n6p. To get the best out of it I will use big PP output cap, it does not need to be super big, a few tens of uF are enough as there is plenty of negative feedback compensating for loss on low frequencies. Thanks to diff input, its very convinient way to implement nfb and play with it. I would like to try switch feedback input divider and change gain based on headphones used. Expected performance is very low THD even into hard loads such as 150-250R headphones...
Inputs / Outputs and extras:
Because of safety there is going to be DC protection circuit with turn on delay to omit any DC on output while powering up and also during playback. As mentioned in intro there is going to be simple input switching as well as DLNA wifi playback support from rasberry powered on demand. There is going to be remote in/out feeds in case of powering up power amps or turning on this preamp by remote source, such as +5V from PC USB f.e. A bit mess for not-involved one but here it is
Front panel stuff:
On front panel there will be small PCB with all the switches, LEDs, VU meter log amp etc. Again a bit of mess but makes connections much more cleaner between front panel and main board. Analogue log VU amps are not my invention but inspired from different service manuals and somehow combined to work for my needs...
More to come (hopefully) soon...
I am building tube OTL headamp, but this one is special in a way that I planned it the best I could in terms of parameters with lots of additions like inputs switching, analog meters, some modes etc. It will be just analogue, very simple, no MCU. Only "digital" switching thingy is raspberry pi on one of the inputs for DLNA, but its powered on demand. Its just a concept and not sure how it will evolve and what the parameters will be, but simulation looks very promising
I am in very begining and gathering all resources atm but for now I will share the schematics what it's all about...
PSU
There is +190V LM317 regulated PSU for tube preamp supported by high voltage mosfet. Very simple and very quiet
Balanced +-12,6V slow turn on regulated PSU by LM317/337 is for tube filaments and VU log amp part OPs
+5V "digital" PSU is just super simple LM1085. Powering all the relays and some cmos. In case of DLNA input selected it is powering rasp pi zero w with boss mini dac.
AMP
I have built some really very good performing OTLs in past, but this time I wanted to do it even better. For this purpose I needed bigger open loop gain to get more feedback and get THD as low as possible. Begins with ecc83 tube diff stage with common cathode CCS with 2N5551 controlled by TL431 reference. Negative voltage for CCS is taken from filament regulated -12,6V psu regulated even more by 7909. Next stage ecc83 again looks like srpp but its not, its normal amp stage with anode load, well tube ccs basically, to get exactly 1/2 voltage on anode for next direct coupled wcf stage by 6n6p. To get the best out of it I will use big PP output cap, it does not need to be super big, a few tens of uF are enough as there is plenty of negative feedback compensating for loss on low frequencies. Thanks to diff input, its very convinient way to implement nfb and play with it. I would like to try switch feedback input divider and change gain based on headphones used. Expected performance is very low THD even into hard loads such as 150-250R headphones...
Inputs / Outputs and extras:
Because of safety there is going to be DC protection circuit with turn on delay to omit any DC on output while powering up and also during playback. As mentioned in intro there is going to be simple input switching as well as DLNA wifi playback support from rasberry powered on demand. There is going to be remote in/out feeds in case of powering up power amps or turning on this preamp by remote source, such as +5V from PC USB f.e. A bit mess for not-involved one but here it is
Front panel stuff:
On front panel there will be small PCB with all the switches, LEDs, VU meter log amp etc. Again a bit of mess but makes connections much more cleaner between front panel and main board. Analogue log VU amps are not my invention but inspired from different service manuals and somehow combined to work for my needs...
More to come (hopefully) soon...
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Could it be possible to ban the use of words like "ultimate" in any audio equipment description? I think it's just a clickbite word that simply attract more attention to many topics than what they deserve...nothing in audio is an ultimate solution to anything.I'd say "complex project" instead...
Month later... finally an update! I have finally received transformer and case:
putting things together...
Rear view of front panel from left to right: alps volume, input leds, rotation switch mounted on PCB which goes on metal stands, below pcb will be hidden mode switches and headphone jack, continuing with backlit VU meters...
Front panel images and details comming soon... right now I'm trying the things and tweaking the amp. Apart small issues all good so far, looks very promising
putting things together...
Rear view of front panel from left to right: alps volume, input leds, rotation switch mounted on PCB which goes on metal stands, below pcb will be hidden mode switches and headphone jack, continuing with backlit VU meters...
Front panel images and details comming soon... right now I'm trying the things and tweaking the amp. Apart small issues all good so far, looks very promising
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My preamp is finished!
I forgot to update progress... met all my expectations, distortion figures correlates to my simulations. Distortion is very hardly measurable by my gear and I have measured ~0,0008% THD into 250R load:
It simply sounds amazing, one of the best I've built. Very versatile, can drive all my headphones with no problem, even low impedance ones.
Here you can see full image gallery: https://drive.google.com/drive/folders/1EqxZDPfVl-CY-Q4GJtL3WduEeCVX4nVC?usp=sharing
Cheers!
I forgot to update progress... met all my expectations, distortion figures correlates to my simulations. Distortion is very hardly measurable by my gear and I have measured ~0,0008% THD into 250R load:
It simply sounds amazing, one of the best I've built. Very versatile, can drive all my headphones with no problem, even low impedance ones.
Here you can see full image gallery: https://drive.google.com/drive/folders/1EqxZDPfVl-CY-Q4GJtL3WduEeCVX4nVC?usp=sharing
Cheers!