TPA3116 / RuneAudio Project

[Downunder55]

Well I checked the circuit, connected it all up, selected a favorite album Steely Dan Then & Now and settled back for .......it sounded just terrible ..... totally crestfallen I packed everything away thinking I would have another go with some new components and my newly improved soldering skill now at level 1 (previously level 0) and hopefully get a better result next time .......... then woke this morning to see Skrodahl post

The volume pot should be installed before the preamp. Like this Source -> Volume -> Preamp -> Amp.

Thankyou, thankyou, thankyou ..... IT WORKS beautifully !!!! ..... Seems to be a real improvement I think everything seems more defined and clearer ..... or maybe this is just my euphoria kicking in .... will need to now do some real listening to compare.

Warning: turning the pre-amp off or on with the amp already on causes a big "POP" through my speakers

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

I wanted to use the white XH2.54 connectors as it allows me to change config easily while still playing, this was the basis for how it was built . Not sure what circuit conventions I have broken in this layout, but I am sure bringing the +ve line over the inputs can't be good. I found a free circuit drawing package online so used this. A red \ is where I cut the copper strip on the board to make the circuit correct. Checked it several time so think it is correct but ......

JFET Preamp.jpg (121.63 KiB) Viewed 2930 times

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.

What does this look like please as I would like to get rid of the batteries, in the original Beavis instructions he didn't like a switch mode PS but that is all I have and your's seems to be good now, do you think the Pre-Amp would run on 19-20V ok ?

[Downunder55]

Every time I build something, I cut myself, and end up with blood on something, hence ......

This must be an "Antipodean" thing (refers to Australia and New Zealand for our overseas friends) cos that happens to me too ..... but then using the soldering iron .......burns are an ever present danger as well !!!!!

[Peter]

@Downunder55: Should the resistor shown on your diagram at right on the G line not be 100k ohm? Feel free to slap me if I am wrong, but the one above it is 100k ohm, and I think it should probably match. Also the Beavis schematic shows only 2 560 ohm resistors, and your image shows 3.

Please, I am just trying to understand, which is hard for someone with little knowledge.

[Downunder55]

@Downunder55: Should the resistor shown on your diagram at right on the G line not be 100k ohm? Feel free to slap me if I am wrong, but the one above it is 100k ohm, and I think it should probably match. Also the Beavis schematic shows only 2 560 ohm resistors, and your image shows 3.

Ooops sorry you are very correct, my built circuit is correct, just my new drawing skills copied a resister and didn't change the value, thanks for picking that up

JFET Preamp v1.jpg (122.33 KiB) Viewed 2926 times

Skrodahl not sure what the protocol is here, should I delete the photo in the previous post so someone doesn't mistakenly use it ?

@Downunder55: Please, I am just trying to understand, which is hard for someone with little knowledge.

That my line .... I too am prodding around in the dark here ...... but it great fun !!!

One other area of clarity is with the Capacitor, the Beavis Parts List refers to a Polarised Capacitor, but the Radio Shack part referenced is a Non Polarised Capacitor ??? ..... I used a NP one and seems to be working ok but not sure if it will make any diff ?

[skrodahl]

Downunder55,

I'm really happy that you got it working, and that you like it. That bodes well for my own version. You would need the volume pot in front of the preamp to make sure you have some attenuation. If not, the full signal will be fed into the preamp all the time. That could lead to clipping of the signal, and it most certainly did in your case since it sounded terrible.

You could, if you want, try to lower the gain of the circuit by increasing R3 and R8 (the 560 ohm resistors from jfet source to ground) a bit. I am using 715 ohm, but you should get the same effect by using anything from 700-820 ohms. Do this only if the sound degrades at high audio volumes of course. That stopped my preamp from clipping even with full input from my HifiBerry DAC.

The PSU filter I'm using is exactly the one from Beavis, it seems to be fine with my switch mode PSU. I'd try that, and see what happens. It looks perfectly fine on the scope. Do not skip the resistor, as it is part of the filter. You're essentially creating a low-pass filter that removes high frequencies. The resistor can be higher, up to 500 ohms or so is no problem, but you don't want it to be too high as it brings down the voltage to the circuit.

Remember also to add the smaller capacitor, as it is more effective at filtering high frequencies. It would preferably be a ceramic, I'm using a polyester film cap. If it seems like the sound quality is worse, add one or two more 220uF electrolytic capacitors in parallel. Switch mode power supplies can be sensitive to capacitive loads, so be conservative when adding filter capacitors.

The circuit should work with anything from 12-30 volts, the PSU that's powering your amp would be fine. The output capacitors don't have to be polarized, non-polarized would probably be better and if you can find a film capacitor at the same value or higher (such as 4.7uF) it would certainly be better. Electrolytic capacitors are not linear with different frequencies, but I doubt that you would notice much difference at all.

