It’s been a productive evening, both USSPA-2 boards are assembled and powered up drama free 
They are being fed +/-17vdc and set up running approximately 60mA bias current. The DC offset is hovering around 0mV. Hopefully I’ll be getting first sound soon..... Too be continued!
They are being fed +/-17vdc and set up running approximately 60mA bias current. The DC offset is hovering around 0mV. Hopefully I’ll be getting first sound soon..... Too be continued!
Attachments
USSPA2 Singing Away!
My new USSPA2 preamp is powered up and singing away! This was a fun, easy and greatly satisfying project. Thank you Fab for putting this small GB together and the detailed build guide. I just introduced the USSPA2 into my main system, without any break-in time I’m very happy with what I am hearing. My speakers are very efficient and reveal any background noise or hum, I hear nothing, zero noise!!
I did decide to go with a two chassis setup. The power supply unit is built with the AMB Labs Sigma22 regulated psu board and a 50W, 2 x 24v R-core transformer. This is adjusted to deliver +/-17vdc to the USSPA2. The umbilical cable is constructed with 3x 18awg fine strand copper wire with silicone jacket for a very flexible 18 inch cable and terminated using Neutrik XLR connectors.
The USSPA2 unit is very basic/no switching capabilities and super short signal wiring. Both USSPA boards are running approximately 60mA bias current and have just about 0vdc offset (-1 to 1mV). I only had an Alps RK27 pot on hand, but this pre deserves a bit better than that. Any suggestions would be welcomed for an upgrade that won’t require a gold ingot for payment
A few details remain, I’m not sure how I want to detail the lid? I think I might plain down the three-quarter inch pine to 3/8 of an inch and router grooves in it for cooling capability. Just a thought....
For now, I’m going to let this preamp sing away and enjoy!
Thanks again Fab
Cheers,
Vunce
My new USSPA2 preamp is powered up and singing away! This was a fun, easy and greatly satisfying project. Thank you Fab for putting this small GB together and the detailed build guide. I just introduced the USSPA2 into my main system, without any break-in time I’m very happy with what I am hearing. My speakers are very efficient and reveal any background noise or hum, I hear nothing, zero noise!!
I did decide to go with a two chassis setup. The power supply unit is built with the AMB Labs Sigma22 regulated psu board and a 50W, 2 x 24v R-core transformer. This is adjusted to deliver +/-17vdc to the USSPA2. The umbilical cable is constructed with 3x 18awg fine strand copper wire with silicone jacket for a very flexible 18 inch cable and terminated using Neutrik XLR connectors.
The USSPA2 unit is very basic/no switching capabilities and super short signal wiring. Both USSPA boards are running approximately 60mA bias current and have just about 0vdc offset (-1 to 1mV). I only had an Alps RK27 pot on hand, but this pre deserves a bit better than that. Any suggestions would be welcomed for an upgrade that won’t require a gold ingot for payment
A few details remain, I’m not sure how I want to detail the lid? I think I might plain down the three-quarter inch pine to 3/8 of an inch and router grooves in it for cooling capability. Just a thought....
For now, I’m going to let this preamp sing away and enjoy!
Thanks again Fab
Cheers,
Vunce
Attachments
Hi Vunce
You are a quite fast and efficient builder
You are welcome and I am glad you liked this project
Keep us posted on further listening experience.
For the volume control the Alps pot is good enough for hifi grade equipment but my preference is for R2R relay attenuator because there is no wiper involved in the audio signal path...
LDR is also another alternative solution.
Fab
You are a quite fast and efficient builder
You are welcome and I am glad you liked this project
Keep us posted on further listening experience.
For the volume control the Alps pot is good enough for hifi grade equipment but my preference is for R2R relay attenuator because there is no wiper involved in the audio signal path...
LDR is also another alternative solution.
Fab
Wow Vunce you got some serious soldering skills and building skils
My experience don't mind me sharing.. Much better sound using a conductive casing, and high quality materials helps and Ground the casing. At very least top and bottom. Can also ground break to direct to ground.
