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Old 11th January 2015, 01:13 AM   #1
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Default Modulus-86 build thread

Thought it might be interesting to start a thread on a build I am just starting. I'm converting my apogee Centaur Minors to open baffle with active amplification and needed 4 channels of the best I could afford of 20-40W per channel. After looking at a number of alternatives I decided to go with the Modulus-86 developed by TomChr, webpage here Modulus-86: Composite amplifier achieving 0.00018 % THD. and DIYaudio thread here Modulus-86: Composite amplifier achieving <0.0004 % THD+N.. My main reason for chosing this was the fact it provides some of the best measured performance I have ever seen from a power amp (comfortably lower distortion than my source or pre-amp), is unfussy about power supply, working to spec with a basic setup and has a balanced input that gives nearly 90dB CMRR even from single ended sources. Basically reference grade for pocket money and perfect for me as I want to play in other areas with my system at the moment.

A few people are thinking of building this so thought I would start a thread on my build and allow people to answer any questions that they have and share their builds.

What I am afraid I cannot do is offer comparisons with xyz. I've been out of DIY for 20 years and getting back in now so I don't have an array of different DIY boards. Also I am not going to claim that this sounds better than any other LM3886 based amp. It will however measure better, much better and certainly well below any sane levels at which transparency is defined. It should have no sound its own.

I'll take some initial pics and start soldering in the morning.

Disclaimer: I have no financial or other interest in the work Tom does. I'm just a customer.
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Old 11th January 2015, 03:12 AM   #2
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Thanks for starting a build thread, Bill, and inviting others to pile on with our own builds. I have my boards and my parts from Tom's Mouser project link included in the kit instruction email. I'm ready to start building tomorrow. I also have Tom's power supply board and Mouser parts.

I need six low distortion amps for 3way active crossover speaker based on Jeff Bagby's Kairos.

I also bought a pair of Twisted Pear Sympatico, and I will compare them. I still need two more channels, or more if one of them doesn't work for me. bought the two different designs out of curiosity, and intending to offer my opinion about which I liked better overall. I've built Twisted Pear kits before.

In the past I have not enjoyed listening to LM4562 opamp, so I'm a little leery of both designs which use these, but Tom's subjective description of his amp's sound did not agree with my negative experiences so I figured what the hell. Then I find out he only built one channel and has not listened to it in stereo. A good engineer's confidence!

So I'm looking forward to hearing them. I will compare the two different designs as stereo amps using the passive crossover of my 2way Kairos, and as individual amps in my 3 way setup with active crossover. I'll also probably built Tom's parallel 4870 board when available.

I have built only one amp before, a point to point perfboard current source LM3875 of a friend's else's design. This should be fun!!
Rich
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Old 11th January 2015, 09:46 PM   #3
tomchr is offline tomchr  United States
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Quote:
Originally Posted by Richidoo View Post
Then I find out he only built one channel and has not listened to it in stereo. A good engineer's confidence!
Guilty as charged...

It helps if you start with a circuit that works well. So I start with measurements. If I can build a circuit that measures well, there is a very high probability that it will sound good as well. Once I have a solid circuit, I press on to the listening test. On my prototypes, I always start out listening in mono. There are two reasons for this: 1) cost (both in time and the additional parts needed for a stereo build). 2) I'm usually curious to hear how it sounds.

I actually find that I get very good correlation between the listening experience in mono and that in stereo. In the past, I have only been pleasantly surprised when building two channels and listening in stereo. Basically, if it sounds good in mono, it will sound amazing in stereo. The Modulus-86 sounded amazing to me in mono. I'm really curious to hear it in stereo!

For the mono test, I mix the left and right channel together and play the stereo signal through the mono amp. It's pretty interesting what one can learn from that.

Now, there are some tracks that are not suitable for mono reproduction. Pink Floyd, "Brain Damage" from Dark Side of the Moon is one of those. The "voices in the head" are actually recorded 180 out of phase in the two channels, hence, they nearly cancel when the left and right channels are added. I'm sure that's not the effect Pink Floyd had in mind...

I do plan to build a stereo setup. That's the whole point of this here exercise... It just happened that work (you know, that day job thing that pays the bills) got super busy just as I launched the MOD86 and the MOD86 gained popularity faster than I had expected. I chose to prioritize customer requests and polishing the documentation over building an amp for myself.

Thanks to billshurv for starting a build thread. It's always good to see my circuits in action.

