Don't throw the transformer away just yet. Math says 328 VA per channel for 14 dB CF music reproduction into 4 Ω. So you're 10 % low. This assumes you drive the amp to clipping for extended periods of time.
For a new build, I think 400-500 VA is more appropriate. In particular as the price difference between 300 and 500 VA is not huge. In a retrofit, I would probably upgrade the amp first. Then the supply if the urge strikes. Then again. I could easily be accused of bias there. 🙂
Ha! Yeah. But you get a much larger share of my time and my gentle and personal care. That is worth far more than a couple of resistors! 🙂
Tom
Thanks! 😀
Yeah. I think I got tired of typing that the Modulus-xxx provides the same stellar performance regardless of supply type and came up with the power supply agnostic term. I can only use "stellar performance" so many times on the same page... 🙂
Tom
This is looking really good! I hope many people order it and build some nice looking amps.
I look forward to see the THD+N vs frequency plots, please use the industry standard 80kHz measurement bandwidth to make comparisons with other designs possible.
I look forward to see the THD+N vs frequency plots, please use the industry standard 80kHz measurement bandwidth to make comparisons with other designs possible.
Or, PSACA as the world is running short of acronyms and keyboard time is always at a premium these days. 😀
To pass the acronym test, I think scottj would want to streamline that to 4 letters or less?
Tom - so 4U height heat sinks recommended for the 686, would 2U work for mono-ed 286s?
Tom - so 4U height heat sinks recommended for the 686, would 2U work for mono-ed 286s?
Yeah. At 0.41 K/W, the 2U Dissipante should be plenty of heat sink for a Mono MOD286.
Tom
It all depends on how uncomfortably hot you want the heat sinks to get and which operating conditions you intend to operate the amp at.
Math says:
Ambient temp: 25 ºC
Max. heat sink temp: 60 ºC
Supply voltage: ±35 V
Crest factor, music: 14 dB
8 Ω, sine wave: 0.32 K/W
8 Ω, music: 0.46 K/W
4 Ω, sine wave: 0.17 K/W
4 Ω, music: 0.25 K/W
That says you want a 4U Dissipante chassis if you want the amp to be able to handle playing music continuously into a 4 Ω load with the peaks reaching clipping levels without having the heat sink temperature exceed 60 ºC.
Of course this will mean that under normal operating conditions, the heat sink might reach lukewarm if that. I consider that good thermal design, but some builders seem to be disappointed by cool heat sinks. 🙂
I chose 60 ºC max as that's really rather hot to touch. It won't burn you instantly, but don't expect to be able to hold your hand on the heat sink for more than a few seconds at a time at that temperature.
In addition to the user's well-being, one should also consider that the SPiKe protection of the LM3886 will start to kick in at the max output power if the heat sink gets much beyond 65-70 ºC.
I linked to the data sheet for the Dissipante heat sinks back in Post #7.
Tom
Last edited:
Power Question
Sorry this sounds probably stupid to you electronic experts....
I wondered about how one achieves 220W at 8ohms with ca. +-35V rail-voltage.
My limited knowledge up to now said, that Poutput = approx. Vrail^2/2/Rspeaker which equals to roughly Poutput = 35V/2/8ohms = 76Watts.
To get to 220W at 8ohms I would have guessed one needs a rail-voltage of ca. +-45V.
Where am I going wrong?
Or would it be even more beneficial to drive a 686 with +-45Vrail?
Thx.
Sorry this sounds probably stupid to you electronic experts....
I wondered about how one achieves 220W at 8ohms with ca. +-35V rail-voltage.
My limited knowledge up to now said, that Poutput = approx. Vrail^2/2/Rspeaker which equals to roughly Poutput = 35V/2/8ohms = 76Watts.
To get to 220W at 8ohms I would have guessed one needs a rail-voltage of ca. +-45V.
Where am I going wrong?
Or would it be even more beneficial to drive a 686 with +-45Vrail?
Thx.
