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DIY 8ch x 100W power amp with microprocessor control

mcdn

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Sometimes either the market doesn't have quite what you want, or you just want to scratch an itch. I wanted an amp with the following features and thought folks here might be interested in what I came up with:
  • 8 channels to drive 4 way active speakers (Linkwitz LXStudio)
  • Really compact to fit in a sideboard
  • 100W per channel at < 0.1% THD
  • 12V trigger in and out
  • Music detection
  • Low/no standby power
The trigger and music detection parts of the requirements became quite complex as I also want to be able to shut it off or turn it on manually, deal with combiations of trigger and music detection events etc, so after pursuing traditional approaches I decided to brute force my way through it by using a microcontroller (specifically a Raspberry Pi Pico) and doing all the logic in software. This dramatically simplified the circuit board.

The key components are:
  • 4 x 3eAudio TPA3255 BTL - while each channel can do 250W into 4 ohms, that's at quite high distortion. I'm only using 100W/ch, if that
  • 1 x Connex SMS800RS - 48V single ended, and 800W continuous power.
  • An auxiliary power board providing +-5v and +-12v, as well as a relay to turn on the main power supply. My word PCBs are cheap to order these days.
  • A controller board to run the show, with Raspberry Pi Pico, 12V trigger optoisolator, RMS signal measuring chip, etc. A cheap PCB and a few passive components, plus a lot of interface pins and an OLED display
Testing was done with a 300W 4.7Ohm dummy load, REW, and a Focusrite Scarlett Solo. Special mention to my Picoscope 2204 which despite only having 8 bit resolution was invaluable in debugging.

Feel free to ask any question or post your thoughts, the whole thing was a lot of fun to do!

On the measurement front, all channels look much like this at 10W into 4.7 ohms. I'm using Archimago's recommended settings for RTA measurement of amps with REW. I'm no expert but seems OK:
1khz 10W.jpg


Front panel (and heroic dummy load):

IMG_0923.jpg


The state machine for the 12v trigger, music and front panel button is like this:

Screen Shot 2021-08-17 at 4.37.39 pm.png

And the guts are like this:

guts.jpg
 
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Colonel7

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What are the dimensions of the box, and who is the vendor? Also, what is the all-in cost?
 
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mcdn

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What are the dimensions of the box, and who is the vendor? Also, what is the all-in cost?

The box is a shallow 2U rackmount unit, so ~450mm wide, 250mm deep, 88mm tall. (or 18x10x3.5 inches if you prefer). I got it from a local electronics shop for about $80 USD: https://www.altronics.com.au/p/h5032-2u-black-aluminium-front-panel-19-inch-rack-case/. I made all the holes in the enclosure myself - please don't ask for a photo, it's not very pretty.

All-in cost would be (in very rough USD), $80 for the case, $260 for the amp modules (4*$65), $120 for the PSU, $120 for all the panel mount connectors, $250 for sundry components (DC power supplies, relays, optocoupler, RMS converter, fan, SMD passives, buttons, LEDs, OLED screen, SMD transistors, plugs, jacks, cables, screws, nuts, etc), $50 for the PCBs, $100 for shipping all the above.

So $980, plus $5 for the insanely capable microcontroller. There's a lesson here about economies of scale...

Of course I haven't included all the random kit I bought along the way, or my time, but it's rather good value for the performance and features I think.
 

Colonel7

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Thanks - nice measurements in a one-box solution. I've seen pics of some really messy amplification dogs for LW designs. What are you using for the crossovers, MiniDsp or something else?
 
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mcdn

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… What are you using for the crossovers, MiniDsp or something else?

MiniDSP 4x10HD. Cabling is quite neat with the miniDSP Phoenix to 4xXLR cables.
 

Carousel

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Hello mcdn. I absolutely love this build. Something very similar would be perfect to power my six ATMOS speakers plus my two smaller surround speakers. I have never done a DIY hi-fi build myself. But my experienced neighbour will supervise!

I will have eight XLR inputs. Is that what you have on your build? From the picture, it looks like you have two 4-plug modules? Is that right? Are these modules available to buy somewhere or did you build them yourself? Appreciate any info or advice you have on these.

