I'm listening in stero today and I'm impressed with the sound quality vs value for money proposition - very listenable with the linear power supply at 19V, slightly on the bright side but with plenty of 'detail'. I'm curently using USB Audio Pro on my smartphone as the source, with 'bit perfect' on, and I've played both 44.1 and 48KHz files without incident. I can't play 88/96KHz files as bit perfect as USB Audio Pro wants to downsample them but I suspect that is a limitation of the USB chip rather than the STA333.
As this is not going to be used as my primary HPA I think I'll stick with what I have, so I won't try to feed the STA333 chip with i2s and I won't transfer the low-noise 5V module from the faulty STA board (I can reuse that for powering the Bluetooth module that I will use in front of my Kubelik).
As this is not going to be used as my primary HPA I think I'll stick with what I have, so I won't try to feed the STA333 chip with i2s and I won't transfer the low-noise 5V module from the faulty STA board (I can reuse that for powering the Bluetooth module that I will use in front of my Kubelik).
Installed the STA board into the chassis and hooked up the transformers via the bipolar caps. I had to remove the USB socket and install some connection pins as there wasn't enough room to clear one of the the transformers.
I just need to drill a couple of panels and some final hook-up wiring to finish the build.
I just need to drill a couple of panels and some final hook-up wiring to finish the build.
That's encouraging. Did you end up with the 200nF cap on the secondary of the trafos? As I recall the roll-off is a little bit premature without it, it has the effect of extending the HF response by forming a resonance with the trafo leakage inductance.
Thanks Richard and thanks for presenting and helping with this interesting little project.
I have some 200nF caps on order as I could only find a couple of 100nF caps in my spares so I've been using them, hooked up with croc-clips, for now.
I think I'll try a filter on the SMPS to see if that improves things using it - I have a few boards to build the LP filter that Nelson Pass designed for his VFET amp and I think I can squeeze it into the little chassis if it helps.
I have some 200nF caps on order as I could only find a couple of 100nF caps in my spares so I've been using them, hooked up with croc-clips, for now.
I think I'll try a filter on the SMPS to see if that improves things using it - I have a few boards to build the LP filter that Nelson Pass designed for his VFET amp and I think I can squeeze it into the little chassis if it helps.
The quality of the supply to the STA333 is paramount as there isn't any feedback in the output stage (unlike a typical classD amp), meaning the PSRR is zero. Its open loop - any and all power supply noise will get to the output. The trafos help with noise reduction but only so far.
P.S. I did eventually twig about the trafo turns number when I scrolled back and looked at your pics. I must have had higher than normal presence of mind to write the numbers on the trafos themselves
P.S. I did eventually twig about the trafo turns number when I scrolled back and looked at your pics. I must have had higher than normal presence of mind to write the numbers on the trafos themselves
So perhaps a battery power supply would be worth consideration?The quality of the supply to the STA333 is paramount as there isn't any feedback in the output stage (unlike a typical classD amp), meaning the PSRR is zero. Its open loop - any and all power supply noise will get to the output. The trafos help with noise reduction but only so far.
P.S. I did eventually twig about the trafo turns number when I scrolled back and looked at your pics. I must have had higher than normal presence of mind to write the numbers on the trafos themselves
I've also been looking at your posts at the beginning of the thread and noted your capacitor bank used for your initial experiments - I assume using a SMPS?
I think a battery supply is definitely worth a punt, though not all batteries are equally low noise.
I very much doubt I was using an SMPSU for the STA333, more likely I was feeding it from my adjustable bench supply, which is linear. In those days though I hadn't learned that my bench supply is really rather noisy, these days I'd use a discrete regulator after it I think.
I very much doubt I was using an SMPSU for the STA333, more likely I was feeding it from my adjustable bench supply, which is linear. In those days though I hadn't learned that my bench supply is really rather noisy, these days I'd use a discrete regulator after it I think.
I don't have a battery supply to hand so I would need to invest but just having a rummage and I have 'rediscovered' a small regulator board I had forgotten about, it uses the TPS7A47 in conjunction with a BJT (D45VH10G) so should be low noise but with ample current so that might be another option to experiment with. I can also explore using a cap bank like the one you used.
I want to keep this project cheap and cheerful, especially as it already sounds pretty good so I will stick with the linear power supply approach and try adding some more capacitance locally to the STA333 chip - I've got a couple of 3300uF 25V caps to hand so I'll try hooking them up as a test.
A few more additions and improvements. I've added a new low noise linear power supply into the chassis and wired in a couple of 3300uF caps to the STA333 board. Also added the loading caps to the output transformer secondaries. I have to say that for the expenditure the sound quality of this project is extraordinary.
I've also made up a rear panel with a USB socket and I'm working on drawing a front panel in CAD with a view to getting it 3D printed.
I've also made up a rear panel with a USB socket and I'm working on drawing a front panel in CAD with a view to getting it 3D printed.