TPA3116 / RuneAudio Project

[Peter]

Now need to have a think about the final box and all those cables, initially might start with making up a new Cat 5 cable as that is the most rigid and bulky.

@downunder55: You could start by removing the outer cover of the cat 5 cable - craft knife and a bit of care will do it - this will substantially reduce the cables bulk.

Looks like you are having fun, Enjoy!

[skrodahl]

I am a minimalist, and quite happy with reducing my system to the bare minimum.

As of now, it is RuneAudio player, an amplifier board, input selector, balance, volume and the RIAA stage. (Balance only for those LPs that aren't perfect...)

That doesn't mean I can't experiment a little. I read about Nelson Pass' jfet BOZ, and found this little circuit that has less gain and a more readily available jfet: http://www.beavishifi.com/projects/JFET_preamplifier/

Again, a lot of other stuff to do, but I think this should be doable.

[Downunder55]

I read about Nelson Pass' jfet BOZ, and found this little circuit that has less gain and a more readily available jfet: http://www.beavishifi.com/projects/JFET_preamplifier/

Skrodahl, would you see this for between the Pi DAC output and the Amp ? what benefits would you expect this to give over just taking the output from the Pi directly into the pot and then to the amp ? .... or were you thinking of an option for an iPod input as well ?

[skrodahl]

I read about Nelson Pass' jfet BOZ, and found this little circuit that has less gain and a more readily available jfet: http://www.beavishifi.com/projects/JFET_preamplifier/

Skrodahl, would you see this for between the Pi DAC output and the Amp ? what benefits would you expect this to give over just taking the output from the Pi directly into the pot and then to the amp ? .... or were you thinking of an option for an iPod input as well ?

A small gain stage preamp between the DAC and the amp. Primarily to make sure the impedances are right both for the DAC and the amp.

I have built other devices with jfet buffers, and they do color the sound in a similar way to what a tube amp or preamp would. I'm not sure if that's a good thing or not, but for a few dollars I reckon it wouldn't hurt to test. I need to do a test setup, run it through the scope and see if there is some clipping going on before I put it to the test.

With a preamp, it would improve any iPod/phone or similar device input as well.

[skrodahl]

So, here it is. My little preamp, or closer to a buffer, built on the jfet transistor MPF102 in class A.

IMG_20140916_151412.jpg (230.63 KiB) Viewed 2731 times

The output from HifiBerry DAC on my RuneAudio player is just above 2.1V with my 1kHz test tone:

IMG_20140916_151451.jpg (240.62 KiB) Viewed 2731 times

Run through the preamp, and it's amplified to almost 2.9V. About 3dB voltage gain in other words:

IMG_20140916_151012.jpg (222.73 KiB) Viewed 2731 times

Spectrum analysis (a previous iteration with less gain) shows 2nd harmonic distortion.

IMG_20140916_144618.jpg (228.17 KiB) Viewed 2731 times

My scope is a Velleman EDU09 kit that I soldered together myself. I'm not at all sure of the scope's accuracy, but it gives clear indications at least.

I don't have the time to do a listening test yet. I need to rearrange components on the board to place inputs/outputs correctly, and I need to install the RIAA stage so I can see where to fit the preamp in my enclosure.

[skrodahl]

My phono stage has to go in before the little preamp.
I have now installed an isolated 12+12V power supply to feed the phono stage, and mounted it.

IMG_20140919_120847.jpg (329.43 KiB) Viewed 2709 times

A little tidying up and soldering the audio connections, and I'm ready to decide how to mount the preamp.

[Downunder55]

I read about Nelson Pass' jfet BOZ, and found this little circuit that has less gain and a more readily available jfet: http://www.beavishifi.com/projects/JFET_preamplifier/

Well I thought I would have a go at this this little thingy as well ....... now got a whole lot of respect for those who can solder, taken me all morning and not very neat on the back at all !

Still need to check I got the circuit correct before I fire it up, but my eyes are just too tired to that do at the moment.

IMG_8002.JPG (92.15 KiB) Viewed 2689 times

Interested to see if it makes a diff, but a whole lot of fun for under $14 !

Thanks Skrodahl for the link.

[skrodahl]

That's a great layout Downunder55, could you perhaps share it?

There were so few components, so I just winged it. The circuit is quite resilient in regards to layout, but I wanted to have a single strip on the stripboard reserved for ground. I get both input and output in between the two channels, so I'm quite happy with the layout. Still, you have proved that it can get considerably smaller.

The two capacitors on the top right of my board, together with a resistor that can't be seen on the picture, form a power supply filter. My PSU (a Dell switch mode charger) was horrendous on the scope, but it cleaned up nicely with that small filter.

It's quite difficult to get a good looking result from soldering on stripboard. Make sure you have no cold solder joints and verify that none of the component leads are touching each other on the top of the board, and you should be good to go.

[Peter]

Sure board has arrived, and is now set up and working great. Curse you skrodahl and Downunder55---- Now I will have to start building a preamp, after your previous encouraging photos and comments! When will the madness stop??

Every time I build something, I cut myself, and end up with blood on something, hence TBA (The Bloody Amplifier), and TBC The Bloody Cables). I suppose TBP (The Bloody Pre-amp) can't be far away.

As ACX said, maybe I should use blood as the new snake oil...I find that, rather than just ferrite beads to reduce noise, some blood smeared on the power cord reduces noise as a result of blood ferritin, just smooths the top ends.

[skrodahl]

Sure board has arrived, and is now set up and working great. Curse you skrodahl and Downunder55---- Now I will have to start building a preamp, after your previous encouraging photos and comments! When will the madness stop??

Haha, I'm so sorry that we are encouraging.

If you decide to invest more blood in building a preamp, I would like to mention one thing that's not entirely clear from this discussion. The volume pot should be installed before the preamp. Like this Source -> Volume -> Preamp -> Amp.

The madness is more likely to escalate, rather than stop, I'm afraid.