Author Topic: Showcase - DIY LM3886 100W+ bridge/parallel build  (Read 749 times)

Offline abza

Showcase - DIY LM3886 100W+ bridge/parallel build
« on: August 26, 2018, 04:13:56 PM »
Hi everyone - fairly new to the forum. Just thought I'd show the results of my latest (successful!) amplifier build. This is one of the few that won't be destined to spend its days in the amplifier graveyard that is my workshop.

I got my hands on 10 original, fresh-from-Texas-Instruments LM3886 Class A/B amplifier chips, and wanted to put as many of them as I could into a single stereo amplifier. I ended up on a parallel + bridged design that uses 8 of the chips in total, leaving me 2 to experiment with and ultimately destroy.

I designed the PCBs and laid them out in Altium, with components calculated and chosen to deliver around ~60W per channel into 8 ohms. They run off +/-28 VDC, with each board capable of handling around 3.5A of current at a maximum.
 
Each PCB has two LM3886es in parallel, which is best suited for 4 ohm loads. However, each channel uses two of the PCBs bridged in a balanced configuration, one board for the positive rail and one for the negative. This brings the output power up to just over 100W per channel into 8 ohms. The final stereo amplifier will therefore have four of these populated boards - two per speaker - and be able to drive 4 or 8 ohm loads comfortably with adequate (read: monster) heatsinking!

Each board accepts one half of a balanced audio signal. To convert an unbalanced line input to a balanced output, I put together a simple DRV134 based board, with integrated 15VDC regulated converter that accepts 36-72V as an input. This means the balanced driver can be powered from the same PSU that drives the amplifiers (28V +28V = 56V) with very high efficiency. Alternatively they can also be driven with a native balanced audio input if available, and this conversion can be skipped.

The final build will have a separate power supply per channel - high currents mean it will be a little less stressful on the PCBs if they only have to handle half of the load. And of course a pair of monster toroidal transformers will take care mains conversion - I got the fine folks at Swiftheat.co.za to roll me up some 500VA 22-0-22V dipped coils.

So far subjective audio tests are very promising, but have yet to hook it up to the scope and scrape some THD values. I doubt there will be any surprises, but fingers crossed!












NAD 3020
Monitor Audio Silver 2, Radius 90, Silver W12

Offline Dubya Jay Dee

Re: Showcase - DIY LM3886 100W+ bridge/parallel build
« Reply #1 on: August 26, 2018, 05:27:38 PM »
Wow beautifully neat looking work!  :clap:

Post those measurements when you've done them. :thumbs:

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Re: Showcase - DIY LM3886 100W+ bridge/parallel build
« Reply #2 on: August 26, 2018, 06:09:42 PM »
Well done so far and keep posting pics

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Offline abza

Re: Showcase - DIY LM3886 100W+ bridge/parallel build
« Reply #3 on: August 28, 2018, 09:48:01 AM »
@Dubya Jay Dee - thanks! I'm going to make it my mission this weekend to get some graphs, will chuck them here when done.

@CAD - shot!
NAD 3020
Monitor Audio Silver 2, Radius 90, Silver W12

Offline abza

Re: Showcase - DIY LM3886 100W+ bridge/parallel build
« Reply #4 on: September 01, 2018, 12:50:17 PM »
Right, first quick tests are in.

I'm testing just a single board, so a parallel LM3886 pair driving an 8 ohm load (not entirely ideal, as a single parallel LM3886 is better suited to 4 ohm loads).

First thing to note is I get massive hiccups attempting to power on with higher volumes present at the input - for now, the volume needs to be low until the amplifier settles (presumably capacitors charging) and then crank it up thereafter. Almost certainly going to have to build a speaker protection/delayed start circuit to accompany all four amplifiers.

Power

Right, so I maxed out my power supply before I could get anywhere NEAR clipping. At 3.2A per rail I was getting consistent, clean power using a 1KHz signal, inputting ~1.6V. Yowza. Here's the results of a more standard 1V 1KHz sine:



30 clean watts, from 2V p2p to 45V p2p.

