While that certainly is true, there is no reason to believe that the performance of the Modulus-86 will in any way be impacted by the interference that does enter the chassis. In fact, I designed the PCB to minimize the impact any RF would have. It could, in theory, be further minimized by switching to SMD components, but we're really getting into dBGF territory here. That's dB relative to one imperial Gnat Fart.
If you really want the chassis to be RF tight, then you need to strip the anodization off. The number of screws holding the chassis together would need to increase dramatically. Ideally, the chassis would be a cast or milled tub with a lid and the interface between the tub and the lid would feature a groove with an RF gasket (wire mesh) in it. The screws holding the box together would likely have to be torqued with a torque wrench to ensure correct gasket pressure. Of course, as you point out, any hole poked through the chassis is an entry point for RF, so you'd use feed-through caps soldered to the enclosure. This also means that all heat must be conducted through the chassis as any ventilation holes would compromise the RF performance of the chassis.
The Modulus-86 or any other amp I offer requires **NONE** of this. I have measured the performance of the Modulus-86 both "naked" on the lab bench and enclosed in an aluminum chassis following the directions I give in the design doc. The only difference I can measure is that the mains hum is slightly (a few dB) lower with the amp in a box. This could be due to the repeatability of the measurement (the mains hum is very far down, thus, hard to measure reliably) or because less 60 Hz hum is coupled to the input wiring of the amp (I use shielded wire in the chassis; short pigtail of regular untwisted wire when measuring the amp naked).
I do understand that part of the attraction of DIYing something is to geek out about one or more technical aspects, but in this case there are much bigger fish to fry.
For example: There needs to be a grommet where the wires go through the metal plates. Otherwise the insulation on the wires will wear and the wire will short to the chassis.
I also support the notion of mounting the mains filter such that the filter can connects to the chassis. If you paid for the filter you may as well get the filtering function...
Also note that even if the filter does make a good connection to the chassis, you still need a short wire from the mains earth terminal to chassis ground for safety.Tom
Relative measuring equipment for it, could be excessively expensive, or currently stuck in development. There could be some delay for it.
I'm new to DiyAudio, and recently completed populating Modulus 286 & SMPS 86 boards. I am not yet decided on casework, but here are a some photos of the completed boards.
IMG_9988.jpg by PScal, on Flickr
IMG_9986.jpg by PScal, on Flickr
IMG_9988.jpg by PScal, on Flickr
IMG_9986.jpg by PScal, on Flickr
Maybe this has been discussed before here but I found this article while in the Benchmark amp site and found it very interesting: https://benchmarkmedia.com/blogs/application_notes/power-amplifiers-the-importance-of-the-first-watt - it concerns how much of the music we listen to is produced at very low power levels; 10 milliwatts and below.
They state:
"We have proven that amplifier crossover distortion can be very audible through loudspeakers when playing pure tones at 0.01 watt. We have also shown that feed-forward error correction is very effective at eliminating the fast transients that are produced by crossover distortion in an amplifier's output stage. In contrast, traditional feedback systems are usually too slow to fully remove these transients. The remaining crossover distortion artifacts may be audible".
I'll admit that some (OK, maybe more) of what was discussed went over my head but anyone care to comment how the Modulus stacks up against the figures for the Benchmark's AHB2 power amplifier in regard to distortion measurements at the 1 watt level as discussed in the article.
They state:
"We have proven that amplifier crossover distortion can be very audible through loudspeakers when playing pure tones at 0.01 watt. We have also shown that feed-forward error correction is very effective at eliminating the fast transients that are produced by crossover distortion in an amplifier's output stage. In contrast, traditional feedback systems are usually too slow to fully remove these transients. The remaining crossover distortion artifacts may be audible".
I'll admit that some (OK, maybe more
) of what was discussed went over my head but anyone care to comment how the Modulus stacks up against the figures for the Benchmark's AHB2 power amplifier in regard to distortion measurements at the 1 watt level as discussed in the article.
I'm new to DiyAudio, and recently completed populating Modulus 286 & SMPS 86 boards. I am not yet decided on casework, but here are a some photos of the completed boards.
IMG_9988.jpg by PScal, on Flickr
IMG_9986.jpg by PScal, on Flickr
Nice power module. I noticed it has the emi filters built in. Personally I noticed change in sound quality for those filters in design as well as component selection.
Also depending on the actual design, there could be some residual ripple occurring at twice the line power frequency of a magnitude of a few mv in the ground. Probably want to check it with a scope if you hear slight hum. If not, then it could also be dithering where you will not hear it.
I will be trying those soon as well.
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For most listening that's certainly true. 10 mW may be a tad low, but 100 mW, sure...
The Modulus-86 measures 0.00006 % THD at 1 W.
The Benchmark ABH-2 is the one that uses the feed-forward scheme (THX patent). As I recall, its specs were close to those of the Modulus-86. On some numbers it was slightly better, on others slightly worse. I seem to recall it having a slightly lower noise floor, just as the noise floor of the Modulus-286 is lower than that of the Modulus-86.
