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Modulus-86  build thread
Modulus-86  build thread
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Old 1st February 2018, 03:29 AM   #3821
Johnny2Bad is offline Johnny2Bad  Canada
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Q: When using a linear power supply, the toroidal transformer is typically mounted to the bottom of the chassis via the centre bolt.

Is this absolutely necessary, or could the transformer be mounted such that it does not connect to the chassis itself but instead to a sub-chassis not grounded to the main chassis? I understand that the centre bolt would still be required as it affects the flux of the transformer.
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Old 1st February 2018, 04:15 AM   #3822
BrianL is offline BrianL  United States
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Modulus-86  build thread
The center bolt is not required for operation of the transformer. In theory, the flux in the center of a toroidal transformer should be zero. What you don't want is a complete loop around the toroid with metal work, bolt, etc. That becomes a shorted turn winding and can cause damage.
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Old 1st February 2018, 05:12 AM   #3823
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by Studley View Post
The bespoke mounting brackets sound like a fine idea.
Yep. That provides a nice solution to the thermal interface as well. As you can imagine, a 200+ W Class AB amp dissipates a bit of power, so getting the thermal system right is a must.

Tom
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Modulus-86, 186, 286, & 686: 40-240W (8Ω) at <-120dB THD. HP-1: 3W/20Ω, -130dB THD, 128dB DNR.
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Old 1st February 2018, 05:41 AM   #3824
HammerSandwich is offline HammerSandwich
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Quote:
Originally Posted by tomchr View Post
Ok, so maybe I'm biased, but I think those of you who want more power have something to look forward to.

Spinning some Dire Straits...
I'm confused. Dire Straits needs more than 25w?
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Old 1st February 2018, 07:13 AM   #3825
HarmonicTHD is offline HarmonicTHD  Switzerland
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Quote:
Originally Posted by tomchr View Post
Yeah. I'll start a thread in the Vendor's Bazar over the weekend.



I'm using a 0.4 K/W heat sink that I bought as scrap aluminum (at scrap aluminum cost) many years ago in Copenhagen. It's about 300x150x50 mm and weighs in at 2.5 kg if I recall correctly.
I need to look at the Dissipante heat sink specs. 4U sounds about right.

If you lower the supply voltage to ±28 V, you can probably get away with the 3U. You'd get around 150 W in that case. That'd be a nice safe and sane build for those who'd like more power than the MOD86 delivers but don't need the 200+ W.



Absolutely. I'll build the boards with the default 26 dB gain. Those who want the slightly lower noise floor can pluck a resistor off the board and reduce the gain to 20 dB.
The gain is adjusted by changing one resistor. This can be done without impacting amplifier stability. It does impact the amplifier noise floor, though, so as the gain is increased, the THD+N gets a bit worse. Figure a 3 dB change in gain -> 3 dB change in THD+N.

The MOD686 has differential (balanced/XLR) input. It can be run single-ended (unbalanced/RCA) as well. I took the lessons learned from the MOD86 and applied them to the '686. Thus, the '686 will start up cleanly (no plop) even with an unbalanced input, unlike the MOD86 where there is a small plop from the input bias current of the THAT1200.

Tom
Hi Tom

This is the link to the Modulus Heat sink specs which I posted a while back in the 286 thread.

http://www.modu.it/CARATTERISICHE_TERMICHE_DISS.pdf

It says 0.41K/W for the 3U. Would that suffice for the 686?

Would be cool though. This way I could upgrade from my 286 to 686.

Thx
SH
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Old 1st February 2018, 09:58 AM   #3826
AndrewT is offline AndrewT  Scotland
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I use a pair of 100W into 8ohms ClassAB amplifiers on a single shared 0.55C/W heatsink, in Scotland where summer rarely, if ever gets above 28degreesC.
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Old 1st February 2018, 03:40 PM   #3827
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by HarmonicTHD View Post
It says 0.41K/W for the 3U. Would that suffice for the 686?

