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

I've decided I'm going to need more room to install speaker protection, so I've installed a pair of 32v switchers instead of transformers and capacitors. I'm getting approximately 200w into 8ohms, and 300w into 4ohms with this combination. I'm pushing the heat-sinks (and my dummy load :)) at this power into 4ohms I know, but I'm never going to go anywhere near 300 watts playing music at home.

Hopefully ill now have enough space to fit a pair of Guardian 686s when available.


Condolences Tom. I lost my Dad in very similar circumstances.
Dave.
 

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The same goes for the Guardian-86 :)

Yep. I wrapped up both projects late at night after receiving the news of my dad's stroke. I basically made use of the time (and chock/trauma energy) from I received the news until I could book airfare. So far, the lacking BOM is the only detail I missed, which is pretty remarkable.

Unfortunately, it leaves my builders dead in the water until I return. Sorry about that.

I've decided I'm going to need more room to install speaker protection, so I've installed a pair of 32v switchers instead of transformers and capacitors.

Nice! ±28-32 V is my favourite operating point with the MOD686. You get a good amount of output power with a thermal system that isn't quite unwieldy yet. Nothing wrong with the ±36 V if you really want to max things out, but it does push the thermal system a bit further. Physics and that... :)

Hopefully ill now have enough space to fit a pair of Guardian 686s when available.

That'd be cool. I seem to recall the board dimensions landing around 3.25 x 2.50 inches. I forget exactly where I ended up with that project, but I seem to recall being ~90 % done with the component placement, so the board size shouldn't change by much from this point.

The Guardian-686 is two Guardian-86 on one circuit board with provisions to operate the board in stereo, dual mono, and bridged configurations. The latter is what you'd use with the MOD686.
As with the Guardian-86, the Guardian-686 will feature both terminal blocks and holes for binding posts (at 0.750"/19mm pitch) located along edge of the board.

Tom
 
Nice one!

Do you perceive any audible differences with these SMPS’s compared to your previous PSU with torroids?


I've decided I'm going to need more room to install speaker protection, so I've installed a pair of 32v switchers instead of transformers and capacitors. I'm getting approximately 200w into 8ohms, and 300w into 4ohms with this combination. I'm pushing the heat-sinks (and my dummy load :)) at this power into 4ohms I know, but I'm never going to go anywhere near 300 watts playing music at home.

Hopefully ill now have enough space to fit a pair of Guardian 686s when available.


Condolences Tom. I lost my Dad in very similar circumstances.
Dave.
 
Nice one!

Do you perceive any audible differences with these SMPS’s compared to your previous PSU with torroids?
To be quite honest and to my relief I would say there was very little difference. If I was pushed to make a decision I would say the switchers sound better.
One plus though, my amplifier is now totality silent, as I was getting some mechanical hum from the toroids.
 
Would you recommend to use the guardian protecting circuit over not using any protection at all?

That really depends on your risk tolerance. The odds of the MOD686 misbehaving are very low, but not zero (as with any circuit). Should the MOD686 misbehave such that the output sticks to the supply rail, the supply will have enough oomph to destroy your speakers. In such a case, a protection circuit would likely save the speakers.

Whether you're comfortable with the risk is for you to decide.

My main concern with the various speaker protection circuits is that some of them impact the performance of the amplifier. That's why I designed the Guardian-86 (and soon -686) to provide as little impact on the signal as possible. I cannot measure or hear any impact of these circuits, so I'm comfortable recommending them for use with the Modulus series of amps.

Tom
 
hi Tom,

Just built up my ISS but I seem to be unable to measure any AC at its output..

Most of it seems to work though:
- latch switch works (i hear the relay latching)
- toggle switch works (i hear the relay latching)
- 5v output measures 5v
- led output works

I've configured the output for 230 VAC by putting the middle bridge in place. When using my multimeter I cannot read any AC output on the 4-pin output

Any ideas?

Thanks
 
You should see 230 V from pin 1 to pin 4 on the output connector. If the relay clicks and you don't see 230 V there, you probably have a measurement error.

I've noticed that some terminal blocks have the screws floating, so if you measure by touching the head of the screws, you'll see 0 V.

Have you tried connecting a known good load, such as an incandescent lightbulb to the output of the ISS (pin 1, 4)?

Tom
 
You should see 230 V from pin 1 to pin 4 on the output connector. If the relay clicks and you don't see 230 V there, you probably have a measurement error.

