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

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

The Modulus-686 is a balanced composite bridge/parallel amplifier. It uses six LM3886 amplifier ICs to provide nearly 400 W into 4 ohm loads at vanishingly low distortion. With a typical output current capability of 33 A, the Modulus-686 easily drives even the most challenging loads, including loads with impedance dips into the 2 ohm range.

You can find the build pictures from a couple of representative builds by following these links. This is not an exhaustive list.
Key Features:
  • Mono construction.
  • 4-layer, gold plated, ROHS compatible circuit board. Professionally assembled in Calgary, Canada.
  • Output power: 380 W (4 ohm) -- 220 W (8 ohm).
  • Ultra-low 0.00026 % THD+N (140 W, 8 ohm, 1 kHz).
  • Ultra-low 0.00029 % THD+N (260 W, 4 ohm, 1 kHz).
  • 16 uV RMS (A-weighted, 20 dB gain, 20 Hz - 20 kHz) output noise.
  • 1 kHz + 5.5 kHz IMD <-110 dBc @ 1 W.
  • Gain: +26 dB. Changeable by resistor option. Minimum gain: +20 dB.
  • On-board EMI/RFI input filter and ESD protection.
  • Power supply agnostic circuit architecture. The Modulus-686 performs identically on an unregulated supply as it does on a well-regulated supply.
  • Board dimensions: 8.25 x 2.30 inches (approx. 210 x 60 mm)
Full set of measurements found in Post #252 and Post #254. The transient response is shown in Post #331.

Also note that the Modulus-686 can be built for lower output power by lowering the supply voltage. For example, by running the Modulus-686 on ±27 V, it will provide 125 W into 8 Ω and 200 W into 4 Ω. Lower output power allows for a smaller heat sink. For a more comprehensive list of output power vs supply voltage, see Post #486.

It is always my goal to offer a positive build experience. Many businesses can deliver a good experience when all goes according to plan. In my view, examining the level of customer service when something doesn't go quite as planned is much more indicative of the quality of a business. Such an example is shown in Post #864. Note that all turned out well in the end. Also note the comparison of the Modulus-686 to a 12000 Euro (~$13600) commercially available amp towards the end of Post #864.

The Modulus-686 combines the learnings from the Modulus-86, -286, and some of the compensation tricks employed in the HP-1 to form a state of the art power amp. It is intended to operate from a +/-36 V supply and will provide the same stellar performance when powered by a switching supply as it does from a regular linear unregulated supply. A Power-86 with a 2x25 VAC, 500 VA transformer or a pair of Mean Well RPS-400-36 would work well for a mono MOD686 build. As seen in the third THD+N plot, the max output power is a little lower with the Power-86 as the rail voltage droops a little under load.

The Modulus-686 is currently available in two board options:
  • All surface mounted devices (SMD) pre-populated.
  • Fully assembled and tested amplifier module.
Update 2018/10/25: Do note that once the current board stock has sold, the SMD pre-populated option will be eliminated and I will only offer the fully assembled module option.
Unlike my past projects, I will not offer bare boards for sale. There are a couple of reasons for this:
  • Focusing on assembled and partially assembled modules allows me to lower the cost to everybody's benefit.
  • Supplying assembled and partially assembled boards allows me to serve my customers better and broaden my customer base.
  • Machine-soldered SMD boards perform much more consistently and reliably than hand soldered boards -- even for competent builders.
So basically, I can deliver a higher quality product by leaving the SMD assembly to the pros. This benefits everybody.

The full description of the Modulus-686 is available on its product description page: Neurochrome :: Modulus-686 :: 220W (8Ω); 380W (4Ω) @ 0.00026% THD+N.. A picture of the prototype is attached here.