~Tom
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Modulus-86: Composite amp achieving 0.000067 % THD. Damn Good 300B, Novar Spud, 21st Century Maida Reg., Filament Reg., etc.
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Last edited by tomchr; 11th January 2015 at 09:53 PM.
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Old 11th January 2015, 10:50 PM   #4
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well when I knocked up a preamp to hold me for 6 months I added a mono switch. I have quite a few mono LPs in my collection so mono so I am happy with that. Gotta love ebay mixed lots, you find some real treasures amongst the coasters
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Old 12th January 2015, 12:34 AM   #5
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Show and tell; excuse the table background. First pic shows the boards. Not the smallest 3886 PCBs out there, but pretty compact. And the box of components from Mouser. Never used the project ordering system before, but it's worked perfectly and all components are labelled by location to make it almost idiot proof. The heatsinks are 10 each off ebay. they need a clean up, but should be more than adequate for my needs.

Still not 100% on the rails to use. The heatsink is around the 0.5K/W mark, so with 2 boards mounted on it could run at +/-28V rails, but as the ribbon tweeter is 3.8Ohms current is king. I need to measure my incoming AC under a couple of situations then will decide on either 18 or 20v secondaries on the toroids. 18V is the current fave as that will give me +/-22V rails. if you look at figure 4 of the LM3886 spec sheet you will see that you benefit considerably in current capability at the limit dropping a few volts. Does that really matter? Unlikely, but as I am right at the limit of impedance that the chip is designed to drive it can't hurt.

Some my comment that my heatsinking is OTT on 22v rails. Possibly, but I know I will be able to add a 3rd board on each side if I need that as the speakers evolve with no worries on thermals.

Doing a sanity check on the power required, the speakers when new have their max output specified with 100W/4Ohms and recommend 150W/4Ohms. The crossover point (750Hz) means that around 40% of the music power is to the ribbon and the rest to the woofer. As I have a sub for the bottom octaves its nearer 30% to the woofer. So 40W/4Ohms into each is fine? In fact that's more than you need. When you go active the splitting of the spectrum gives you a doubling of effective power. so 2*25W/4Ohms into the active system will give me the same as 100W into the Passive version. I will be around 40W/4Ohms per channel, so considerable headroom. If I wasn't needing to add some boost to the bass to compensate for baffle losses I could drop to 20v rails no problem.
Attached Images
File Type: jpg board.jpg (135.1 KB, 1722 views)
File Type: jpg mouser.jpg (94.2 KB, 1595 views)
File Type: jpg heatsink.jpg (118.9 KB, 1541 views)
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Old 12th January 2015, 05:33 PM   #6
jb74 is offline jb74  United States
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Default Modulus-86 Power Supply

Couple of photos of my assembled power supply.

Since both in and out connections are very similar, I marked a photo
of the side where the transformer connects to avoid possible confusion.

I had the hardware on hand & decided to attach the heat sink with bolts
& washers instead of solder. (Bottom view.jpg)

Some of the leads haven't been trimmed in the photo.

jb74
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File Type: jpg Transformer in.jpg (181.0 KB, 1478 views)
File Type: jpg Bottom view.jpg (215.8 KB, 1431 views)
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Old 13th January 2015, 11:17 AM   #7
jb74 is offline jb74  United States
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Default Modulus-86 Power Supply

Referring to the my previous post I should add that #6 washers have
too small an interior diameter to fit around the shoulders of the heat
sink that protrude through the pcb.


I simply found lock washers that would work - #10.
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Old 13th January 2015, 03:54 PM   #8
tomchr is offline tomchr  United States
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You can also use a 2-part epoxy to glue the bushings of the heat sink to the board. The bushings really put your soldering iron to the test... I like the solution with the #10 lock washers, though.

~Tom
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Modulus-86: Composite amp achieving 0.000067 % THD. Damn Good 300B, Novar Spud, 21st Century Maida Reg., Filament Reg., etc.
Neurochrome : : Audio - http://www.neurochrome.com - Engineering : : Done : : Right
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Old 13th January 2015, 03:56 PM   #9
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Quote:
Originally Posted by tomchr View Post
The bushings really put your soldering iron to the test...

~Tom
Is that a challenge? Always wanted to see what a metcal could really do
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Old 13th January 2015, 07:44 PM   #10
tomchr is offline tomchr  United States
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I use a 6.3 mm wide chisel tip on my Metcal MX500. That'll solder those heat sink studs...
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Modulus-86: Composite amp achieving 0.000067 % THD. Damn Good 300B, Novar Spud, 21st Century Maida Reg., Filament Reg., etc.
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