I've noticed that some terminal blocks have the screws floating, so if you measure by touching the head of the screws, you'll see 0 V.

Have you tried connecting a known good load, such as an incandescent lightbulb to the output of the ISS (pin 1, 4)?

Tom

I’ll measure pin 1 and 4 today and will also try connecting a light bulb.

The 4 pin output connector reads:
Pin 1: ac
Pin 2 and 3: 0
Pin 4: ac

So i was under the impression i should be able to find 230v on pins 1&2 and 3&4, so i guess that is incorrect?

As i have 4 SMPS’s to connect, do all 4 have to go to pin 1 and 4?

Thanks
 
The 4 pin output connector reads:
Pin 1: ac
Pin 2 and 3: 0
Pin 4: ac

That's exactly what you want in a 230 V system.

So i was under the impression i should be able to find 230v on pins 1&2 and 3&4, so i guess that is incorrect?

For 230 V operation, you'll find the AC mains on pins 1 and 4. For 120 V operation, you'll find the mains at pins 1-2 and 3-4. Recall, the 4-pin output connector is intended for connecting to a power transformer with two primary windings which need to be in parallel for 120 V operation and in series for 230 V operation.

As i have 4 SMPS’s to connect, do all 4 have to go to pin 1 and 4?

Correct. That will work on all mains voltages.

Sounds like I should expand the documentation a bit regarding how to connect the board. Thanks for mentioning this.

Tom
 
Tom,

Hope you don’t mind me asking a few more questions.

Do you know of any decent in-chassis ‘power strips/deviders’ that provide a decent solution to split the ac mains coming from the iss into 4 different ac leads for my smps’s?

I can think of many solutions, like:
- daisy chaining the smps’s and connecting the last one to the iss
- using wago 222 devices
- using barrier strip with jumper wire

But none of those sound like decent to me. I almost thought i would need to create a small pcb with some faston connectors for this, but i cannot imagine no good solution exists for this purpose. However i cannot find it.

Also, i seem to be really unable to attach the megafit connectors to my speakerwire. It was a longshot as the speakerwire was too thick (5.3 mm2), but even with soldering on the crimp terminal it would not go in to the connector, got stuck at the last mm or so.

So now i am thinking of the following:
- mod686 to guardian686: using molex precrimped leads
- guardian686 to my terminals: use my speaker cable attached to the terminal on the guardian686

It is a bit of a bummer to not be able to use my kinber 8tc between the mod686 and guardian 686 but the megafit connectors seem unforgiving.

The iss works by the way, perfect!
 
Do you know of any decent in-chassis ‘power strips/deviders’ that provide a decent solution to split the ac mains coming from the iss into 4 different ac leads for my smps’s?

The Mean Well SMPSes use JST connectors. You can get the matching female housings and pre-crimped leads at Digikey. Just google the part number mentioned in the Mean Well data sheet. I'd go that route. If you can get all four wires into the terminal blocks on the ISS, that's probably the best way to go. Alternatively, a barrier block or "euro terminals" (Mouser P/N 571-1776296-4 for example) would work. I'm not a fan of the Wago devices.
You can also daisy chain the supplies in groups of two and put two wires into the terminal blocks of the ISS.

But none of those sound like decent to me. I almost thought i would need to create a small pcb with some faston connectors for this, but i cannot imagine no good solution exists for this purpose.

That seems overkill. I'm also not a fan of faston/QC connectors. They're great if you have the right tools and never need to disconnect them. For DIY stuff I much prefer connectors with a lower insertion/removal force. If I was to make a PCB, I'd stick with the JST connectors and use pre-crimped leads.

So now i am thinking of the following:
- mod686 to guardian686: using molex precrimped leads
- guardian686 to my terminals: use my speaker cable attached to the terminal on the guardian686

Sounds good to me.

It is a bit of a bummer to not be able to use my kinber 8tc between the mod686 and guardian 686 but the megafit connectors seem unforgiving.

Even if you could make it fit, you'd have had issues with the bending radius on the heavy gauge cable. For the 15-20 cm of wire inside the chassis, there's really no reason to use a super heavy gauge wire. The AWG12 or AWG16 pre-crimped MegaFit leads are fine.

The iss works by the way, perfect!

Awesome!