The Modulus-686 circuit boards have arrived from the manufacturer. You can order yours here: https://www.neurochrome.com/product/modulus-686/

Thanks,

Tom
 

Attachments

  • Modulus-686 PROTO_ THD+N vs Output Power (8 ohm, 20 dB gain, MW SE-600-36, 1 kHz, 20 kHz BW).png
    Modulus-686 PROTO_ THD+N vs Output Power (8 ohm, 20 dB gain, MW SE-600-36, 1 kHz, 20 kHz BW).png
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  • Modulus-686 PROTO_ THD+N vs Output Power (4 ohm, 20 dB gain, MW SE-600-36, 1 kHz, 20 kHz BW).png
    Modulus-686 PROTO_ THD+N vs Output Power (4 ohm, 20 dB gain, MW SE-600-36, 1 kHz, 20 kHz BW).png
    50.9 KB · Views: 8,442
  • Modulus-686 PROTO_ IMD 917 Hz + 5.5 kHz @ 1 W, 8 ohm.png
    Modulus-686 PROTO_ IMD 917 Hz + 5.5 kHz @ 1 W, 8 ohm.png
    39.6 KB · Views: 8,342
  • Modulus-686 PROTO_ THD+N vs Output Power, Power Supply (8 ohm, 1 kHz, 20 kHz BW).png
    Modulus-686 PROTO_ THD+N vs Output Power, Power Supply (8 ohm, 1 kHz, 20 kHz BW).png
    53.9 KB · Views: 8,600
  • MOD686_BlockDiagram.png
    MOD686_BlockDiagram.png
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  • MOD686_Prototype.jpg
    MOD686_Prototype.jpg
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Tom, to confirm, its $249 for 1 channel, so need to order 2, yes?

Yep. You'll need two for a stereo amp.

Tried to order but it says no shipping options available so I can't complete the order.

Argh!! What a horrible time for my website to act up. I'm sorry about that. Would you try to hit Reload in your browser when you view the cart and shipping calculator? If that does not fix it, please toss me an email (tomchr@neuro...)

The "No Shipping Available" happens sometimes after I install the latest bug fixes in the various code pieces on my website, which I did just the other day. Hitting Reload will usually fix it.

Unfortunately I am not able to reproduce the issue on my end at this time. If the issue persists after you hit Reload, please let me know. It's a lot more fun to run a business when your customers can buy stuff from you. :)

When reusing my Pwr86 supply, do I need bigger caps?

Nope. My measurement was done with the stock Power-86 and an Antek AN-5225 transformer.

You'll need a large heat sink for the MOD686. My test heat sink is 0.4 K/W and got quite toasty (60-65 ºC) during the torture test of the MOD686. You might be able to squeeze by with a 3U ModuShop/DIY Audio Store Dissipante series chassis. I'd feel more comfortable with a 4U. You can see their heat sink specs here: http://www.modu.it/CARATTERISICHE_TERMICHE_DISS.pdf

Will the PSSR and th CMRR similarly high as for the 286?

It should be about the same. Maybe slightly lower, but not meaningfully so.

Given that the 686 can provide up to 33A, does the board-side output connector have to be increased in its amp spec?

33 A peak is 23 A RMS, so the output connectors should be fine even if you decide to run the full current continuously. For music reproduction you'll have plenty of margin, even if you hit the 33 A peaks...

closer look seems to show only two caps, 6 chips, 2 inductors and 3 connectors are through hole. So it makes sense to just get the boards fully assembled. Just wondering though, lead free solder I suppose?

You are correct. Two inductors, two caps, three connectors, and six LM3886es are the thru-hole components. The board is ROHS compatible, so gold plated with lead-free solder.

You can wind the output inductor yourself. I'll give instructions for how to do that in the design doc. Or you can buy the inductor from me. I had a bunch custom wound. They were definitely not cheap, but man it sure is nice with a plug-n-play solution.

Tom
 
26 dB will be the default gain.

Tweakers can change the gain by changing a resistor. 20 dB is the minimum supported gain and is reached by removing the gain resistor.

I suppose some will want to know what the maximum is. For all practical purposes, there isn't much of a limit. You'll start to reduce the bandwidth of the amp somewhere around 50-60 dB. At some point the OPA1642 will run out of loop gain. The max gain is basically limited by the amount of noise you are willing to tolerate in your system.
I don't see any reason at all to go beyond 33-34 dB as that will allow a 900 mV source, such as a tablet or phone, to drive the amp to clipping. You better not have a finger slip when you're trying to turn up the volume in your music app.

I will always recommend to go with the lowest gain possible as that gives the best noise performance. Go with 20 dB if your source has enough oomph. 26 dB is a happy medium that gives good noise performance and allows common consumer line level to drive the amp to clipping.

