Raspi 2 and HiFiberry DAC+ coupled to a LM3886T Gain Clone Amp

[sunilmani]

Hello All Audio enthusiasts,

I have been dabbling with electronics on and off but you may consider me as a novice in this area. I was playing around with Raspi2 & HiFiBerry DAC+ and felt the need to have a single box that would house both the media player and a good amplifier. I started with listing down the requirements and they are as follows
1. Should be able to play Audio and Video from net
2. Should be able to play content stored on my HDD connected to network
3. Should be able hook on to net using cable or wifi
4. Should be able to play content from mobile/ipad/laptop (if possible like chrome cast)
5. The media player to be entirely controlled, including volume from Android Phone/iPad device.

For the past 7 months have been working towards getting the components in place for this build. Have designed the box and got it fabricated - this was a big task. Gathered components for the amplifier; procured some from the US. Got made to order Torroidal transformer which turned out to be so large that had to re-design the box . It was my first time - ordering a torroidal. I just requested for number of tapping and VA ratings and forgot the size completely and that landed me in a soup. Finding a fabricator to make just 1 box was a herculean task. Thanks to sadik from this group who shared snaps of his enclosure which helped me with my design. After much delay - almost 1.5 months, tomorrow i should be getting the fabricated box as per my specs and design.

Starting my journey of assembling the box and amplifier and the media player. Wanted to share it with this forum . Looking forward to the support and guidance from the senior Audiophiles in this forum to help me realize this dream build.

 

[sunilmani]

The Amplifier design was one i found on the net - designed by Jeff Rowland. PCB for the same were readily available so went for the same. The circuit diagram depicts the configuration. I have managed to procure the PCB as well from Aliexpress for the same.Few details of the same -
1. LM3886T - i have the coated one and non coated. Plan to go with the non coated as heat dissipation would be better.
2. Power Supply - Toroidal - 25-0-25 250VA + 12-0 (12VA)
Capacitor 10000 (Nover) X2 for each rail
35 Amp bridge rectifier.
3.Raspi2 + HiFiBerry Dac+ would also get inside the box
4. Component placement within the box is as per the dig.

Appreciate any thoughts/feedback on the stuff shared.

 

[Thinkdifferent]

Hello Mr. Mani,

Very interesting, I'd also be interested to know/learn, and will follow your journey to learn.

Good Luck
TD

 

[sunilmani]

I am sharing info in chunks....typing in as an when time permits.

I managed to lay hands on Auricap XO caps (1 uF). One of my friends was coming from US and was kind enough to get a pair for me. Now, for the input, as per the circuit we have 3.3uF (C3).

1> If i were to calculate the cut off frequency it would come to 1 hz ( fc=1/2*pi*R*C ) with a 3.3uF. I plan to replace the 3.3 with a 1 uF which would push the cut off frequency to 3 Hz ???
Plan to use those Auricap XO caps here. Let me know if issues are seen with this.


2> Thiele network -read about this on the forum. Shall i be using a 0.7uH inductor in shunt to 0.22Ohm resistors? The 0.22 ohms BTW are non inductive cement resistors

Appreciate expert opinion on the above two points.

Regards
-Sunil

 

[sunilmani]

On thinking further i feel the Thiele network should be in series at the output. If the network is used as shunt for the 0.22 ohm resistors, its not serving the purpose of blocking high frequencies as they can still get through the 0.22 ohms. So, in series with the output would be a more appropriate position?

 

[Aniket]

Hi Sunil,

Good to know your project. Very enthusiastic mate.

Few things i would like to ask, are you making a boombox or something or kind of an integrated amp.?
you are using lm3886 in parallel configuration, do you really need 150W+, if you do then there are better discrete amplifier designs out there. i personally like lm3886 but it cant beat any good discrete amp on sound quality.

Now few things i would advice, the schematic in post 2 is flawed. always refer to the datasheet for correct implementation of the chip, thoroughly read and understand each page in the datasheet. secondly, before finalizing any schematic and pcb have a look at this : http://www.ti.com/lit/an/snaa021b/snaa021b.pdf

Best wishes for your build.

Regards,
Aniket

 

[jls001]

On thinking further i feel the Thiele network should be in series at the output. If the network is used as shunt for the 0.22 ohm resistors, its not serving the purpose of blocking high frequencies as they can still get through the 0.22 ohms. So, in series with the output would be a more appropriate position?

Do refer to this page about your doubt on Thiele network and Zobel network:

LM3886 Chip Amp Stability Analysis

AFAIK, Thiele compensation is not always needed.

 

[Audiodoc]

The 3886 based amps sound as good or better than discrete designs if used in the correct topology. The MyRef and Modulus designs are good examples so is the Jeff Rowland and gaincard or peter daniels simplistic designs using good components. Paralleling two chips will help in case your speakers have a impedance less than 8 ohms.

Modulus-86: Composite amplifier achieving 0.00018 % THD.