That's probably mostly the marketing department that wrote that. It may also be that in many discrete amps (which is what the ABH-2 is likely to be compared against) the loop gain and loop bandwidth are rather low. In that case their statement makes sense.
In case of the Modulus, the loop gain is extremely high and the loop bandwidth is a couple of MHz, which is why the Modulus rivals the performance of the ABH-2.
Tom
Most SMPS designs, regardless of safety class and power level, have a rather big capacitor (~1nF) across the transformer that connects primary and secondary side. The primary side always will have significant LF and RF ripple and the ripple current that makes it to the secondary wants to return to PE. Therefore it is often a good idea to connect one output of an SMPS to PE (capacitively, at least) and to use a substantial common-mode choke (no PE connection) on the input side. By doing so, the ripple current's source impedance is increased and the ripple current returns via a direct low impedance path so there is only very little ripple voltage to be found on the local PE vs other PEs on the same power strip.
Actually, some SMPS(+Amp) makers like ICEpower offer Class-II products that must be earthed (at least for RF frequencies) on the secondary side to in order to be EMC compliant. This is typically stated in a footnote in the EMC report that often gets overlooked....
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The great thing about buying from Tom is that you have an 'Engineered product'. As in he has actually designed and measured for a particular performance. With SMPS +Mod its a combined performance. There is unlikely to be anything any of us can do to improve what comes out, but many ways to make it worse. If your personal subjective bubble likes that, then fine, but recommending people to start messing with things when the product is as good as this is should be given a caveat.
I was working on a filter for a switching module, getting EMC levels to comply for class II was much easier, but I could not find a two line power cord already compliant with safety standards, trying to certify a new cord seemed a bad idea. One reason was precisely due to the secondary connected to chassis ground. When that ground is connected to PE, it actually causes more noise to be measured at the power line. But if you leave it Class II design, the chassis ground needs to be totally isolated, otherwise you will feel the leakage current when you touch it. When you start mixing different classes with each other, things get complicated.
But if I power filter is designed for Class II, it may not pass EMC requirements when the PE is connected. Whether EMI radiation will pass or not depends on measurement method. I spent some time trying to understand the different methods because the test methods used effected cost.
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When I see a certified report on the it, I will believe it. DIY does not require certification.
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According to the DEQX website regarding active speakers that need separate power amplifiers for each bass, midrange/tweeter drivers (2 way in my case) they state the following:
"An active loudspeaker uses line-level active crossover filters that drive separate power amplifiers for each bass, midrange and tweeter driver........Bass requires very high power at moderate resolution, mids require good resolution at medium power, and highs require the highest resolution but at lower power.
Where practical to do so, best performance is more likely and cost effective using three amplifiers each suited for their limited frequency range compared to very high-end amplifiers that can deliver all 10 octaves perfectly. Even identical amplifiers operating over limited frequency ranges perform better."
Tom, what are your thoughts on this and if you agree, is it possible to modify the Modulus amp to operate over a limited frequency range as discussed above?
"An active loudspeaker uses line-level active crossover filters that drive separate power amplifiers for each bass, midrange and tweeter driver........Bass requires very high power at moderate resolution, mids require good resolution at medium power, and highs require the highest resolution but at lower power.
Where practical to do so, best performance is more likely and cost effective using three amplifiers each suited for their limited frequency range compared to very high-end amplifiers that can deliver all 10 octaves perfectly. Even identical amplifiers operating over limited frequency ranges perform better."
Tom, what are your thoughts on this and if you agree, is it possible to modify the Modulus amp to operate over a limited frequency range as discussed above?
First of all, the amount of power depends on bandwidth. So talking about power requirements should relate with crossover points. Second, low frequencies require higher voltage, but most drivers for home usage bottom out within 25 volts peak. So unless you are using very large drivers, possibly 25" or more, then then you need amps that can output higher voltages. When you are trying to keep costs to minimum on commercial products, it is possible that you have to do some give and take.
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True. It also depends on the power spectral density of the music. Specifically, the power required is the integral of the power spectral density over the bandwidth of interest. Contrary to common opinion, this usually means that the woofer/subwoofer actually needs lower power than the tweeter as the integration bandwidth of a subwoofer (maybe 100 Hz) is much, much lower than that of a tweeter (18 kHz in some cases). Tweeters are usually more efficient, so it is likely that it ends up being a wash in the end.
Given your previous statement that the power depends on the bandwidth, above doesn't make sense. P = V^2/R. Thus, if the power depends on the bandwidth, so does the voltage. As you said, you need to do the math before you can make any statement about the power requirements.
Also, 25 V peak into 4 Ω is 78 W. Surely, there are drivers smaller than 25" that can handle more than 78 W. The SB Acoustics 8" SB23MFCL45-4, for example, can handle 150 W.
Now you're contradicting yourself. Above sentence says if you're using large drivers (25"+) you can get away with lower voltage amps. For smaller drivers (<25") you need higher voltage amps. That's interesting given how you in the previous sentence said that most smaller drivers can't handle more than 25 V.
Tom
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