Think its the bridged feature where it see only halv the load (Bridged and paralleled amplifiers - Wikipedia)
Tom, as this ain't a ClassA toaster - which can be found elsewhere - I think your conservatively operating well below the chip's inherent thermal protection is probably wise.
No worries.
Good question.
Pout = Vout^2/R, with the output voltage in V RMS.
Pout = Vout^2/(2R), with the output voltage in Vpeak.
So, you'll need sqrt(2*8*220) = 59.3 V peak across the load. That seems a bit of a stretch for an amp running on ±35 V ... unless you split the amp into two halves and drive one towards +35 V and the other towards -35 V it the same time and present the difference across the load. Then suddenly ±70 V swing across the load is possible.
That was a cool party trick! What's the drawback? One of the drawbacks is that instead of building one amplifier, you now need to build two. And as pointed out above, each amp half needs to be able to drive half the load impedance, so each amp half will need to be able to drive a 2 Ω load for the full amp to be able to support 4 Ω load impedances. This is no small order.
Each set of three LM3886es in the MOD686 is an amp capable of driving a 2 Ω load (and lower, actually).
40 V peak swing for 100 W.
50 V peak swing for 150 W.
60 V peak swing for 200 W.
Rough math here (except for 100 W, which is 40 Vp exactly).
Not if you want the LM3886es to survive. Their ABSMAX rating with signal swing is ±42 V. Their performance starts to suffer once you get much past ±35-36 V.
Now on the topic of driving low-impedance loads: I did once by accident drive a 1 Ω with the MOD686. I forget the resulting output power as the amp shut down pretty quickly. A few hundred watt, I think. With 2 Ω load I seem to recall seeing 600 W out for long enough to read the display on the power meter before the amp overheated and shut down. Note that driving a resistive 2 Ω load is rather different from driving a reactive load that happens to dip into the 1-2 Ω range. I'm generally not overly concerned about those dips as they tend to be pretty narrow band.
Oh, and by the way: The amp survived the "1 Ω incident" just fine. As soon as I turned the signal generator off, the amp came back to life.
Tom
Last edited:
Okedoke. Thx Tom and Byrtt for the detailed explanation.
So what happens to my speakers if one of the three LM3886 in one of the amp halves goes into thermal protection, while the remaining five keep humming along?
Will the output experience a speaker damaging offset?
I would hate to let the magic smoke out.
So what happens to my speakers if one of the three LM3886 in one of the amp halves goes into thermal protection, while the remaining five keep humming along?
Will the output experience a speaker damaging offset?
I would hate to let the magic smoke out.
In that specific scenario, the remaining two LM3886es would carry the load and quickly overheat and turn off. In reality, the entire amp turns off and the output signal goes to zero.
I have not seen any ill effects in the lab when the various protection circuits in the LM3886 have engaged. That does sound worth looking at, though. I'll add it to the list.
I did look at the clipping behaviour of the Modulus-686. It has the cleanest clipping of any amp I make. I'll see if I can get the scope shots cleaned up and posted tonight.
Just an FYI: I have obviously evaluated the prototype build in great detail, but would prefer to focus my measurement efforts on the final builds. The performance with machine soldered SMD components is a lot more consistent and repeatable than hand soldering - especially as my prototype now has a couple of fly-by-wire components dangling off of it. Expect lots more measurements to come once I get the assembled boards in.
I should have the bare boards next week and hopefully the first few samples from the assembly house soon thereafter.
Tom
Damn you! Now you got me curious... 🙂 That's not a bad thing and I've been procrastinating on a short paper that I have due on Thursday anyway. I needed a procrastination task.
I took a look at the THD+N vs Output Power into a 2 Ω load. I had to use a linear supply as the Mean Well SE-600-36 with its 16.6 A output current rating runs out of juice on the peaks. So the THD+N vs power graph is with the Power-86 in its stock configuration powered by an Antek AN-5225 power transformer. The plot is attached here.
The THD+N is a bit higher than I'd like. I may be able to improve on that. Stay tuned for the final boards.