THD

Rock, rock bottom, at least in terms of what my equipment can measure. The lab power supply is likely to be cleaner than the average amplifier PSU, so the results may be inflated somewhat.



-68.41dB equates to around 0.037% THD.

So far so good. Next challenge is to sort out the aggressive power-on issue, and finish designing the power supply!
NAD 3020
Monitor Audio Silver 2, Radius 90, Silver W12

Offline Curlycat

Re: Showcase - DIY LM3886 100W+ bridge/parallel build
« Reply #5 on: September 01, 2018, 01:00:49 PM »
Impressive  :thumbs:

Online 2wice

Re: Showcase - DIY LM3886 100W+ bridge/parallel build
« Reply #6 on: September 01, 2018, 01:38:22 PM »
Very nice!
Keep us updated.  :thumbs:

Offline Dubya Jay Dee

Re: Showcase - DIY LM3886 100W+ bridge/parallel build
« Reply #7 on: September 01, 2018, 04:16:21 PM »
 :clap: :2thumbs:

Offline abza

Re: Showcase - DIY LM3886 100W+ bridge/parallel build
« Reply #8 on: September 03, 2018, 09:39:56 AM »
@2wice , @Curlycat - cheers, thanks!
NAD 3020
Monitor Audio Silver 2, Radius 90, Silver W12

Offline abza

Re: Showcase - DIY LM3886 100W+ bridge/parallel build
« Reply #9 on: September 03, 2018, 11:23:58 AM »
@Dubya Jay Dee - thanks!
NAD 3020
Monitor Audio Silver 2, Radius 90, Silver W12

Offline abza

Re: Showcase - DIY LM3886 100W+ bridge/parallel build
« Reply #10 on: November 09, 2018, 11:17:57 AM »
Update 09/11/2018

Right, redesigned the power supply. Here's a little step-by-step for a single supply - each channel gets a dedicated board:



Started with the smaller caps, resistors and SMD status LEDs:



Then on to the (huge) caps (24700uf per rail = 49400uf per board = 98800uf for the amplifier. Ouch!):



Finally prepping the diodes. They won't get very hot, so I let two diodes share a single tower heatsink, using some SilPads which I cut for insulation:



Final PSU:



Testing will be done this weekend. It will no doubt require a soft-start circuit to prevent it popping fuses like it ain't a thang, so will build one of those as well and test it driving a dummy load.

NAD 3020
Monitor Audio Silver 2, Radius 90, Silver W12

Offline Curlycat

Re: Showcase - DIY LM3886 100W+ bridge/parallel build
« Reply #11 on: November 09, 2018, 02:43:38 PM »
Wow! Keep those pictures coming.  :2thumbs:

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Re: Showcase - DIY LM3886 100W+ bridge/parallel build
« Reply #12 on: November 09, 2018, 04:50:37 PM »
Damn !! Nicely done abza  :thumbs:
The way of the warrior is NO match for the way of the wife.
Light travels faster than sound. This is why some people appear bright until you hear them speak.

Offline abza

Re: Showcase - DIY LM3886 100W+ bridge/parallel build
« Reply #13 on: November 14, 2018, 04:19:31 PM »
Alright, soft start board finished:



It's the DIYaudio design, using a bank of high power resistors to throttle the current for ~200ms until the relay triggers, allowing the current to pass unimpeded. More than enough time to let the PSU caps charge.

Here's the test setup, along with one of Swiftheat's toroids:



Testing today, fingers crossed. Looking for a nice, healthy +/-28V on the output :)

NAD 3020
Monitor Audio Silver 2, Radius 90, Silver W12

Offline rakumatr

Re: Showcase - DIY LM3886 100W+ bridge/parallel build
« Reply #14 on: November 14, 2018, 04:21:13 PM »
Amazing workmanship

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