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Modulus-686: 380W (4Ω); 220W (8Ω) Balanced Composite Power Amp with extremely low THD

One quick question:

On your website, you suggest a 2 x 25VAC transformer for the Modulus 686.

National Semiconductor, in their publication for the BPA200, specifies a Toroid Corp of Maryland 2 x 30VAC secondary transformer.

Some years ago, I assembled most of the parts to construct a BPA200, including a Toroid Corp of Maryland 820VA 30VAC x 2 transformer, to use with a stereo version of the BPA200. Per the Toroid Corp website, the DC output voltage of this transformer should be about 38.5V, when used with a full wave rectifier, and a "large capacitor".

My question is, would the transformer I currently possess be suitable to power a pair of Modulus 686 amplifiers, or does the extra 5 volts in your opinion represent a show stopper?
 
The LM3886 data sheet specifies an ABS MAX of ±42 V with signal applied. If you go beyond ±42 V, the reliability of the LM3886 will be affected. This could mean anything from, "the IC will last 8 years instead of 10 years" to "the IC will last 10 ms".

You will also notice that all the parameters in the data sheet are given for the ±28 V operating point. The data sheet only mentions ±35 V for the power rating into 8 Ω.

The way I read this is: For the best performance, operate the LM3886 with ±28 V for 4 Ω load and ±35 V with 8 Ω load (or if you want to support both 4 Ω and 8 Ω loads, then run it at ±28 V and accept lower output power into 8 Ω). If you go beyond ±28/±35 V with 4,8 Ω loads, respectively, accept degraded performance.

This is consistent with my experience in the lab. I generally find that ±28-30 V is the maximum supply voltage that results in good performance with 4 Ω load. If you push beyond that, the SPiKe protection kicks in a bit earlier (higher supply voltage -> greater power dissipation -> greater temperature differential across the thermal impedance of the package -> SPiKe engages earlier). This results in degraded performance (higher THD, lower max output power).

With the bridge-parallel approach I take in the Modulus-686, you'll get the full performance with a ±36 V power supply. That's why I suggest that as the design target. Pushing beyond ±36 V will not provide any benefit, so why go there?!

FWIW, I run my "daily driver" Modulus-686 on ±27 V using a pair of Mean Well RPS-400-27-C switching supplies. That provides 130 W (8 Ω), 200 W (4 Ω) and will fit in a ModuShop Mini Dissipante 3U x 330 x 300 mm chassis. I love the form factor and 130 W is plenty of power for me (even with a sizeable listening space and mid-low efficiency KEF R700 8 Ω speakers).
If you want to push to ±36 V, you need a 4U x 300 mm heat sink (assuming the ModuShop Dissipante heat sinks).

Tom
 
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Hello,
Newcomer to this forum, I am awaiting receipt of the Modulus 686 kit and I have received most of the components to complete the assembly.
So I will certainly open a “FAQ bar” to benefit from each other's experiences.
You can find a bit of me by following this link JustLiSten | Living Leedh
If I decided to undertake this realization it was following an experience of presentation of a Leedh E2 glass speaker model (glass diaphragm technology, ironless motor, ferrofluid seal), from a French manufacturer and friend that I accompanied to a presentation of his latest model at Munich High End 2016. Munich High-End 2016 | Living Leedh
These fabulous speakers were powered by a prototype amplifier based on 3868 ( << L’Ampli >>, un prototype | Living Leedh )
This amp will not have a follow-up but I could appreciate the sound. Convinced tubist, I find that the 3886-based amps sound a bit like tubes with more power qualities.
See you soon, with all my questions
Best regards
JLS
 
I framed the inside of my enclosure with some aluminum angle and bent up some perforated aluminum sheet. Now the electronics are fully encased.



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I decided to build monoblocks with my Mod686 boards. I like the tall and thin layout. I was going with a 5U x 300mm heat sink, but putting the amp board, Power 86, and 400VA antek transformer in a space that is 150mm wide, 300mm long, 5U tall (210mm). I will have to put the transformer vertically and in close proximity to the amp board. Does the amp board being in close proximity to the transformer or the power 86 board have any interference with the output signal?
 
For a pair of mono's, I presume I need the following for each mono (using SMPS)?

1 x Modulus 686
1 x Meanwell controller
2 x Meanwell RPS-400-27
1 x Guardian 686

Correct. With the RPS-400-27(-C), you can get away with the 3U x 300 mm ModuShop heat sinks (Mini Dissipante becomes an option). With the 36 V RPS-400-36(-C), you'll need 4U heat sinks.

Also note that about half of builders have had issues with the 36 V RPS-400-36 whining at light load. It all cases I'm aware of, this has been resolved by turning the output voltage to ±40 V. I would not recommend planning for that operating point, but am merely mentioning it in case you end up with a whining RPS-400-36.

I have not had any issues with any of the ten (10) RPS-400-27(-C) I've used so far.

I really do recommend the caged version (-C suffix), by the way. It's very nice to work with and limits the amount of exposed mains.

Another option for those who want ±36 V would be the Connex SMPS800RE. Just have Cristi make you a custom ±36 V version. He charges an additional $8 for that.
Note that I have not tested the SMPS800RE with the Modulus-686, but see no reason why it would not work. As long as you get the ±36 V custom version, that is.

Tom