Tom
I took a look at the THD+N vs Output Power into a 2 Ω load. I had to use a linear supply as the Mean Well SE-600-36 with its 16.6 A output current rating runs out of juice on the peaks. So the THD+N vs power graph is with the Power-86 in its stock configuration powered by an Antek AN-5225 power transformer. The plot is attached here.
The THD+N is a bit higher than I'd like. I may be able to improve on that. Stay tuned for the final boards.
Tom
Attachments
Last edited:
I thought of putting all the plots into one post, but figured I'd better put the plots that shows the amp in overload condition separate from those that show the amp under normal working conditions.
Here are the overload plots of the power vs time, DC offset vs time, and THD+N vs time. All plots were done with 2 Ω load.
I ran three conditions:
The amplifier is obviously not producing a clean output when it is driven into hard clipping, when the LM3886 thermal protection kicks in, or when the supply current limits. That should be obvious. However, importantly, the amplifier does recover well from all those conditions as evidenced by the THD+N returning to the same value as before the fault condition. Also note that the DC output voltage never exceeds 1.0 V. This isn't even enough to trigger the DC protection in a speaker protection circuit.
It is interesting to note that after running at 200 W for 15 seconds, the heat sink is so hot that the amp starts to overheat at 100 W (into 2 Ω, remember!) already. This is with a 0.4 K/W heat sink.
I stand by my claim that this amp will drive 8 Ω and 4 Ω rated speakers - even if they should have impedance dips into the 2 Ω range.
Tom
Here are the overload plots of the power vs time, DC offset vs time, and THD+N vs time. All plots were done with 2 Ω load.
I ran three conditions:
- 2 W -> 600 W step: Hard clipping recovery. (Power-86 + Antek AN-5225).
- 100 W -> 200 W step: Triggers thermal overload. (Power-86 + Antek AN-5225).
- 10 W -> 100 W step: Triggers supply brownout. (Mean Well SE-600-36).
The amplifier is obviously not producing a clean output when it is driven into hard clipping, when the LM3886 thermal protection kicks in, or when the supply current limits. That should be obvious. However, importantly, the amplifier does recover well from all those conditions as evidenced by the THD+N returning to the same value as before the fault condition. Also note that the DC output voltage never exceeds 1.0 V. This isn't even enough to trigger the DC protection in a speaker protection circuit.
It is interesting to note that after running at 200 W for 15 seconds, the heat sink is so hot that the amp starts to overheat at 100 W (into 2 Ω, remember!) already. This is with a 0.4 K/W heat sink.
I stand by my claim that this amp will drive 8 Ω and 4 Ω rated speakers - even if they should have impedance dips into the 2 Ω range.
Tom
Attachments
-
Modulus-686 PROTO_ Supply Brownout Recovery - Output Power vs Time (2 ohm, MW SE-600-36).PNG -
Modulus-686 PROTO_ Supply Brownout Recovery - Output DC Level vs Time (2 ohm, MW SE-600-36).PNG -
Modulus-686 PROTO_ Thermal Overload Recovery - THD+N vs Time (2 ohm, Power-86 + AN-5225).PNG -
Modulus-686 PROTO_ Thermal Overload Recovery - Output Power vs Time (2 ohm, Power-86 + AN-5225).PNG -
Modulus-686 PROTO_ Thermal Overload Recovery - Output DC Level vs Time (2 ohm, Power-86 + AN-522.PNG -
Modulus-686 PROTO_ Clipping Recovery - THD+N vs Time (2 ohm, Power-86 + AN-5225).PNG -
Modulus-686 PROTO_ Clipping Recovery - Output Power vs Time (2 ohm, Power-86 + AN-5225).PNG -
Modulus-686 PROTO_ Clipping Recovery - Output DC Level vs Time (2 ohm, Power-86 + AN-5225).PNG -
Modulus-686 PROTO_ Supply Brownout Recovery - THD+N vs Time (2 ohm, MW-600-36).PNG
Last edited:
