Got a Free Project Enclosure! What to Build in it?

[TimW]

I just got this very interesting custom made enclosure. The maker intended it to be a preamplifier using a Nelson Pass design but never got around to building that. What he did end up putting in it was a miniDSP along with the simple power supply that is still in the enclosure.

The front panel is a solid piece of wood that the maker must have machined and then stained. The back panel is a chunk of aluminum that he said he had machined by a friend, it is attached to the other pieces with a wooden frame. He must have had the back and top panels anodized this blue color after cutting them to size. I am very impressed by the craftsmanship.

There is this nice wooden connector plate that was made for a couple of USB connectors. He made it removable so that different connectors could be used. There is also 6 pairs of holes for audio connectors to mount, right now there are 3 pair of fancy Rhodium plated Cardas RCA jacks there.

The sides of the enclosure are made from Conrad heatsinks. Actually it was one MF35-151.5 model heatsink that has been cut in half to create 2 7/8" (73 mm) tall heatsinks. This should make thermal resistance just slightly worse than that of the MF35-75 model which is 0.32 C/Watt for 80° C rise.

The simple power supply he built in this enclosure must have powered the miniDSP unit through a regulator. It has a nice cast cover over the AnTek AS-1212 transformer, but no fuse protection! Needless to say I will probably not be using this power supply, at least not without the addition of a fuse.

At first I was considering using this enclosure just to hold a miniDSP 2x4 HD, making it prettier and more convenient to connect. Then I thought about also mounting power supplies in it for external amplifiers or other equipment, but that doesn't make a whole lot of sense. I would really like to make use of the nice heatsinks and build something suited for this enclosure.

So my current plan is to install some Neurochrome amplifiers in this thing. I could make use of a stereo power amplifier in my current systems so that's one option. But I'm more interested in making a 4 channel amp for an active two way system. I have DSP units lying around from previous DIY active speaker projects and I've been itching to build some LXmini's...

@tomchr I've been perusing your site and considering my options. I like the Modulus-86 specs but building 4 channels would be a bit pricey. For that reason I might go with the LM3886 Done Right if I go with 4 channels rather than 2. But what I really want to ask you is how much amplifier do you think I can stuff in this case? Do you think these heatsinks will handle 4 Modulus-86 or LM3886 amps? Will they even handle two? And if I do go with 4 amps, do you strongly recommend the Power-686 over the Power-86? Not sure how many parts will fit in the case but in any layout I plan to also install your Intelligent Soft Start board. Would you strongly recommend the Guardian-86 for use with your amplifiers?

I also have this AnTek AS-3222 transformer laying around that just barely fits in the case.

Is this suitable for multiple Neurochrome Modulus-86 or LM3886 amps?

 

[TimW]

I drew this up using actual dimensions. The Guardian-686's are mounted vertically in this view. I show two Power-86's because I could not fit a Power-686. After reading the power supply design portion of the Taming the LM3886 Chip Amplifier article on the Neurochrome website, I think the AnTek AS-3222 should be sufficient for normal use. I'm hoping @tomchr can help me figure out if these Conrad heatsinks are sufficient because I don't even know where to begin on that.

 

[tomchr]

The AS-3222 is perfect for a 4-channel Modulus-86 (or LM3886DR) amp.

I would strongly encourage you to pursue the Modulus-86 instead of the LM3886DR. While the LM3886DR is the best I can do with the 'naked' LM3886 (i.e. without error correction), the error correction of the Modulus series amps takes the performance to a whole new level. I do get that the Modulus-86 is more expensive. Tradeoffs, tradeoffs.

I do give you the math for the heat sink calculations here: https://neurochrome.com/pages/thermal-design
I'll save you the trouble, though. Yes. 0.32 K/W is plenty for two channels of Modulus-86 per heat sink.

In your chassis layout, be mindful that you need space for the wire entries to the boards. You can scrap the transformer 'bell' if you don't need/like it. If you can fit it in, great, but I wouldn't make it a priority.

I like that chassis. The Tektronix blue and perf plate looks nice.

Tom

 

[TimW]

Thanks Tom.

I knew I would end up going with the Modulus-86 over the LM3886DR. I just thought I would ask on the off chance you might say there's not much of a difference and my wallet could catch a break haha. The measurements are just so impressive!

I considered building the Akitika GT-102 for a while because I liked the ready to assemble kit aspect of it. The Stereophile measurements looked pretty good too, except the thing clearly couldn't handle a 4 ohm load properly. Comparing to measurements of the Modulus-86 though shows that you have come up with a much better design with the same chip amp. Much lower distortion and no apparent 4 ohm load issues. It's funny because Akitika blames this issue on "the safe-operating area circuits built into the LM3886."

I will probably start by making this a 2 channel amp that I can use now. When I start work on a pair of LXmini's or other diy active speaker I'll add 2 more channels.

 

[tomchr]

I knew I would end up going with the Modulus-86 over the LM3886DR. I just thought I would ask on the off chance you might say there's not much of a difference and my wallet could catch a break haha. The measurements are just so impressive!

The Modulus-86 has about 100x improvement in THD+N in the high end of the audible frequency range over the LM3886DR. I strongly suspect this is why the Modulus-86 sounds much cleaner. There's no "chipamps harshness". It's just the source material, amplified.

I considered building the Akitika GT-102 for a while because I liked the ready to assemble kit aspect of it. The Stereophile measurements looked pretty good too, except the thing clearly couldn't handle a 4 ohm load properly. Comparing to measurements of the Modulus-86 though shows that you have come up with a much better design with the same chip amp. Much lower distortion and no apparent 4 ohm load issues. It's funny because Akitika blames this issue on "the safe-operating area circuits built into the LM3886."

Comparing the Modulus-86 to the LM3886DR or Akitika is not an exact apples-to-apples comparison, though. My yardstick for the LM3886DR (and any other 'naked' LM3886 amp) is the LM3886 data sheet. An amp implemented using just the LM3886 (no error connection) should show the same performance as TI shows in the data sheet. My LM3886DR actually shows slightly better performance than what you see in the data sheet, but I think that's probably due to improvements in instrumentation.

The Modulus-86 adds error correction. That's why it sounds better and why it measures much better. I use an LME49720 to perform error correction on the LM3886, which drives the THD into the abyss. It turns out that improves a range of other parameters as well.

I will probably start by making this a 2 channel amp that I can use now. When I start work on a pair of LXmini's or other diy active speaker I'll add 2 more channels.

That sounds like a great idea. I suggest that you drill the necessary holes in the heat sink and chassis bottom now, so all you have to do is to build another pair of MOD86es and drop them into the chassis at a later date.

I use a 4xMOD86 amp powered by an Antek AS-3222 and Power-86 with my LXmini by the way. Works great. I started out with the MiniDSP 4x10HD for the XO and moved to the MiniDSP SHD once it became available. I implemented the analog sections of the SHD for MiniDSP and they were kind enough to send me one once production started.

Tom

Edit: Fixed typo.

 

[TimW]

Wow I didn't realize you were involved in the design of the SHD. I have one as well and I think it sounds great. I was impressed by pictures of the inside that were recently posted here. Are you the one who put all those caps in there?

Do you use one of your Guardian protection circuits with your LXmini amp? I'm considering excluding it from my build for cost and space reasons.

 

[HammerSandwich]

Comparing the Modulus-86 to the LM3886DR or Akita...

Hey! Let's keep it classy, instead of literally calling your competition a dog.

 

[tomchr]

Hey! Let's keep it classy, instead of literally calling your competition a dog.

Please re-read my sentence. I am NOT comparing my competition to a dog. I am comparing the Modulus-86 to the LM3886DR (which is my own product) and I am equating the LM3886DR (which is my own product) to the Akitika (which is a competing product). I'm basing my claims on measured data. Is that wrong or unprofessional?

Tom

Edit: Fixed typo.

 

[tomchr]

Wow I didn't realize you were involved in the design of the SHD. I have one as well and I think it sounds great. I was impressed by pictures of the inside that were recently posted here. Are you the one who put all those caps in there?

I recommended that they use the bright green Nichicon Muse UES-series, yes. I'm assuming those are the caps you're referring to.

Do you use one of your Guardian protection circuits with your LXmini amp? I'm considering excluding it from my build for cost and space reasons.

I don't use the Guardian-86 in my LXmini amp. I built that amp in 2015 - well before the Guardian-86 was introduced. I go without speaker protection in my Modulus-686 amps for the same reason.
For a DIY project, it's really up to you. The LM3886 is a rugged chip with quite a bit of internal protection circuitry. If I was to build another amp for my own use, I'd probably go without the protection circuit to save time and cost. If I was to put an amp on the market as a commercial/retail product I would most definitely include a protection circuit.

Tom

 

[HammerSandwich]

Is that wrong or unprofessional?

I just meant to tease you, Tom, and hoped the wink would make this clear. Tease you about what? What is an Akita? What is an Akitika?

We cool?

 

[tomchr]

I wish people would bother to communicate directly. If you have something insightful to say, say it. If I've said something wrong, and you need to correct me, do so (politely, please). Example: "I think you misspelled Akitika in your post. I suggest you fix that."

I've fixed the typo. Thanks for pointing it out. Good now?

Tom

 

[TimW]

Alright no protection circuits on the output for me then. I will also plan to use just one Power-86. I assume I should use larger capacitors with the Power-86 when powering 4 Modulus-86's?

 

[tomchr]

Not necessarily. I used 2x22000 uF. The board supports 40 mm diameter caps with 10 mm pin spacing (snap-in types). You can use larger caps if you wish.

Tom

 

[A800]

Maybe I got an idea...
Oh wait...
it's too small for a decent sub.

 

[TimW]

Not much to do right now. So I've been making fast progress.

Got the Hammond transformer because my AnTek was too tall. Had to modify the mounting hardware to even get this one to fit.

Would have used Neutrik XLR and speakON connectors but I didn't want to modify the rear panel.

Waiting on standoffs and wire now. I plan to run the speaker output cables under the MOD-86 boards to save space. Hopefully this doesn't cause noise issues. I will use Canare 4S8 cable to hopefully mitigate that issue, but it may be too large, in which case I will use Mogami W3082.

Trying to figure out how I should solder the LM3886 chips in place at the correct height and angle. Maybe I need to make a jig?

 

[tomchr]

Hopefully there's still room for the rubber pads on the top/bottom of the transformer.

No issue running the output cables below the boards.

One way I've used in the past to get the LM3886es aligned, was to mount the PCB on standoffs in the position I wanted. I then marked the position of the LM3886 on the heat sink (use the middle pin in the back row of pins to find the centre). Once you get a mark on the heat sink in a known spot, you can take everything apart and measure to get the mounting hole for the LM3886 in the right spot. I use the package dimensions in the LM3886 data sheet and add 0.5 mm such that the bends in the IC pins are a little bit up from the board. As long as you wait to solder the LM3886 after it's been mounted to the heat sink, you don't have to be super precise. So even though this method is a bit of a kludge, it does work. In the ideal world, you would have designed the chassis in a CAD tool, and everything would just be perfect, but the world isn't always ideal.
Anyway. Once you get the mounting hole for the LM3886 drilled and tapped, assemble the amp, and mount the LM3886 to the heat sink with the proper hardware (you can skip the thermal pad for now). Solder a couple of pins (preferably 2-3 pins total with at least one pin in each row soldered) on the LM3886 from the top. Then remove the PCB from the chassis and finish the solder job. Then mount the PCB in the chassis for the last time. This time with proper hardware and thermal pad (assuming you use the non-isolated LM3886T).
This method is not the most convenient, but I do find that I get good results every time. I'd rather spend an extra hour on a weekend-long project than to have a crappy thermal interface.

Tom

 

[TimW]

Finished for now. Tom was very helpful by quickly answering all of my questions and helping solve the issues I ran into.

This is the most quiet amp I have ever heard. Granted I'm not currently using very efficient speakers with it, these are Pioneer SP-BS22-LR's modified to match Dennis Murphy's affordable accuracy monitors. But I can't hear any noise coming from the tweeter, even with my ear pressed up against it. And this is with single ended connections to an Allo Boss streamer. I measure 0.0 mV of DC on the output. No hint of distortion either, even at volumes that were loud enough to piss off my roommate.

Unfortunately the transformer does have a mechanical hum to it, but it's very quiet when under load. Tom has told me it may be due to DC on my mains and has shown me a circuit I could install to eliminate it. I may do that eventually. I'm also not super happy with the way my input wires run but oh well. Still need to build two more channels and install my trigger input as well.

 

[tomchr]

That's fantastic. Thank you for sharing. You amp turned out really nice.

One question: It's hard to tell from the photos, but what did you do to insulate the LM3886 mounting screw from the LM3886 metal tab? Is there a shoulder washer there? There should be...

I'm normally not a fan of blue LEDs, but I really like the bi-colour blue/yellow LED in the power switch. I'd be tempted to put a dimmer on it to get the blue LED to match with the blue of the chassis. I might make a circuit available for that some day. It just dawned on me that such a dimmer would have to support bi-colour LEDs. Hmmm... I can come up with something.

Regarding your comment about the input wiring: I wouldn't worry too much about it. Just keep the wiring towards the chassis walls, like you have done, and you'll be fine.

Enjoy the amp.

Tom

 

[TimW]

Yes there are shoulder washers, they are brown and blend in with the screws.

I'm not usually a fan of blue LED's either but when it's dim and diffused through a ring I don't mind it. I would have used an RGB switch to get white if I could have. It would be great if your dimmer supported RGB to get infinite color options.

 

[tomchr]

To support RGB, you'd need three dimmers. One for each colour. Six if you want to support one colour for ON and another for OFF. But it can certainly be done.

Tom