Would be cool though. This way I could upgrade from my 286 to 686.
That'll work. If you run the amp for hours at the worst case dissipation (150-200 W, 4 Ω, sine wave) expect to see heat sink temperatures around 65 șC, which is quite hot. The amp will survive just fine though.
For music reproduction, your heat sink will be more than lukewarm but not by much.

Curious: Did you build your MOD286 as mono modules or stereo?

Tom
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Old 1st February 2018, 04:03 PM   #3828
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by HammerSandwich View Post
Ahhh... Should I bite or should I not.

If you agree with Bruno's assumptions about listening distance and SPL, you will find his conclusion valid as well. Change any of the input variables and the result will change. For example, what if you prefer 91 dB SPL at the listening position? That'll bump the 25 W up to 100 W. Or have a large room and sit further away from the speakers? I give you the math here, so you can easily play with the numbers yourself.

As Bruno concludes in the last line, "the occasional clipping event isn't going to spoil the fun." Do you agree? Or would you rather have a system that doesn't clip?

There are also perfectly valid use cases where you do need the power. For example, the woofers in the Linkwitz LX521.4 and Orion speakers require significant power to be available if you want to avoid clipping and rude noises emitted by the speakers. That's common with active speakers where significant boost is added in the filter chain. In fact, I recently spoke with a guy who makes sound systems for recording studios. He needed 1 kW on the woofer channel to be able to support the bass boost of his active speakers ... and this was for near field listening ... and he'd done the math. Now, he was aiming for rather high SPLs, so residential users can likely dial that back some, but still...

Also, don't forget that needs and wants aren't always connected.

Tom
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Modulus-86, 186, 286, & 686: 40-240W (8Ω) at <-120dB THD. HP-1: 3W/20Ω, -130dB THD, 128dB DNR.
Neurochrome : : Audio - www.neurochrome.com - Engineering : : Done : : Right
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Old 1st February 2018, 05:02 PM   #3829
HammerSandwich is offline HammerSandwich
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Quote:
Originally Posted by tomchr View Post
Ahhh... Should I bite or should I not.
I'll add a smiley next time - as I should have here. (Not that there's anything wrong with having the extra details you added.)

Quote:
As Bruno concludes in the last line, "the occasional clipping event isn't going to spoil the fun." Do you agree? Or would you rather have a system that doesn't clip?
I'd much prefer never to clip. But, in fairness to BP, the full sentence shows he's no longer talking about 25 watts:
Quote:
Originally Posted by Bruno Putzeys
The only reason why you can say that more than a few 100W is overdoing it, is because at such levels the occasional clipping event isn't going to spoil the fun.
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Old 1st February 2018, 07:03 PM   #3830
tomchr is offline tomchr  Canada
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Either way, I think we can agree that occasional clipping will have the same effect on the two amps, only it will occur at power levels 10 dB lower on the 25 W amp than on a 250 W amp.

I do suspect that the harshness that comes on many amps when the volume is cranked hard is due to the occasional clipping. At clipping, the distortion spectrum becomes a rather tall forest of harmonics of all orders. In a multi-tone environment that would create all sorts of fun IMD products, which seem to be rather strongly correlated to the listening experience. If you get fewer clipping events, the harshness should go away.

I will also note that a local buddy of mine runs into rude woofer noises with the Parallel-86 on his LX521.4s. I'm suspecting the recording is to blame as the rude noises are only present briefly in one channel on one track. He isn't convinced and suspects that the amp is clipping, which could be the case. As I pointed out earlier: Once you start adding boost to a woofer, you will need to have some power available to drive the cone excursion.

For less demanding speakers, such as the LXmini, I still think the Modulus-86 or Modulus-286 MONO (if you need a little extra headroom) are good candidates.

Tom
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Modulus-86, 186, 286, & 686: 40-240W (8Ω) at <-120dB THD. HP-1: 3W/20Ω, -130dB THD, 128dB DNR.
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