Tom
 
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26 dB will be the default gain.

Tweakers can change the gain by changing a resistor. 20 dB is the minimum supported gain and is reached by removing the gain resistor.

I suppose some will want to know what the maximum is. For all practical purposes, there isn't much of a limit. You'll start to reduce the bandwidth of the amp somewhere around 50-60 dB. At some point the OPA1642 will run out of loop gain. The max gain is basically limited by the amount of noise you are willing to tolerate in your system.
I don't see any reason at all to go beyond 33-34 dB as that will allow a 900 mV source, such as a tablet or phone, to drive the amp to clipping. You better not have a finger slip when you're trying to turn up the volume in your music app.

I will always recommend to go with the lowest gain possible as that gives the best noise performance. Go with 20 dB if your source has enough oomph. 26 dB is a happy medium that gives good noise performance and allows common consumer line level to drive the amp to clipping.

Tom

My source provides 4VRMS, so I guess the 20dB option for me? Do I need to remove the resistor or can the 20dB option be ordered directly?

One further question. Do I need a soft start board for the 500VA or will the household circuit breaker be able to handle it?

Thx
 
............. Do I need a soft start board for the 500VA or will the household circuit breaker be able to handle it?...........
My answer is very definitely.
A 500VA drawing maximum power from a 220Vac supply will run on a T2.5A fuse.
But you don't run your transformer at maximum output. You will find it operates on a T2A fuse without blowing even when having a drunken party.

Relying on a big fuse (or distribution board MCB/fuse) increases massively the time that a fault could draw enormous current until the fuse/breaker opens.

It's that time delay that increases the risk of fire, especially if left unattended while you go and answer the phone, or hang out the washing, or etc.

A close rated fuse is safer. And that needs a soft start.
 
My source provides 4VRMS, so I guess the 20dB option for me? Do I need to remove the resistor or can the 20dB option be ordered directly?

If you let me know at the time you order (please via email AND the notes field on the order) I can certainly pluck those resistors off for you and deliver 20 dB boards for you. No additional charge. :)

One further question. Do I need a soft start board for the 500VA or will the household circuit breaker be able to handle it?

Your house circuit breaker will be fine. As Andrew points out, it's the rating of the fuse in the amp that you have to worry about. To make the fuse survive the inrush, you need to size it up a bit. This means it won't provide as good protection in case of "soft faults". I'm not overly concerned about that. If you want protection against soft faults (like arcing), you don't use a fuse anyway. You use an arc-fault interrupter or some other circuit. The fuse is to protect against the cases where wiring has frayed and shorted to ground or your supply caps have shorted out. For that, a fuse that's large enough to survive the inrush will work just fine.

In my view, the purpose of a soft start is to extend the life of the transformer. The loud hum you hear right when the transformer starts up is the primary stretching. Not by much, but enough that over 25-30 years it'll wear through the enamel on the wiring. Then the primary shorts out and the fuse blows.

A soft start board is definitely on my radar. For such a simple circuit, it's actually surprisingly hard to get it right. Many of the existing circuits out there drive low-voltage relay coils directly from the mains, which I really don't like. Others forget that the peak power dissipated in the inrush limiter (resistor or NTC) is enormous. Anyway. It's just one of these little five-component circuits that requires lots of thought and data sheet reading.

Another circuit on my radar is a speaker protection circuit. That one I'm further along with.

I use the size which was recommended for Tom’s Pwr86 supply, which has 2x22.000uF as main caps. So would that throw the circuit breaker?

It hasn't for me. The lighting circuits in my lab are on a 15 A breaker.

What about driving electrostats which impedance dips below 1 Ohm at 20kHz? Would that be a problem for either stability or performance of your amps?

I've tested the stability with 8 Ω || 1 uF. No issues. Usually what saves electrostatic speakers is that the impedance rises above 20 kHz, so it's not a pure capacitor.

It's that time delay that increases the risk of fire, especially if left unattended while you go and answer the phone, or hang out the washing, or etc.

I think that's overdramatizing things a bit.

I do agree that you should always have a fuse in the amp itself and that this fuse should be rated for the lowest current practically possible. This may mean that the fuse is rated for a higher current if you don't use a soft start than if you do.

