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Neurochrome LM3886DR Build
Neurochrome LM3886DR Build
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Old 29th August 2016, 11:51 AM   #21
TioFrancotirador is offline TioFrancotirador  Poland
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Thanks Tom for quick response.

Out of curiosity again:
1. In your design you are using a bit different values for Thiele Network, why?
2. In some designs they recommend putting NFB resistor directly on chip pin. In your PCB one lead of nfb resistor is close to the chip pin while the other has a thin trace leading to output (before Thiele Network). I bet there is reason for this.
Can you elaborate on those points. Thanks.
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Old 29th August 2016, 04:39 PM   #22
tomchr is offline tomchr  Canada
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Most designs use a *way* under-damped Thiele network. That results in excessive ringing with a capacitive load. I aim for a clean response with a 8 Ω || 100 nF load.

Yeah. I'm actually one who recommends using short traces on the feedback resistor. If you are not using the Thiele network, the trace inductance in series with the feedback resistor can cause stability issues with capacitive load. When the Thiele network in place these issues go away as the capacitive load is effectively isolated from the feedback network.
The voltage at the point where the feedback trace taps off is the voltage that is controlled by the amplifier. In the ideal world, this point would be right on the voice coil of the speaker and there would be a similar arrangement on the ground side of the voice coil. Unfortunately, this puts the feedback on the outside of the Thiele network which may cause some issues. The best point is therefore the point where the output voltage enters the Thiele network.

Tom
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MOD686: Up to 240W/8Ω, <0.00025% THD. MOD86: 40W/8Ω, 0.000054% THD. HP-1: 3W/20Ω, -130dBc THD, 128dB DNR.
Neurochrome : : Audio - www.neurochrome.com - Engineering : : Done : : Right
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Old 29th August 2016, 07:04 PM   #23
TioFrancotirador is offline TioFrancotirador  Poland
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Thanks Tom,

Can you advise me then what parameters for Thiele network should I use for my setup:
4ohm load, 10feet long cables per channel, awg12 solid core copper cable with very thin insulator (like the one you used for creating Thiele network).
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Old 30th August 2016, 12:26 AM   #24
tomchr is offline tomchr  Canada
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The Thiele network does not depend on the load. My recommendation is to use the one I specify in the LM3886DR documentation.

Tom
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MOD686: Up to 240W/8Ω, <0.00025% THD. MOD86: 40W/8Ω, 0.000054% THD. HP-1: 3W/20Ω, -130dBc THD, 128dB DNR.
Neurochrome : : Audio - www.neurochrome.com - Engineering : : Done : : Right
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Old 4th September 2016, 03:29 AM   #25
dreadnoughtstrength is offline dreadnoughtstrength  Australia
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I've been absolutely flat out with work, but slowly chipping away at getting this finished - thankfully a lot of the work is getting me familiar with CAD which I will use to the benefit of this project.

I ended up buying a new toroid as the other one is going towards a build with an integrated pre-amp. Nothing fancy, 2x25v/300VA.

20160903_195342.jpg

Got the two eBay regulator modules (set to 30v output) set up on an old heatsink with a mica washer, along with one amp module (was doing this at 8pm at night so didn't bother hooking up both).

20160903_195348.jpg

Ran a 50Hz sine wave (as a stress test) through one of the spare speakers for 10 minutes continuously @ ~30w output. Zero voltage sag even when increasing this to 50w output. Pretty happy for $30 worth of Chinese power supplies!

Good news is there is no audible background noise, and both the regulators stayed completely cool in this time, with the LM3886 getting fairly warm to the touch (but still cool enough to hold my finger onto). I know this is a fairly big chunk of heatsink and only one module was running, but on the final assembly the results should be similar.
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Old 4th September 2016, 05:09 AM   #26
tomchr is offline tomchr  Canada
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For the final assembly, don't forget the thermal goop on those mica washers. Or better yet, use one of the SilPad thermal pads intended for the purpose.

Tom
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MOD686: Up to 240W/8Ω, <0.00025% THD. MOD86: 40W/8Ω, 0.000054% THD. HP-1: 3W/20Ω, -130dBc THD, 128dB DNR.
Neurochrome : : Audio - www.neurochrome.com - Engineering : : Done : : Right
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Old 8th September 2016, 07:05 AM   #27
dreadnoughtstrength is offline dreadnoughtstrength  Australia
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14192576_10153729408710025_6351314383394461376_n.jpg

Slab #1 poured - 300x140x20mm (~12"x5.5"x7/8")

Will get started on machining soon.
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Old 8th September 2016, 10:06 AM   #28
TioFrancotirador is offline TioFrancotirador  Poland
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Hi Tom,

Are there any guidelines for power rating of Thiele network resistor? I know 2W is suggested but is the 1W enough?
I got the following resistor under my hand that I could choose:
1R5 5W wire-wound "non-inductive" design - afraid of its inductance anyway
10R 2W metal film - afraid that value is to much
2R2 1W metal film - afraid that power rating is to low

Which one would be the best to use in my case?
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Old 8th September 2016, 12:02 PM   #29
AndrewT is offline AndrewT  Scotland
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if you never apply HF test signals then the correctly rated Zobel and/or output damping resistors will never operate hot.

It's the HF that passes easily through the capacitances that will heat the resistors.
Take a 4r7 Zobel resistor in series with a 100nF capacitor.
The capacitor reactance @ 20kHz is ~80ohms
If you combine that with the 4r7 resistor you get a total impedance of sqrt (4.7 + 79.6) ~ 79.7ohms.
The current passing when 20Vac is applied is ~251mAac and the power dissipated in the Zobel resistor is <300mW

But apply a 50kHz squarewave and the Zobel resistor will smoke very quickly and the esr heating in the capacitor may well damage the capacitor as well.

BTW,
1r0 600mW 1% resistors can be stacked in either parallel, or series, or a series parallel, combination to make accurate high power "specials" that are cheaper and quicker (but not nearly as pretty !) than buying in two values of a special when you have to pay postage for low quantities and wait days, or weeks, for the delivery.
I am nearly 40% through my box of 1000 1r0 600mW 1% metal film I bought about 5years ago.
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Last edited by AndrewT; 8th September 2016 at 12:09 PM.
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Old 8th September 2016, 04:25 PM   #30
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by TioFrancotirador View Post
1R5 5W wire-wound "non-inductive" design - afraid of its inductance anyway
10R 2W metal film - afraid that value is to much
2R2 1W metal film - afraid that power rating is to low
Of those choices, 1.5 Ω, 5 W is the best option. I generally stick with 2 W types. During sine wave testing above the audio band those resistors do get hot. Same for the Zobel resistor as Andrew mentioned.

Tom
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MOD686: Up to 240W/8Ω, <0.00025% THD. MOD86: 40W/8Ω, 0.000054% THD. HP-1: 3W/20Ω, -130dBc THD, 128dB DNR.
Neurochrome : : Audio - www.neurochrome.com - Engineering : : Done : : Right

Last edited by tomchr; 8th September 2016 at 04:27 PM.
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