Neurochrome Audio: High performance DIY audio circuits.

 

[sunilmani]

@Aniket - Thanks a lot for your insights.

Few things i would like to ask, are you making a boombox or something or kind of an integrated amp.?

Planning to make a media player integrated amplifier.

you are using lm3886 in parallel configuration, do you really need 150W+, if you do then there are better discrete amplifier designs out there. i personally like lm3886 but it cant beat any good discrete amp on sound quality.

I always wanted to make one with 3886T. Had built a smaller one earlier and it had sounded good. The power delivered with the current design is around 60W into 8 Ohms and 120W into 4 Ohms.

Now few things i would advice, the schematic in post 2 is flawed. always refer to the datasheet for correct implementation of the chip, thoroughly read and understand each page in the datasheet. secondly, before finalizing any schematic and pcb have a look at this : http://www.ti.com/lit/an/snaa021b/snaa021b.pdf

The links that you have shared is a treasure trove of knowledge. Thanks for sharing i went through it and that answered many questions for me.

The question - 1 that i had raised is answered. Will be going ahead with 1uF cap.
question - 2 seems the Thiele network is not really mandatory. Can live without one. So, going ahead without one right now.

 

[sunilmani]

Do refer to this page about your doubt on Thiele network and Zobel network:

LM3886 Chip Amp Stability Analysis

AFAIK, Thiele compensation is not always needed.

Thanks for sharing. I did go through this earlier and based on my limited knowledge do agree to your point that Thiele not being mandatory.

 

[sunilmani]

The 3886 based amps sound as good or better than discrete designs if used in the correct topology. The MyRef and Modulus designs are good examples so is the Jeff Rowland and gaincard or peter daniels simplistic designs using good components. Paralleling two chips will help in case your speakers have a impedance less than 8 ohms.

Modulus-86: Composite amplifier achieving 0.00018 % THD.

Neurochrome Audio: High performance DIY audio circuits.

Heartening to see that 3886 is used here as well. I loved what i heard while listening to 3886T.

 

[sunilmani]

Appreciate comments on the layout within the box. Apart from the toroidal, others i can play around. But since fixing most of the components would be imperative before i power up, in case i have done some obvious mistakes, do let me know. Layout

Thanks
-Sunil

 

[Aniket]

Hi Sunil,


for the layout, if you can, move the toroid in front and place rectifier in middle of the two modules, i suppose rectifier board has the main filter caps as well. so, in this way you have the shortest run of the power rails from psu to amp modules.

Just my opinion

Regards,
Aniket

 

[jls001]

Regarding layout, do try as far as feasible to minimise running input signal wires near the internal power cables (especially in parallel) so that interferences from power lines to signal lines is avoided.

 

[sunilmani]

Hi Sunil,


for the layout, if you can, move the toroid in front and place rectifier in middle of the two modules, i suppose rectifier board has the main filter caps as well. so, in this way you have the shortest run of the power rails from psu to amp modules.

Just my opinion

Regards,
Aniket

As part of the box design I have added in an additional Aluminium bar right at the center on which the Toroid is to be mounted. The fabrication of the box is done and hence the Toroidal cannot be moved. Sharing some snaps in the subsequent response.

 

[sunilmani]

Regarding layout, do try as far as feasible to minimise running input signal wires near the internal power cables (especially in parallel) so that interferences from power lines to signal lines is avoided.

Agreed. I am taking utmost care of for it. Once the wiring has been done will share those snaps as well.

 

[sunilmani]

Yesterday I could not get my box as the fabrication was not complete :sad: . Spent a good 6 hours with the person. Hopefully...hopefully next weekend it gets done. meanwhile assembled the rest of the boards. The main LM3886T board is still WIP.
Some snaps of the boards in progress

3886T and Main supply board.
The bulky Toroidal
Soft Start Board
The Box Front View..This is still in works.
The Box...Side view
Raspi2+HifiberryDac+


One of the challenges I am facing on main 3886 board is to get matching resistors.

Since the configuration is parallel, their gain should be identical to the extent possible and the factors affecting the gain are RF(200K) and Ri(10K). I have been trying to find a pair of identical match for the board, but it has been in vain. Any suggestions here? The only option I see is to buy a bunch more of resistors and hope that I turn lucky. If anyone has better suggestions, do let me know.

Regards,
Sunil

 

[ajeshkumarts]

How are you going to power the Pi + Dac?

 

[sunilmani]

How are you going to power the Pi + Dac?

The toroid has a 12-0 (X2) tapping as well. Will be coupling with 7805 + 2n3055 for a 5V 3 Amp regulated supply. Was thinking of the DC-DC buck circuit due to its efficiency; but want to keep any and all noise causing circuits away.

 

[jls001]

One of the challenges I am facing on main 3886 board is to get matching resistors.

Are these resistors regular 1/8W or 1/4W? If yes, they're cheap enough to buy 10-20 pieces each of the same value and hand match them to within a few ohms.