Since I will have eight speaker binding posts, I will have to save some room on the back of the case. Hopefully I can drill straight and/or can use a predrilled template from ghentaudio like: https://ghentaudio.com/kit/alu-part/drr-sp4-1.html

The only "optional" feature I would like to use would be a trigger on/off. Otherwise I do not need a display or much more than an on-off switch.

Appreciate any info or advice you can give me on how to set up the power supply to feed the four amp modules. Do I need any extra modules like you are using?

Note: I will be using an Arcam AV40 to feed the unit with signal via XLR, using Dirac to room balance. I have four Dali Alteca C1 4-Ohm speakers in the corners of the room. These sit on top of full-sized Dali speakers powered by NAD Class D amps. In the middle of the room, I have four B&W M1 8-Ohm speakers, one up high and one a meter or so high on each side of the room. The four Dali Alteca C1s and the four B&W M1s will be powerer by the DIY build.

Thank you in advance for any and all pointers! You are an inspiration!
 
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mcdn

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Hi @Carousel , thank you for your kind words and enthusiasm!

The XLR inputs are individual components, mounted on two 4-input carrier boards that I designed. The amp modules I used are TPA3255 ones from 3e-audio (https://www.3e-audio.com/amplifier-kits/tpa3255-2ch-260w/). They have been unstocked for a while and I've no idea if they'll come back in. The control board and software is my own design and not something that's in fit state to recommend to anyone else - for example the music detection is unreliable due to noise issues. The logic around music detection, 12v triggers, soft start, mute during power-up, run-on timers and so on is fairly complex. I used a Connex SMPS800 for the power supply, as I genuinely don't need more than 100w/ch.

Since Buckeye Amps started up I really don't think there's any justification any more for a full DIY build. Their latest lineup (https://www.buckeyeamp.com/shop/p/hypex-nc252mp-amplifier-8-channel) has all the MCU controlled goodness like low current trigger input, and while the NC252MP is a more expensive module than the 3e-Audio TPA3255-260, it's also a highly reputable and robust design with easy replacement in case of failure and much more total power output than my build. You will have to spend a lot on time, equipment and parts to do a DIY build right (soldering iron, stand drill, decent case, dummy load, etc etc), and an 8-channel amp with trigger is a very complex first build! This is the 4th amp I've built, and I had other reasons for wanting to learn and use the various tools involved.

So I don't want to discourage you, but I do want to warn you that it is a much harder thing to do than it looks. You can't just buy a case, drill some holes and mount some modules and expect to get the result you are after. For a two or four channel build with no trigger it's possible, but for more complex/bigger requirements I wouldn't recommend it.
 

Carousel

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Thanks @mcdn for the advice and info. That is why I have the retired German heavy-industry electrical engineer neighbour along for the mission. He builds his own amps and speakers and cables. But IC modules and programming are not his forte. Hence he was leery of helping me, having seen the special items you have!

But I also only need 100W per channel on these secondary ATMOS speakers. Nord Acoustics does have an 8-channel NC122 powered amp listed on their website. And these amp modules are scarce when I look for them online. But I can get four of the 3e-audio TPA3255 modules for the price of two Ncores (if they were available)!

The neighbour is cool on simply helping me put in 8 XLR and 8 binding posts. That is his technology. He thinks of the build I have in mind as four stereo amps. But he asked me to get a plan for the power supply as, at least for Class D, he usually uses modules with their own onboard power, not one power supply to four modules. He is concerned about balance.

So I am not totally scared off yet! Any last recommendations for how to do the power supply without your custom modules?! I could wait to see if amp supply comes back for four NC122's with onboard power. Maybe that would be better. But it could be too quiet here for too long too. Thanks again!
 

Sokel

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Building a similar solution with a friend (he's in HT thing) with a little different components (4x 300a2 along with the Ice Power 1500/57 power supply or something similar and suitable) with approximately the same cost but 8 x300 watt (at least at spec's) I find your built super interesting.
Trigger and control are already integrated but would be a nice touch to put a little screen like yours and maybe DSP through an RPi.
 