Tom
 
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Tom, congratulations on your Power-686. Also congratulations on your change of heart about providing CRC snubbers for the transformer secondaries.

I think you and your customers will quickly discover that there is not a huge amount of unit-to-unit variability in transformer secondary leakage inductance, even though this is not a measured specification on the transformer datasheet. Thus all customers who optimize a CRC snubber for Antek transformer # ABCDE, will get almost identical results (on their Cheapomodo tester).

So, if you feel like it, you can maintain a two column list which shows customer-measured optimum snubbers. Column1 is transformer make+model, Column2 is optimum snubber for that transformer. It might save people a couple hours of lab work. More importantly, it might allow builders to order CRC components even before their transformer is delivered, secure in the knowledge that another Power-686 customer has determined the optimum component values. Call it a transformer database for that up-to-date feeling.

If you're interested in viewing the before-and-after-snubbing performance, I've discovered that a battery powered, handheld oscilloscope is just the perfect tool for the job. Look at the God-awful kludge Bob Cordell had to perform (Figure 16.10 in his book) to get something halfway useful with a conventional grounded scope. Whereas with a battery powered scope, the entire instrument is floating, independent of "ground", and you can perform differential measurements with ease. So the waveform you actually want to see (across the two AC inputs to the bridge rectifier) is easily probed even though neither of the two nodes is "ground". Word to the wise: get a scope whose bandwidth is at least 10 MHz, not because you need this high bandwidth to see snubbers in action (you don't), but instead because you need very high performance scope triggering. They don't build great triggering circuitry in low bandwidth handheld scopes.

Couple years ago I published an article in Linear Audio volume 10 (here is a link) whose Appendix contains a circuit analysis of transformer + CRC snubber behavior using Laplace Transforms and such. It derives a rule of thumb for CRC capacitor selection, as a function of the desired damping factor zeta (zeta=0.5/Q).

BTW I still have a small number of fully assembled & tested RingNot power supply boards, which include discrete diodes and CRC snubbers. It's function compatible to the Power-86 (little brother of Power-686), both are designed to drive two LM3886 chipamp boards. $50 for the stuffed, soldered, and tested board, which compares favorably to the price of bare PCBs for chipamp PSUs.

~
 
I haven't had a change of heart, actually. I still don't see any need for snubbers beyond the simple capacitor, which works with all transformers. But I do realize that some builders do see a need for fancier snubbers. I chose to add the two footprints necessary and optimize the routing so that I could cater to those builders as well.

You are correct that the CRC snubber will need to be optimized for the specific transformer and secondary wiring. Thus, I leave the optimization of the CRC snubber as an exercise for the advanced builders.

Tom
 
Tom,
I have mod86. Willing to get mod286, but cannot wait that long :), so I want to get mod686. The only concern I have is heat dissipation and power consumption.
For my daily normal use the heatsinks of my mod86 are warm.
Assuming I will replace mod86 with mod686 and keep same listening conditions (use same speakers, same listening levels) then, should I also expected the total heat generated from mod686 to be the same as from mod86? (heat will be balanced across 6 lm3886(mod686) instead of cumulated on one lm3886(mod86))?
Should I also assume that the power consumption for same listening conditions will be also the same (or almost the same), so that I can resue trafo (300VA) and caps 2x22000uF from my mod86 build?
 
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I want to get mod686. The only concern I have is heat dissipation and power consumption.
For my daily normal use the heatsinks of my mod86 are warm.
Assuming I will replace mod86 with mod686 and keep same listening conditions (use same speakers, same listening levels) then, should I also expected the total heat generated from mod686 to be the same as from mod86? (heat will be balanced across 6 lm3886(mod686) instead of cumulated on one lm3886(mod86))?

You'll get six times the idle dissipation, so on ±30 V rails, you'll get ~18-20 W instead of 3ish W per channel. If you leave your MOD86 amp on at idle (no music) for a few hours and the heat sinks are more than 2-3 ºC warmer than the room, I'd get larger heat sinks for the MOD686.

Should I also assume that the power consumption for same listening conditions will be also the same (or almost the same), so that I can resue trafo (300VA) and caps 2x22000uF from my mod86 build?

I'm assuming 2x22 VAC @ 300 VA here. That's a wee bit underpowered if you intend to run the amp to clipping. If you're not changing your listening level by much, the 300 VA and 2x22000 uF will probably be OK.

Tom