In either case, the rating of the fuse will be lower than that of the circuit breaker in your household breaker panel. The fuse in the amp should also be a type where a wire melts and breaks rather than a thermal circuit breaker. There we certainly agree.
A typical glass fuse of the slow-blow/time-delay type will typically last a minute or so at 100 % overload (AFAIR). A typical thermal circuit breaker will take an hour to trip under those conditions. A glass fuse will blow within a few ms on a hard fault.

Always use a fuse. Get a fused IEC inlet. That makes things really easy. Use a slow-blow/time-delay fuse with as low a current rating as practically possible. This means the fuse should not blow when the amp is turned on and should not blow when the amp delivers the full rated power into the lowest specified load impedance.

Once I get a little further with the design documentation, I will make specific recommendations regarding the fuse rating with and without a soft start.

Tom
 
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Another circuit on my radar is a speaker protection circuit. That one I'm further along with.
Don't suppose you'd hazard a guess as to the likely size of the board (or tentative timeline :)) for this, would you, Tom?

I'm finally building up my 2 x Mod-86 / Power-86 and have all parts but the enclosure. I will definitely order the protection board and would hate to find that my enclosure or layout doesn't easily accommodate retrofitting it.
 
If you let me know at the time you order (please via email AND the notes field on the order) I can certainly pluck those resistors off for you and deliver 20 dB boards for you. No additional charge. :)



Your house circuit breaker will be fine. As Andrew points out, it's the rating of the fuse in the amp that you have to worry about. To make the fuse survive the inrush, you need to size it up a bit. This means it won't provide as good protection in case of "soft faults". I'm not overly concerned about that. If you want protection against soft faults (like arcing), you don't use a fuse anyway. You use an arc-fault interrupter or some other circuit. The fuse is to protect against the cases where wiring has frayed and shorted to ground or your supply caps have shorted out. For that, a fuse that's large enough to survive the inrush will work just fine.

In my view, the purpose of a soft start is to extend the life of the transformer. The loud hum you hear right when the transformer starts up is the primary stretching. Not by much, but enough that over 25-30 years it'll wear through the enamel on the wiring. Then the primary shorts out and the fuse blows.

A soft start board is definitely on my radar. For such a simple circuit, it's actually surprisingly hard to get it right. Many of the existing circuits out there drive low-voltage relay coils directly from the mains, which I really don't like. Others forget that the peak power dissipated in the inrush limiter (resistor or NTC) is enormous. Anyway. It's just one of these little five-component circuits that requires lots of thought and data sheet reading.

Another circuit on my radar is a speaker protection circuit. That one I'm further along with.



It hasn't for me. The lighting circuits in my lab are on a 15 A breaker.



I've tested the stability with 8 Ω || 1 uF. No issues. Usually what saves electrostatic speakers is that the impedance rises above 20 kHz, so it's not a pure capacitor.



I think that's overdramatizing things a bit.

I do agree that you should always have a fuse in the amp itself and that this fuse should be rated for the lowest current practically possible. This may mean that the fuse is rated for a higher current if you don't use a soft start than if you do.

In either case, the rating of the fuse will be lower than that of the circuit breaker in your household breaker panel. The fuse in the amp should also be a type where a wire melts and breaks rather than a thermal circuit breaker. There we certainly agree.
A typical glass fuse of the slow-blow/time-delay type will typically last a minute or so at 100 % overload (AFAIR). A typical thermal circuit breaker will take an hour to trip under those conditions. A glass fuse will blow within a few ms on a hard fault.

Always use a fuse. Get a fused IEC inlet. That makes things really easy. Use a slow-blow/time-delay fuse with as low a current rating as practically possible. This means the fuse should not blow when the amp is turned on and should not blow when the amp delivers the full rated power into the lowest specified load impedance.

Once I get a little further with the design documentation, I will make specific recommendations regarding the fuse rating with and without a soft start.

Tom

Thanks for all the info.

As I want upgrade my two 286 to 686 I can now start ordering the 500VA, throw out the 300VA, possibly replace the 3A SB fuse in the IEC inlet and then order the 20dB 686 boards. BTW thanks for the option to take out the gain resistor. Actually I would get less components for the same price ;-)