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mcdn

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Your neighbour is right about issues when using a single supply for 8 highly correlated outputs - it's usual to wire half the modules inverted, then invert again at the speaker outputs. The 'NCxxxMP' variants from Hypex with their integrated power supplies and amp modules are preferred for large builds for this reason, as well as the fact that a single power supply capable of delivering 1kw is very expensive.

So, if you do go for this yourself you'll use 4 stereo NC122MP modules. If you are OK for these to run 'on' all the time then just wire them up to a mains switch and you are done. Idle losses are about 7.5W per unit so would be 30w overall.

Adding a trigger input is a whole other dimension.
 
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mcdn

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Building a similar solution with a friend (he's in HT thing) with a little different components (4x 300a2 along with the Ice Power 1500/57 power supply or something similar and suitable) with approximately the same cost but 8 x300 watt (at least at spec's) I find your built super interesting.
Trigger and control are already integrated but would be a nice touch to put a little screen like yours and maybe DSP through an RPi.
The raspberry pi only has one I2S interface, and while it has in and out data lines they have to be clocked the same, so the opportunity for DSP is limited to 2 channel in/out only. I'd love to know if there's a similar general purpose linux device with multiple I2S IOs.
 

Sokel

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The raspberry pi only has one I2S interface, and while it has in and out data lines they have to be clocked the same, so the opportunity for DSP is limited to 2 channel in/out only. I'd love to know if there's a similar general purpose linux device with multiple I2S IOs.
That's the details I'm wondering about too.Simple status report/viewing of the components is super easy to do with Ice Power,some control too,but DSP is a whole other matter.
A lot of searching is about to be done!
 

Carousel

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OK. You scared me away. I ordered an 8-channel Ncore 250W amp from Nord Acoustics. It is beautiful, has triggers and has modules for switching between XLR and RCA input. Costs more and is not as much fun (conceptually) though! At least the neighbour has a DIY tube amp kit coming from Canada for his beryllium super tweeters that I can help him with.
 

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Thank you @fluid for that. It is very topical for me. I was looking at such Hypex charts with my "math degree yes, electrical engineering degree no" background. Although I plan to feed the 8-channel surround amp with XLR to XLR from an Arcam AV40, I need to figure out how to get signal to the main five speakers. For their amps, I am already using the XLR inputs to feed directly from my exaSound S88 DAC. (Note: This system is mainly used for music; the family ATMOS set up is upstairs.) So I only have the RCA inputs available for the main five speakers still. I have high-quality RCA cables that I can use with the RCA preamp outs of the AV40. But I was wondering if I should go with either 1-meter XLR-to-RCA cables or use an XLR-to-RCA converter plug at the AV40 and then use my good RCA cables. I have been through many many forum postings on the subject and was about to write to Arcam. Most of what I read came down on the side of not using plug converters and sticking with an XLR-to-RCA cable (with appropriate pin drops / combos / etc). Sounds like that is your recommendation too. Correct?
 

fluid

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Most of what I read came down on the side of not using plug converters and sticking with an XLR-to-RCA cable (with appropriate pin drops / combos / etc). Sounds like that is your recommendation too. Correct?
If you want to connect a device with RCA output to an amp with XLR input then yes a pseudo balanced cable is the best way. They are pretty simple to make if time consuming, I make all my own cables mostly so I have no idea if you can buy one that is wired correctly but it shouldn't be too hard to modify a commercial XLR to RCA (as long as they are not moulded) if it wasn't right.

I did this for my own DIY 8 channel Hypex amp and Najda DSP board that was RCA output and it was a super quiet active system, which is not always easy to achieve.
 

Sokel

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I did the same with an RCA active crossover before I get my balanced one.Never bothered me.Pseudobalanced cables are super easy to make.
 

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@fluid @Sokel

Yes, my neighbour offered to help me make my own cables. Could help make up for the deficit feeling in my soul for buying, not making an amp myself.

Note though that I am going from XLR-or-RCA preamp output to RCA power amp input for five of the channels. So my question is about going the other way.
 
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