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

I've noted that there was a bracket revision on the 686 recently. It lit looks to be a nice simplification. Are there any circuit changes that you might have implemented with the latest batch of boards and its effect on performance?

Yep. I made some improvements in the input section, which lowered the noise and THD+N by about 2 dB. I also improved the 1+5.5 kHz IMD at 1 W.

Will there be a notification on when the sale price of the assembled 686 will end? Perhaps some kind of notification might sway some who might have been sitting on the fence.

As I pointed out in my most recent newsletter, the sale pricing will go away once I have modules in stock. My current estimate for when that will happen is on November 18. It could be a few days before or a few days after.
The sale is my reward to those who are willing to support me by helping to put a dent in the rather large credit card bill before it comes due.

If you're on the fence, you may want to get off the fence and onto a computer to place your order. :)

Also note that since you're in Canada, I can take payment via eTransfer in CAD at the current exchange rate. That'll save you a 3-4% exchange fee, plus you get a better exchange rate. Just toss me an email if that has your interest. You can do that through the Contact Us form on my website or by adding @neurochrome.com to my user ID here.

One final question. I have a couple of spare 120V primary 500VA toroids with each one outputting 35+35 VAC secondaries. I was thinking that I could connect each primary 120V coil in series and thereby have 18 +18+18+18 VAC leads that could power the 2x 686.

That should give you about ±24 V rectified, which will give you 100 W into 8 Ω give/take. The 3U x 300 mm Dissipante or Mini Dissipante from ModuShop would be a good chassis for such an amp.
I also recommend using the Intelligent Soft Start (ISS). The ISS limits the inrush of the transformers, which extends the service life of your transformers and also prevents nuisance blows of the mains fuse. The ISS is currently on intro sale. Same deal as with the MOD686: Sale will end when boards arrive in stock (by early December in case of the ISS).

If that is possible, what estimates of output power could one expect and I imagine the driving low 2ohm impedance loads would be a cakewalk with 25-26 VDC rails.

Yep. That should work well.

Any consideration for Phoenix Contact PCB terminal blocks?

I originally went with connectors on the MOD686 because people requested them. They were tired of connecting/disconnecting the screw terminals during prototyping. But, humans being humans, the first question after I changed to connectors is, "why didn't you use screw terminals?" I can't make everybody happy all the time I guess. :)

Tom
 

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I originally went with connectors on the MOD686 because people requested them. They were tired of connecting/disconnecting the screw terminals during prototyping. But, humans being humans, the first question after I changed to connectors is, "why didn't you use screw terminals?" I can't make everybody happy all the time I guess. :)

Tom
Spring-cage, not screw terminals. https://www.phoenixcontact.com/onli..._P-15-03/c252a030-3764-4828-a692-d0ba1dd45a27

https://www.phoenixcontact.com/onli...Rsk5-d_QPMnfBo/#Z7_82KCHG41MGCF90QL9V1UF79NV5

What confuses me, though, is if people were tired of using screw terminals during prototyping, what else were they going to use? Re-soldering? Wire nuts???
 
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Spring-cage, not screw terminals. https://www.phoenixcontact.com/onli..._P-15-03/c252a030-3764-4828-a692-d0ba1dd45a27

https://www.phoenixcontact.com/onli...Rsk5-d_QPMnfBo/#Z7_82KCHG41MGCF90QL9V1UF79NV5

What confuses me, though, is if people were tired of using screw terminals during prototyping, what else were they going to use? Re-soldering? Wire nuts???
Alligator clips
https://www.amazon.ca/WGGE-WG-026-P...ocphy=9000792&hvtargid=pla-568154706689&psc=1
Built in fusing
 
What confuses me, though, is if people were tired of using screw terminals during prototyping, what else were they going to use? Re-soldering? Wire nuts???

Well, they wanted connectors so they could plug/unplug a few times during the build. I don't know if people build on a piece of plywood first and then move the amp to a chassis. Any other amplifier module on the market uses connectors. I'm sure OEMs prefer them as well. Recall, I'm not marketing exclusively to DIYers.

If you really really don't want the connectors, you can always solder wire directly to the bottom of the PCB. I don't recommend trying to remove the connectors as you'll likely destroy the PCB in the process.

Tom
 
Tom, how does the rev 1.1 measure up against the benchmark amp? Have you taken the numbers crown back?

We're neck-and-neck. The AHB-2 is lower noise. The 686 provides higher output power than each channel in the AHB-2. Benchmark doesn't specify IMD of any kind for the AHB-2.

Tom, I signed up for your newsletter months ago but I've never received a newsletter.

That's odd. Try signing up again and/or checking your spam folder.

You can sign up for my newsletter here: Neurochrome Newsletter
Traditionally I've averaged a newsletter every two months, so I won't drown you in spam.

Tom
 
Can you point me to a specific product data sheet from this category? I am unable to see specific specs anywhere -- I'm probably not familiar with their product organisation sitemap.

Are you referring to this one? 1700053 | Phoenix Contact, FRONT 2.5-V/SA10 5mm Pitch, 1 Way PCB Terminal Strip | RS Components
 
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Oh, I see what the problem is. The product filters are not persistent through the site URL, so even after I filter for spring-cage connections the link doesn't give the right search results.

Here are a few examples:

https://www.phoenixcontact.com/online/portal/us?uri=pxc-oc-itemdetail:pid=1904558&library=usen&tab=1
https://www.mouser.ca/ProductDetail/Phoenix-Contact/1904558?qs=71zq8cT40IRQNJUy17xiUQ==

https://www.phoenixcontact.com/online/portal/ca?uri=pxc-oc-itemdetail:pid=1933998&library=caen&tab=1
https://www.mouser.ca/ProductDetail/Phoenix-Contact/1933998?qs=6i0QExrCb7MiXKwjl/FnoQ==

If you search for product series ZFKDSA on Mouser you should get the results you're interested in. Wago also makes these, and as a matter of fact, I believe they are the ones who invented this style of terminal connector (they call it "CAGE CLAMP").
 
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I have a single ended pre I'd like to try with the Modulus 686. Is there any benefit to use Tom's Universal Buffer between the pre and the 686 to go from single ended to balanced rather than to just use a single-ended to pseudo differential cable?

I've used my amps with both a pseudo-differential cable and with XLR cables. I don't perceive any differences as long as the cables are short. Some (fewer than 1% I think) have had issues with hum when using pseudo-differential connections. That hum has gone away when converting to balanced signalling.

I think a lot depends on the complexity of the overall system. Stereo builds rarely have issues. Multi-channel builds seem to attract more issues, maybe because you get many more opportunities for ground loops.

Tom
 
That is the reason that I've gone balanced. There is ten channels feeding my Linkwitz LX521.4's. Four amps in each of two boxes and two amps in a third. Feeding that lot from an active crossover single ended was a recipe for hunt the hum. So went balanced from the get go. And have had no problems - plug it together and away you go.

Craig
 

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