All in all, this is a very resilient circuit in so many ways. The only thing I wouldn't change is the bias resistor from V+ to jfet drain. At least not without proper measurements. Even then, as long as you're within a reasonable similar value you'd be fine.

Finally, thanks for sharing the stripboard layout. I'd keep the original picture, it's in context and mistakes are what makes us improve.

BTW, isn't it übercool and satisfying to know that you have built this yourself?

[StivVid]

All worked as planned, just one little annoying issue that is a small hum/scratching sound with no music playing, that only marginally increases with increased volume. The sound changes with Pi activity !

So far have identified after much testing:

1. If the Pi is powered from another power source the noise goes away

2. If the RCA connectors are disconnected the sound goes away

3. if I connect the ground (a mounting hole) on the AMP to the metal surround of the USB or LAN connectors on the Pi the noise reduces but is not fully eliminated and to a lesser degree from the Pi to the small power board

This has me thinking I have a grounding issue ? ....... any thoughts ?

Hi, guys!

It seems we've all had a very similar idea. In fact, I've bought the same USB power supply board as Downunder55 and am having the same ground loop issue. At first, I was using a Sure (like a Mean Well) power supply and thought maybe the power supply was the culprit. I'm now using a laptop power supply and the noise is still there. It's a hum with a crackling that coincides with Raspberry Pi activity--anytime the LEDs are flashing on the Pi, the crackling is very audible.

I've noticed that there are pin outs on the USB board that supply 5V while bypassing the USB connector. I'm wondering/hoping that this 5V output might be isolated better? Gonna give it a try. Let you know what happens.

I've been doing a little research on this problem, and am finding that USB is very noisy unless its power supply is isolated from the main power supply's ground. At least that's how I understand it. There are isolated power supplies for USB that solve the issue, but they're expensive and don't supply any more juice than about 500mA. Not enough, I'm afraid.

The only solution I've found so far is to run the Pi off a separate power supply, which isn't the elegant solution I was looking for.

[skrodahl]

The only solution I've found so far is to run the Pi off a separate power supply, which isn't the elegant solution I was looking for.

We all have different goals, but running the Pi off a separate power supply was exactly the elegant solution I was looking for. I put my RaspBerry Pi in a separate enclosure, and made sure there is no connection between ground and the chassis, and connectors/chassis. With HifiBerry, which has place on the PCB for power in, it was a breeze:

IMG_20140714_114926 (1).jpg (430.46 KiB) Viewed 2885 times

(It's an old photo, from when I just got it working. It has changed a bit since then, but the layout of the components remains the same.)

This is the PSU:
http://www.ebay.com/itm/5V-2A-10W-AC-DC ... 35cb4fae5f

Here's the case:
http://www.ebay.com/itm/0905-Full-Alumi ... 2c72bf76e6

I also got this MicroSD adapter to make the Pi fit in the enclosure:
http://www.ebay.com/itm/T-flash-TF-Card ... 5d43981e18

[StivVid]

So... You've connected your power supply to the HifiBerry and the Pi is powered by it? I would have thought the HifiBerry would be getting its power from the Pi, not vice-versa.

I'm using the IQaudio Pi-DAC. I've got power running to the Raspberry Pi's micro USB power input, and the Pi-DAC gets its power from the P5 header of the Raspberry Pi.

[skrodahl]

So... You've connected your power supply to the HifiBerry and the Pi is powered by it? I would have thought the HifiBerry would be getting its power from the Pi, not vice-versa.

I'm using the IQaudio Pi-DAC. I've got power running to the Raspberry Pi's micro USB power input, and the Pi-DAC gets its power from the P5 header of the Raspberry Pi.

You can input power to the GPIO +5v/GND pins as well as get power from them. The HifiBerry just presents these pins as soldering holes in the PCB. You can do the same when using the Pi-DAC as well, but you have to use the GPIO pins directly.

You just need to make sure that the power supply that you're using is regulated and adjusted to exactly 5 volts.

[skrodahl]

This build is really taking its time... Or, rather, I don't have too much time to build.

My previous build post showed the phono stage going in. Now it is in, it's powered up and it didn't blow up on me. The phono input is connected, now it needs to be connected to the input selector.

I did some tidying up, placed the PSU straight and freed some space. Tomorrow could be the day when the phono stage is finally getting tested. And, if I didn't fsck anything up, it will be time to decide how to mount the preamp.

IMG_20140923_184139.jpg (501.61 KiB) Viewed 2860 times

The preamp can only fit above the phono stage, and I'm thinking something in the way of these (only longer):

49431-8933-3.jpg (4.29 KiB) Viewed 2860 times

That means that I also have to wire the two remaining inputs before mounting the preamp, and use zip-ties to organize and hold all the cables in place using "zip-tie anchors" like these:

8872652734494.jpg (34.63 KiB) Viewed 2860 times