Keep us posted on sanken.. Mine on the way.
Cheers
My experience don't mind me sharing.. Much better sound using a conductive casing, and high quality materials helps and Ground the casing. At very least top and bottom. Can also ground break to direct to ground.
Keep us posted on sanken.. Mine on the way.
Cheers
Thanks for the kind words Fellas
Fab,
Thanks for the alternative volume control types. I’ve been reading about R2R relay attenuators. This topic gets very involved, many ways to implement this type of volume control
Now, With the cold weather coming and shorter days (more nighttime for diya!) maybe I can finally get your USSA5 amplifier built!!
AnthonyA,
If it wasn’t for you starting this thread, I don’t think I would have found it in time to hop onboard this small GB, THANKS!!
I do have plans to use a single aluminum chassis with source switching capabilities for the USSPA2, but I will not receive the enclosure for some time yet. I didn’t want to wait that long before I heard this preamp, that’s why I built it this way. But, it is working out so well I do not know if there is a need to change it unless I really want switching capabilities. Even though the preamp section is in a wooden case there is no noise issues whatsoever, it is dead silent at idle.
Black boards = Black background
Cheers,
Vunce
Fab,
Thanks for the alternative volume control types. I’ve been reading about R2R relay attenuators. This topic gets very involved, many ways to implement this type of volume control
Now, With the cold weather coming and shorter days (more nighttime for diya!) maybe I can finally get your USSA5 amplifier built!!
AnthonyA,
If it wasn’t for you starting this thread, I don’t think I would have found it in time to hop onboard this small GB, THANKS!!
I do have plans to use a single aluminum chassis with source switching capabilities for the USSPA2, but I will not receive the enclosure for some time yet. I didn’t want to wait that long before I heard this preamp, that’s why I built it this way. But, it is working out so well I do not know if there is a need to change it unless I really want switching capabilities. Even though the preamp section is in a wooden case there is no noise issues whatsoever, it is dead silent at idle.
Black boards = Black background
Cheers,
Vunce
Hi Vunce
For R2R attenuator I use this great site to calculate the resistors values. You can set input and output impedance, number of steps, select tolerance type resistors, ....Well done.
Logarithmic Attenuator Calculator
For input impedance I use 10k ohms.
Looking forward to seeing the progress on your USSA5
Fab
For R2R attenuator I use this great site to calculate the resistors values. You can set input and output impedance, number of steps, select tolerance type resistors, ....Well done.
Logarithmic Attenuator Calculator
For input impedance I use 10k ohms.
Looking forward to seeing the progress on your USSA5
Fab
H2
You are welcome !
You have to re-evaluate your priorities...
Leaving out R19/20 will not give lowest distortion unless you are very lucky
These resistors may either decrease or lower H2....but you need an THD analyser to adjust it or use a 1K pot in lieu of either R19 or R20 and listen...
But I have found that the resulting level of H2 is not detrimental at all and is even often looked for (just look on the H2 thread of the Pass forum).
I have attached distortion profile of the USSA-3 power amp having an adjustment pot for H2 level as an example.
Fab
You are welcome !
You have to re-evaluate your priorities...
Leaving out R19/20 will not give lowest distortion unless you are very lucky
These resistors may either decrease or lower H2....but you need an THD analyser to adjust it or use a 1K pot in lieu of either R19 or R20 and listen...
But I have found that the resulting level of H2 is not detrimental at all and is even often looked for (just look on the H2 thread of the Pass forum).
I have attached distortion profile of the USSA-3 power amp having an adjustment pot for H2 level as an example.
Fab
Attachments
Jfet source
For the ones still looking for genuine brand new jfet for the USSPA , follow this link to the Diyaudio store:
JFETs from Linear Systems
I still have several pcb available from my last order.
Fab
For the ones still looking for genuine brand new jfet for the USSPA , follow this link to the Diyaudio store:
JFETs from Linear Systems
I still have several pcb available from my last order.
Fab
